Journal of Applied Physiology, Vol 70, Issue 2 732-739, Copyright © 1991 by American Physiological Society
Effect of increased inspired CO2 on respiratory dead space in ponies
T. F. Lowry, H. V. Forster, M. A. Forster, C. L. Murphy, A. G. Brice and L. G. Pan
Department of Physiology, Medical College of Wisconsin, Milwaukee 53226.
The objective of the present study was to determine the effect of elevated
inspired CO2 on respiratory dead space (VD) of 12 normal, 8 carotid
body-denervated (CBD), 7 hilar nerve-denervated (HND), and 6 CBD+HND
ponies. The Fowler technique was used to determine VD on a breath-by-breath
basis while the ponies breathed room air and inspired CO2 at 3 and 6%.
During room air breathing, tidal volume (VT) and VD were greater in HND
ponies than in normal and CBD ponies (P less than 0.05), and VT was less
and VD/VT was greater after CBD than before CBD. For all groups. VD, VT,
and breathing frequency (f) increased and VD/VT decreased significantly (P
less than 0.01) with increasing inspired CO2. During CO2 breathing, VT and
VD were higher (P less than 0.05) in the HND ponies than in all other
groups, the decrease (P less than 0.05) in VD/VT was greatest in the
CBD+HND group, and f was lower in the HND and HND+CBD than in the normal
and CBD ponies. In addition, when inspired CO2 was increased from 0 to 6%,
the decrease in VD/VT was greater and the increase in arterial PCO2 was
less (P less than 0.05) after CBD than before CBD. For 70% of the ponies in
all groups, VD increased linearly with increases in VT; for most of the
remainder, VD tended to plateau at higher values of VT.
