Cardiovascular and hormonal responses to bicycle exercise during lower body negative pressure.

UNLABELLED

The purpose was to combine bicycle exercise and LBNP as a model to investigate the mechanisms regulating circulation during gravitational stresses.

METHODS

Cardiac Output (CO) by acetylene-, argon-, oxygen rebreathing; Heart Rate (HR) from ECG; Mean Arterial Pressure (MAP) by arm cuff; Plasma Nor-Epinephrine (NE), Epinephrine (E), and Lactate (HLa) by isotope and enzymatic techniques respectively. Total Peripheral Resistance (TPR) = MAP/CO and stroke volume (SV) = CO/HR. Six subjects exercised at 180-200 Watts in the upright position on a mechanically braked computer controlled small bicycle ergometer placed in a LBNP box. The above parameters were measured at rest and at 5 and 9 min of exercise without and with LBNP at -30 or -40 mmHg (depending on LBNP tolerance).

RESULTS AND CONCLUSIONS

In the face of an increased sympathetic nervous activity (NE+E,TPR, and HR increased) during exercise in LBNP, there was a decrease in CO and SV indicating that the venous return was insufficient. However, HLa was unchanged demonstrating that the blood flow to the working muscles did not suffer.