The effects of a 10-day period of head-down tilt on the cardiovascular responses to intravenous saline loading.

We tested the hypothesis that adaptation to microgravity, simulated by a 10-day period of head-down tilt (HDT), alters the responses to an intravenous fluid load by causing a larger fraction of the infused volume to be retained and magnifying the acute hemodynamic effects. HDT caused a significant (p<0.01) decrease in blood volume (-0.72 liters) and weight (-1.6 kg). Rapid infusion (22 ml/kg over 20 min.) of isotonic saline before, during, and after HDT produced a transient blood volume expansion with 18% of the infusate retained intravascularly after 2 hours. HDT had no effect on this response. Control hemodynamics were significantly different with lower cardiac output and higher total peripheral resistance (TPR) during and after HDT. Saline caused significant increases in cardiac output, heart rate, and stroke volume and a decrease in TPR. The magnitude and time course of these changes were not altered by HDT. The results refute the hypothesis and suggest that during HDT new set points or operating points were established for the control of intravascular volume and hemodynamic state.