Effect of lower body negative pressure release on hyperpnea induced by inhaled gas

Transient breath-by-breath ventilation (VI) and end-tidal gases were measured on 6 men in response to: (1) the release of lower body negative pressure (LBR); (2) the one-min inhalation of a gas mixture (GAS) containing 5.4% CO2, 14% O2, balance N2; (3) the two combined (LBR + GAS) with LBR preceding GAS inspiration by 5 sec. With LBR a + 29% peak response in VI occurred after 20 sec whereas with GAS and LBR + GAS, VI tended to level off after 25 sec at + 100% and + 180%, respectively. Simultaneously, PETCO2 had risen 2, 5 and 8 mm Hg and PETO2 had fallen 7, 14 and 19 mm Hg, respectively. LBR accelerated and potentiated the VI response to GAS. Leg volume indicated that 50% of the pooled blood returned from the legs after 4 sec, presumably with high PCO2 and low PO2 contributing an endogenous respiratory stimulus to the exogenous one of GAS. VI with LBR + GAS was greater than the sum of each, suggesting an enhancement of the chemoreceptor response by returning pooled blood.