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Effects of gravity and blood volume shifts on cardiogenic oscillations in respired gas

Section of Environmental Physiology, Department of Physiology and Pharmacology, Karolinska Institutet, Stockholm, Sweden

Submitted 4 March 2005 ; accepted in final form 19 April 2005

During the cardiac cycle, cardiogenic oscillations of expired gas (x) concentrations (COS_[x]) are generated. At the same time, there are heart-synchronous cardiogenic oscillations of airway flow (COS_flow), where inflow occurs during systole. We hypothesized that both phenomena, although primarily generated by the heartbeat, would react differently to the cephalad blood shift caused by inflation of an anti-gravity (anti-G) suit and to changes in gravity. Twelve seated subjects performed a rebreathing-breath-holding-expiration maneuver with a gas mixture containing O₂ and He at normal (1 G) and moderately increased gravity (2 G); an anti-G suit was inflated to 85 mmHg in each condition. When the anti-G suit was inflated, COS_flow amplitude increased (P = 0.0028) at 1 G to 186% of the control value without inflation (1-G control) and at 2 G to 203% of the control value without inflation (2-G control). In contrast, the amplitude of COS of the concentration of the blood-soluble gas O₂ (COS_[O2/He]), an index of the differences in pulmonary perfusion between lung units, declined to 75% of the 1-G control value and to 74% of the 2-G control value (P = 0.0030). There were no significant changes in COS_flow or COS_[O2/He] amplitudes with gravity. We conclude that the heart-synchronous mechanical agitation of the lungs, as expressed by COS_flow, is highly dependent on peripheral-to-central blood shifts. In contrast, COS_{[blood-soluble gas]} appears relatively independent of this mechanical agitation and seems to be determined mainly by differences in intrapulmonary perfusion.

human; stroke volume; centrifuge; anti-gravity suit