O2-chemoreflex drive of ventilation in awake birds at rest

Three varieties of birds (a non-flyer, the chicken; a diver, the duck; a flyer, the pigeon) were studied in resting conditions at neutral ambient temperature, while awake, either intact or sham operated, or after bilateral chronic carotid body denervation. Tidal volume (VT), ventilatory period (T) and minute volume (V = VT/T) measured plethysmographically, were recorded breath-by-breath in steady state at two levels of oxygenation, and in the course of transient pure O2 inhalation (60-sec O2-test). O2 partial pressures in the inspired and expired gases (PIO2 and PETO2), and in the arterial blood (PaO2) were also measured. In intact and sham operated birds, the abrupt switch from a normoxic gas mixture (FIO2 = 0.21) to pure O2 resulted shortly in rapid increases of PIO2, PETO2 and paO2, and in a fall of V which was completed within 20--30 sec and accounted for about 30% of the control minute volume. In the animals previously made hypoxic (FIO2 = 0.12) and hyperventilating, the O2-test provoked a 50--60% fall of V. In chickens, a decreased VT and an increased T contributed to the transient ventilatory changes; in ducks, mainly VT changed, and in the pigeon only T changed. In the birds with denervated carotid bodies, there were no ventilatory responses to hypoxia and to the O2-test. A few of these birds developed a tachypneic response to hypoxia with no apparent change in the effective pulmonary ventilation, which was partly overcome during the O2-test. It is concluded that in the three varieties of birds, the O2-chemoreflex drive from the carotid bodies controlled about 30% of the resting minute volume near sea level, and 50--60% in hypoxic conditions duplicating an altitude exposure to 4000 m.