EFFECTS OF DEEP BREATHS ON SUBSEQUENT VENTILATION IN MAN DURING REST AND EXERCISE

1. We examined the effects of twenty-four to thirty inspiratory capaci ty (IC), expiratory capacity (EC) and vital capacity (VC) breaths on s ubsequent breathing pattern in five normal subjects at rest. 2. During IC breaths and following EC and VC breaths at rest, end-tidal CO2 pre ssure (P-ET,P-CO2) fell by 7.5, 8.5 and 9.5 mmHg, respectively. In the group analysis significant inhibition of ventilation of 1.5 l min(-1) was seen after the IC breath but not after EC or VC breaths. 3. We re peated the study with five normal subjects under conditions of higher ventilatory drive, namely 50 W exercise (one subject was common to bot h groups). 4. During exercise, the drop in P-ET,P-CO2 was smaller (4.0 , 3.5 and 4.0 mmHg, respectively, with IC, EC and VC breaths) but vent ilation was inhibited to a greater extent. Ventilatory undershoot was seen after all three types of deep breaths. 5. We propose that the exp iration to residual volume in EC and VC breaths abolished the hypocapn ic inhibition of ventilation at rest, possibly by a deflation reflex w hich was not sufficiently powerful to overcome the ventilatory undersh oot during exercise. Our results also support the view that the slope of the CO2 response curve is steeper near the control point during exe rcise.