Role of tracheal and bronchial circulation in respiratory heat exchange

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Abstract
Due to their anatomic configuration, the vessels supplying the central airways may be ideally suited for regulation of respiratory heat loss. Blood flow to the trachea, bronchi and lung parenchyma was measured in 10 anesthetized supine open-chest dogs. They were hyperventilated (frequency, 40; tidal volume 30-35 ml/kg) for 30 min on warm humidified air, cold (-20.degree. C) dry air and warm humidified air. End-tidal CO2 was kept constant by adding CO2 to the inspired ventilator line. Five minutes before the end of each period of hyperventilation, measurements of vascular pressures (pulmonary arterial, left atrial and systemic), cardiac output (CO), arterial blood gases and inspired, expired and tracheal gas temperatures were made. Then using a modification of the reference flow technique, 113Sn-, 153Gd-, and 103Ru-labeled microspheres were injected into the left atrium to make separate measurements of airway blood flow at each intervention. After the last measurements had been made, the dogs were killed and the lungs, including the trachea, were excised. Blood flow to the trachea, bronchi and lung parenchyma was calculated. There was no change in parenchymal blood flow, but there was an increase in tracheal and bronchial blood flow in all dogs (P < 0.01) from 4.48 .+-. 0.69 ml/min (0.22 .+-. 0.01% CO) during warm air hyperventilation to 7.06 .+-. 0.97 ml/min (0.37 .+-. 0.05% CO) during cold air hyperventilation.