The arterial blood pressure is one of the most common and most unreliable measurements in modern medicine. The folly of the blood pressure measurement is demonstrated in the following scenario. The most common medical disorder in the United States is hypertension, which affects 20% of Americans over the age of 6 years (1,2). About 80% of patients with hypertension have no evidence of end-organ involvement (1), which means that in the majority of cases of hypertension, the presence of the disorder is based solely on the blood pressure measurement. The standard method for measuring the blood pressure (i.e., sphygmomanometry) is well recognized for its inaccuracy (3), as pointed out by the warning issued by the American Heart Association in the introductory statement. Thus, the most common medical disorder in the United States (high blood pressure) owes its existence to a technique that was disputed by expert opinion almost 50 years ago.
This chapter describes the methods used to monitor the arterial blood pressure in critically ill patients. The first section describes some of the shortcomings of indirect pressure recordings, and the final section describes the direct method of recording pressures with intravascular catheters.
Indirect measurements of arterial pressure are obtained with a device that consists of a cloth cuff with an inflatable rubber bladder on the inner surface. The cloth cuff is wrapped around the arm or leg, and the bladder is inflated to generate a pressure that compresses the underlying artery and veins. The bladder is then slowly deflated, allowing the compressed artery to open, and the arterial pressure is determined either by detecting sounds that are generated (auscultation method) or by recording vascular pulsations (oscillometric method). The accuracy of these indirect pressure determinations is influenced by the size of the inflatable bladder relative to the girth of the compressed limb.
DIMENSIONS OF THE CUFF BLADDER
The inflatable cuff should produce uniform compression of the underlying artery to ensure optimal recordings when the artery is allowed to open. The ability of cuff inflation to produce uniform arterial compression is a function of the size of the inflatable bladder relative to the size of the limb. Figure 9.1 shows the optimal dimensions of the cuff bladder for indirect measurements of brachial artery pressure. The length of the bladder should be at least 80% of the circumference of the upper arm, and the width of the bladder should be at least 40% of the upper arm circumference (3,4). If the bladder size is too small for the arm circumference, the indirect pressure measurements will be falsely elevated (1-5).
The use of inappropriate-sized cuffs is one of the most common sources of error in indirect blood pressure measurements (1-5), so attention to this matter is important. A simple bedside method for evaluating cuff size is described below.
The following maneuver is recommended for each patient before recording indirect pressures in the arm or leg. First, align the cuff so that its long axis is parallel to the long axis of the arm. Then turn the cuff around so that the inner surface (bladder side) is facing outward. Now wrap the cuff around the upper arm. The bladder (width) should encircle half of the upper arm (circumference). If the bladder encompasses less than half of the upper arm with this maneuver, select a larger cuff for the pressure measurements. No change in cuff size is necessary if the bladder encircles more than half of the upper arm with this maneuver because large cuffs (on thin limbs) do not produce considerable errors in blood pressure recording.
The following is a brief description of two indirect methods of blood pressure recording.
The standard method of measuring blood pressure involves manual inflation of an arm cuff placed over the brachial artery. The cuff is then gradually deflated, and the pressure is determined by sounds (called Korotkoff sounds) that are generated when the artery begins to open. The details of this method are not be presented in detail here; see References 3-5 for a review.
The Korotkoff Sounds
One of the fundamental problems of the auscultatory method is the ability to hear the Korotkoff sounds. The threshold frequency for sound detection by the human ear is 16 Hz, and the frequency range of the Korotkoff sounds is just above this threshold at 25 to 50 Hz (6). (Human speech occurs at frequencies of 120 to 250 Hz, whereas optimum sound detection by the human ear occurs at frequencies of 2000 to 3000 Hz.) Thus, the human ear is almost deaf to the sounds it must hear to measure blood pressure.
Bell-shaped transducer stethoscope heads are designed to detect lower-frequency sounds than flat, diaphragm-shaped transducer heads. Therefore, to optimize detection of the low-frequency Korotkoff sounds, the American Heart Association recommends that a bell-shaped stethoscope head be used to measure blood pressure (2,4). This preference often is not appreciated, as illustrated by the fact that some stethoscopes are manufactured without a bell-shaped head.
Low Flow States
When systemic blood flow is reduced, the auscultatory method can significantly underestimate the actual blood pressure. This is illustrated in Table 9.1, which shows the difference between systolic pressures measured by auscultation of Korotkoff sounds and systolic pressures recorded with intraarterial catheters in hypotensive patients with low cardiac outputs. In half of the patients, the auscultatory method underestimated the actual systolic blood pressure by at least 30 mm Hg. According to the American Association for Medical Instrumentation, indirect pressure measurements should be within 5 mm Hg of directly recorded pressures to be considered accurate (7). Thus, there was not a single accurate pressure recording with the auscultatory method in the study results shown in Table 9.1.
The poor performance of the auscultatory method in low flow states is not surprising because Korotkoff sounds are generated by the flow of blood through partially constricted arteries. Thus, as flow diminishes, the Korotkoff sounds become less audible, and the earliest sounds that signal the systolic pressure might not be detected. The potential for large measurement errors such as those shown in Table 9.1 is the reason the auscultatory method should never be used to monitor arterial pressures in hemodynamically unstable patients.
The oscillometric method is based on the principle of plethysmography to detect pulsatile pressure changes in a nearby artery. When an arm cuff is inflated, pulsatile pressure changes in an underlying artery produce periodic pressure changes in the inflated cuff. The oscillometric method thus measures periodic pressure changes (i.e., oscillations) in an inflated cuff as an indirect measure of pulsatile pressure in an underlying artery (8). The most recognized oscillometric device is the Dinamap (device for indirect assessment of mean arterial pressure), first introduced for clinical use in 1976. The original device could detect only mean arterial pressures, but more modern devices are capable of measuring both systolic and diastolic pressures.
Although considered more reliable than the auscultatory method, the oscillometric method also suffers from a limited and variable accuracy. This is demonstrated in Figure 9.2, which shows a comparison of systolic pressures measured with an automated oscillometric device and systolic pressures recorded with brachial artery catheters in patients undergoing cardiopulmonary bypass surgery. The dark line is the line of unity, where pressures obtained with both recording techniques are identical. The lighter lines are placed 5 mm Hg above and below the line of unity, and (because indirect pressures should be within 5 mm Hg of directly recorded pressures) the area bounded by the lighter lines represents the zone of acceptable accuracy for oscillometric pressure measurements. Note that most of the points fall outside the zone bounded by the lighter lines, (filled squares) indicating that a majority of the oscillometric measurements are inaccurate.
Automated oscillometric devices have gained widespread popularity in recent years, both in hospitals and in outpatient clinics, so it is important to be aware of their limitations.
Direct recording of intravascular pressures is recommended for all patients in the ICU who require careful monitoring of arterial pressure. Unfortunately, direct arterial pressure monitoring has its own shortcomings. The following description is intended to help reduce errors in interpretation of directly recorded arterial pressures.
PRESSURE VERSUS FLOW WAVES
Although there is a tendency to consider arterial pressure an index of blood flow, pressure and flow are distinct physical entities. Ejection of the stroke volume generates both a pressure wave and a flow wave. Under normal conditions, the pressure wave travels 20 times faster than the flow wave (10 m/second versus 0.5 m/second), and thus the pulse pressure recorded in a peripheral artery precedes the corresponding stroke volume by a matter of seconds (9). When vascular impedance (i.e., compliance and resistance) is increased, the velocity of transmission of the pressure wave is increased while the velocity of transmission of the flow wave is decreased. (When vascular impedance is reduced, pressure can be diminished while flow is enhanced.) Thus, when vascular impedance is abnormal, the arterial pressure is not a reliable index of arterial flow. This discrepancy between pressure and flow is one of the major limitations of arterial pressure monitoring.
THE ARTERIAL PRESSURE WAVEFORM
The contour of the arterial pressure waveform changes as the pressure wave moves away from the proximal aorta. This is shown in Figure 9.3. 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 that the mean arterial pressure remains unchanged. Therefore, the mean arterial pressure is a more accurate measure of central aortic pressure.
The increase in systolic pressure in peripheral arteries is the result of pressure waves that are reflected back from the periphery (10). These reflected waves originate from vascular bifurcations and from narrowed blood vessels. As the pressure wave moves peripherally, wave reflections become more prominent, and the reflected waves add to the systolic pressure wave and amplify the systolic pressure. Amplification of the systolic pressure is particularly prominent when the arteries are noncompliant, causing reflected waves to bounce back faster. This is the mechanism for systolic hypertension in the elderly (10). Because a large proportion of patients in the ICU are elderly, systolic pressure amplification is probably commonplace in the ICU.
Fluid-filled recording systems can produce artifacts that further distort the arterial pressure waveform. Failure to recognize recording system artifacts can lead to errors in interpretation.
Vascular pressures are recorded by fluid-filled plastic tubes that connect the arterial catheters to the pressure transducers. This fluid-filled system can oscillate spontaneously, and the oscillations can distort the arterial pressure waveform (11,12).
The performance of a resonant system is defined by the resonant frequency and the damping factor of the system. The resonant frequency is the inherent frequency of oscillations produced in the system when it 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 an overdamped system.
Three waveforms obtained from different recording systems are shown in Figure 9.4. The waveform 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. The recording systems used in clinical practice are naturally underdamped, and these systems can amplify the systolic pressure by as much as 25 mm Hg (13). The systolic amplification can be minimized by limiting the length of the connector tubing between the catheter and the pressure transducer.
The waveform in panel C of Figure 9.4 shows an attenuated systolic peak with a gradual upslope and downslope and a narrow pulse pressure. This waveform is from an overdamped system. Overdamping reduces the gain of the system and is sometimes the result of air bubbles trapped in the connector tubing or in the dome of the pressure transducer. Flushing the hydraulic system to evacuate air bubbles should improve an overdamped signal.
Unfortunately, it is not always possible to identify underdamped and overdamped systems using the arterial pressure waveform. The test described in the next section can help in this regard.
THE FLUSH TEST
A brief flush to the catheter-tubing system can be applied to determine whether the recording system is distorting the pressure waveform (12,14). Most commercially available transducer systems are equipped with a one-way valve that can be used to deliver a flush from a pressurized source. Figure 9.4 shows the results of a flush test in three different situations. In each case, the pressure increases abruptly when the flush is applied. However, the response at the end of the flush differs in each panel. In panel A, the flush is followed by a few oscillating waveforms. The frequency of these oscillations is the resonant frequency (f) of the recording system, which is calculated as the reciprocal of the time period between the oscillations. When using standard strip-chart recording paper divided into 1-mm segments, f can be determined by measuring the distance between oscillations and dividing this into the paper speed (11); that is, f (in Hz) = paper speed (in mm/second) divided by the distance between oscillations (in mm). In the example shown in panel A, the distance (d) between oscillations is 1.0 mm and the paper speed is 25 mm/second, so f = 25 Hz (25 mm/second divided by 1.0 mm).
Signal distortion is minimal when the resonant frequency of the recording system is five times greater than the major frequency in the arterial pressure waveform. Because the major frequency in the arterial pulse is approximately 5 Hz (15), the resonant frequency of the recording system in panel A (25 Hz) is five times greater than the frequency in the incoming waveform, and the system will not distort the incoming waveform.
The flush test in panel B of Figure 9.4 reveals a resonant frequency of 12.5 Hz (f = 25/2). This is too close to the frequency of arterial pressure waveforms, so this system will distort the incoming signal and produce systolic amplification.
The flush test shown in panel C of Figure 9.4 does not produce any oscillations. This indicates that the system is overdamped, and this system will produce a spuriously low pressure recording. When an overdamped system is discovered, the system should be flushed thoroughly (including all stopcocks in the system) to release any trapped air bubbles. If this does not correct the problem, the arterial catheter should be repositioned or changed.
MEAN ARTERIAL PRESSURE
The mean arterial pressure has two features that make it superior to the systolic pressure for arterial pressure monitoring. First, the mean pressure is the true driving pressure for peripheral blood flow. Second, the mean pressure does not change as the pressure waveform moves distally, nor is it altered by distortions generated by recording systems (11).
The mean arterial pressure can be measured or estimated. Most electronic pressure monitoring devices can measure mean arterial pressure by integrating the area under the pressure waveform and dividing this by the duration of the cardiac cycle. The electronic measurement is preferred to the estimated mean pressure, which is derived as the diastolic pressure plus one-third of the pulse pressure. This formula is based on the assumption that diastole represents two-thirds of the cardiac cycle, which corresponds to a heart rate of 60 beats/minute. Therefore, heart rates faster than 60 beats/minute, which are common in critically ill patients, lead to errors in the estimated mean arterial pressure.
In most circumstances, the mean pressures in the aorta, radial artery, and femoral artery are within 3 mm Hg of each other. However in patients undergoing cardiopulmonary bypass surgery, the mean radial artery pressure can be significantly (more than 5 mm Hg) lower than the mean pressures in the aorta and femoral artery (16). This condition may be caused by a selective decrease in vascular resistance in the hand, because compression of the wrist often abolishes the pressure difference. An increase in radial artery pressure of at least 5 mm Hg when the wrist is compressed (distal to the radial artery catheter) suggests a discrepancy between radial artery pressure and pressures in other regions of the circulation (17).