EKG/ECG

EKG Findings Related to Myocardial Infarction

July 15, 2017 Cardiology, EKG/ECG No comments , , , , , ,

In most infarctions, the EKG will reveal the correct diagnosis. An EKG should be performed immediately on anyone in whom an infarction is even remotely suspected. However, the initial EKG may not always be diagnostic, and the evolution of electrocardiographic changes varies from person to person; therefore, it is necessary to obtain serial cardiograms once the patient is admitted to the hospital. During an acute myocardial infarction, the EKG evolves through three stages: 1) T-wave peaking followed by T-wave inversion, 2) ST-segment elevation, 3) the appearance of new Q waves.

T Wave

With the onset of infarction, the T waves become tall and narrow, a phenomenon called peaking. These peaked T waves are often referred to as hyperacute T waves. Shortly afterward, usually a few hours later, the T waves invert. These T-wave changes reflect myocardial ischemia, the lack of adequate blood flow to the myocardium. Ischemia is potentially reversible: if blood flow is restored or the oxygen demands of the heart are eased, the T waves will revert to normal. On the other hand, if actual myocardial cell death (true infarction) has occurred, T-wave inversion will persist for months to years. T-wave inversion by itself is indicative only of ischemia and is not diagnostic of myocardial infarction. T-wave inversion is a very nonspecific finding. Many things can cause a T wave to flip. One helpful diagnostic feature is that the T waves of myocardial ischemia are inverted symmetrically, whereas in most other circumstances, they are asymmetric, with a gentle downslope and rapid upslope. In patients whose T waves are already inverted, ischemia may cause them to revert to normal, a phenomenon called pseudonormalization. Recognition of pseudonormalization requires comparing the current EKG with a previous tracing. In young persons without any cardiac-related symptoms, T-wave inversion isolated to one geographic region of the heart, particularly one or two midprecordial leads, for example, V3 and V4, is usually a normal variant.

  • Peak and inversion
  • Only reflecting ischemia
  • Symmetrical
  • Pseudonormalization

ST Segment

ST-segment elevation is the second change that occurs acutely in the evolution of an infarction. ST-segment elevation signifies myocardial injury. Injury probably reflects a degree of cellular damage beyond that of mere ischemia, but it, too, is potentially reversible, and in some cases the ST segments may rapidly return to normal. In most instances, however, ST-segment elevation is a reliable sign that true infarction has occurred and that the complete electrocardiographic picture of infarction will evolve unless there is immediate and aggressive therapeutic intervention.

Even in the setting of a true infarction, the ST segments usually return to baseline within a few hours. Persistent ST-segment elevation often indicates the formation of a ventricular aneurysm, a weakening and bulging out of the ventricular wall. Like T-wave inversion, ST-segment elevation can be seen in a number of other conditions in addition to an evolving myocardial infarction. There is even a type of ST-segment elevation that can be seen in normal hearts. This phenomenon has been referred to as early depolarization or J point elevation.

The J point, or junction point, is the place where the ST segment takes off from the QRS complex. J point elevation is very common in young, healthy individuals. The ST segment usually returns to baseline with exercise. J point elevation has long been thought to have no pathologic implications. However, some research has reported a slightly increased risk of death from cardiac causes in patients with J point elevation in the inferior leads. How can the ST-segment elevation of myocardial injury be distinguished from that of J point elevation? With myocardial injury, the elevated ST segment has a distinctive configuration. It is bowed upward and tends to merge imperceptibly with the T wave. In J point elevation, the T wave maintains its independent waveform.

  • Elevation
  • Reflecting true infarction
  • Return to baseline within a few hours
  • J point elevation

Q Waves

The appearance of new Q waves indicates that irreversible myocardial cell death has occurred. The presence of Q waves is diagnostic of myocardial infarction. Q waves usually appear within several hours of the onset of infarction, but in some patients they may take several days to evolve. The ST segment usually has returned to baseline by the time Q waves have appeared. Q waves usually persist for the lifetime of the patient.

Other leads, located some distance from the site of infarction, will see an appear increase in the electrical forces moving toward them. They will record tall positive R waves. These opposing changes seen by distant leads are called reciprocal changes. The concept of reciprocity applies not only to Q waves but also to ST-segment and T-wave changes. Thus, a lead distant from an infarct may record ST-segment depression.

Small Q waves can be seen in the left lateral leads (I, aVL, V5, and V6) and occasionally in the inferior leads (especially II and III) of perfectly normal hearts. These Q waves are caused by the early left-to-right depolarization of the interventricular septum. Pathologic Q waves signifying infarction are wider and deeper. They are often referred to as significant Q waves. The criteria fro significance include: 1) The wave must be greater than 0.04 seconds in duration, 2) the depth of the Q wave must be at least one-third the height of the R wave in the same QRS complex. Because lead aVR occupies a unique position on the frontal plane, it normally has a very deep Q wave. Lead aVR should not be considered when using Q waves to look for possible infarction.

Acute Potassium Disorders

July 15, 2017 Cardiology, Clinical Skills, Critical Care, Differential Diagnosis, EKG/ECG No comments , , , , ,

Disorders of potassium homeostasis are common in hospitalized patients and may be associated with severe adverse clinical outcomes, including death. Prevention and proper treatment of hyper- and hypokalemia depend on an understanding of the underlying physiology.

The total body potassium content of a 70-kg adult is about 3500 mmol (136.5 g), of which only 2% (about 70 mmol / 2.73 g) is extracellular. It is not surprising that the extracellular potassium concentration is tightly regulated. In fact, two separate and cooperative systems participate in potassium homeostasis. One system regulates external potassium balance: the total body parity of potassium elimination with potassium intake. The other system regulates internal potassium balance: the distribution of potassium between the intracellular and extracellular fluid compartments. This latter system provides a short-term defense against changes in the plasma potassium concentration that might otherwise result from total body potassium losses or gains.

Disorders of Potassium Homeostasis

Disorders of potassium homeostasis may be conveniently divided according to the duration of the disturbance: acute (<48 hours’ duration) or chronic.

Acute Hyperkalemia

Excessive potassium intake. Given an acute potassium load, a normal individual will excrete about 50% in the urine and transport about 90% of the remainder into cells over 4 to 6 hours. It is possible to overwhelm this adaptive mechanism such that if too much potassium is taken in too quickly, significant hyperkalemia will result. Such events are almost always iatrogenic. One’s ability to tolerate a potassium load declines with disordered internal balance and impaired renal potassium excretory capacity. In such circumstances, an otherwise tolerable increase in potassium intake may cause clinically significant hyperkalemia: Doses of oral potassium supplements as small as 30 to 45 mmol have resulted in severe hyperkalemia in patients with impaired external or internal potassium homeostasis.

KCl, used as a supplement, is the drug most commonly implicated in acute hyperkalemia. Banked blood represents a trivial potassium load under most circumstances, because a unit of fresh banked blood, either whole or packed cells, contains only 7 mmol (273 mg) of potassium. Thus, severe hyperkalemia would result only from massive transfusion of compatible blood. However, the potassium concentration in banked blood does increase substantially as the blood ages.

Patients undergoing open heart surgery are exposed to cardioplegic, solutions containing KCl typically at about 16 mmol/L, which may lead to clinically significant hyperkalemia in the postoperative period, especially in patients with diabetes mellitus with or without renal failure.

Abnormal potassium distribution. Acute hyperkalemia may result from sudden redistribution of intracellular potassium to the extracellular space. If only 2% of intracellular potassium were to leak unopposed from cells, serum potassium level would immediately double. Fortunately, such dramatic circumstances are rarely encountered. Nevertheless, smaller degrees of potassium redistribution commonly result in clinically significant hyperkalemia.

Among the most impressive syndromes associated with acute hyperkalemia are those involving rapid cell lysis. The tumor lysis syndrome results from treatment of chemosensitive bulky tumors with release of intracellular contents, including potassium, into the ECF. Extreme hyperkalemia even causing sudden death has featured prominently in some series of patients. Most of such patients were in renal failure from acute uric acid nephropathy, thus impairing their ability to excrete the potassium load. Rhabdomylosis, either traumatic or nontraumatic, may result in sudden massive influx of potassium to the extracellular space. Other circumstances that may result in redistributive hyperkalemia include severe extensive burns, hemolytic transfusion reactions, and mesenteric ischemia or infarction.

Pharmacologic agents. Two drugs may rarely cause acute hyperkalemia by redistribution: digitalis glycosides and succinylcholine. Massive digitalis overdose has been associated with extreme hyperkalemia. Succinylcholine depolarizes the motor end plate and in normal individuals causes a trivial amount of potassium leak from muscle, resulting in an increase in serum potassium level by about 0.5 mmol/L. In patients with neuromuscular disorders, muscle damage, or prolonged immobilization, however, muscle depolarization may be more widespread, causing severe hyperkalemia. Prolonged use of nondepolarizing non depolarizing neuromuscular blockers in critically ill patients may predispose to succinylcholine-induced hyperkalemia.

Hyperkalemic periodic paralysis. This rare syndrome of episodic hyperkalemia and paralysis is caused by a mutation of the skeletal muscle sodium channel, inherited in an autosomal dominant pattern. Attacks may be precipitated by exercise, fasting, exposure to cold, and potassium administration, and prevented by frequent carbohydrate snacks. Attacks are usually brief and treatment consists of carbohydrate ingestion. Severe attacks may require intravenous glucose infusion.

Acute renal failure. Hyperkalemia accompanies acute renal failure in 30% to 50% of cases. It is seen most commonly in oliguric renal failure. Contributing factors include tissue destruction and increased catabolism.

Pseudohyperkalemia. It refers to a measured potassium level that is higher than that circulating in the patient’s blood. It has a number of possible causes. First, it may be caused by efflux of potassium out of blood cells in the test tube after phlebotomy. This may be seen in a serum specimen in cases of thrombocytosis or leukocytosis, when the clot causes cell lysis in vitro. These days, many clinical laboratories measure electrolytes in plasma (unclotted) specimens. Even under these conditions, extreme leukocytosis may cause pseudohyperkalemia if the specimen is chilled for a long time before the plasma is separated, leading to passive potassium leak from cells. Hemolysis during specimen collection with false raise [K+] or plasma potassium concentration by liberating intraerythrocyte to potassium. Second, if the patient’s arm is exercised by fist clenching with a tourniquet in place before the specimen is drawn, the sampled blood potassium concentration will rise significantly as a result of local muscle release of intracellular potassium.

Acute Hypokalemia

Treatment of diabetic ketoacidosis. It is well recognized that patients presenting in DKA are always severely depleted in total body potassium as a result of glucose-driven osmotic diuresis, poor nutrition, and vomiting during the development of DKA. Paradoxically, most patients in DKA have a normal serum potassium level upon admission. Insulin deficiency and hyperglycemia appear to account for the preservation of a normal [K+] despite severe total body potassium depletion. Once therapy for DKA is instituted, however, [K+] typically plummets as potassium is rapidly taken up by cells. Potassium replacement at rates up to 120 mmol (4.68 g) per hour have been reported, with total potassium supplementation of 600 to 800 mmol (23.5 to 31.2 g) within the first 24 hours of treatment. Hypokalemia in this setting may lead to respiratory arrest.

Refeeding. A situation analogous to DKA arises during aggressive refeeding after prolonged starvation or with aggressive “hyperalimentation” of chronically ill patients. The glucose-stimulated hyperinsulinemia and tissue anabolism shift potassium into cells, rapidly depleting extracellular potassium.

Pharmacologic agents. Specific beta2-adrenergic receptor agonists may cause electrophysiologically significant hypokalemia, especially when given to patients who are potassium depleted from the use of diuretic drugs. Epinephrine, given intravenously in a dose about 5% of that recommended for cardiac resuscitation, cause a fall in [K+] by about 1 mmol/L. A rare cause of severe hypokalemia is poisoning with soluble barium salts such as chloride, carbonate, hydroxide, and sulfide. Soluble barium salts are used in pesticides and some depilatories, which may be ingested accidentally or intentionally. Thiopentone, a barbiturate used to induce coma for refractory intracranial hypertension, is associated with redistributive hypokalemia in the majority of treated patients within 12 hours of initiating therapy.

Hypokalemic periodic paralysis. Three forms of this rare syndrome have been described: familial, sporadic, and thyrotoxic. All have in common attacks of muscle weakness accompanied by acute hypokalemia caused by cellular potassium uptake.

Pseudohypokalemia. Severe leukocytosis may cause spuriously low plasma potassium concentrations if blood cells are left in contact with the plasma for a long time at room temperature or higher. This phenomenon results from ongoing cell metabolism in vitro with glucose and potassium uptake. Unexpected hypokalemia and hypoglycemia in the setting of leukocytosis should alert the clinician to this phenomenon.

Summary

Potassium exchange between ECF and ICF: insulin, epinephrine, and [H+]

Potassium renal secretion: [K+], dietary intake of potassium, AngII, aldosterone, tubular sodium delivered to principal cells (at distal nephron).

EKG Changes of Potassium Disturbances

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Hyperkalemia

Hyperkalemia produces a progressive evaluation of changes in the EKG that can culminate in ventricular fibrillation and death. The presence of electrocardiographic changes is a better measure of clinically significant potassium toxicity than the serum potassium level. As the potassium begins to rise, the T waves across the entire 12-lead EKG begin to peak. This effect can easily be confused with the peaked T waves of an acute myocardial infarction. One difference is that the changes in an infarction are confined to those leads overlying the area of the infarct, whereas in hyperkalemia, the changes are diffuse. With a further increase in the serum potassium, the PR interval becomes prolonged, and the P wave gradually flattens and then disappears. Ultimately, the QRS complex widens until it merges with the T wave, forming a sine wave pattern. Ventricular fibrillation may eventually develop.

It is important to note that whereas these changes frequently do occur in the order described as the serum potassium rises, they do not always do so. Progression to ventricular fibrillation can occur with devastating suddenness. Any change in the EKG due to hyperkalemia mandates immediate clinical attention.

Hypokalemia

With hypokalemia, the EKG may again be a better measure of serious toxicity than the serum potassium level. Three changes can be seen, occurring in no particular order, including: ST-segment depression, flattening of the T wave with prolongation of the QT interval, and appearance of a U wave. The term U wave is given to a wave appearing after T wave in the cardiac cycle. It is usually has the same axis as the T wave and is often best seen in the anterior leads. Its precise physiologic meaning is not fully understood. Although U waves are the most characteristic feature of hypokalemia, they are not in and of themselves diagnostic. Other conditions can produce prominent U waves, and U waves can sometimes be seen in patients with normal hearts and normal serum potassium levels. Rarely, severe hypokalemia can cause ST-segment elevation. Whenever you see ST-segment elevation or depression on an EKG, you first instinct should always be to suspect some form of cardiac ischemia, but always keep hypokalemia in your differential diagnosis.

[Clinical Art][Circulation] Interpretation of Hemodynamic Waveforms

October 20, 2016 Cardiology, Critical Care, EKG/ECG, Hemodynamics, Mechanical Ventilation No comments , , , , , , , , , , , , , , , , , , , , , , , , ,

1st_ceb_insigniaBasic Knowledge

Mechanism of Hemodynamic Monitoring

The rapidly occurring events (represent mechanical forces) of cardiac chambers and vessels during cardaic cycle require conversion to an electrical signal to be transmitted and subsequently translated into an interpretable, graphic format. The pressure transducer is the essential component that translates the mechanical forces to electrical signals. The transducer may be located at the tip of the catheter (micromanometer) within the chamber or, more commonly, the pressure transducer is outside of the body, and a pressure waveform is transmitted from the catheter tip to the transducer through a column of fluid. These transducers consist of a diaphragm or membrane attached to a strain-gauge-Wheatstone bridge arrangement. When a fluid wave strikes the diaphragm, an electrical current is generated with a magnitude dependent on the strength of the force that deflects the membrane. The output current is amplified and displayed as pressure versus time.


Clinical Art

Pre-operations Before Recording

Old generations of transducers required calibration against a mercury manometer; fortunately, the factory-calibrated, disposable, fluid-filled transducers in clinical use today no longer need this. Table-mounted transducers do require balancing or "zeroing," which refers to the establishment of a reference point for subsequent pressure measurements. The reference or "zero" position should be determined before any measurements are made. By convention, it is defined at the patient's midchest in the anteroposterior dimension at the level of the sternal angle of Louis (fourth intercostal space). This site is an estimation of the location of the right atrium and is also known as the phlebostatic axis. A table-mounted transducer is placed at this level and the stopcock is opened to air (atmospheric pressure) and set to zero by the hemodynamic system. The system is now ready for presure measurements. It is important to emphasize that the pre-operation of the hemodynamic monitor is very important, because if the "zero" level is not properly set and the transducer not appropriately balanced, the hemodynamic data recorded would be misleading, even fatal.

Interpretation of pressure waveforms requires a consistent and systematic approach in Table 2-1. Careful scrutiny of the waveform ensures a high-fidelity recording without over- or under-damping. Each pressure event should be timed with EKG.

Table 2-1 A Systematic Approach to Hemodynamic Interpretation
1.Establish the zero level and balance transducer
2.Confirm the scale of the recording
3.Collect hemodynamics in a systematic method using established protocols
4.Critically assess the pressure waveforms for proper fidelity
5.Carefully time pressure events with the EKG
6.Review the tracings for common artifacts

At present, in the clinical setting, 3 pressure waveforms can be obtained at bedside with invasive hemodynaic monitoring devices (central venous cathether/CVC and pulmonary artery catheter/PAC), including right atrial pressure/Prapulmonary artery pressure/Ppa, and pulmonary artery wedge pressure/Ppw. The pressurewave form is recorded along with a synchronized EKG.

Normal Pressure Waveform

Atrial Pressure

The goal of measuring the atrial pressure is to measure the pressure in the ventricles  at the end of diastole, to idenfify a "filling pressure". The goal for any atrial pressure measurement is to obtain the measurement at the every end of diastole, when the atrial pressure is closest to the ventricular pressure. The normal Pra is 2-8 mm Hg and is characterized by a and v waves and x and y descents. The causes of a, v waves and x, y descents are listed below.

PS: The Rationale Reason for the Formation of Pra waveform

a wave represents the pressure rise within the right atrium due to atrial contraction follows the P wave on the EKG by about 80 msec
descet represents the pressure decay following the a wave and reflect both atrial relaxation and the sudden downward motion of the atrioventricular junction that occurs because of early ventricular systole  
c wave is sometimes observed after the a wave and is due to the sudden motion of the tricuspid annulus toward the right atrium at the onset of ventricular systole the c wave follows the a wave by the same time as the PR interval on the EKG
v wave when the tricuspid valve is closed, the pressure rise responsible for the v wave is due to passive venous filling of the atrium, represent atrial diastole. the peak of the right atrial v wave corresponds with the end of T wave on the surface EKG; the ORS alawys appears before the v wave is produced
y wave is due to rapid emptying of the right atrium when the tricuspid valve opens  

Atrial waveform interpretation in detail

v wave

The atrial pressures initially increase during systole as the contracting ventricles return blood to the atria, refilling the upper chambers. This rise in the atrial pressure is identified as the "v" wave. The upstroke of the v wave is the rise in atrial pressure as a result of atrial filling. Because it is produced as a result of ventricular contraction, its location is relative to the QRS on the EKG. Ejection eventually leads to the return of blood to the atria (left ventricular contraction refills the right atrium and produces the right atrial v wave; right ventricular contraction refills the left atrium and produces the left atrial v wave). Thus, the QRS causes the v wave, however, the QRS always appears before the v wave is produced.

PA

The normal pulmonary artery systolic pressure/Ppas is 15-30 mm Hg, the normal diastolic pressure/Ppad is 4-12 mm Hg, and the mean 9-18 mm Hg. The components pulmonary artery pressure include a rapid rise in pressure, systolic peak, a pressure decay associated with a well-defined dicrotic notch from pulmonic valve closure, and a diastolci trough.

PA and arterial pressure waveforms have similar morphology. Systole begins with the opening of the pulmonic valves. Prior to opening of the pulmonary valve, the pulmonary artery pressure is very low (the pulmonary vascular system does not need a high pressure system to perfuse). As the ventricles contact, they eject blood into the pulmonary artery. This causes an immediate rise in the arterial pressure. As blood enters the great vessels, the pressure rise quickly and steadily, producing a steep vertical rise. Late in systole, the rate of ejection slows as the pressure gradient between the right ventricle and pulmonary artery narrows. Although blood is still moving from the ventricle to the great vessels, the rate of movement is slowed to the point where the pressure begins to decline. This cause the early downslope in the arterial tracing that represents this period of reduced ejection. Like the right atrial v wave, the pulmonary artery systolic wave typically coincides with the T wave of the EKG.

Later, the ventricle begins to relax, causing the ventricular pressure to drop below the pressure in great vessels. This causes the pulmonic valves to close, producing a small rise in the PA pressure, known as the dicrotic notch. Following closure of the semi-lunar valves, the pulmonary artery continues to fall as blood leaves the great vessels to perfuse the tissues and lungs.

PAWP

The normal mean pulmonary artery wedge pressure/Ppw is obtained when the inflated catheter obstructs forward flow within a branch of the pulmonary artery, creating a static column of blood between the tip of the catheter and the j point in the pulmonary venous bed where it intersects with flowing blood. The Ppw tracing contains the same sequence of waves and descents as the Pra tracing. However, when referenced to the ECG, the waves and descents of the Ppw will be seen later than those of the Pra, because the pressure waves from the left atrium must travel back through the pulmonary vasculature and a longer length of catheter. Therefore, in the Ppw tracing, the a wave usually appears after the QRS complex, and the v wave is seen after the T wave.
screen-shot-2016-10-19-at-2-19-49-pmInterpretation of CVP and PAWP measurements

Correlation to the EKG

The easiest wave to evaluate an atrial tracing is to first locate the v wave. Generally, it will appear immeidately after the peak of T wave on a CVP waveform, however, it will be 80-120 ms after the T wave on a PAWP tracing. You can generally identify the v wave by ruling out other waves. It must be after the peak of the T wave. Once the v wave is identified, the a and c can be determined.

Observe the EKG rhythm. If the patient has a sinus rhythm, an a wave should be present. The a should be in the PR interval for a CVP. It is later in the PAWP, appearing within or even afte QRS. If the patient does not have a P wave, the a wave will be absent. If the P wave is not synchronized to the QRS, very large a wave may be present. These large a waves may appear as one very large wave during a cardiac cycle. The large a waves are called cannon a waves. They are actually exaggerated atrial pressures that occur when the atria contract against a closed AV valve, adding to the pressure that is already being generated due to the c or v wave.

If present, the c wave is generally within the QRS for a CVP. It will be after the QRS for a PAWP.

Where to Measure CVP and PAWP

At the very end of ventricular diastole, the atrial pressure equilibrates with the ventricular pressure, at the very end of ventricular filling. Measurement of the atrial pressure at the end of diastole provides the best opportunity to capture ventricular filling pressure. The location on the atrial pressure wave that best reflects end-diastolic pressure is the point just prior to the c wave. However, c wave is often absent or difficult to find, espeically true in the PAWP waveform, which is subject to considerable movement artifact from right ventricular systole and breathing. If we cannot use the mehtod based on c wave to measure the filling pressure, instead we can use other two ways to capture the filling pressure, where the second method for identification of the end-diastolic pressure is to take the mean of the highest and lowest a wave pressure; and the thrid method is used if the a wave is hard to interpret or absent, that is, the end-diastolic pressure can be estimated by identifying the Z point. Draw a line from the end of the QRS to the atrial tracing. The point where the line intersects with the waveform is the Z line. Note that for a PAWP waveform the Z line should be estimated as 0.08-0.12 seconds to the left of the end of the QRS (Z point is delayed 0.08-0.12 seconds from the QRS on the PAWP).


Respiratory Influences on Hemodynamic Data: Transmural Pressure

The Pra and Ppw are used as surrogates for RV and LV filling pressure (so the preload), but remember that when evaluating the patient's preload the end-diastolic volume of the ventricles should also be included in the interpretation. Here in this section we focus our discussion on the respiratory influecnes on the recorded hemodynamic data. OK, it is the transmural (intravascular minus pleural) pressure that represents the distending pressure for cardiac filling. During normal breathing, Ppl is slightly negative at end-expiration and intrathoracic vascular pressures measured at this point in respiratory cycle provide the best estimate of transmural pressure. Either a strip recording or the cursor method should be used to define the end-expiratory pressure.

One error is the assumption that during mechanical ventilation the lowest point in the pressure tracing reflects end expiration. While this is true during controlled ventilation, inspiratory efforts that trigger mechanical breaths produce a nadir in the pressure tracing. Identification of end expiration in the Ppw tracing is aided by the knowledge that expiration is usually longer than inspiration, two exceptions being marked tachypnea and inverse-ratio ventilation. Identification of end expiration from the pressure tracing should not be difficult when interpreted in relationship to the patient's ventilatory pattern. When confusion occurs, a simultaneous airway pressure tracing may be used.

The Pra and Ppw will overestimate transmural pressure if intrathoracic pressure is positive at end expiration. This can occur from an increase in end-expiratory lung volume due to applied positive end-expiratory pressure (PEEP) or auto-PEEP, or from increased intra-abdominal pressure due to active expiration or intra-abdominal hypertension.


Common Errors and Artifacts

screen-shot-2016-10-20-at-7-58-42-pmMost errors in the collection and interpretation of hemodynamic data are listed in Table 2-2.

Probably the most commonly observed artifacts relate to an improper degree of damping. The over-damped tracing indicates the presence of excessive friction absorbing the force of the pressure wave somewhere in the line from the catheter tip to the transducer. The tracing lacks proper fidelity and appears smooth and rounded because of loss of frequency response. This will result in loss of data and will falsely lower peak pressures. Typically, the dicrotic notch on the aortic or pulmonary artery waveforms is absent, and the right atrial or PAPW waveforms will lack distinct a and v waves.

Under-damping causes overshoot or ring artifact. This artifact typically appears as one or more narrow "spikes" overshooting the true pressure during the systolic pressure rise with similar, negatively directed waves overshooting the true pressure contour during the downstroke. This artifact may lead to overestimation of the peak pressure and underestimation of the pressure nadir. Tiny air bubbles that oscillate rapidly back and forth, transmitting energy back to the transducer, cause this artifact. Flushing the catheter or transducer often corrects this artifact; alternatively, introduction of a filter to the hemodynamic system may be necessary to eliminate this artifact.

Related to overshoot or ring artifact is catheter whip or fling artifact. This artifact is created by acceleration of the fluid within the catheter from rapid catheter motion and is commonly seen with balloon-tipped catheters in hyperdynamic hearts or balloon-tipped catheters placed in the pulmonary artery with extraneous loops. Similar to ring artifact, catheter whip causes overestimation of the systolic pressure and underestimation of the diastolic pressure. This artifact is difficult to remedy; eliminating the extra loops or deflation of the balloon can improve the appearance and limit this artifact.

Catheter malposition creates several interesting artifacts.