Failure of Hypoxic Pulmonary Vasoconstriction in the Canine Asthma Model

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Abstract
Measurements of respiratory mechanics, arterial blood gases, and pulmonary vascular resistance were made before and 15 min after inhalation challenge with Ascaris suum extract in dogs with natural sensitivity to this antigen. 25 of 47 dogs were treated before inhalation challenge with a prostaglandin inhibitor (90 mg/kg of aspirin or 2 mg/kg of indomethacin by intravenous infusion). In response to the challenge, bronchospasm developed in approximately half (responders) of each group reflected by decreases in mean specific respiratory system conductance and arterial oxygen tension. While the dogs were breathing room air, pulmonary vascular resistance remained unchanged after antigen challenge in the responders not given aspirin or indomethacin, but increased significantly and was associated with a lesser degree of arterial hypoxemia in the responders pretreated with either of the prostaglandin inhibitors. Prevention of arterial hypoxemia by oxygen breathing blocked an increase in pulmonary vascular resistance in four pretreated responders. No changes in respiratory mechanics, pulmonary hemodynamics, or arterial blood gases were noted in the 21 dogs who did not develop bronchospasm regardless of whether or not they were pretreated. 12 additional dogs in whom arterial hypoxemia was produced by 10% oxygen breathing, showed an increase in pulmonary vascular resistance that was not potentiated by pretreatment with aspirin in 6. We conclude that in acute experimental canine asthma, vasodilator prostaglandins appear to blunt the hypoxic pulmonary vasoconstrictor response, thereby further compromising gas exchange but preventing the development of pulmonary hypertension.