Short-term cardiovascular adaptations to vertical head-down suspension.

Inversion devices have been advocated as means of therapy for patients with low back pain. The present investigation was undertaken to determine the cardiovascular responses to vertical head-down suspension with such a device. Eight men were monitored for three minutes in the upright and supine position, and for ten minutes in the vertical head-down position. Stroke volume (SV), heart rate (HR), cardiac output (QT), segmental arm blood flow (ABF), and segmental leg blood flow (LBF) were measured by impedance plethysmography. Moving from the upright to the supine positions resulted in an SV increase from 93.1 +/- 18.8 to 138.2 +/- 28.2 ml.beat-1 (p less than 0.01). During the first minute of suspension a further increase in SV to a maximum of 178.6 +/- 45.0 ml.beat-1 was noted. By the end of the suspension period, SV had declined to 159.6 +/- 34.2 ml.beat-1. Resumption of the upright position was associated with a reduction in SV to 94.3 +/- 15.8 ml.beat-1. Changes in QT across the supine and vertical head-down positions reflected the changes in SV. The vertical head-down position was also associated with a gradual reduction in both ABF and LBF. These data suggest that the primary effect of inverted suspension was a transient increase in venous return, which requires an effective ventricular response. An element of caution should be exercised in using these devices to avoid aggravating concurrent pathologic conditions.