MECHANISM FOR INCREASE IN TRACHEOBRONCHIAL BLOOD-FLOW INDUCED BY HYPERVENTILATION OF DRY AIR IN DOGS

To test whether the consistent increase in tracheal and bronchial blood flow observed in dogs during hyperventilation of dry air might be the result of release of mediators such as vasodilatory prostaglandins or neuropeptides, we studied two groups of mechanically ventilated dogs. Group 1 (n = 6) was hyperventilated for four 30-min periods with 1) warm humid air (38-40°C, 100% relative humidity), 2) warm dry air (38-40°C, 0% relative humidity), 3) warm humid air, and 4) warm dry air. After period 2, a loading dose of indomethacin (4 mg/kg iv) was given over 15 min followed by a constant infusion (4 mg·kg-1·h-1). Group 2 (n = 10) was hyperventilated for four 15- to 20-min periods by use of the protocol described above. After period 3 (group 2a) or period 2 (group 2b), topical 4% lidocaine hydrochloride solution was instilled into the trachea and main stem bronchi. Five minutes before the end of each period of hyperventilation, cardiac output and vascular pressures were measured. To determine airway blood flow, differently labeled radioactive microspheres were injected into the left atrium. After the last measurements, dogs were killed and the lungs excised. Blood flow to the trachea, main stem bronchi, and parenchyma (group 1 only) was calculated. Results showed that hyperventilation of dry air produced a significant increase in blood flow to the trachea and bronchi (period 2). In group 1, this increase was attenuated (P<0.02) after administration of indomethacin. In group 2 topical application of lidocaine did not significantly affect base-line blood flow and produced an attenuation of the increase in blood flow to the trachea (P<0.01) but not to the bronchi. This study suggests that local release of vasodilatory prostaglandins and neuropeptides may be important in the local control of tracheal blood flow.