Postinversion responses to inversion in normal subjects

The purpose of this study was to determine cardiac output and related cardiovascular responses during postinversion by comparing preinversion (baseline data) to postinversion data in healthy, normal subjects. Each of 20 subjects (means = 22 years) was inverted for five minutes. Cardiac output was measured noninvasively with the Beckman MMC and CO2 rebreathing program. ANOVA with repeated measures was used to determine significance of change between preinversion and postinversion values. The alpha level was set at 0.05 for statistical significance. During postinversion stand, there were (a) significant decreases in oxygen uptake (p less than 0.0008), cardiac output (p less than 0.0005), and stroke volume (p less than 0.0018); (b) significant increases in arteriovenous oxygen difference (p less than 0.0281), peripheral vascular resistance (p less than 0.0001), and diastolic blood pressure (p less than 0.0087); and (c) nonsignificant changes in heart rate, systolic blood pressure, and double product from the preinversion baseline standing position. The results demonstrate little if any need for concern for a subject's return to the upright position.