The Valsalva maneuver: a test of autonomic nervous system function in pregnancy.

Four hundred forty-two studies of the Valsalva maneuver were performed on 282 pregnant subjects and 37 nonpregnant female control subjects. A fetal monitor was used for graphic presentation of the subject's beat-to-beat heart rate changes. The Valsalva ratio was calculated, defined as maximum tachycardia divided by maximum bradycardia during a Valsalva maneuver. The data from the studies on pregnant subjects were grouped according to weeks of gestation: less than 13, 13 to 16, 17 to 20, 21 to 24, 25 to 28, 29 to 32, 33 to 36, and greater than 36. The mean Valsalva ratio for control subjects is higher than the mean for every gestation group, and there is a definite downward trend in the means for each successive gestation group through 29 to 32 weeks. The tendency for pregnant women to have an "abnormal" Valsalva ratio (that is, less than 1.5) also appears to be related to gestation. This exploratory study demonstrates that the Valsalva ratio is sensitive to physiologic changes associated with pregnancy. This test might have use as a noninvasive indicator of certain types of abnormalities associated with pregnancy.