Ventilatory and circulatory responses to hyperthermia in the mute swan (Cygnus olor)

Ventilatory parameters of mute swans were measured at thermoneutral conditions and during heat stress. Deep body temperature increased from 39.5 to 41.1 degrees C. Breathing frequency increased 29 times, compared to the thermoneutral condition. Tidal volume decreased to 18% of the pre-panting value, and the total ventilatory volume increased by 5.4 times. End-tidal PCO2 and PO2 values decreased and increased, respectively. The swans developed a slight respiratory alkalosis; arterial PCO2 decreased from an average of 27.1 to 25.7 mmHG and arterial pH increased from 7.501 to 7.559. Cardiac output, heart rate and stroke volume were 106%, 154%, and 70%, respectively, of the values at thermoneutrality. Mean arterial blood pressure and total peripheral resistance were slightly reduced. It is concluded that the increased ventilation during panting mainly constitutes dead space ventilation resulting from the great reduction in tidal volume. Parabronchial ventilation remains nearly unchanged, resulting in only a slight hypocapnic alkalosis.