Abstract
To clarify the control mechanism of ventilation during posture change, ventilatory parameters, PETCO2' and ventilatory response to CO2 were examined in 11 healthy male subjects at supine (0°) and 75° head-up tilt positions. Minute expiratory ventilation (VE), tidal volume (VT), respiratory frequency (f), end-tidal and transcutaneous PCO2 and CO2 output (VCO2), and ventilatory response to CO2 were measured during a steady state condition. VE (VA) and VT increased significantly at 75° tilt with significant decrease in PETCO2 from 40.1 mmHg (0°) to about 36.1 mmHg (75°). Transcutaneous PCO2 also decreased during tilt, by 3.3 mmHg. Physiological dead space (VD/VT) and VCO2, however, remained unchanged, and ventilatory equivalent (VE/VCO2, VA/VCO2) increased significantly. The CO2-ventilatory response curve shifted upward (or leftward) without significant change in the response slope. At 75° tilt, EMG activity of gastrocnemius muscle increased. These findings suggested that PETCO2 decreased because of increased VE (VA) with a leftward shift of CO2-ventilatory response curve. Various signals such as afferents from lower extremities might have net stimulatory effects on a CO2-ventilation control system to reset the controlled level of PETCO2 to a lower range, but without significant change in CO2-ventilatory response during upright position.
