Effects of glyceryl trinitrate (nitroglycerin) on pulmonary vascular pressures in standing thoroughbred horses

Strenuously exercising Thoroughbreds exhibit a dramatic increase in pulmonary capillary blood pressure, which contributes to stress failure of pulmonary capillaries resulting in exercise induced pulmonary haemorrhage (EIPH). One strategy to prevent EIPH is, therefore, to lower the pulmonary capillary blood pressure of exercising horses. Recent work in several species suggests that nitric oxide plays a significant role in maintaining low vascular resistance in the pulmonary circulation; however, the effects of nitrovasodilators (which work via the same mechanism as nitric oxide) on equine pulmonary circulation have not been examined. The present study examined the effects of glyceryl trinitrate (nitroglycerin) on right atrial and pulmonary vascular pressures in 7 healthy sound Thoroughbred horses. Freshly prepared nitroglycerin solution was infused for 240 s into the right atrium of quietly standing Thoroughbreds at dose rates of 350, 700, 1400 and 2100 g/min in a randomised manner. All infusions were performed in duplicate. Heart rate, right atrial, pulmonary artery, pulmonary capillary and pulmonary artery wedge pressures were determined preinfusion, at 30 s intervals during nitroglycerin infusions and at 60 s post infusion. Measurements were made using catheter mounted manometers whose in vivo signals had been matched with fluid-filled systems referenced at the level of the point of the shoulder. It was observed that nitroglycerin infusions caused a dose related increase in heart rate while dose related reductions occurred in the mean right atrial, pulmonary artery, pulmonary artery wedge and pulmonary capillary pressures. At 2100 micrograms/min, nitroglycerin induced reduction in pulmonary artery wedge pressure was significantly greater than that in the pulmonary artery pressure.(ABSTRACT TRUNCATED AT 250 WORDS)