Wave reflections and global arterial compliance during normal human pregnancy

Biomechanical remodeling of the murine descending thoracic aorta during late-gestation pregnancy

With the rise in maternal mortality rates and the growing body of epidemiological evidence linking pregnancy history to maternal cardiovascular health, it is essential to comprehend the vascular remodeling that occurs during gestation. The maternal body undergoes significant hemodynamic alterations which are believed to induce structural remodeling of the cardiovascular system. Yet, the effects of pregnancy on vascular structure and function have not been fully elucidated. Such a knowledge gap has limited our understanding of the etiology of pregnancy-induced cardiovascular disease. Towards bridging this gap, we measured the biaxial mechanical response of the murine descending thoracic aorta during a normotensive late-gestation pregnancy. Non-invasive hemodynamic measurements confirmed a 50% increase in cardiac output in the pregnant group, with no changes in peripheral blood pressure. Pregnancy was associated with significant wall thickening ( ∼14%), an increase in luminal diameter ( ∼6%), and material softening in both circumferential and axial directions. This expansive remodeling of the tissue resulted in a reduction in tensile wall stress and intrinsic tissue stiffness. Collectively, our data indicate that an increase in the geometry of the vessel may occur to accommodate for the increase in cardiac output and blood flow that occurs in pregnancy. Similarly, wall thickening accompanied by increased luminal diameter, without a change in blood pressure may be a necessary mechanism to decrease the tensile wall stress, and avoid pathophysiological events following late gestation.

A mathematical model of vascular and hemodynamics changes in early and late forms of preeclampsia

Preeclampsia-eclampsia syndrome is a leading cause of maternal mortality. The precise etiology of preeclampsia is still not well-defined and different forms exist, including early and late forms or preeclampsia, which may arise via distinctly different mechanisms. Low-dose aspirin administered at the end of the first trimester in women identified as high risk has been shown to reduce the incidence of early, but not late, preeclampsia; however, current risk factors show only fair predictive capability. There is a pressing need to develop accurate descriptions for the different forms of preeclampsia. This paper presents 1D fluid, solid, growth, and remodeling models for pregnancies complicated with early and late forms of preeclampsia. Simulations affirm a broad set of literature results that early forms of preeclampsia are characterized by elevated uterine artery pulsatility index (UA-PI) and total peripheral resistance (TPR) and lower cardiac output (CO), with modestly increased mean arterial blood pressure (MAP) in the first half of pregnancy, with elevation of TPR and MAP beginning at 20 weeks. Conversely, late forms of preeclampsia are characterized by only slightly elevated UA-PI and normal pre-term TPR, and slightly elevated MAP and CO throughout pregnancy, with increased TPR and MAP beginning after 34 weeks. Results suggest that preexisting arterial stiffness may be elevated in women that develop both early forms and late forms of preeclampsia; however, data that verify these results are lacking in the literature. Pulse wave velocity increases in early- and late-preeclampsia, coincident with increases in blood pressure; however, these increases are mainly due to the strain-stiffening response of larger arteries, rather than arterial remodeling-derived changes in material properties. These simulations affirm that early forms of preeclampsia may be associated with abnormal placentation, whereas late forms may be more closely associated with preexisting maternal cardiovascular factors; simulations also highlight several critical gaps in available data.

Second systolic peak in fetal middle cerebral artery Doppler after intrauterine transfusion

OBJECTIVE

To evaluate functional relationship between fetal circulatory response to intrauterine transfusion (IUT) as a circulatory challenge and appearance of second systolic peak (P2) in middle cerebral artery (MCA) based on hemodynamic principles.

METHODS

According to the concept of pulse wave (PW) propagation and reflection in adults, PWs arrive twice at cerebral circulation, as primary wave caused by left ventricle ejection and secondary after reflection in peripheral arteries. Thus adults show a biphasic contour of systolic blood flow in cerebral arteries. Similar waveforms may appear in fetal MCA-Doppler, as a response to IUT as a circulatory challenge. This is a proof-of-principle study, applying classical hemodynamic principles to fetal circulation. Accordingly, appearance of MCA-P2 may indicate vasoconstriction with increased PW reflection and timing of P2(Δt) should agree with the additional PW travel time down to reflection and return (Tr). To test this agreement, we searched our database for IUTs performed for severe fetal anemia, and compared Δt, obtained by Doppler, with Tr, obtained by hemodynamic calculation using human fetal data. Level of agreement was assessed using Bland-Altman-Plots.

RESULTS

We identified 21 fetuses with adequate Doppler quality for Δt evaluation. In four cases (19%) MCA-P2 was observed before the intervention, and in 17 interventions (81%) thereafter; a highly significant association between IUT and P2 appearance (p < 0.001). In these 17 interventions good agreement of P2 timing was found between Doppler assessment: Δt = 80 ± 8 ms, and hemodynamic calculation: Tr = 76 ± 4 ms.

CONCLUSION

P2 appearance in fetal MCA-Doppler seems to indicate PW reflection due to increased vasoconstriction after IUT. Thus hemodynamic considerations might enable Doppler monitoring of fetal vasoconstriction.

Center-To-Periphery Arterial Stiffness Gradient Is Attenuated and/or Reversed in Pregnancy-Associated Hypertension

Background: Non-pregnant (NP) women have a progressive increase in arterial stiffness from central-to-peripheral arteries [“stiffness gradient” (SG)], which is of physiologic importance since excessive pulsatility is filtered by the creation of wave reflections. If the aorta gets stiff with minimal or no change in the periphery, the SG is dissipated transmitting pressure disturbances to the microcirculation. It remains unknown the status of the SG in both women with healthy pregnancies (HP) and complicated by pregnancy-associated hypertension (PAH).

Objective: To determine whether HP and PAH are associated with changes in SG. Secondarily, we aim at identifying potential differences between the subgroups of PAH (pre-eclampsia and gestational hypertension).

Methods: HP (n = 10), PAH (n = 16), and healthy NP women (n = 401, to be matched for age, and cardiovascular risk with the pregnant women) were included. Carotid-to-femoral (cfPWV) and carotid-to-radial pulse wave velocity (crPWV), common carotid artery (CCA) and brachial artery (BA) diameters and elastic modulus (EM), and regional (cfPWV/crPWV or “PWV ratio”) and local (CCA EM/BA EM or “EM ratio”) SG were quantified.

Results: HP showed no changes in PWV ratio compared with NP, in the presence of significantly lower cfPWV and crPWV. HP exhibited higher arterial diameters and lower CCA EM/BA EM compared to NP, without differences with PAH. PAH was associated with a significant increase in the PWV ratio that exceeded the levels of both NP and HP, explained by a lower (although significant) reduction of cfPWV with respect to that observed in HP with respect to NP, and a higher reduction in crPWV with respect to that observed between HP and NP. The blunted reduction in cfPWV observed in PAH coincided with an increase in the CCA EM.

Conclusions: Compared with NP, HP was associated with unchanged PWV ratio but with a reduction in CCA EM/BA EM, in the setting of a generalized drop in arterial stiffness. Compared with NP and HP, PAH was associated with an “exaggerated rise” in the PWV ratio without changes in CCA EM/BA EM, in the setting of a blunt reduction in cfPWV but exaggerated crPWV drop. The SG attenuation/reversal in PAH was mainly driven by pre-eclampsia.

Role of arterial impairment in preeclampsia: should the paradigm shift?

Preeclampsia is a worldwide pregnancy complication with serious short- and long-term maternal and neonatal consequences. Our understanding of preeclampsia pathophysiology has significantly evolved over the last decades with the recognition that impaired arterial function and structure may occur early in the course of pregnancy, preceding the clinic-humoral syndrome and driving long-term cardiovascular disease risk in the future of these patients. Although an early abnormal placentation may be the inciting event for a large proportion of cases, there is growing evidence that challenges the placental hypothesis in all affected women, since placental histopathology lesions thought to be characteristic are neither sensitive nor specific markers for the disorder. Recent hemodynamic investigations and studies on left ventricular function and structure in women with preeclampsia further challenge this universal paradigm and propose that placental dysfunction could be secondary to a maternal cardiovascular maladaptation to pregnancy in certain patients. Supporting this hypothesis, certain vascular features, which are characteristically enhanced in normal pregnancy allowing a healthy vascular adaptation, are absent in preeclampsia and comparable to the nonpregnant population. However, arterial biomechanics in preeclampsia may only not cope with hemodynamic demands of pregnancy but also impose additional detrimental loads to the maternal heart ("impaired left-ventricle-aorta coupling") and transmit pressure and flow disturbances into the fetoplacental circulation ("impaired large arteries-microcirculation coupling"). In this review, we analyze the major role of the arterial dysfunction in the cardiovascular maladaptation hypothesis of preeclampsia, shed light on its potential etiopathogenic link, and discuss the complementary nature of the placental and cardiovascular theories.

Wave reflections and global arterial compliance during normal human pregnancy

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.

Maternal Cardiovascular Dysregulation During Early Pregnancy After In Vitro Fertilization Cycles in the Absence of a Corpus Luteum

Commonly used in vitro fertilization protocols produce pregnancies without a corpus luteum (CL), a major source of reproductive hormones. In vitro fertilization pregnancies without a CL showed deficient gestational increases of central (aortic) arterial compliance during the first trimester and were at increased risk for developing preeclampsia. Here, we investigated whether there was generalized impairment of cardiovascular adaptation in in vitro fertilization pregnancies without a CL compared with pregnancies conceived spontaneously or through ovarian stimulation, which lead to 1 and >1 CL, respectively (n=19-26 participants per cohort). Prototypical maternal cardiovascular adaptations of gestation were serially evaluated noninvasively, initially during the follicular phase before conception, 6× in pregnancy, and then, on average, 1.6 years post-partum. The expected increases of cardiac output, left atrial dimension, peak left ventricular filling velocity in early diastole (E wave velocity), peripheral/central arterial pulse pressure ratio, and global AC, as well as decrease in augmentation index were significantly attenuated or absent during the first trimester in women who conceived without a CL, when compared with the 1 and >1 CL cohorts, which were comparable. Thereafter, these cardiovascular measures showed recovery in the 0 CL group except for E wave velocity, which remained depressed. These results provided strong support for a critical role of CL factor(s) in the transformation of the maternal cardiovascular system in early gestation. Regimens that lead to the development of a CL or replacement of missing CL factor(s) may be indicated to improve cardiovascular function and reduce preeclampsia risk in in vitro fertilization pregnancies.