Journal of Applied Physiology, Vol 38, Issue 6 965-968, Copyright © 1975 by American Physiological Society
Effect of resistive loading on ventilatory response to hypoxia
A. S. Rebuck and E. F. Juniper
Ventilatory responses to hypoxia, with and without an inspiratory resistive
load, were measured in eight normal subjects, using a rebreathing
technique. During the studies, the end-tidal P-CO2 was kept constant at
mixed venous level (Pv-CO2) by drawing expired gas through a variable
CO2-absorbing bypass. The initial bag O2 concentration was 24% and
rebreathing was continued until the O2 concentration in the bag fell to 6%
or the subject's arterial oxygen saturation (Sa-O2), monitored continuously
by ear oximetry, fell to 70%. Studies with and without the load were
performed in a formally randomized order for each subject. Linear
regressions for rise in ventilation against fall in Sa-O2 were calculated.
The range of unloaded responses was 0.78-3.59 1/min per 1% fall in Sa-O2
and loaded responses 0.37-1.68 1/min per 1% fall in Sa-O2. In each subject,
the slope of the response curve during loading fell by an almost constant
fraction of the unloaded response, such that the ratio of loaded to
unloaded slope in all subjects ranged from 0.41 to 0.48. However, the
extrapolated intercept of the response curve on the Sa-O2 axis did not
alter significantly indicating that the P-CO2 did not alter between
experiments. These results suggest that the change in ventilatory response
to hypoxia during inspiratory resistive loading is related to the
mechanical load applied, with the loaded slope being directly proportional
to the unloaded one.
