Pulmonary vascular pressures of strenuously exercising Thoroughbreds during intravenous infusion of nitroglycerin

Abstract

Objective - To determine whether intravenous infusion of nitroglycerin would modify pulmonary arterial, capillary, or venous hypertension in strenuously exercising Thoroughbreds. Animals - 5 healthy Thoroughbred horses. Procedure - Right atrial, right ventricular, and pulmonary vascular pressures were measured. Each horse was used in a control treatment (not medicated) and a nitroglycerin infusion (20 μg/kg of body weight/min) at rest and during exercise on a treadmill. Sequence of treatments was randomized for each horse, and treatments were separated by a 7-day interval. Galloping at 14.2 m/s on a 5% uphill grade elicited maximal heart rate (mean ± SEM, 212 ± 2 beats/min) and could not be sustained for > 90 seconds. Nitroglycerin dosage was selected, because maximal pulmonary and systemic hemodynamic effects of IV nitroglycerin were elicited at 5 μg/kg/min and increasing the dosage to 20 μg/kg/min did not cause adverse effects. Results - In the control treatment, exercise performed at maximal heart rate resulted in a significant increase in right atrial as well as pulmonary arterial, capillary, and wedge pressures. Nitroglycerin infusion in standing horses significantly decreased right atrial and pulmonary vascular pressures, whereas heart rate increased. Exercise in nitroglycerin-infused horses also resulted in a significant increase in right atrial as well as pulmonary arterial, capillary, and wedge pressures, and these values were not significantly different from data for the control treatment. All horses experienced exercise-induced pulmonary hemorrhage for both treatments. Conclusions and Clinical Relevance - IV administration of nitroglycerin does not modify exercise-induced pulmonary hypertension and is unlikely to affect the incidence or severity of exercise-induced pulmonary hemorrhage in Thoroughbreds.