Noninvasive estimation of baroreflex sensitivity using pressure pulse amplification

Quantification of peripheral and central blood pressure variability using a time-frequency method

Systolic blood pressure variability (BPV) is associated with cardiovascular events. As the beat-to-beat variation of blood pressure is due to interaction of several cardiovascular control systems operating with different response times, assessment of BPV by spectral analysis using the continuous measurement of arterial pressure in the finger is used to differentiate the contribution of these systems in regulating blood pressure. However, as baroreceptors are centrally located, this study considered applying a continuous aortic pressure signal estimated noninvasively from finger pressure for assessment of systolic BPV by a time-frequency method using Short Time Fourier Transform (STFT). The average ratio of low frequency and high frequency power band (LF_PB/HF_PB) was computed by time-frequency decomposition of peripheral systolic pressure (pSBP) and derived central aortic systolic blood pressure (cSBP) in 30 healthy subjects (25-62 years) as a marker of balance between cardiovascular control systems contributing in low and high frequency blood pressure variability. The results showed that the BPV assessed from finger pressure (pBPV) overestimated the BPV values compared to that assessed from central aortic pressure (cBPV) for identical cardiac cycles (P<;0.001), with the overestimation being greater at higher power.

Assessment of baroreflex sensitivity by continuous noninvasive monitoring of peripheral and central aortic pressure

Noninvasive assessment of baroreceptor sensitivity (BRS) facilitates clinical investigation of autonomic function. The spontaneous sequence method estimates BRS using the continuous measurement of arterial pressure in the finger. Since the baroreceptors are centrally located (aortic arch, carotid arteries), this study assessed the use of a continuous aortic pressure signal derived from the peripheral pressure pulse to compute the BRS from changes in systolic pressure (SBP) and pulse interval (PI). BRS computed from central aortic (cBRS) and peripheral pressure (pBRS) was calculated in 12 healthy subjects (25-62 years, 7 females). The difference between pBRS and cBRS was calculated for four levels of pulse lags between changes in SBP and PI. For each lag and for the pooled data for all lags, cBRS was significantly correlated with pBRS (r(2)=0.82). The within subject difference ranged from -41.2% to 59.2%. This difference was not related to age, gender of hemodynamic parameters (systolic or diastolic pressure, heart rate, aortic pulse wave velocity). However 18.2% of the variance was due to the difference in the number of spontaneous pulse sequences used to determine values of cBRS and pBRS. The differences between pBRS and cBRS are in the range of values of BRS as those found, in other studies, to discriminate between patient groups with different levels of autonomic function. Findings of this study suggest that, given the heart rate dependent amplification of the arterial pressure pulse between the central aorta and the peripheral limbs, BRS determined from central aortic pressure derived from the peripheral pulse may provide an improved method for noninvasive assessment of baroreceptor function.

Relating drug-induced changes in carotid artery mechanics to cardiovagal and sympathetic baroreflex control

Previous evidence indicates that sensitivity of the baroreflex cardiovagal and sympathetic arms is dissociated. In addition, pharmacologic assessment of baroreflex sensitivity (BRS) has revealed that cardiovagal, but not sympathetic, BRS is greater when blood pressure is increasing versus falling. The origin of this hysteresis is unknown. In this study, carotid artery distensibility and absolute distension (diameter) were assessed to test the hypothesis that vessel mechanics in barosensitive regions affect the BRS of cardiovagal, but not sympathetic, outflow. R-R interval (i.e. time between successive R waves), finger arterial blood pressure, muscle sympathetic nerve activity, and carotid artery dimensions (B-mode imaging) were measured during sequential infusions of sodium nitroprusside (SNP) and phenylephrine (PHE). Systolic and diastolic common carotid artery diameters and pulse pressure were recorded to calculate distensibility of this vessel under each drug condition. Cardiovagal BRS was greater when blood pressure was increasing versus decreasing (p < 0.01). Sympathetic BRS was not affected by direction of pressure change. Distensibility did not differ between SNP and PHE injections. However, compared with SNP, infusion of PHE resulted in larger absolute systolic and diastolic carotid diameters (p < 0.001). Therefore, cardiovagal reflex hysteresis was related to drug-induced changes in common carotid artery diameter but not distensibility. The lack of sympathetic hysteresis in this model suggests a relative insensitivity of this baroreflex component to carotid artery dimensions and provides a possible mechanism for the dissociation between cardiovagal and sympathetic BRS.

Augmentation index as a measure of peripheral vascular disease state

Pulse pressure, especially in central arteries, is an independent predictor of adverse cardiovascular events in patients with increased elastic artery stiffness (or elastance). The central arterial pressure wave is composed of a forward traveling wave generated by left ventricular ejection and a later arriving reflected wave from the periphery. Increased stiffness of elastic arteries is the primary cause of increased pulse pressure in subjects with degeneration and hyperplasia of the arterial wall. As stiffness increases, transmission velocity of both forward and reflected waves increase, which causes the reflected wave to arrive earlier in the central aorta and augments pressure in late systole [ie, augmentation index = (augmented pressure/pulse pressure) increases]. These changes in wave reflection properties are associated with vascular disease and aging and cause an increase in left ventricular afterload, myocardial mass, and oxygen consumption. Vasoactive drugs have little direct effect on large elastic arteries but can markedly change wave reflection amplitude and augmentation index by altering stiffness of the muscular arteries and modifying transmission velocity of the reflected wave from the periphery to the heart. This change in amplitude and timing of the reflected wave causes a generalized change in central arterial systolic and pulse pressure that is not detected by cuff pressure measurements in the brachial artery.