A comparison of breathing pattern between transient and steady state hypoxia in awake Dogs

To quantitatively examine and compare the breathing patterns during transient and steady state hypoxia, studies were performed repeatedly in five awake, chronic dogs. Transient hypoxia of 30 s was induced with 5-7 consecutive breaths of nitrogen; steady state isocapnic hypoxia with step changes in end-tidal O₂ concentration (range 5-8.5%) and maintained for 6-9 min at each level. Linear relationships were found between minute ventilation ( {Mathematical expression} _I) and tidal volume (V_T) in both types of hypoxia. However, a breakpoint emerged in the {Mathematical expression} _I-V_T line of steady state hypoxia when V_T and {Mathematical expression} _I reached 317% and 509% (mean values), respectively, of their baseline values. The slope of steady state_I-V_T line below the breakpoint was 22.7 min^-1 (weighted mean) which was not significantly different (P>0.1) from that of transient hypoxia (17.3 min^-1). During steady state hypoxia both durations of inspiration and expiration were shorter than those during transient hypoxia in the same range of V_T in four of these dogs. These differences in respiratory volume-timing relationship may be related to the central effect(s) of steady state hypoxia.