EFFECTS OF GLYCERYL TRINITRATE (NITROGLYCERIN) ON PULMONARY VASCULAR PRESSURES IN STANDING THOROUGHBRED HORSES

Strenuously exercising Thoroughbreds exhibit a dramatic increase in pu lmonary capillary blood pressure, which contributes to stress failure of pulmonary capillaries resulting in exercise induced pulmonary haemo rrhage (EIPH). One strategy to prevent EIPH is, therefore, to lower th e pulmonary capillary blood pressure of exercising horses. Recent work in several species suggests that nitric oxide plays a significant rol e in maintaining low vascular resistance in the pulmonary circulation; however, the effects of nitrovasodilators (which work via the same me chanism as nitric oxide) on equine pulmonary circulation have not been examined, The present study examined the effects of glyceryl trinitra te (nitroglycerin) on right atrial and pulmonary vascular pressures in 7 healthy sound Thoroughbred horses, Freshly prepared nitroglycerin s olution was infused for 240 s into the right atrium of quietly standin g Thoroughbreds at dose rates of 350, 700, 1400 and 2100 g/min in a ra ndomised manner, Ah infusions were performed in duplicate, Heart rate, right atrial, pulmonary artery, pulmonary capillary and pulmonary art ery wedge pressures were determined preinfusion, at 30 s intervals dur ing nitroglycerin infusions and at 60 s post infusion, Measurements we re made using catheter mounted manometers whose in vivo signals had be en matched with fluid-filled systems referenced at the level of the po int of the shoulder, It was observed that nitroglycerin infusions caus ed 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 mu g/min, nitroglyce rin induced reduction in pulmonary artery wedge pressure was significa ntly greater than that in the pulmonary artery pressure, It is conclud ed that (i) pulmonary vascular bed of quietly standing horses has a ce rtain basal vasomotor tone which was unmasked by nitroglycerin infusio ns (ii) nitroglycerin may have a more potent dilator effect on equine pulmonary veins than on the arteries.