General Obstetrics and Gynecology Obstetrics
Hemodynamic changes and baroreflex gain in the supine hypotensive syndrome

https://doi.org/10.1067/mob.2002.127304Get rights and content

Abstract

Objective: The purpose of this study was to determine whether supine hypotensive syndrome is caused by a decreased baroreflex gain that results in inadequate compensatory responses to restore cardiac output. Study Design: Ten third-trimester gravid women with symptoms that were suggestive of supine hypotensive syndrome and 10 control subjects without symptoms were studied. Patients were studied initially for 30 minutes in the lateral recumbent position with continuous heart rate, blood pressure, external cardiotocography, and cardiac output determinations every 5 minutes. After 30 minutes, the patient was repositioned supine for up to an additional 30 minutes, and all measurements were repeated similarly. This lateral-supine sequence was repeated twice or until symptoms precluded further testing. Results: The two groups were demographically similar. With maternal position change to supine, cardiac output (−2.7 L/min vs −1.0 L/min, P =.04) and mean arterial blood pressure (−10.7 mm Hg vs −2.5 mm Hg, P =.03) were respectively lower, and heart rate (+26.9 vs +14.4 beats/min, P =.04) was higher in the subjects with supine hypotensive syndrome than in the control subjects. Baroreflex gain, however, did not differ between the subjects with supine hypotensive syndrome and control subjects (P =.26). Conclusion: We were unable to demonstrate a decrease in baroreflex gain as a possible cause of supine hypotensive syndrome. Inadequate pelvic collateral circulation that leads to a greater decrease in cardiac output may be causative in the syndrome. (Am J Obstet Gynecol 2002;187:1636-41.)

Section snippets

Subjects

Pregnant subjects were recruited from the Women and Infants Hospital obstetric clinic and associated ultrasound facilities. Entry criteria were fulfilled by response to a 10-question survey. The questionnaire identified two groups of subjects: women who had symptoms of supine hypotension (ie, dizziness, nausea, and anxiety in relation to supine positioning that is temporally related to the second and third trimesters) and women who had none of these symptoms. Potential candidates were

Results

Results of a comparison of demographic variables for SHS and non-SHS groups are presented in Table I, with no differences noted between the groups.

. Demographic characteristics of SHS and control subjects

GroupAge (y)Body mass index (kg/m2)Gestational age (wk)Hemoglobin (g/dL)Hematocrit (%)
SHS (n = 10)28 ± 6.931.7 ± 5.833.6 ± 3.711.9 ± 1.635.7 ± 4.1
Control (n = 10)27 ± 7.333.2 ± 6.133.9 ± 4.111.5 ± 0.833.7 ± 1.7
P value.6.56.95.62.35

Values are presented as mean ± SD.

Each group contained one smoker.

Comment

The sequence of events that precede symptoms of SHS is purported to be as follows: On supine positioning, there is a degree of compression of the inferior vena cava by the gravid uterus that produces a variable decrement in venous return. This, in turn, results in decreased stroke volume, which is sensed as decreased pressure in the aortic arch baroreceptors. Tachycardia develops in an attempt to restore cardiac output. However, with the persistence of caval compression, cardiac output is

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