Autonomic Hemodynamic Control in Pregnancy-Induced Hypertension

Heart Rate Variability and Cardiovascular Reflex Tests for Assessment of Autonomic Functions in Preeclampsia

Alterations in the autonomic cardiovascular control have been implicated to play an important etiologic role in preeclampsia. The present study was designed to evaluate autonomic functions in preeclamptic pregnant women and compare the values with normotensive pregnant and healthy nonpregnant controls. Assessment of autonomic functions was done by cardiovascular reflex tests and by analysis of heart rate variability (HRV). Cardiovascular reflex tests included deep breathing test (DBT) and lying to standing test (LST). HRV was analyzed in both time and frequency domain for quantifying the tone of autonomic nervous system to the heart. The time domain measures included standard deviation of normal R-R intervals (SDNN) and square root of mean squared differences of successive R-R intervals (RMSSD). In the frequency domain we measured total power (TP), high frequency (HF) power, low frequency (LF) power, and LF/HF ratio. Cardiovascular reflex tests showed a significant parasympathetic deficit in preeclamptic women. Among parameters of HRV, preeclamptic group had lower values of SDNN, RMSSD, TP, HF, and LF (ms²) and higher value of LF in normalised units along with high LF/HF ratio compared to normotensive pregnant and nonpregnant controls. Furthermore, normotensive pregnant women had lower values of SDNN, TP, and LF component in both absolute power and normalised units compared to nonpregnant females. The results confirm that normal pregnancy is associated with autonomic disturbances which get exaggerated in the state of preeclampsia.

Baroreflex Control of Sympathetic Nerve Activity in Hypertensive Pregnant Rats with Reduced Uterine Perfusion

OBJECTIVE

Baroreflex sensitivity is reduced in women with preeclampsia. The aim of this study was to determine whether baroreflex control of renal sympathetic nerve activity (RSNA) is altered in pregnant rats with reduced uterine perfusion (a model of human preeclampsia).

METHODS

Uterine perfusion was reduced in the third trimester by clipping the distal aorta and uterine branches of the ovarian arteries. RSNA baroreflex parameters were compared at term gestation in rats with reduced uterine perfusion (n = 12), in normal pregnant rats (n = 14) and in nonpregnant rats (n = 14).

RESULTS

Reduced perfusion rats were hypertensive (123.6 +/- 2.3 mm Hg), and normal pregnant rats were hypotensive (97.7 +/- 2.2 mm Hg), compared with controls. In rats with reduced perfusion, the baroreflex was shifted to a higher set-point, and maximum and minimum RSNA were increased compared with normal pregnant rats.

CONCLUSION

The blunted baroreflex gain of normal pregnancy is maintained in rats with reduced uterine perfusion, but a hypertensive shift in baroreflex function exists in this rat model that is associated with a reversal of the reflex maximum and minimum RSNA observed in normal pregnancy.

Baroreflex sensitivity, heart rate, and blood pressure variability in hypertensive pregnancy disorders

Hypertensive pregnancy disorders are a leading cause of perinatal and maternal morbidity and mortality. Heart rate variability (HRV), blood pressure variability (BPV), and baroreflex sensitivity (BRS) are relevant predictors of cardiovascular risk in humans. The aim of the study was to evaluate whether HRV, BPV, and BRS differ between distinct hypertensive pregnancy disorders. Continuous heart rate and blood pressure recordings were performed in 80 healthy pregnant women as controls (CON), 19 with chronic hypertension (CH), 18 with pregnancy-induced hypertension (PIH), and 44 with pre-eclampsia (PE). The data were assessed by time and frequency domain analysis, nonlinear dynamics, and BRS. BPV is markedly altered in all three groups with hypertensive disorders compared to healthy pregnancies, whereby changes were most pronounced in PE patients. Interestingly, this increase in PE patients did not lead to elevated spontaneous baroreflex events, while BPV changes in both the other hypertensive groups were paralleled by alterations in baroreflex parameters. The HRV is unaltered in CH and PE but significantly impaired in PIH. We conclude that parameters of the HRV, BPV, and BRS differ between various hypertensive pregnancy disorders. Thus, distinct clinical manifestations of hypertension in pregnancy have different pathophysiological, regulatory, and compensatory mechanisms.

Serial assessment of cardiovascular control shows early signs of developing pre-eclampsia

PURPOSE

To evaluate whether differences in autonomic cardiovascular control between normal pregnant women and women who develop pre-eclampsia later in pregnancy can be detected even before or early in pregnancy.

DESIGN

We studied 42 women, 21 multigravid with a history of pre-eclampsia and 21 primigravid, before pregnancy, at 6, 8, 12, 16, 20 and 32 weeks gestation and 15 weeks after delivery.

METHODS

The outcome of pregnancy was classified after delivery as normal pregnancy (NP group) or pre-eclampsia (PE group). Continuous heart rate and blood pressure were recorded by Portapres (TNO, Amsterdam, The Netherlands) during orthostatic stress, during rest in a supine and sitting position, and during paced breathing for periods of 1 min at breathing frequencies of 6, 10 and 15 breaths/min. Baroreflex gain from heart rate and blood pressure variability and the phase angle between both signals at low (approximately 0.1 Hz) and high frequency (respiratory rate) were analyzed by spectral analysis.

RESULTS

Eight women were diagnosed with pre-eclampsia. Subgroups did not differ in age, weight or height. The PE group showed a significantly higher mean arterial pressure before and during pregnancy [analysis of variance (ANOVA), P = 0.001], a significantly larger initial blood pressure drop to orthostatic stress before and in the first half of pregnancy (ANOVA, P = 0.002) and a significantly larger negative phase difference during supine rest at low frequency from 8 weeks onward (ANOVA P = 0.003).

CONCLUSIONS

These findings are compatible with increased resting sympathetic activity and decreased circulating volume, already present before and early in pregnancy, in women who will later develop pre-eclampsia.

Non-invasive assessment of autonomic cardiovascular control in normal human pregnancy and pregnancy- associated hypertensive disorders: a review

PURPOSE

Pre-eclampsia is a major complication of pregnancy. Although the disorder usually becomes apparent only in the third trimester of pregnancy, evidence is available that underlying pathophysiological abnormalities are already present early in pregnancy. The association between alterations in autonomic cardiovascular control and the development of hypertension in pregnancy has been investigated for some time. Non-invasive methods are especially of interest, since they have the advantage of minimal risk for the mother and the conceptus and enable repeated measurements during pregnancy. If non-invasive tests for autonomic cardiovascular control could demonstrate the increased sympathetic activity, as observed by microneurography than this method is a candidate for early identification of pre-eclampsia. Therefore, the literature on non-invasive testing of autonomic cardiovascular control in normal pregnancies and pre-eclampsia was summarized.

DATA IDENTIFICATION AND SELECTION

Medline was searched and 36 articles on autonomic cardiovascular control in human pregnancy by non-invasive test methods were reviewed. For each test method, data of different studies were summarized to evaluate if the method could discriminate between healthy pregnancy and pre-eclampsia.

CONCLUSION

Although small differences have been observed between normal pregnancy and pre-eclampsia in individual studies using non-invasive methods, the consistency in the available data is insufficient to discriminate between normal pregnancy and pre-eclampsia. The failure to demonstrate the increased sympathetic activity, as observed by direct microneurography, might be due to methodological factors of the non-invasive studies. Alternatively, sympathetic activity to resistance vessels in skeletal muscle may not be a proper reflection of autonomic cardiovascular control in pregnancy. Well-designed longitudinal research could be useful to test these suppositions.

Comparison of vagal baroreflex function in nonpregnant women and in women with normal pregnancy, preeclampsia, or gestational hypertension

OBJECTIVE

Our aim was to compare baroreflex function among nonpregnant women and among women with normal pregnancy, preeclampsia, or gestational hypertension.

STUDY DESIGN

Baroreflex function was tested in 20 women with preeclampsia, in 20 age- and gestational age-matched normotensive gravid women, in 20 age-matched nonpregnant women, and in 20 nonmatched women with gestational hypertension. The baroreflex was measured by several modalities.

RESULTS

Vagal baroreflex gain measured by cross-spectral analysis of parallel spontaneous heart rate and blood pressure changes is significantly decreased in normal pregnancy (15.8 +/- 7.2 vs 10.8 +/- 4.1 ms/mm Hg; P = 0.001), in comparison with vagal baroreflex gain in nonpregnant women. Baroreflex gain is further reduced in preeclamptic pregnancy (10.8 +/- 4.1 vs 7.2 +/- 2.6 ms/mm Hg; P = 0.003) and in gestational hypertension (10.8 +/- 4.1 vs 6.5 +/- 2.7 ms/mm Hg; P = 0.001), compared with that in normal pregnancy. Similar differences were seen with other baroreflex testing modalities.

CONCLUSIONS

The normal reduction of baroreflex gain in pregnancy is further depressed in subjects with hypertensive disorders of pregnancy.

Preeclamptic pregnancy is associated with increased sympathetic and decreased parasympathetic control of HR

Previous work from our laboratory using heart rate variability (HRV) has demonstrated that women before menopause have a more dominant parasympathetic and less effective sympathetic regulations of heart rate compared with men. Because it is still not clear whether normal or preeclamptic pregnancy coincides with alternations in the autonomic functions, we evaluated the changes of HRV in 17 nonpregnant, 17 normotensive pregnant, and 11 preeclamptic women who were clinically diagnosed without history of diabetic neuropathy, cardiac arrhythmia, and other cardiovascular diseases. Frequency-domain analysis of short-term, stationary R-R intervals was performed to evaluate the total variance, low-frequency power (LF; 0.04-0.15 Hz), high-frequency power (HF; 0.15-0.40 Hz), ratio of LF to HF (LF/HF), and LF in normalized units (LF%). Natural logarithm transformation was applied to variance, LF, HF, and LF/HF for the adjustment of the skewness of distribution. We found that the normal pregnant group had a lower R-R value and HF but had a higher LF/HF and LF% compared with the nonpregnant group. The preeclamptic group had lower HF but higher LF/HF compared with either the normal pregnant or nonpregnant group. Our results suggest that normal pregnancy is associated with a facilitation of sympathetic regulation and an attenuation of parasympathetic influence of heart rate, and such alterations are enhanced in preeclamptic pregnancy.

24-hour ECG frequency-domain measures in preeclamptic and healthy pregnant women during and after pregnancy

OBJECTIVE

The aim of the present study was to evaluate the autonomic balance in women with preeclampsia and in healthy women during and after pregnancy by means of a 24-h ECG Holter recording combined with power spectral analysis.

METHODS

Fifteen preeclamptic and 15 healthy women underwent 24-h Holter monitoring in the 32nd-36th week of gestation and 3-6 months postpartum. The power spectrum of the maternal electrocardiogram was analyzed with an autoregressive algorithm.

MAIN OUTCOME MEASURES

The power spectrum contains two major components: a low-frequency peak, primarily attributed to sympathetic tone, and a high-frequency peak, reflecting vagal tone.

RESULTS

The power spectrum of maternal heart rate variability did not differ between preeclamptic and normal women during pregnancy. After delivery, the amplitude of all components became significantly higher than those during pregnancy, with one exception: the high-frequency component in the patients who had been preeclamptic. In a comparison of the two groups, the high-frequency component after delivery was significantly lower in women who had preeclampsia than in normal healthy women (p = 0.03).

CONCLUSIONS

During pregnancy, the power spectrum is reduced and cannot be used to distinguish between patients with preeclampsia and normal healthy women. Three to 6 months after delivery, the high-frequency component is still reduced in the preeclamptic group of women. This indicates an impaired vagal modulation even in the nonpregnant state in this group of women, unlike those who had a normotensive pregnancy.

Valsalva manoeuvre can be used to study baroreflex sensitivity in pregnancy

OBJECTIVE

The aim of this study was to assess whether baroreflex sensitivity can be measured in a non-invasive manner with the Valsalva manoeuvre in pregnancy.

STUDY DESIGN

Baroreflex sensitivity was measured from the reflex response to phenylephrine injection and phase four of the Valsalva manoeuvre in nine pregnant women at 27 (range 24-33) gestational weeks.

RESULTS

Both the phenylephrine test and the Valsalva manoeuvre yielded similar estimates of baroreflex sensitivity (9.3 (4.1) ms/mmHg vs. 8.0 (5.2) ms/mmHg, Pearson's correlation coefficient r = 0.81, P < 0.008, linear regression BRSValsalva (ms/mmHg) = 1.03 x BRSPhenylephrine + 1.59). Comparable changes in heart rate and blood pressure were obtained with the phenylephrine test and the Valsalva manoeuvre.

CONCLUSION

The physiological challenge caused by the Valsalva manoeuvre can be used to measure baroreflex sensitivity in pregnancy. A possibility to study baroreflex function non-invasively, without pharmacological intervention, benefits future research of blood pressure regulation in pregnancy.

Heart rate and blood pressure variabilities are increased in pregnancy-induced hypertension

OBJECTIVE

Our purpose was to study whether cardiovascular changes in pregnancy-induced hypertension are associated with the increase in sympathetic control of hemodynamics and change in sympathovagal balance.

STUDY DESIGN

Fourteen women with pregnancy-induced hypertension and 16 women with uncomplicated pregnancies of similar duration were studied. Electrocardiographic signals and arterial blood pressure (Finapres monitor, Ohmeda) were continuously measured noninvasively throughout the study. Heart rate and blood pressure were measured while the subject was breathing (1) with her normal tidal volume at a frequency of 15 breaths per minute and (2) as deeply as possible at a frequency of six breaths per minute. Heart rate and systolic blood pressure variability were calculated with use of the autoregressive model of spectral analysis.

RESULTS

Heart rate and systolic blood pressure variabilities were significantly increased in women with pregnancy-induced hypertension compared with normotensive pregnant women. This increase was greatest in the high frequency component of heart rate variability (p = 0.02) while the women were breathing with a normal tidal volume. Further, the medium frequency (p = 0.03) and high-frequency variabilities (p = 0.03) of systolic blood pressure were significantly increased in women with preeclampsia compared with normotensive pregnant subjects.

CONCLUSIONS

Neural control of the heart rate and blood pressure are disturbed in pregnancy-induced hypertension, as shown by increased heart rate and blood pressure variability. Both the sympathetic and parasympathetic control of the heart rate and blood pressure appear to be increased. The maladaptation of the cardiovascular system in women with pregnancy-induced hypertension is manifested as a lack of the physiologic decline in cardiovascular oscillations.

Circadian rhythm of frequency-domain measures of heart rate variability in pregnancy

OBJECTIVE

To examine frequency domain measures of heart rate variability and their circadian rhythms in pregnancy.

DESIGN

A longitudinal study.

SETTING

University hospital in Turku, Finland.

PARTICIPANTS

Sixteen healthy women between 11 and 27 weeks of pregnancy; 12 women before pregnancy; and four women postpartum.

MAIN OUTCOME MEASURES

Heart rate variability as measured in frequency domain from 24-hour ambulatory electrocardiography.

RESULTS

Pregnancy was associated with a lower standard deviation of R-R intervals (P < 0.01), with reduced very low (P < 0.05), low (P < 0.01), and high frequency (P < 0.05) power spectral components of heart rate variability. The high frequency power was lower at night in pregnancy, but similar in the daytime in pregnant and nonpregnant women.

CONCLUSIONS

Pregnancy is associated with an overall reduction in heart rate variability, most markedly reflected in the low frequency component. This suggests altered baroreflex or sympathetic modulation of heart rate, and decreased vagal activation at night.

Preeclampsia -- a state of sympathetic overactivity

BACKGROUND

Preeclampsia is characterized by a marked increase in peripheral vascular resistance leading to an increase in blood pressure, but the triggering mechanisms are unclear.

METHODS

To determine whether augmented sympathetic vasoconstrictor activity may be an important mechanism in mediating the increase in vasomotor tone, we measured postganglionic sympathetic-nerve activity in the blood vessels of skeletal muscle by means of intraneural microelectrodes in nine women with preeclampsia, eight normotensive pregnant women, six normotensive nonpregnant women, and seven nonpregnant women with hypertension, both at rest and during noninvasive cardiovascular-reflex testing (with the Valsalva maneuver and the cold pressor test).

RESULTS

The mean (+/-SE) rate of sympathetic-nerve activity in the normotensive pregnant women (10+/-1 bursts per minute) was not significantly different from that in normotensive nonpregnant women (12+/-2 bursts per minute) or hypertensive nonpregnant women (15+/-3 bursts per minute). In contrast, the rate of sympathetic-nerve activity in the patients with preeclampsia (33+/-3 bursts per minute) was more than three times as high as that in the normotensive pregnant women (P<0.05) and more than twice as high as in the group of nonpregnant women with hypertension (P<0.05). Hemodynamic and sympathetic-nerve responses to both reflex tests did not differ significantly among the four groups. Six patients with preeclampsia were studied again after delivery; mean sympathetic-nerve activity at that time had decreased significantly from the value during pregnancy (from 36+/-4 to 13+/-2 bursts per minute, P<0.01), as had mean arterial pressure (from 118+/-3 to 96+/-1 mm Hg, P<0.01).

CONCLUSIONS

Preeclampsia is a state of sympathetic overactivity, which reverts to normal after delivery. Our data indicate that the increases in peripheral vascular resistance and blood pressure that characterize this disorder are mediated, at least in part, by a substantial increase in sympathetic vasoconstrictor activity.

Autonomic cardiovascular control in pregnancy

Pregnancy is associated with profound adaptive changes in the maternal hemodynamics. Although the autonomic nervous system plays a central role in the adaptation of the cardiovascular system to various needs, its role in the adaptation of the circulation to the demands of pregnancy is poorly understood. This paper reviews the literature of autonomic cardiovascular control in pregnancy as studied with the cardiovascular reflex tests. A Medline search and manual cross-referencing for prior publications were used. All papers found on the hemodynamic effects of the Valsalva maneuver, the orthostatic test, the deep breathing test, the isometric handgrip test and maternal heart rate variability in pregnancy were reviewed and all publications that studied short-term changes in maternal heart rate and blood pressure were included. The beginning of pregnancy is associated with sympathetic reactivity, whereas the latter half of pregnancy is characterized by increased hemodynamic stability during orthostatic stress. The heart rate response to the Valsalva maneuver is blunted in mid-pregnancy, possibly due to changes in the baroreflex and increased maternal blood volume. Heart rate variability is significantly reduced in the second trimester. Cardiovascular reflex tests can be used to study drug effects on maternal circulation non-invasively.