Hemodynamic changes and baroreflex gain in the supine hypotensive syndrome

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.

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.

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).

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.