Aortic pH oscillations in conscious humans and anaesthetised cats and rabbits

Respiratory oscillations in arterial blood gas composition influence breathing in cats and dogs. Their role in the control of breathing in humans in less certain. To determine whether oscillations are very small or absent in mammals who are large or breathe fast, aortic pH oscillations, recorded with a tridodecylamine based hydrogen-ion selective electrode, were compared in humans (n = 13), cats (n = 7) and rabbits (n = 4) over a wide range of ventilation. For comparison, data were analysed in terms of the ratio of tidal volume to functional residual capacity (VT/FRC). During spontaneous breathing in rabbits, cats and humans (mean respiratory frequency fR = 61, 20.4 and and 17.5 min-1), mean VT/FRC were 1.35, 0.63 and 0.36 respectively. Corresponding pH amplitudes (pHamp) of 0.009 (0.004), 0.016 and 0.013 (0.005) pH units (mean +/- 1SD) were not significantly different. The pHamp decreased exponentially with increasing FR in each species and pHamp increased linearly with increasing VT in the 3 cats in which this was studied. The study confirms the dependence of pHamp on FR and VT and its comparability among species despite differences in body size. It also demonstrates that oscillations can be recorded in humans at FR in excess of 20 min-1.