Intraarterial pressure is typically measured from the radial, brachial, axillary, or femoral arteries.
The contour of the arterial pressure waveform changes as it moves away from the proximal aorta. This i shown in Figure 7.4. Note that as the pressure wave moves toward the periphery, the systolic pressure gradually increases and the systolic portion of the waveform narrows. The systolic pressure can increase as much as 20 mm Hg from the proximal aorta to the radial or femoral arteries. This increase in peak systolic pressure is offset by the narrowing of the systolic pressure wave, so the mean arterial pressure remains unchanged.
The increase in systolic pressure in peripheral arteries is the result of pressure waves that are reflected back from vascular bifurcations and narrowed blood vessels. Reflected waves move faster when the arteries are stiff (the rigidity of arteries increases as with age), and they reach the arterial pressure waveform before it has time to decrement; the convergence of antegrade and retrograde pressure waves serves to heighten the peak of antegrade pressure waveform. Amplification of the systolic pressure by reflected waves is the mechanism for systolic hypertension in the elderly. Because systolic amplification is the result of retrograde pressure waves, it does not promote systemic blood flow.
Mean Arterial Pressure
The mean arterial pressure (MAP) is the time-averaged pressure in the major arteries, and is the principal driving force for systemic blood flow. The MAP is measured electronically as the area under the arterial pressure wave, divided by the duration of the cardiac cycle.
Steady flow (Q) through a closed hydraulic circuit is directly related to the pressure gradient across the circuit (Pin – Pout), and inversely related to the resistance to flow (R) through the circuit. Therefore we get the following formula:
Q = (Pin – Pout)/R [Equation 1]
If the hydraulic circuit is the circulatory system, volumetric flow becomes cardiac output (CO), the inflow pressure is the mean arterial pressure (MAP), the outflow pressure is the mean right atrial pressure (mRAP), and the resistance to flow is the systemic vascular resistance (SVR). The equation 1 relationship can be demonstrated as below in circulatory system:
CO = (MAP – mRAP) /SVR
In normal conditions, mRAP=RAP=CVP=RVEDP
Fluid-filled recording systems can produce artifacts that further distort the arterial pressure waveform. Failure to recognize these artifacts can lead to errors in blood pressure management.
Vascular pressures are transmitted through fluid-filled plastic tubes that connect the arterial catheter to the pressure transducer. This fluid-filled system can oscillate spontaneously, and the oscillations can distort the arterial pressure waveform. The performance of a resonant system is defined by two factors: the resonant frequency and the damping factor. The resonant frequency is the frequency of oscillations that occur when the system is disturbed. When the frequency of an incoming signal approaches the resonant frequency of the system, the resident oscillations add to the incoming signal and amplify it. This type of system is called an underdamped system. The damping factor is a measure of the tendency for the system to attenuate the incoming signal. A resonant system with a high damping factor is called overdamped system.
Three waveforms from different recording systems are shown in Figure 7.5. The wave form in panel A, with the rounded peak and the dicrotic notch, is the normal waveform expected from a recording system with no distortion. The waveform in panel B, with the sharp systolic peak, is from an underdamped recording system. Underdamped systems are popular for pressure recording because their rapid response characteristics, but these systems can amplify the systolic pressure by as much as 25 mm Hg. The final waveform in panel C has an attenuated peak and a narrow pulse pressure. This waveform is from an overdamped system. Overdamping reduces the gain of the system and attenuates the pressure waveform. Overdamping can be the result of partial obstruction of the catheter with a thrombus, or air bubbles in the recording circuit.
Fast Flush Test
A pressurized flush of the catheter-tubing system can also help to identify a recording circuit that is distorting the pressure waveform. Most commercially available transducer system are equipped with a one-way valve that can be used to deliver a flush from a pressurized source. Figure 7.5 shows the results of a flush test in three different situations; the response when the flush is released will help characterize the system.
In panel A, the flush release is followed by a high-frequency burst. This is the normal behavior of a fluid-filled system. In panel B, the flush release produces a more sluggish frequency response. This is characteristic of an underdamped system, which will produce some degree of systolic amplification. The flush release in panel C does not produce oscillations. This is a sign of an over damped system, which will attenuate the arterial pressure waveform and produce a spuriously low systolic pressure.
Central Venous Pressure
When the PA catheter is properly placed, the proximal port of the catheter should be situated in the right atrium, and the pressure recorded from this port should be the right atrial pressure. The pressure in the right atrium is the same as the pressure in the superior vena cava, and these pressures are collectively called the central venous pressure/CVP. In the absence of tricuspid valve dysfunction, the RAP should be equivalent to the right-ventricular end-diastolic pressure (RVEDP), and RAP is the same as the pressure in the superior vena cava.
Pulmonary Wedge Pressure
The wedge pressure is obtained by slowly inflating the balloon at the tip of the RA catheter until the pulsatile pressure disappears, as shown in Figure 8.3. Note that the wedge pressure is at the same level as the diastolic pressure in the pulmonary artery. This relationship is altered in pulmonary hypertension, where the wedge pressure is lower than the pulmonary artery diastolic pressure.
The wedge pressure represents the venous pressure on the left side of the heart, and the magnified section of the wedge pressure in Figure 8.3 shows a typical venous contour that is similar to the venous pressure on the right side of the heart. The a wave is produced by left atrial contraction, the c wave is produced by closure of the mitral valve, and the v wave is produced by systolic contraction of the left ventricle against a closed mitral valve.
When the balloon on the PA catheter is inflated to obstruct flow (Q=0), there is a static column of blood between the tip of the catheter and the left atrium, and the wedge pressure at the tip of the catheter (Pw) is equivalent to the pulmonary capillary pressure (Pc) and the pressure in the left atrium (P[LA]). To summarize: if Q=0, then Pw=Pc=P[LA]. If the mitral valve is behaving normally, the left atrial pressure will be equivalent to the end-diastolic pressure (the filling pressure) of the left ventricle. Therefore, in the absence of mitral valve disease, the wedge pressure is a measure of left ventricular end-diastolic pressure/filling pressure.
The wedge pressure will reflect the left atrial pressure only if the pulmonary capillary pressure is greater than the alveolar pressure (Pc > Pa); otherwise the wedge pressure will reflect the alveolar pressure. Respiratory variations in the wedge pressure suggest that the catheter tip is in a region where alveolar pressure exceeds capillary pressure. In this situation, the wedge pressure should be measured at the end of expiration, when the alveolar pressure is closest to atmospheric (zero) pressure.
In addition to respiratory variations, the CVP and wedge pressures can vary spontaneously, independent of any change in the factors that influence these pressures. In general, a change in wedge pressure should exceed 4 mm Hg to be considered a clinically significant change.
Mean Pulmonary Artery Pressure
CO = (mPAP – mLAP) /PVR
The PVR is a global measure of the relationship between pressure and flow in the lungs. Because the pulmonary artery wedge pressure (PAWP) is equivalent to the left atrial pressure, the formula above could be written as:
CO = (mPAP – PAWP)/PVR
Ventricular End-Diastolic Pressure
Ventricular end-diastolic volume is not easily measured at the bedside, and the end-diastolic pressure (RVEDP and LVEDP) is used as the clinical measure of ventricular preload. Although the end-diastolic pressure is the clinical measure of preload, clinical studies have shown a poor correlation between end-diastolic pressure and end-diastolic volume (preload). Studies indicate that ventricular filling pressures (i.e., CVP and PAWP) are unreliable as surrogate measures of ventricular filling.
The poor correlation between end-diastolic pressures and volume is particularly noteworthy because the subjects were healthy adults with normal cardiac function. When ventricular distensibility is impaired, which is common in critically ill patients, the discrepancy between end-diastolic pressures and volumes will be greater than usual.
Despite the shortcomings of EDP as a measure of ventricular filling, CVP monitoring continues to be a popular practice in ICUs.
The reference ranges for the CVP and wedge pressure are shown in Table 9.1. Note that the very low pressure range for the CVP, which helps to promote venous return to the heart. Note also that the sedge pressure is slightly higher than the CVP; the higher pressure in the left atrium closes the flap over the foramen oval and prevents right-to-left shunting in patients with a patent foramen ovale.