CARDIORESPIRATORY CHANGES DURING THE ONSET OF HEAD-DOWN TILT

Background: During spaceflight, changes in the cardiovascular system a nd in pulmonary mechanics take place but no apparent impairment of res piratory function occurs. However, little is known about the first hou rs in microgravity. Hypothesis: The changes occurring at the same time in the cardiovascular and pulmonary systems could interact and lead t o a transient impairment of blood gases at the onset of microgravity. Methods: Cardiovascular and respiratory changes were studied during 6 degrees head-down tilt (HDT), a now well-known method for simulation o f microgravity. After a baseline standing position, 10 men were expose d to 4 h of 6 degrees HDT. Hemodynamic parameters were measured by tho racic electrical bioimpedance. Ventilatory parameters were studied by spirographic measurements and mass spectrometer analysis of expired ga ses. Arterial blood parameters were analyzed by specific electrodes. R esults: Immediately after tilting, stroke volume and cardiac output in creased, as measured by thoracic bio-impedance, while heart rate and t horacic fluid index decreased. Blood gas analysis showed hypercapnia, acidosis and a tendency to hypoxia. These changes were related to hypo ventilation shown by the decrease in minute ventilation. After usually less than 30 min, all the parameters reached a steady state. Return t o the standing position provoked reverse variations with orthostatic i ntolerance in 4 subjects. Conclusion: Marked changes in both the cardi ovascular and respiratory systems occur within the first minutes of HD T (i.e., transition to simulated microgravity).