Forced expiratory flow-volume curves during the application of lower-body negative pressure

Ten normal subjects were studied during supine rest and quiet standing, and when exposed, supine, to lower-body negative pressure (LBNP) of 30, 40 and 50 mmHg, each for a period of 7 min, in random order. Their partial and complete flow-volume curves, heart rate and blood pressure were recorded during the last 3 min in each condition. The expected reflex cardiovascular responses to the decrease in central blood volume during standing and during LBNP were seen. The forced vital capacity was somewhat greater during standing and during LBNP than while supine. The airflow variables measured from the flow-volume curve-except MEF25% (partial)--were significantly increased during progressive LBNP but did not reach the raised values found when the posture changed from supine to standing. The observations suggest that besides the redistribution of the central blood volume to the periphery, other factors must contribute to the increase in airflows during standing.