Characteristics of Aortic Diastolic Pressure Decay with Application to the Continuous Monitoring of Changes in Peripheral Vascular Resistance

Abstract

Large changes in stroke volume and peripheral vascular resistance were induced by varying the heart rate and by intra-aortically infusing acetylcholine or angiotensin II in six dogs with heart block and electromagnetic flowmeters chronically implanted around their ascending aortas. Changes in stroke volume, aortic and atrial pressures, and systemic resistance were monitored continuously for 3-6 hours under morphine-pentobarbital anesthesia. The characteristics of diastolic pressure decay at heart rates ranging from 60 beats/min to 200 beats/min and during transient periods of asystole were studied, especially with reference to the distortions caused by reflected pressure waves. The diastolic phase of pressure pulses recorded over a segment of the thoracic aorta several centimeters long centered about 4 cm cephalad to the dorsal insertion of the diaphragm could be closely approximated by a straight line on a semilogarithmic scale. Under the conditions of these experiments, changes in the slope of that line and of its reciprocal, the time constant, correlated well with concomitant variations in peripheral vascular resistance. This relationship appears to be of practical value for continuous monitoring of systemic resistance directly from the diastolic segments of pressure pulses recorded from the lower thoracic aorta. KEY WORDSatrial-ventricular pacing dog vasoactive drugs peripheral vascular resistance central aortic pressure decay • The contour of the arterial pressure pulse varies dramatically during its transmission down the arterial tree, and the pressure decay during diastole is generally not a monotonic function of time (1-4). However, a relatively smooth diastolic pressure decay has been described for aortic pressure pulses recorded near the dorsal insertion of the diaphragm (5, 6); this observation has been confirmed repeatedly in our laboratory. An initial objective of this study was to determine how uniformly an approximately monotonic diastolic decay of the aortic pressure pulse could be recorded from this particular site in the aorta in different dogs and during widely different hemodynamic conditions. Secondarily, a time-independent method of characterizing this pressure decay, presumably caused by drainage of blood from the large arteries to the periphery, was sought. If such a time-independent characterization could be established, it would allow extrapolation into the immediately preceding or subsequent systolic periods of the decrement in aortic pressure due to peripheral flow. If possible, this extrapolation would provide a means of "correcting" aortic pressures during systole for the drainage of blood from the great arteries during ventricular ejection. This increment in aortic pressure due to ventricular ejection might be directly related to individual values of stroke volume by, at worst, a small number of calibration factors or, at best, a single easily determined constant multiplier.When pressure pulses were recorded from the above-mentioned specific aortic location in preliminary experiment...

Continuous Cardiac Output Monitoring by Peripheral Blood Pressure Waveform Analysis

“…Although the correlation between CO error and MAP is positive, the degree of the correlation is mild (Table II). This result suggests that the AC in our swine model may be nearly linear over a wide pressure range, which is consistent with the canine studies of Bourgeois et al [8], [9] and a human study in which differential compliance was measured in the excised aorta at autopsy [10]. Previous studies reporting more significant degrees of nonlinearity may be a consequence of errors associated with the estimation of AC.…”

Aortic compliance in human hypertension.

“…1 - 13 The accuracy of the values of compliance quoted in these studies is, however, questionable and has not been validated because it is extremely difficult to accurately measure the total arterial blood volume. In addition, compliance is a nonlinear function of pressure.…”

Blind identification of the aortic pressure waveform from multiple peripheral artery pressure waveforms

“…Although this reference measurement may be viewed as a potential limitation of our study, we were interested in this particular site because the diastolic pressure intervals have been shown to best resemble pure exponential decays (4). Furthermore, the time constants of these decays have been shown to be extremely strong predictors of relative changes in total peripheral resistance (4). In this way, total peripheral resistance may potentially be monitored from only arterial pressure without the need for a cardiac output measurement.…”

“…Perhaps, as a result, central measurements of systolic pressure and pulse pressure have been shown to be superior in predicting patient outcome compared with corresponding measurements made in more peripheral arteries (20,26,27). Moreover, aortic pressure (particularly in the descending thoracic aorta) is less complicated by wave reflections than peripheral artery pressure (4,17), and the entire waveform usually reveals the cardiac ejection interval through the dicrotic notch (6).…”