Month: August 2016

EKG – Supraventricular Arrhythmias

August 27, 2016 Uncategorized No comments , , , ,

Mechanism for Arrhythmias ("HIS DEBS" rule)

  • Hypoxia
  • Ischemia and irritability
  • Sympathetic stimulaiton
  • Drugs
  • Electrolyte disturbances
  • Bradycardia
  • Stretch

Arrhythmias that originate in the atria or the AV node, the supraventricular arrhythmias. Atrial arrhythmias can consist of a single beat or a sustained rhythm disturbance lasting for a few seconds or many years.

Atrial and Junctional Premature Beats

Screen Shot 2016-08-27 at 1.15.26 PMSingle ectopic supraventricular betas can originate in the atria or in the vicinity of the AV node. The former are called atrial premature beats (or premature atrial contractions, PACs) and the latter, junctional premature beats. These are common phenomena, neither indicating underling cardiac disease nor requiring treatment. They can, however, initiate more sustained arrhythmias.

An atrial premature beat can be distinguished from a normal sinus beat by the contour of the P wave and by the timing of the beat. Because an atrial premature beat originates at an atrial site distant from the sinus node, atrial depolarization does not occur in the usual manner, and the configuration of the resultant P wave differs from that of the sinus P waves. If the site of origin of the atrial premature beta is far from the sinus node, the axis of the atrial premature beat will also differ from that of the normal P waves. An atrial premature beat comes too early; that is, it intrudes itself before the next anticipated sinus wave.

With junctional premature beats, there is usually no visible P wave, but sometimes, a retrograde P wave may be seen. This is just like the case with the junctional escape beats seen with sinus arrest. What is the difference between a junctional premature beat and a junctional escape beat? They look exactly alike, but the junctional premature beat occurs early, prematurely, interposing itself into the normal sinus rhythm. An escape beat occurs late, following a pause when the sinus node has failed to fire.

Both atrial and junctional premature beats are usually conducted normally to the ventricles, and the resultant QRS complex is therefore narrow. Some times, an atrial premature beat may occur sufficiently early that the AV node will not have recovered (i.e., repolarized) from the previous conducted beat and will therefore be unable to conduct the atrial premature beat into the venticles. The EKG may then show only a P wave without an ensuing QRS complex. This beat is then termed blocked atrial premature contraction.

Atrial Fibrillation

In atrial fibrillation, atrial activity is completely chaotic, and the AV node may bebombarded with more than 500 impulses per minute. Whereas in atrial flutter a single constant reentrant circuit is responsible for the regular saw-toothed pattern on the EKG, in atrial fibrillation multiple reentrant circuits whirl around in totally unpredictable fashion. No true P waves can be seen. Instead, the baseline appears flat or undultates slightly. The AV node, faced with this extraordinary blitz of atrial impulses, allow only occasional impulses to pass through at variable intervals, genrating an irregularly irregular ventricular rate, usually between 120 and 180 beats per minute. However, slower or faster ventricular responses can often be seen.

This irregularly irregular appearance of QRS complexes in the absence of discrete P waves is the key to identifying atrial fibrillation. The wavelike forms that may often be seen on close inspection of the undulating baseline are called fibrillation waves.

Carotid message may slow the ventricular rate in atrial fibrillation, but it is rarely used in this setting because the diagnosis is usually obvious.

The Physical Examination – Heart

August 25, 2016 Cardiology, Clinical Skills, Differential Diagnosis No comments , , , , , , ,

The Splitting of Heart Sounds

While these events are occurring on the left side of the heart, similar changes are occuring on the right side, which involves the right atrium, tricuspid valve, RV, pulmonic valve, and pulmonary arteries. Right ventricular and pulmonary arterial pressures are significantly lower than corresponding pressures on the left side. Note that right-sided cardiac events usually occur slightly later than those on the left. Instead of a hearing a single heart sound for S2, you may hear two discernible components, the first from left-sided aortic valve closure, or A2, and the second from right-sided closure of the pulmonic valve, or P2.

The second heart sound, S2, and its two compnents, A2 and P2, are caused primarily by closure of the aortic and pulmonic valves, respectively. During inspiration, the right heart filling time is increased, which increases right ventricular stroke volume and the duration of right ventricular ejection compared with the neighboring left ventricle. This delays the closure of the pulmonic valve, P2, splitting S2 into its two audible components. During expiration, these two components fuse into a single sound, S2.

Of the two components of the S2, A2 is normally louder, reflecting the high pressure in the aorta. It is heard throughout the precordium. In contrast, P2 is relatively soft, reflecting the lower pressure in the pulmonary artery.

S1 also has two components, an earlier mitral and a later tricuspid sound. The mitral sound – the principal component of S1 – is much louder, again reflecting the higher pressures on the left side of the heart. It can be heard throughout the precordium and is loudest at the cardiac apex. Splitting of S1 does not vary with respiration.

Auscultation

Auscultation of heart sounds and murmurs is a pre-eminent skill that leads directly to important clinical diagnoses. The ACC and the AHA has deemed cardiac auscultation as "the most widely used method of screening for valvular heart disease." Review the six auscultatory areas in Figure 9-41, with the follwing caveats: 1) many authorities discourage designations such as "aortic area," because murmurs may be loudest in other areas, and 2) these areas do not apply to patients with cardiac dilatation or hypertrophy, anomalies of the great vessels, or dextrocardia.Screen Shot 2016-08-25 at 6.57.34 PM

Throughout your examination, take your time at each of the six auscultatory areas. Concentrate on each of the events in the cardiac cycle, listening carefully to S1, then S2, then other sounds and murmurs occurring in systole and diastole.

Known Your Stethoscope

It is important to understand the uses of both the diaphragm and the bell.

The diaphragm. The diaphragm is better for picking up the relatively high-pitched sounds of S1 and S2, the murmurs of aortic and mitral regurgitation, and pericardial friction rubs. Listen throughout the precordium with the diaphragm, pressing it firmly against the chest.

The bell. The bell is more sensitive to the low-pitched sounds of S3 and S4 and the murmur of mitral stenosis. Apply the bell lightly, with just enough pressure to produce an air seal with its full rim. Use the bell at the apex, then move medially along the lower sternal border. Resting the heel of your hand on the chest like a fulcrum may help you to maintain light pressure.

Identifying Systole and Diastole

To facilitate the correct identification of systole and diastole, as you auscultate the chest, palpate the right carotid artery in the lower third of the neck with your left index and middle fingers – S1 falls just before the carotid upstroke and S2 follows the carotid upstroke.

Identifying Heart Murmurs

Timing

First decide if you are hearing a systolic murmur, falling between S1 and S2, or a diastolic murmur, falling between S2 and S1. Palpating the carotid pulse as you listen can help you with timing. Murmurs that coincide with the carotid upstroke are systolic.

Location of Maximal Intensity

This is determined by the site where the murmur originates. Find the location by exploring the area where you hear the murmur. Describe where you hear it best in terms of the intercostal space and its proximity to the sternum, the apex, or its measured distance from the midclavicular, midsternal, or one of the axillary lines.

Radiation for Transmission from the Point of Maximal Intensity

This reflects not only the site of origin but also the intensity of the murmur, the direction of blood flow, and bone conduction in the thorax. Explore the area around a murmur and determine where else you can hear it.

Intensity

This is usually graded on a six point and expressed as a fraction. The numerator describes the intensity of the murmur whereever it is loudest; the denominator indicates the scale you are using. Intensity is influenced by the thickness of the chest wall and the presence of intervening tissue.

Screen Shot 2016-08-25 at 7.57.09 PMPitch

This is categorized as high, medium, or low.

Quality

This is described in terms such as blowing, harsh, rumbling, and musical.

Hemostasis Mechanism – Platelet Structure and Function

August 24, 2016 Cardiology, Hematology No comments , , , , , , , , , , , , , , , , , , , , , ,

Platelet Granules and Organelles

Platelets possess secretory granules and mechanisms for cargo release to amplify responses to stimuli and influence the surrounding environment. Platelet granule structures include 𝛼- and dense granules, lysosomes, and peroxisomes. 𝛼-Granules and the dense bodies are the main secretory granules that release cargo (e.g., fibrinogen and adenosine diphosphate [ADP]) upon platelet activation.

Platelet granule secretion begins with a dramatic increase in platelet metabolic activity, set off by a wave of calcium release and marked by increased adenosine triphosphate (ATP) production. After platelet stimulation by agonists, a "contractile ring" develops around centralized granules, the granules fuse with the surface membranes, and then they extrude their contents. Granule secretion in platelets is a graded process that depends on the number, concentration, and nature of the original stimulus/stimuli, either strong (e.g., thrombin and collagen) or weak (e.g., ADP and epinephrine).

𝛼-Granules

What the 𝛼-granules have include:

  • 𝛽-thromboglobulin
  • PF4
  • thrombospondin
  • vWF
  • Fibrinogen
  • Other plasma proteins (small amount)
  • Growth factors
  • 𝛼IIb𝛽3 [platelet receptor]

𝛼-Granules, with a cross-sectional diameter of approximately 300 nm and numbering approximately 50 per platelet, are the predominant platelet granules. They are approximately spherical in shape, with an outer membrane enclosing two distinct intragranular zones that vary in electron density. The larger, electron-dense region is often eccentrically placed and consists of a nucleoid material that is rich in platelet-specific proteins such as 𝛽-thromboglobulin. The second zone, of lower electron density, lies in the periphery adjacent to the granule membrane and contains tubular structures with adhesive GPs such as von Willebrand factor (vWF) and multimerin, along with factor V. Platelets take up plasma proteins and store them in their 𝛼-granules.

Three proteins, 𝛽-thromboglobulin, PF4, and thrombospondin, are synthesized in megakaryocytes and highly concentrated in 𝛼-granules. The first two, 𝛽-thromboglobulin and PF4, show homology in amino acid sequence and share the additional features of localization in the dense nucleoid of 𝛼-granules, heparin-binding properties, and membership in the CXC family of chemokines. Together, they constitute approximately 5% of total platelet protein, and they can serve as useful markers for platelet activation in serum or plasma. Thrombospondin may comprise up to 20% of the total platelet protein released in response to thrombin, and likely participates in multiple biologic prcesses.

vWF is also synthesized by megakaryocytes and is present in the tubular structures of the 𝛼-granule peripheral zone, similar to its localization within Weibel-Palade bodies of vascular endothelial cells. Factor V and multimerin, a factor V/Va-binding protein, co-localize with vWF in platelets but not in endothelial cells. Fibrinogen is also found in 𝛼-granules, but is incorporated actively from plasma and not synthesized by megakaryocytes. In fact, small amounts of virtually all plasma proteins, such as albumin, immunoglobulin G (IgG), fibronectin, and 𝛽-amyloid protein precursor, may be taken up into the platelet 𝛼-granules. 𝛼-Granules also contain many growth factors, including platelet-derived growth factor, transforming growth factor-𝛽1 (TGF-𝛽1), and vascular endothelial growth factor. These signaling molecules may contribute to the mitogenic activity of platelets.

Platelet 𝛼-granules serve as an important reservoir for 𝛼(IIb)𝛽3 that contributes significantly to the surface fibrinogen receptors present on activated platelets. The 𝛼-granule membrane protein, P-selectin (granule membrane protein-140) is translocated to the plasma membrane after platelet activation. Finally, a number of additional proteins have been located to the surface of 𝛼-granules alone, including CD9, platelet endothelial cell adhesion molecule-1 (PECAM-1), Rap 1b, GPIb-IX-V, and osteonectin.

The platelets and megakaryocytes of patients with gray platelet syndrome have decreased numbers of 𝛼-granules and reduced levels of some proteins. It is proposed that there is incorrect targeting of 𝛼-granule proteins to the 𝛼-granule in the megakaryocyte in this disease.

Dense Bodies

Dense bodies contain a large reservoir of ADP, a crtitical agnoist for platelet activation that amplifies the effect of other stimuli. In addition to this nonmetabolic pool of ADP, the  dense bodies are rich in ATP, pyrophosphate, calcium, and serotonin (5-hydroxytryptamine), with lesser amounts of guanosine triphosphate (GTP), guanosine diphosphate (GDP), and magnesium. The adenine nucleotides are synthesized and segregated by megakaryocytes, whereas serotonin is incorporated into dense granules from the plasma by circulating platelets. There is more ADP than ATP in dense bodies, and both can lead to adenosine monophosphate (AMP). In turn, AMP can be dephosphorylated to adenosine or cyclized to produce cyclic AMP, an inhibitor of the platelet-stimulatory response. The dense granule membrane contains P-selectin and granulophysin.

Compared ADP/ATP within the metabolic/cytoplasmic pool (at least two different pools: metabolic pool and cytoplasmic pool), ADP/ATP in storage pool (dense bodies) contains approximately two-thirds of the total platelet nucleotides, mainly in the form of ADP and ATP, and is metabolically inactive, does not rapidly incorporate exogenous adenine or phosphate, and equilibrates slowly with the metabolic pool. Nucleotides in this pool (storage pool) are extruded fromt the platelet during the release reaction and cannot be replenished after release.

The ATP (metabolic pool) that is broken down to provide energy for the release reaction is not rephosphorylated; rather, it is irreversibly degraded to hypoxanthine, which diffuses out of the cell.

Lysosomes

Lysosomes are small, acidified vesicles, approximately 200 nm in diameter, that contain acid hydrolases with pH optima of 3.5 to 5.5, including 𝛽-glucuronidase, cathepsins, aryl sulfatase, 𝛽-hexosaminidase, 𝛽-galactosidase, heparitinase, and 𝛽-glycerophosphatase. Additional protein found in lysosomes include cathepsin D and lysosome-associated membrane proteins (LAMP-1/LAMP-2, which are expressed on the plasma membrane after activation). Lysosomal constituents are released more slowly and incompletely (maximally, 60% of the granules) than 𝛼-granules or dense-body components after platelet stimulation, and their release also requires stronger agonists such as thrombin or collagen.

Organelles

Peroxisomes are rare, small granules, demonstrable with alkaline diaminobenzidine as a result of their catalase activity. The structure may participate in the synthesis of platelet-activating factor.

Mitochondria in platelets are similar, with the exception of their smaller size, to those in other cell types. There are approximately seven per human platelet, and they serve as the site for the actions of the respiratory chain and the citric acid cycle. Glycogen is found in small particles or in masses of closely associated particles, playing an essential role in platelet metabolism.

Platelet Kinetics

Approximately one-third of the total platelet mass appears to pool in the spleen. The splenic pool exchanges freely with the platelets in the peripheral circulation. Administration of epinephrine, which evacuates platelets from the spleen, increases the peripheral platelet count 30% to 50%, and platelet counts in asplenic patients are not affected by epinephrine. Some studies suggest that the splenic pool consists of the youngest, largest platelets. Pathophysiologi states can result in 80% to 90% of platelets being sequestered in the spleen, resulting in thrombocytopenia.

Other organs that have pool of platelets (accounting for about 16% of total platelets) include the lungs and the liver and so on.

The life span of platelet has been estimated to be 8 to 12 days in humans. In steady state, when platelet production equals destruction, platelet turnover has been estmated at 1.2 to 1.5 x 1011 cells per day.

PS: Details of various platelet products can be found in thread "Platelet Transfusion for Patients w/ Cancer" at http://www.tomhsiung.com/wordpress/2013/04/platelet-transfusion-for-patients-with-cancer-part-one/


Platelet Adhesion and Activation

Part I – Adhesion

  • Adhesive ligands: vWF, collagen, fibronectin, thrombospondin, laminin (perphaps)
  • Platelet surface receptors: GPIb/V/IX complex, GPVI, 𝛼IIb𝛽3, ð›¼2𝛽1, ð›¼5𝛽1, ð›¼6𝛽1
  • Interaction of GPVI with collagen activates platelet intergrins
  • At low shear conditions, fibrinogen is the primary ligand (interacting with 𝛼IIb𝛽3), but other ligands may also be involved

Platelet adhesion to exposed subendothelium is a complex multistep process that involves a diverse array of adhesive ligands (vWF, collagen, fibronectin, thrombospondin, and perphaps laminin) and surface receptors (GPIb/V/IX, GPVI, integrins 𝛼IIb𝛽3, 𝛼2𝛽1, 𝛼5𝛽1, and 𝛼6𝛽1). The specific ligand/receptor palyers in primary platelet adhesion are largely dependent on the arterial flow conditions present. In high shear conditions, platelet tethering is dependent on the unique shear-dependent interaction between GPIb/V/IX and subendothelial vWF, derived either from plasma or released by endothelial cells and/or platelets. A tether forms between GPIb and vWF that either halts platelet movement or reduces it such that other interactions can proceed. Subsequent interactions are mediated by GPVI binding to glycineproline-hydroxyproline sites on collagen and perhaps to exposed laminin. The interaction of GPVI with collagen strongly activates platelets such that 𝛼IIb𝛽3, can engage in high-affinity interactions with ligands. At low shear conditions, fibrinogen is thought to be the primary ligand supporting platelet plug formation through its interaction with 𝛼IIb𝛽3, although thrombus formation can take place in the absence of vWF and fibrinogen, so other ligands may also be involved.

Following initial platelet adhesion, subsequent platelet-platelet interactions are intially mediated by two receptors, GPIb/V/IX and 𝛼IIb𝛽3, and their respective contributions are dependent on the flow conditions present. In high shear stress conditions, GPIb/V/IX receptor and vWF ligand action are predominant, with fibrinogen playing a stabilizing role.

Platelet Glycoprotein Ib Complex-von Willebrand Factor Interaction and Signaling

Screen Shot 2016-08-18 at 4.01.30 PMThe interaction of the platelet GPIb "complex" (the polypeptides GPIb𝛼, GPIb𝛽, GPIX, and GPV) with its primary ligand. vWF, is the receptor-ligand pairing that initiates platelet adhesion followed by a cascade of events leading to pathologic thrombosis or physiologic hemostasis. A unique aspect of this receptor-ligand interaction is that it requires the presence of high arterial shear rates to take place, thus explaining the predisposition of platelet-rich "white clots" in the arterial circulation over clots found in the venous circulation, with its relatively lower shear forces, in which clot formation takes place independent of the GPIb complex.

The binding site for vWF is present in the N-terminal 282 residues of GPIb. Important to the interactions are a cluster between residues Asp 252 and Arg 293 containing sulfated tyrosine residues and important anionic residues, a disulfide loop between Cys 209 and Cys 248, and an N-terminal flanking sequence of the leucine-rich repeats (LRG). Mutations involving single amino acid residues within these LRGs account for some cases of the congential bleeding disorder Bernard-Soulier syndrome, in which the GPIb complex binds poorly, or not at all, to vWF.

Unlike other receptors, GPIb does not require platelet activation for its interactions with vWF. In vitro, the interactions of vWF and binding with the GPIb complex occur with generally very low affinity in the absence of shear. The presence of the vancomycin-like antibiotic ristocetin or viper venom proteins, such as botrocetin, promotes the interactions. Mobilization may uncoil vWF to promote interactions with GPIb. The addition of shear, in a parallel-plate flow system, results in platelet interaction with subendothelial vWF that occurs in a biphasic fashion. Likewise, the rate of translocation of platelets from blood to the endothelial cell surface, which is dependent, increases linearly up to wall shear rates of 1,500 s-1, whereas the translocation rate remains relatively constant with the wall shear rate between 1,500 and 6,000 s-1. Thus, the presence of shear is important for promoting the interactions between the GPIb complex and vWF. Studies of real-time thrombus formation in the absence of platelet GPIb complex and in blood from individuals with severe (type 3) von Willebrand disease indicate that GPIb and vWF interaction are required for platelet surface interaction at high shear rates (>1,210 s-1), whereas GPIb deficiency results in poor platelet adhesion at lower shear. Shear accelerates thrombus formation likely by promoting this receptor-ligand interaction.

When the GPIb complex interacts with its vWF ligand under conditions of elevated shear stress, signals are initiated that activate integrin 𝛼IIb𝛽3. The pathways involved lead to a) elevation of intracellular calcium; b) activation of a tyrosine kinase signaling pathway that incorporates nonreceptor tyrosine kinases such as Src, Fyn, Lyn, and Syk, phospholipase C(gamma)2, and adaptor protein such as SHC, LAT, and SLP-76; c) inside-out signaling through the 𝛼IIb𝛽3 integrin followed by platelet aggregation; and d) activation of protein kinase C (PKC), protein kinase G (PKG), and phosphoinositide 3-kinase (PI3K), …… and so on.

Once vWF binds to GPIb-V-IX, signaling complexes form in the vicinity of the GPIb𝛼 cytoplasmic tail consisting of cytoskeletal proteins such as 14-3-3ζ  as well as signaling protein like Src and PI 3-kinase. This process leads to Syk activation, protein tyrosine phosphorylation, and recruitment of other cytoplasmic proteins with pleckstrin homology domains that can support interactions with 3-phosphorylated phosphoinositides and ultimately activation of integrin 𝛼IIb𝛽3.

Glycoprotein Ib Complex Interaction with Thrombin and Other Molecules

The GPIb complex serves as an 𝛼-thrombin binding site on platelets, although the physiologic relevance of the interactions is not clear. The density of GPIb complexes (~20,000/platelet) far exceeds the number of thrombin binding sites reported on platelets (~6,000/platelet). Studies have identified interaction of the GPIb complex with ligands other than vWF. These include a study a reversible association of GPIb with P-selectin, which is examined in more detail in the section "Platelets and Endothelium." The interaction of platelet GPIb with the neutrophil adhesion receptor 𝛼(M)𝛽2 (Mac-1) is discussed in the section "Platelets and White Blood Cells." Additionally, GPIb reportedly interacts with high-moecular weight kininogen, factor XII, and factor XI.

Platelet-Collagen Interaction and Signaling

  • Receptors: GPVI, ð›¼2b𝛽1
  • Ligands: collagens

Collagens, one of the most thrombogenic substance in vessels, are very important activators of platelets in the vascular subendothelium and vessel wall, and thus are prime targets for therapeutic intervention in patients experience a pathologic arterial thrombotic event such as MI or stroke. Platelets have two major surface receptors for collagen, the immunoglobulin superfamily member GPVI and the integrin 𝛼2𝛽1. The former is considered to be the primary palyer in platelet adhesion. In additon to these two surface receptors, the GPIb complex can also be considered an indirect collagen receptor because its subendothelial vWF ligand essentially acts as bridging molecule between platelets and collagen by fixing itself to the latter, which, in turn, acts as scaffolding for the multimers. Collagen adhesion also results in indirect activation of the protease-activated receptor 1 via MMP-1. Other molecules, such as CD 36, may also sustain collagen interaction.

Glycoprotein VI receptor

GPVI is the main receptor involved in collagen-mediated platelet activation. Studies of mice lacking platelet GPVI show that they lose collagen-induced platelet activation due to a defect in platelet adhesion. Thus, GPVI appears to serve as teh initial receptor involved in platelet adhesion, and it activates integrin binding. GPVI alone supports adhesion to insoluble collagens, and works with 𝛼2𝛽1 to promote platelet adhesion to soluble collagen microfibrils. GPVI can also be engaged by collagen-related peptides (arranged in triple helical structures with sequences similar to collagen) and teh snake venom convulxin, which elicit signals through GPVI.

Synergism between GPVI pathways and those related to other adhesion receptors such as GPIb-V-IX and soluble agonists released by activated platelets are likely necessary for the full repertoire of platelet-collagen signaling. Exposure of platelets to collagen surfaces likely results in GPVI clustering that in turn triggers the tyrosine phosphorylation of the FcRγ chain. The GPVI/FcRγ-chain complex leads to platelet activation through a pathway that has many aspects in common with signaling by immune receptors, such as the Fc receptor family and the B- and T-cell antigen receptors.

α2β1 receptor

The first platelet collagen receptor identified was the integrin 𝛼2𝛽1 receptor, also known as platelet GPIa/IIa and lymphocyte VLA-2.

When compared to vWF, collagen is a more efficient substrate when it comes to supporting stable platelet adhesion and thrombus formation. The fact that initial platelet tethering to collagen under high shear flow first requires interaction between vWF and platelet GPIb serves to underscore the importance of the two major collagen receptors, GPVI and 𝛼2𝛽1, in promoting platelet adhesion and activation under shear conditions.

In addition to GPVI, the α2β1 receptor also propagates signaling. The use of α2β1 selective ligands has demonstrated calcium-dependent spreading and tyrosine phosphorylation of several proteins when interaction with platelets takes place.

Physiologic Inhibition of Platelet Adhesion

Negative regulation of platelets is essential to set the stimulus threshold for thrombus formation, determine final clot size and stability, and prevent uncontrolled thrombosis. The mechanisms behind the negative regulation of platelet activation are described later, and in this respect, roles of players such as nitric oxide and prostacyclin have been well characterized. Platelet activation can also be inhibited by signaling through the adhesion moleculde PECAM-1 (CD31). Expressed on a number of blood cells and endothelial cells, PECAM has a wide array of regulatory functions in processes such as apoptosis and cell activation. Following homophilic interactions and/or clustering, PECAM-1 is tyrosine phosphorylated in its cytoplasmic tail ITIM domain. Phosphorylation of PECAM-1 recruits and activates the SH2 domain-containing protein-tyrosine phosphatase, SHP-2. Studies suggest that the PECAM-1/SH-2 complex functions to counteract platelet activating, most particularly for collagen by inhibiting GPVI/FcRγ chain signaling.


Part II – Activation

PAR Thrombin Interactions

  • See Figure 16.9

Platelet thrombin receptors/platelet protease-activated receptors/PARs  and signaling

Screen Shot 2016-08-21 at 12.46.55 PMPARs are G-protein-coupled receptors that use a unique mechanism to convert an extracellular protein cleavage event into an intracellular activation signal. In this case, the ligand is already part of the receptor per se, by virtue of the fact that it is represented by the amino acid sequence SFLLRN (residues 42 through 47) and is unmashed as a new amino terminus after thrombin cleaves the peptide bond between Arg 41 and Ser 42 (Figure 16.9). This "tethered ligand" then proceeds to irreversibly dock with the body of its down receptor to effect transmembrane signaling, as shown in Figure 16.9.

Thrombin signaling in platelet is mediated, at least in part, by four members of a family of G-protein-coupled PARs (PAR-1, -2, -3, and -4). Human platelets express PAR-1 and PAR-4, and activation of either is sufficient to trigger platelet aggregation. PAR-1, -3, and -4 can be activated by thrombin, whereas PAR-2 can be activated by trypsin, tryptase, and coagulation factors VIIa and Xa. Presumably, other proteases are capable  of recognizing the active sites of these receptors and can thus also trigger PAR signaling.

Once activated, PAR-1 is rapidly uncoupled from signaling and internalized into the cell. It is then transported to lysosomes and degraded. Platelet presumably have no need for a thrombin receptor recycling mechanism, becuase once activated, they are irreversibly incorporated into blood clots. Conversely, in cell lines with characteristics similar to megakaryocytes, new protein synthesis is needed for recovery of PAR-1 signaling, and in endothelial cells, sensitivity to thrombin is maintained by delivery of naive PAR-1 to the cell surface from a preformed intracellular pool.

Platelet ADP (Purinergic) Receptors and Signaling

  • P2Y1
  • P2Y12
  • P2X

Evidence that ADP plays an important role in both the formation of the platelet plug and the pathogenesis of arterial thrombosis has been accumulating since its initial characterization in 1960 as a factor derived from red blood cells that influences platelet adhesion. ADP is present in high concentratons (molar) in platelet-dense granules and is released when platelet stimulation takes place with other agonists, such as collagen; thus, ADP serves to further amplify the biochemical and physiologic changes associated with platelet activation and aggregation. Inhibitors of this ADP-associated aggregation include commonly used clinical agents, including ticlopidine, clopidogrel, prasugrel, and ticagrelor, proven to be very effective antithrombotic drugs.

Adenine nucleotides interact with P2 receptors that are ubiquitous among different cell types and have been found to regulate a wide range physiologic processes. They are divided into two groups, the G-protein-coupled superfamily named P2Y and the ligand-gated ion channel superfamily termed P2X. Two G-protein-coupled (P2Y) receptors contribute to platelet aggregation. The P2Y1 receptor initiates aggregation through mobilization of calcium stores, and the P2Y12 receptor is coupled to inhibition of adenylate cyclase and is essential for a full aggregation response to ADP with stabilization of the platelet plug.

PS: ADP >>> P2Y12 >>> inhibition of adenylate cyclase >>> decreased cAMP production >>> decreased intensity of aggregation

Inhibition of either P2Y1 or P2Y12 receptors is sufficient to block ADP-mediated platelet aggregation, and coactivation of both receptors is therefore necessary, through the G proteins Gq and Gi, respectively, for ADP to activate and aggregate the platelet.

Although considered a weak agonist in comparison to collagen or thrombin, ADP clearly palys an important role in thrombus stabilization, likely by contributing to the recruittment of additional platelets to growing thrombi. Aggregation is often reversible when platelets are stimulated by ADP alone. In addition, low concentrations of ADP serve to amplify the effects of both strong and weaker agonists, the latter inlcuding serotonin and adrenaline, among others.

Platelet Activation by Soluble Agonists

Epinephrine

Epinephrine is unique among platelet agonists because it is considered to be  capable of stimulating secretion and aggregation, but not cytoskeletal reorganization responsible for shape change. Platelet responses to epinephrine are mediated through 𝛼2-adrenergic receptors, and these responses have been found to vary among individuals, with some donors with otherwise normal platelets manifesting delayed or absent responses.

Arachidonic acid, thromboxane A2, and thromboxane receptors

After platelet stimulation by a number of agonists, arachidonic acid is generated directly by phospholipase A from its membrane phospholipid precursors (PC, PS, and PI) and indirectly by PLC generation of DAG followed by DAG lipase action. Most platelet agonists are believed to activate this pathway. Three known eicosanoid subsetsl of biochemical compounds are known to be derived from the formation of arachidonic acid – the prostanoids, leukotrienes, and epoxides. Although all three of these pathways are present in platelets, most arachidonic acid ends up being metabolized to thromboxane A2 (TxA2).

TxA2 is produced in platelets from arachidonic acid through the generation of PGH2 by the enzyme cyclo-oxygenase, which is irreversibly inhibited by aspirin through acetylation of a serine residue near its C terminus. PGE2 and PGI2 act to inhibit platelet activation by generating intracellular cAMP, whereas TxA2 activates platelets. Platelets primarily synthesize thromboxane, and endothelial cells mainly synthesize prostaglandins such as PGI2.

Like ADP and epinephrine, TxA2 is also capable of activating nearby platelets after its release into plasma. It has a very short half-life of 30 seconds before its conversion to the inactive metabolite thromboxane B2 prevents widespread platelet activation beyond the vicinity of thrombus formation. Both arachidonic acid and analogs of TxA2 have been found to activate and aggregate platelets by mediating shape change and phosphorylation of signaling enzymes. The thromboxane receptor (TP) is a member of the seven-transmembrane G-protein-coupled receptor family and has been localized to the plasma membrane. Two isoforms of the receptor have been identified in platelets TP𝛼 and TP𝛽 – and they activate platelets through ghe Gq pathway.

Physiologic Inhibition of Platelet Activation

One of the many remarkable features of platelets is their ability to remain in a physiologic resting state and resist becoming activated while navigating the heart, arterial, and venous circulations. Indeed, the pathologic consequences associated with widespread inappropriate platelet activation are life- and limb-threatening in the settings of well-characterized clinical disorders, such as thrombotic thrombocytopenic purpura and heparin-induced thrombocytopenia. The mechanisms responsible for maintaining the fine balance of keeping platelets in a resting state until they encounter a genuine need  to undergo adhesion, activation, and aggregation at the site of vascular injury are nearly as diverse as those responsible for mediating these physiologic phenomena.

Some general mechanisms involved in physiologic inhibition of platelet activation include phenomena such as a) generation of negative-regulating molecules by the platelet (e.g., cAMP), endothelium (e.g., PGI2, nitric oxide, heparan sulfate), and at distant sites (e.g., antithrombin); b) barrier of endothelial cells that prevents direct contact of circulating platelets with collagen; c) ecto-ADPase (CD39) expression by endothelial cells that metabolizes ADP secreted from platelets; d) tendency for blood flow to wash away unbound thrombin and other soluble mediators from the site of platelet plug formation; e) brief half-life of certain key platelet activators such as TxA2; f) tight regulation of the affinity state of receptors such as 𝛼IIb𝛽3; g) downregulation of signaling receptors to limit their actions; and h) inhibitory pathways mediated by ITIM-containing and/or contact-dependent adhesion receptors, such as PECAM, CECAM-1, JAM-A, and potentially others.

Receptor downregulation and desensitization

Signaling through G-protein-coupled receptors present on the surface of platelets is limited by their phosphorylation, which triggers desensitization, that is, uncoupling from G proteins, and internalization via Claritin-mediated endocytosis (for detail about G-protein coupled receptors please refers to thread "G Protein-Coupled Receptors and Second Messengers" at http://www.tomhsiung.com/wordpress/2014/09/g-protein-coupled-receptors-and-second-messengers/). G-protein kinases and 𝛽-arrestin are central to these processes. In addition, G-protein-coupled receptors interact with a myriad of other molecules that finely tune their signaling, including regulators of G-protein signaling (RGS) and GPCR-associated sorting proteins.

Inhibitory prostaglandins

Generation of the prostaglandins from arachidonic acid metabolism, such as PGI2 and PGE2 (at high concentrations), results in inhibition of platelet activation and aggregation, and counterbalances the actions of thromboxanes derived from the same pathway. While PGI2 and PGD2 inhibit platelet function at low doses, PGE2 displays a biphasic reponse, and inhibits platelet function only at higher concentrations, likely via the EP4 receptor. The inhibitory effects are mediated via G-protein-coupled receptors (IP and EP receptors, respectively) that couple to the 𝛼 subunits of Gs to regulate adenylate cyclase-mediated generation of cAMP. cAMP levels in platelets are also governed by the activity of phosphodiesterase, the enzyme responsible for cAMP metabolism. This enzyme activity is inhibited drugs such as the weak antiplatelet agent dipyridamole, the bronchodilator theophylline, and sildenafil, used to treat erectile dysfunction in men.

Nitric oxide

NO is generated by endothelial cells and platelets from L-arginine in response to shear stress forces and other platelet agonists, such as thrombin and ADP. The bulk of the evidence suggests that at high concentrations NO functions to inhibit platelet activation through the cyclic guanosine monophosphate (cGMP) second messenger generated by guanylate cyclase activation. Elevations in cGMP, by modulating phosphodiesterase activity, can raise intraplatelet cAMP. Paradoxically, low levels of NO may elicit platelet activation pathways. Endothelial NO synthase activity is enhanced during platelet activation, presumably as an additional means for limiting platelet aggregation.


Platelet Aggregation: 𝛼IIb𝛽3 (GPIIb/IIIa) Receptor and Its Signaling Mechanisms

Platelet aggregation is a complex phenomenon that is the end result of a series of adhesion- and activation-related processes. Essential components of this process include an agonist, calcium, and the adhesive proteins fibrinogen and vWF. Divalent cations, such as calcium and magnesium, are required for platelet aggregation in trace amounts, and these alter the specificity of the integrin ð›¼IIb𝛽3 for its ligands. Fibrinogen and vWF play dominant roles in platelet aggregation through binding to ð›¼IIb𝛽3, and also by the ability of the former to generate polymerized fibrin as support for the platelets in a thrombus.

The signaling pathways of 𝛼IIb𝛽3 are complex. Central concepts of the signaling pathway include inside-out signaling, which involves the processes termed affinity and avidity modulation, and outside-in signaling, in which messages are transmitted to the inside of the platelet via the events occurring outside the membrane through 𝛼IIb𝛽3 activation. Primary platelet agonists such as ADP, thrombin, and matrix proteins collagen and vWF affect platelet aggregation through a process known as inside-out signaling. In the inside-out signaling, agonist-dependent intracellular signals stimulate the interaction of key regulatory ligands (such as talin) with integrin cytoplasmic tails. This leads to conformational changes in the extracellular domain that result in increased affinity for adhesive ligands such as fibrinogen, vWF, and fibronectin. In the outside-in signaling, extracellular ligand binding, initially reversible, becomes progressively irreversible and promotes integrin clustering and further conformational changes that are transmitted to the cytoplasmic tail. This results in the recruitment and/or activation of enzymes, adaptors, and effectors to form integrin-based signaling complexes.


Brief Review of Physiology of Platelet

Following injury to the blood vessel, platelets interact with collagen fibrils in the exposed subendothelium by a process (adhesion) that involves, among other events, the interaction of a plasma protein ,vWF, and a specific glycoprotein (GP) complex on the platelet surface, GP Ib-IX-V (GPIb-IX). This interaction is particularly important for platelet adhesion under conditions of high shear stress. After adherence to the vessel wall via vWF and the long GP Ib-IX-V receptor, other platelet receptors interact with proteins of the subendothelial matrix. Hereby collagen provides not only a surface for adhesion but also serves as a strong stimulus for platelet activation.

Activated platelets release the contents of their granules (secretion), including ADP and serotonin from the dense granules, which causes the recruitment of additional platelets. These additional platelets form clumps at the site of vessel injury, a process called aggregation (cohesion). Aggregation involves binding of fibrinogen to specific platelet surface receptors, a complex composed of GPIIb-IIIa (integrin 𝛼IIb𝛽3), an integrin that normally exists in a resting (low-affinity) state but that transforms into an activated (high-affinity) state when stimulated by the appropriate signal transduction cascade. GPIIb-IIIa is platelet specific and has the ability to bind vWF as well. Although resting platelets do not bind fibrinogen, platelet activation induces a conformational change in the GPIIb-IIIa complex that leads to fibrinogen binding.

Moreover, platelets play a major role in coagulation mechanisms; several key enzymatic reactions occur on the platelet membrane lipoprotein surface. During platelet activation, the negatively charged phospholipids, especially PS, become exposed on the platelet surface, and essential step for accelerating specific coagulation reactions by promoting the binding of coagulation factors involved in thrombin generation.

A number of physiologic agonists interact with specific receptors on the platelet surface to induce responses, including a change in platelet shape from discoid to spherical, aggregation, secretion, and thromboxane A2 production. Other agonists, such as prostacyclin, inhibit these responses. Binding of agonists to platelet receptors initiates the production or release of several intracellular messenger molecules, including products of hydrolysis of phosphoinositide (PI) by phospholipase C, TxA2, and cyclic nucleotides. These induce or modulate the various platelet responses of Ca2+ mobilization, protein phosphorylation, aggregation, secretion, and thromboxane production.

The TDM of Vancomycin

August 16, 2016 Critical Care, Infectious Diseases, Pharmacokinetics No comments , , , , ,

Question #1. B.C., a 65-year old, 45-kg man with a serum creatinine concentration of 2.2 mg/dL, is being treated for a presumed hospital-acquired, MRSA infection. Design a dosing regimen that will produce peak concentration less than 40 to 50 mg/L and through concentrations of 5 to 15 mg/L.

Target Plasma Concentration

Screen Shot 2016-08-16 at 10.37.03 AM

Clearance and Volume of Distribution

The first step in calculating an appropriate dosing regimen for B.C. is to estimate his pharmacokinetic parameters (i.e., volume of distribution, clearance, elimination rate constant, and half-life).

The volume of distribution for B.C. can be calculated by using Equation 13.1.

V (L) = 0.17 (age in years) + 0.22 (TBW in kg) + 15

So, B.C.'s expected volume of distribution would be: V (L) = 0.17 (65 yrs) + 0.22 (45 kg) + 15 = 36.0 L [Equation 13.1]

Using Equation 13.2 and Equation 13.4 to calculated B.C.'s expected creatinine clearance and vancomycin clearance.

Clcr for males (mL/min) = (140 – Age)(Weight in kg) / [(72)(SCrss)] [Equation 13.2]

Vancomycin Cl ≈ Clcr [Equation 13.4]

For B.C. the vancomycin Cl ≈ (140 – 65 yrs)(45 kg) / [(72)(2.2 mg/dL)] = 21.3 mL/min = 1.28 L/hr

The calculated vancomycin clearance of 1.28 L/hr and the volume of distribution of 36.0 L then can be used to estimate the elimination rate constant of 0.036 hr-1. And the corresponding vancomycin half-life can be calculated, which equals (0.693)(V) / Cl = 19.5 hr.

Loading Dose

In clinical practice, loading doses of vancomycin are seldom administered. This is probably because most clinicians prescribe about 15 mg/kg as their maintenance dose.

C0 = (S)(F)(Loading Dose) / V = (1)(1)(15 mg/kg x 45 kg) / 36 L = 18.8 mg/L ≈ 20 mg/L (Equation 13.8)

If you want to administer a loading dose, the loading dose = (V)(C) / [(S)(F)] = (36.0 L)(30 mg/L) / [(1)(1)] = 1080 mg or ≈ 1000 mg.

Steady-State

During the steady-state, Css max = Css min + [(S)(F)(Dose) / V] (Equation 13.5). This equation is based on several conditions including: 1) Steady state has been achieved; 2) the measured plasma concentration is a trough concentration; and 3) the bolus dose is an acceptable model (infusion time <1/6 half-life).

In the clinical setting, trough concentrations are often obtained slightly before the true trough. Because vancomycin has a realtive long half-life, most plasma concentrations obtained within 1 hour of the true trough can be assumed to have met condition 2 above.

Since vancomycin follows a multicompartmental model, it is difficult to avoid the distribution phase when obtaining peak plamsa concentrations. If peak levels are to be measured, samples should be obtained at least 1 or possibly 2 hours after the end of the infusion period. It is difficult to evaluate the appropriateness of a dosing regimen that is based on plasma samples obtained before steady state. Additional plasma concentrations are required to more accurately estimate a paient's apparent clearance and half-life, and to ensure that any dosing adjustments based on a non-steady-state trough concentration actually achieve the targeted steady-state concentrations.

Maintenance Dose

The maintenance dose can be calculated by a number of methods. One approach might be to first approximate the hourly infusion rate required to maintain the desired average concentration. Then, the hourly infusion rate can be multiplied by an appropriate dosing interval to calculate a reasonable dose to be given on an intermittent basis. For example, if an average concentraion of 20 mg/L is selected (approximately halfway between the desired peak concentration of ≈ 30 mg/L and trough concentration of ≈ 10 mg/L), the hourly administration rate would be 25.6 mg/hr.

Maintenance Dose = (Cl)(Css ave)(tau) / [(S)(F)] 

For this patient the 24 hour dose should be (1.28 L/hr)(20 mg/L)(24 hr) / [(1)(1)] = 614 mg ≈ 600 mg

– or –

Maintenance delivery rate = Dose/tau = (Cl)(Css ave) / [(S)(F)]

For this patient the maintenance deliver rate = (1.28 L/hr)(20 mg/L) / [(1)(1)] = 25.6 mg/hr

The second approach that can be used to calculate the maintenance dose is to select a desired peak and trough concentration that is consistent with the therapeutic range and B.C.'s vancomcin half-life. For example, it steady-state peak concentrations of 30 mg/L are desired, it would take approximately two half-lives for that peak level to fall to 7.5 mg/L. Since the vancomycin half-life in B.C. is approximately 1 day, the dosing interval would be 48 hours. The dose to be administered every 48 hours can be calculated as follows using Equation 13.5:

Dose = (V)(Css max – Css min) / [(S)(F)] = (36.0 L)(30 mg/L – 7.5 mg/L) / [(1)(1)] = 810 mg ≈ 800 mg

The peak and trough concentrations that are expected using this dosing regimen can be calcualted by using Equations 13.12 and 13.14, respectively.

Css max = (S)(F)(Dose) / {V x [1- e(-k*tau)]} = 27.0 mg/L (Equation 13.12)

Note that although 27 mg/L is an acceptable peak, the actual clinical peak would normally be obtained approximately 1 hour after the end of a 1-hour infusion, or 2 hours after this calculated peak concentration, and would be about 25 mg/L, as calculated by Equation 13.13.

C2 = C1[e(-k*t)] = 25.1 mg/L (Equation 13.13)

The calculated trough concentration would be about 5 mg/L.

Css min = (S)(F)(Dose / V)[e(-k*tau)] / [1 – e(-k*tau)] = (Css max)[e(-k*tau)] = 4.8 mg/L (Equation 13.14 and 13.15)

This process of checking the expected peak and trough concentrations is most appropriate when the dose or the dosing interval has been changed from a calculated value (e.g., twice the half-life) to a practical value (e.g., 8, 12, 18, 24, 36, or 48 hours). Many institutions generally prefer not to use dosing intervals of 18 or 36 hours because the time of day whent the next dose is to be given changes, potentially resulting in dosing errors. If different plasma vancomycin concentrations are desired, Equations 13.12 and 13.14 can be used target specific vancomycin concentrations by adjusting the dose and/or the dosing interval.

A third alternative is to rearrange Equation 13.14, such that the dose can be calculated:

Dose = (Css min)(V)[1 – e(-k*tau)] / {(S)(F)[e(-k*tau)]} (Equation 13.16)

Pathophysiology of The Circulation

August 8, 2016 Cardiology, Critical Care, Hemodynamics, Physiology and Pathophysiology No comments , , , , , , , , , , , , ,

The Diastolic V-P Curve

Screen Shot 2016-07-25 at 5.40.29 PMFigure 31-4B plots LVEDV against LVEDP. As ventricular volume increases from zero, the transmural pressure of the ventricle does not exceed zero until about 50 mL (the unstressed volume) is added. Then LVEDP increases in a curvilinear manner with ventricular volume (the stressed volume) first as a large change in volume for a small change in pressure and then as a small change in volume for a large change in pressure. If the pericardium is removed, these V-P characteristics are more linear such that the large change in LVEDP at higher values of LVEDV is no longer evident. Thus the pericardium acts like a membrane with a large unstressed volume loosely surrounding the heart up to a given ventricular volume, but at greater LVEDV the pericardium becomes very stiff. At higher heart volumes, most of the pressure across the heart is across the pericardium, accounting for the very steep rise in the diastolic V-P relation.

Abnormality

  • Below factors shift the diastolic V-P curv up left
  • Diastolic dysfunction also suppress the cardiac function curve

In the presence of pericardial effusion, the volume at which the pericardium becomes a limiting membrane is reduced by the volume of the effusion. When the effusion is large enough, reduced end-diastolic volumes are associated with quite large end-DPs. In turn, pericardial pressure decreased VR by increasing Pra, thus keeping end-diastolic volume and QT abnormally low. Other common causes of diastolic dysfunction in critically ill patients signaled by high left atrial pressure and low ventricular end-diastolic volume are listed in Table 31-1.

Screen Shot 2016-07-25 at 6.31.10 PMThe End-Systolic V-P Curve

The contracting ventricle shortens against the aortic afterload pressure until its volume reaches the end-systolic volume; at that lower volume, the maximum pressure that can be generated is equal to the afterload pressure, so the aortic valve closes and ejection is over. If the afterload pressure were decreased, the ventricle could eject further to a lower end-systolic volume, where the maximum generated pressure equals the reduced afterload; hence, SV would increase.

The line connecting all end-systolic V-P points is an indicator of the pumping function or contractility of the heart because this line defines the volume to which the ventricle can shorten against each afterload for a given contractile state. Agents that enhance contractility shift the end-systolic V-P relation up and to the left; then the ventricle can shorten to a smaller end-systolic volume for each afterload, thereby increasing SV at a given LVEDV/LVEDP. Conversely, negative inotropic agents, myocardial ischemia, hypoxia, and acidemia depress the end-systolic V-P relation down and to the right. Then end-systolic volume is increased for a given pressure afterload, thereby reducing the SV at a given filling pressure.

Of note that diastolic dysfunction not only affect diastolic V-P curve (upper-left shift), but also we affect the end-systolic V-P curve/contractility (right shift), as decreased preload and resultant decreased contractility.


Control of Cardiac Output by The Systemic Vessels

The heart is a mechanical pump that generates flow in the circulation. Because QT is the product of HR and SV, it is often erroneously assumed that the heart controls QT. In fact VR to the right heart is controlled by the systemic vessels, so the heart is more accurately described as a mechanical pump having diastolic and systolic properties that determine how it accommodates the VR.

We use the classical Guyton view that mean systemic pressure (Pms), right atrial pressure (Pra), and resistance to venous return (RVR) govern VR. This conceptual model draws attention to how the resistance and capacitance of systemic vessels and their distribution exert control on the VR, especially through baroreceptor reflexes. This model also provides a graphical solution for the unique values of Pra and VR at the intersection of cardiac function and VR curves in health and in diverse critical illnesses.

We choose to downplay several potential shortfalls of this interpretation, which some regard as fatal flaws. Their analyses and interpretation of Guyton's experiments suggest that Pra is not the "back pressure" impeding VR, that Pms is an imaginative concept that ought not be interpreted as the pressure driving VR, and that Pms – Pra is the result of VR, not its cause. Our comparison of these two viewpoints reveals that the first provides more useful concepts for explaining the pathophysiology and treatment of the circulation in critical illness, so we build our discussion on Guyton's view.

Mean Systemic Pressure

When the heart stops beating, pressure equalizes throughout the vascular system, and its new value is the Pms (10-15 mm Hg). This pressure is much lower than the arterial pressure and is closer to the Pra. When flow stops, blood drains from the high-pressure, low-volume arterial system into the high-volume, low pressure venous system, which accommodates the displaced volume with little change in pressure. When the heart begins to beat again, the left heart pumps blood from the central circulation into the systemic circuit, thus increasing pressure there. At the same time, the right heart pumps blood into the lungs, thereby decreasing its pressure (Pra) with respect to Pms, so blood flows from the venous reservoir into the right atrium. Pressure on the venous side decreases slightly below Pms, whereas pressure on the arterial side increases considerably above Pms with succeeding heartbeats. This continues until a steady state is reached, when arterial pressure has increased enough to drive the whole SV of each succeeding heartbeat through the high arterial resistance into the venous reservoir. The Pms does not change between the state of no flow and the new state of steady flow because neither the vascular volume nor the compliance of the vessels has changed. What has changed is the distribution of the vascular volume from the compliant vein to the stiff arteries; this volume shift creates the pressure difference driving flow through the circuit.

Pms is the driving pressure for VR to the right atrium when circulation resumes. It can be increased to increase VR by increasing the vascular volume or by decreasing the unstressed volume and compliance of the vessels. The latter two mechanisms are mediated by baroreceptor reflexes responding to hypotension by increasing venous tone and usually occur together. The unstressed volume may also be reduced by raising the legs of a supine patient or applying military antishock trousers; both methods return a portion of the unstressed vascular volume from the large veins in the legs to the stressed volume, thereby increasing Pms and VR. When the heart has an improvement in inotropic state or a reduction in afterload, blood is shift from the central compartment to the stressed volume of the systemic circuit, thereby increasing Pms and VR; moreover, improved ventricular pumping function decreases Pra to increase VR further.

Venous Return and Cardiac Function Curves

Screen Shot 2016-07-26 at 7.07.47 PMBefore the heart was started in the discussion above, Pra was equal to the pressure throughout the vascular system, Pms. With each succeeding heartbeat, Pra decreases below Pms and VR increases. This sequence is repeated in a more controlled, steady state by replacing the heart with a pump set to keep Pra at a given value while VR is measured. Typical data are plotted in Figure 31-6. As Pra is decreased from 12 to 0 mm Hg (indicated by the thin continuous line), VR is progressively increased with the driving pressure (Pms – Pra). The slope of the relation between VR and Pms – Pra is the resistance to VR (RVR = delta[Pms – Pra]/deltaVR). When Pra falls below zero, VR does not increase further because flow becomes limited while entering the thorax. This occurs when the pressure in these collapsible great veins decreases below the atmospheric pressure outside the veins. Further decreases in Pra and CVP are associated with progressive collapse of the vein rather than with an increase in VR.

For a given stressed vascular volume and compliance, Pms is set and RVR is relative constant. In the absence of pulmonary hypertension or right heart dysfunction, LV function will determine Pra and, hence, VR to the right heart, along the VR curve. The corresponding cardiac function curve is drawn as the thick line. QT is described by the cardiac function curve, drawn as a thick continuous line relating Pra (abscisa) to QT (ordinate), in Figure 31-6. The heart is able to eject a larger SV and QT when the end-DP is greater because more distended ventricles eject to about the same end-systolic volume as less distended ventricles do. Accordingly, as Pra decrease, QT decreases along the cardiac function curve. However, VR increaseas Pra decreases until VR equals QT at a unique value of Pra, indicated by the intersection of the cardiac function and VR curves in Figure 31-6 (see point A in both panels).

When QT is insufficient, VR can be increased in several ways. A new steady state of increased VR is achieved by increasing Pms with no change in RVR, indicated by the interrupted VR curve in the left panel of Figure 31-6. This new VR curve intersects the same cardiac function curve at a higher value of QT at point B. This method of increasing VR is associated with an increase in Pra. Due to the steep slope of the cardiac function curve in normal hearts, large increase in VR occur with only small increases in Pra. Alternatively, VR can be increased by enhanced cardiac function by increasing contractility or decreasing afterload of th heart. This is depicted as an upward shift of the cardiac function curve, as in the right panel of Figure 31-6, such that greater QT occurs at each Pra. The increase on each VR curve by this mechanism is associated with a reduction in Pra. Further, in the normal heart, only a small change in VR is possible (from point A to point B in the right panel), and greater reductions in Pra do not increase QT further because VR becomes flow limited as Pra decreases to below zero. This explains why inotropic agents that ehance contractility are ineffective in hypovolemic shock.

Screen Shot 2016-07-30 at 2.20.14 PMWhen cardiac pumping function is depressed, as depicted by the interrupted line in Figure 31-7, VR is decreased from point A to point B for the same value of Pms as Pra increases. The patient must then retain fluid or initiate cardiac reflexes to increase Pms toward the new value required t omaintain adequate QT, as in chronic congestive heart failure. This is associated with a large increase in Pra from point B to point C, which in turn causes jugular venous distention, hepatomegaly, and peripheral edema. Diuretic reduction of vascular volumes will correct these abnormalities at the expense of decreasing Pms and VR. In contrast, inotropic and vasodilator drugs, which improve depressed cardiac function by shifting the interrupted cardiac function curve upward, increase QT and decrease Pra more effectively than in patients with normal cardiac function.

Resistance to Venous Return

At a given Pms and Pra, VR is increased by reduced RVR. The RVR is an average of all of the regional resistances. Each regional resistance (R) is weighted by its contribution to the entire systemic vascular compliance (C/CT) and to the fraction of the cardiac output draining from that region.

In most conditoins, RVR remains relatively constant, increasing only slightly with large adrenergic stimulation; even then the increase in regional resistances is offset by redistribution of blood flow to peripheral beds having low resistance and/or compliance. One illustraion of this effect is the opening of an abdominal arteriovenous fistula between the aorta and the inferior vena cava, which doubles VR at the same values of Pms and Pra (Figure 31-8). Consider aliquots of blood leaving the left heart simultaneously; the aliquot traversing the fistula returns to the right heart before the aliquot perfusing the lower body returns. When a greater fraction of the QT traverses the open fistula having a very low compliance and resistance, more blood returns to the heart because RVR decreases. This manifestation of reduced RVR may account for poorly explained hemodynamic changes in septic shock, when high QT is associated with increased blood flow to skeletal muscle, as if some metabolic stimulus increasese the fraction of QT perfusing the low resistance and low compliance skeletal muscle bed, thereby reducing RVR and increasing VR. For another example, systemic hypoxemia triples VR. It does so by increasing Pms through venoconstriction to cause 70% of this increase, while redistribution of QT toward vascular beds having reduced capacitance and resistance account for 30% of the change.

Screen Shot 2016-07-30 at 3.41.07 PMNote in Figure 31-8 that increased VR from A to B is associated with increased Pra when RVR is reduced without changing the cardiac function curve. In fact, Pra does not increase, and VR actually increases from A to C, as if arteriovenous shunting improved cardiac function from the continuous to the interrupted cardiac function curve shown in the figure. One explanation is that reduced SVR associated with arteriovenous shunting lowers the afterload on the left ventricle to improve cardiac function.