Wave reflections and global arterial compliance during normal human pregnancy

Abstract

Profound changes occur in the maternal circulation during pregnancy. Routine measures of arterial function - central systolic pressure (CSP) and augmentation index (AIx) - decline during normal human pregnancy. The objectives of this study were twofold: (1) explore wave reflection indices besides CSP and AIx that are not routinely reported, if at all, during normal human pregnancy; and (2) compare wave reflection indices and global arterial compliance (gAC) obtained from carotid artery pressure waveforms (CAPW) as a surrogate for aortic pressure waveforms (AOPW) versus AOPW synthesized from radial artery pressure waveforms (RAPW) using a generalized transfer function. To our knowledge, a comparison of these two methods has not been previously evaluated in the context of pregnancy. Ten healthy women with normal singleton pregnancies were studied using applanation tonometry (SphygmoCor) at pre-conception, and then during 10-12 and 33-35 gestational weeks. CSP and AIx declined, and gAC increased during pregnancy as previously reported. As a consequence of the rise in gAC, the return of reflected waves of lesser magnitude from peripheral reflection sites to the aorta was delayed that, in turn, reduced systolic duration of reflected waves, augmentation index, central systolic pressure, LV wasted energy due to reflected waves, and increased brachial-central pulse pressure. For several wave reflection indices, those derived from CAPW as a surrogate for AOPW versus RAPW using a generalized transfer function registered greater gestational increases of arterial compliance. This discordance may reflect imprecision of the generalized transfer function for some waveform parameters, though potential divergence of carotid artery and aortic pressure waveforms during pregnancy cannot be excluded.

Keywords: Applanation tonometry; SphygmoCor; maternal cardiovascular function; pulse wave analysis; pulse wave velocity.

Figure 1

Central aortic pressure waveform synthesized from a radial pressure waveform. P _i indicates the merging (or inflection) point of the forward traveling and reflected (or backward traveling) waves. The early part of the ascending aortic pressure (i.e., forward traveling) wave with amplitude (P1) is generated by left ventricular (LV) ejection. The later part of the pressure wave with amplitude (AP) is the reflected wave arriving during systole and adding to the forward traveling pressure wave. Thus, pulse pressure (PP) = P1 + AP and augmentation index (AIx) = AP/PP. Tr is the sum of the travel time of the forward traveling wave from the LV to the periphery and the backward traveling reflected wave from the periphery to the LV; SDR is systolic duration of the reflected wave; ED is ejection duration (or systolic pressure time, SPT); DPTI is diastolic pressure time integral (or index) and DPT is diastolic pressure time. The area under the systolic portion of the reflected wave (dark shaded area) is defined as LV wasted energy (LVEw). Systolic pressure time index (SPTI) = ΔSPTI + LVEw. From Nichols et al. (2015) with permission.

Figure 2

Estimation of global arterial compliance using the area method (AC _area). A central aortic pressure waveform obtained from the carotid artery pressure waveform as a surrogate or synthesized from the radial artery pressure waveform using a generalized transfer function is depicted. PB and PD define the beginning and end of the diastolic portion of the aortic pressure waveform. Ad is the area under the curve defined by these boundaries. AC _area = Ad/[SVR(PB‐PD)], where SVR is systemic vascular resistance calculated by MAP/CO, and CO obtained by echocardiography (see Materials and Methods for further details). Based on Liu et al. (1986) and Poppas et al. (1997).

Figure 3

Central aortic systolic pressure for each subject (N = 10) using (A) carotid artery or (B) aortic pressure waveforms, the latter derived from radial artery pressure waveforms using a generalized transfer function. (C) Mean ± 1.96SE.

Figure 4

Augmentation index normalized to heart rate of 75 b/min for each subject (N = 10) using (A) carotid artery or (B) aortic pressure waveforms, the latter derived from radial artery pressure waveforms using a generalized transfer function. (C) Mean ± 1.96SE. Method: P = 0.003; Time: P = 0.004; Method × Time: P = 0.03.

Figure 5

Round‐trip travel time of the pressure wave to and from major reflecting sites for each subject (N = 10) using (A) carotid artery or (B) aortic pressure waveforms, the latter derived from radial artery pressure waveforms using a generalized transfer function. (C) Mean ± 1.96SE. Time: P = 0.010; Method × Time: P = 0.10

Figure 6

Systolic duration of the reflected wave for each subject (N = 10) using (A) carotid artery or (B) aortic pressure waveforms, the latter derived from radial artery pressure waveforms using a generalized transfer function. (C) Mean ± 1.96SE. Method: P = 0.001; Time: P = 0.012; Method × Time: P = 0.03.

Figure 7

Left ventricular wasted energy for each subject (N = 10) using (A) carotid artery or (B) aortic pressure waveforms, the latter derived from radial artery pressure waveforms using a generalized transfer function. (C) Mean ± 1.96SE. Method: P = 0.01; Time: P = 0.001; Method × Time: P = 0.13.

Figure 8

Global arterial compliance for each subject (N = 10) using (A) carotid artery or (B) aortic pressure waveforms, the latter derived from radial artery pressure waveforms using a generalized transfer function. (C) Mean ± 1.96SE. Method: P = 0.01; Time: P = 0.13.