Effect of adrenergic blockade on cardiovascular responses to increased airway pressure

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Abstract
Increases in airway pressure in dogs (produced by adjusting tidal volume) caused a decrease in cardiac output and an increase in heart rate without change in arterial pressure. After administration of an alpha adrenergic blocking agent the same increases in airway pressure caused a greater reduction in cardiac output and marked decrease in arterial pressure; the increase in rate tended to be greater. After administration of a beta adrenergic blocking agent increases in airway pressure did not cause a reduction in arterial pressure, the decrease in cardiac output was not augmented, and the increase in heart rate was antagonized. Increases in right atrial pressure were not altered significantly by the blocking agents. Circulatory compensation for effects of increased intrathoracic pressure is mediated predominantly through alpha adrenergic receptors. Increments in airway pressure were accompanied by reductions in end-trial CO2 concentration. Effects of restoration of end-tidal CO2 on circulatory responses to increased intrathoracic pressure were measured also. Cardiac output and heart rate were higher at any level of airway pressure during hypocapnia. These responses persisted alter alpha or beta blocking agents, suggesting that some of the effects of hypocapnia were mediated through nonadrenergic mechanisms.