Ventilation-perfusion inequality during normoxic and hypoxic exercise in the emu

Many avian species exhibit an extraordinary ability to exercise under hypoxic condition compared with mammals, and more efficient pulmonary O-2 transport has been hypothesized to contribute to this avian advantage. We studied six emus (Dromaius novaehollandaie, 4-6 mo old, 25-40 kg) at rest and during treadmill exercise in normoxia and hypoxia (inspired O-2 fraction approximate to0.13). The multiple inert gas elimination technique was used to measure ventilation-perfusion ((V)over dot/(Q)over dot) distribution of the lung and calculate cardiac output and parabronchial ventilation. In both normoxia and hypoxia, exercise increased arterial PO2 and decreased arterial PCO2, reflecting hyperventilation, whereas pH remained unchanged. The (V)over dot/(Q)over dot distribution was unimodal, with a log standard deviation of perfusion distribution = 0.60 +/- 0.06 at rest; this did not change significantly with either exercise or hypoxia. Intrapulmonary shunt was <1% of the cardiac output in all conditions. CO2 elimination was enhanced by hypoxia and exercise, but O-2 exchange was not affected by exercise in normoxia or hypoxia. The stability of (V)over dot/(Q)over dot matching under conditions of hypoxia and exercise may be advantageous for birds flying at altitude.