Effect of increased inspired CO2 on respiratory dead space in ponies

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.