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Summary

High-intensity exercise results in a dramatic increase in mean pulmonary capillary blood pressure of horses, and administration of furosemide 4 hours before exertion significantly attenuates this exercise-induced increment. To test whether this effect of furosemide is mediated via release of prostaglandins, right atrial and pulmonary vascular pressures were measured in 8 healthy, sound, exercise-trained Thoroughbreds at rest and during incremental-step exercise on a treadmill. Horses were studied on 3 separate occasions: after IV administration of saline (0.9% NaCl) solution, after administration of furosemide (250 mg, IV, 4 hours before exercise) alone, and after administration of flunixin meglumine (1.1 mg/kg, IV, q 8 h for 3 days) and furosemide (250 mg, IV, 4 hours before exercise; last dose of flunixin meglumine was administered 90 seconds after furosemide injection). Experiments on each horse were separated by at least 7 days and were performed in random order. At rest and at the highest workload (14.5 m/s on a 5% uphill incline), mean pulmonary capillary blood pressure recorded after administration of furosemide alone was not significantly different from that recorded after administration of flunixin meglumine and furosemide. However, these values were significantly (P< 0.05) less than corresponding values of mean pulmonary capillary blood pressure recorded after administration of saline solution. Thus, it was concluded that furosemide-induced attenuation of the increment in pulmonary capillary blood pressure during strenuous exercise is probably not mediated via prostaglandin production.

Free access
in American Journal of Veterinary Research
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SUMMARY

Using catheter mounted microtip manometers, right atrial, pulmonary artery, and pulmonary artery wedge pressures were studied in 8 horses while they were standing quietly (rest), and during galloping at treadmill speeds of 8, 10, and 13 m/s. At rest, mean (± sem) heart rate, mean right atrial pressure, mean pulmonary artery pressure, and mean pulmonary artery wedge pressure were 37 (± 2) beats/min, 8 (± 2) mm of Hg, 31 (± 2) mm of Hg, and 18 (± 2) mm of Hg, respectively. Exercise at treadmill belt speed of 8 m/s resulted in significant (P < 0.05) increments in heart rate, right atrial pressure, pulmonary artery systolic, mean, diastolic and pulse pressures, and pulmonary artery wedge pressure. All these variables registered further significant (P < 0.05) increments as work intensity increased to 10 m/s, and then to 13 m/s. Pulmonary artery diastolic pressure was, however, not different among the 3 work intensities. During exercise at belt speed of 13 m/s, heart rate, mean right atrial pressure, mean pulmonary artery pressure, pulmonary artery pulse pressure, and mean pulmonary artery wedge pressure were 213 (± 5) beats/min, 44 (± 4) mm of Hg, 89 (± 5) mm of Hg, 69 (± 4) mm of Hg, and 56 (± 4) mm of Hg, respectively. Assuming mean intravascular pulmonary capillary pressure to be halfway between the mean pulmonary arterial and venous pressures, its value during exercise at 13 m/s may have approached 72.5 mm of Hg. Transmural pressure (intravascular minus alveolar pressure) across pulmonary capillaries may be even higher because of the large negative pleural pressure swings in galloping horses. High transmural pressures may cause stress failure of pulmonary capillaries, resulting in exercise-induced pulmonary hemorrhage.

Free access
in American Journal of Veterinary Research

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. (Am J Vet Res 1999;60: 1436–1440)

Free access
in American Journal of Veterinary Research

SUMMARY

Objective

To examine regional distribution of blood flow in the brain of horses at rest and during exercise.

Animals

9 clinically normal horses.

Procedure

Regional brain blood flow was measured using radionuclide-labeled 15-μm-diameter microspheres injected into the left ventricle, while reference blood samples were obtained from the aorta.

Results

At rest, cerebral cortex and caudate nuclei received significantly higher blood flow, compared with cerebral white matter. A similar perfusion heterogeneity existed in the cerebellum. In the brain stem, a gradual tapering of blood flow from thalamus-hypothalamus towards medulla was observed in standing horses. Progressive significant increases in heart rate and in aortic and right atrial pressures occurred during exercise at 8 and 13 m/s, and horses developed significant arterial hypoxemia and hypercapnia. Cerebral and cerebellar gray- to white-matter perfusion heterogeneity was maintained during exercise, indicating differential metabolic O2 needs. Despite arterial hypoxemia, hypercapnia, and hypertension, exercise did not result in significant changes in blood flow to the cerebral cortex and caudate nuclei whereas, in cerebral white matter, a significant decrease in blood flow was observed. In all cerebral tissues, vascular resistance increased during exercise, indicating autoregulation of cerebral blood flow. In the cerebellar cortex, blood flow increased significantly with strenuous exercise as vasodilation occurred. Vascular resistance in cerebellar white matter increased during exercise at 13 m/s. Blood flow in the medulla, pons, midbrain, and thalamus-hypothalamus was not significantly altered during exercise from that at rest.

Conclusion

Despite arterial hypoxemia, hypercapnia, and hypertension, autoregulation of cerebral and cerebellar blood flow is maintained in horses during exercise. (Am J Vet Res 1998;59:893–897)

Free access
in American Journal of Veterinary Research

SUMMARY

The effect of age on maximal heart rate induced by iv infusion of isoproterenol was studied in 19 healthy, sedentary, normothermic horses ranging in age from 0.25 to 9.90 years. Isoproterenol was administered iv (1.0 μg/kg of body weight/min) for 3 minutes, and the heart rate attained during the last 30 seconds of the infusion was determined. Linear regression of the maximal heart rate on age suggested that the rate decreased with age in a trend described by the equation: maximal heart rate (beats/min) = 209.63 − 3.28 × age (years). The regression coefficient (r) for this relation was 0.769 (P < 0.001). These data indicate that as healthy horses age, their β-adrenoceptor-mediated maximal chronotropic response is diminished.

Free access
in American Journal of Veterinary Research

Summary

The heart rate (hr) induced by maximal β-adrenergic activation, which was elicited by infusion of isoproterenol, was studied in 8 healthy horses before (control) and after hyperthermia was induced by iv administration of 2,4-dinitrophenol (dnp). Isoproterenol was administered iv at 1.0 μg·kg-1·min-1 for 3 minutes, and the hr was determined during the final 30 seconds of the infusion. As the rectal temperature increased (P < 0.001) from 38.2 ± 0.1 C (mean ± SEM; normothermic control) to 40.1 ± 0.1 C at 60 minutes after dnp administration, the isoproterenol-induced hr also increased from 198 ± 4 beats/min (control) to 214 ± 4 beats/min (P < 0.001). It appeared that the values of hr achieved with maximal β-adrenergic activation were augmented by the hypermetabolic, hyperthermic state induced by dnp.

Free access
in American Journal of Veterinary Research

Summary

Right atrial, pulmonary artery, pulmonary capillary, pulmonary artery wedge, and systemic blood pressures of strenuously exercising horses increase markedly. As a consequence, myocardial metabolic O2 demand in exercising horses must be high. Experiments were, therefore, carried out on 9 healthy, exercise-conditioned horses (2.5 to 8 years old; 481 ± 16 kg) to ascertain the regional distribution of myocardial blood supply in the atria and ventricles at rest and during exercise. Blood flow was measured, using 15-μm-diameter radionuclide-labeled microspheres that were injected into the left ventricle while reference blood samples were being withdrawn at a constant rate from the thoracic aorta. Myocardial blood flow was determined at rest and during 2 exercise bouts performed on a high-speed treadmill at 8 and 13 m/s (0% grade). The sequence of exercise bouts was randomized among horses, and a 60-minute rest period was permitted between exercise bouts. There was considerable heterogeneity in the distribution of myocardial perfusion in the atria and the ventricles at rest; the right atrial myocardium received significantly (P < 0.05) less perfusion than did the left atrium, and these values were significantly (P < 0.05) less than those for the respective ventricular myocardium. The right ventricular myocardial blood flow also was significantly less than that in the left ventricle. With exercise, myocardial blood flow in all regions increased progressively with increasing work intensity and marked coronary vasodilation was observed in all cardiac regions. During exercise at 8 or 13 m/s, right and left atrial myocardial blood flows (per unit weight basis) were not different from each other. Although at treadmill speed of 8 m/s, left ventricular myocardial blood flow exceeded that in the right ventricle, this was not the case at 13 m/s, when perfusion values (per unit weight basis) became similar. These data suggested that, in exercising horses, myocardial metabolic O2 requirements increase markedly in all regions. However, the right atrial and right ventricular myocardial blood flows increased out of proportion to those in the left atrium and left ventricle, respectively.

Free access
in American Journal of Veterinary Research

Abstract

Objective

To determine the effects of phenylbutazone administration on heart rate and right atrial and pulmonary vascular pressures in Thoroughbreds during rest and during exercise performed at maximal heart rate.

Animals

7 healthy, exercise-conditioned Thoroughbreds.

Procedure

Horses were studied on 3 occasions: without medication (control), after IV administration of phen-ylbutazone (4.4 mg/kg of body weight) at 12-hour intervals for 2 days and a final dose given 1 hour before exercise, and after IV administration of phenylbutazone for 2 days in the same manner, but with the final dose given 24 hours before exercise. Horses were studied at rest and during exercise performed at maximal heart rate on a treadmill. Right atrial and pulmonary vascular pressures were measured with catheter-tip manometers referenced at the point of the shoulder.

Results

We did not detect significant differences in heart rate or right atrial and pulmonary vascular pressures among values recorded when horses were not given medication and values recorded when phenylbutazone was administered by either regimen. Exercise-induced pulmonary hemorrhage occurred in 6 of the 7 horses re-gardless of whether phenylbutazone was administered or the dosage regimen used.

Conclusions

In these Thoroughbreds, phenylbutazone treatment did not modify heart rate or right atrial and pulmonary vascular pressures at rest or during exercise capable of eliciting exercise-induced pulmonary hemorrhage. Thus, because phenylbutazone is a potent inhibitor of cyclooxygenase, prostaglandins probably do not play a role in mediating exercise-induced pulmonary hypertension in horses.

Clinical Relevance

Phenylbutazone administration did not modify the pulmonary capillary hypertension in the strenuously exercising Thoroughbreds, and therefore, is unlikely to alter the prevalence or severity of exercise-induced pulmonary hemorrhage in Thoroughbred race-horses. (Am J Vet Res 1996;57:1354-1358)

Free access
in American Journal of Veterinary Research

SUMMARY

Using radionuclide-labeled 15-μm-diameter microspheres injected into the left ventricle, we examined blood flow to the thyroid gland, adrenal glands, kidneys, and various gastrointestinal tract tissues in 9 healthy horses while they were standing quietly (rest) and during exercise at 2 work intensities (8 and 13 m/s). Hemodynamic measurements were made during steady-state conditions, as judged by the stability of heart rate as well as aortic, pulmonary, and right atrial pressures. The similarity of blood flow values for the left and the right kidneys during each of the 3 conditions indicated adequate mixing of microspheres with blood. In standing horses, of all tissues examined, the thyroid gland had the highest blood flow (1,655.2 ± 338.5 ml/min/100 g)—being about threefold that in the kidneys. Adrenal blood flow, by contrast, was only 25% of that in the kidneys (589.5 ± 50.4 ml/min/100 g). Among the gastrointestinal tract tissues, glandular stomach and pancreas had the highest blood flows (214.3 ± 21.6 and 197.6 ± 23.4 ml/min/100 g, respectively). Small intestinal perfusion was not different from that in the ventral colon and cecum, but their values exceeded those for the dorsal and small colons. Exercise at 8 and 13 m/s caused significant increase in adrenal blood flow as vascular resistance decreased significantly. In the kidneys, blood flow was only insignificantly affected during exercise at 8 m/s, but at 13 m/s there was a profound reduction in renal blood flow as intense renal vasoconstriction occurred. Vasoconstriction also caused thyroid and pancreatic blood flow to decrease significantly at both levels of exertion. Significant vasoconstriction occurring in all gastrointestional tract tissues at 8 and 13 m/s caused blood flow to be diverted away from these vascular beds. Thus, our data indicated that renal, adrenal, and splanchnic organ/tissue blood flow responses of strenuously exercising horses closely resemble those described for exercising ponies.

Free access
in American Journal of Veterinary Research

SUMMARY

Experiments were carried out on 8 healthy ponies to examine the effects of prolonged submaximal exercise on regional distribution of brain blood flow. Brain blood flow was ascertained by use of 15-μm-diameter radionuclide-labeled microspheres injected into the left ventricle. The reference blood was withdrawn from the thoracic aorta at a constant rate of 21.0 ml/min. Hemodynamic data were obtained with the ponies at rest (control), and at 5, 15, and 26 minutes of exercise performed at a speed setting of 13 mph on a treadmill with a fixed incline of 7%. Exercise lasted for 30 minutes and was carried out at an ambient temperature of 20 C. Heart rate, mean arterial pressure, and core temperature increased significantly with exercise. With the ponies at rest, a marked heterogeneity of perfusion was observed within the brain; the cerebral, as well as cerebellar gray matter, had greater blood flow than in the respective white matter, and a gradually decreasing gradient of blood flow existed from thalamus-hypothalamus to medulla. This pattern of perfusion heterogeneity was preserved during exercise. Regional brain blood flow at 5 and 15 minutes of exercise remained similar to resting values. However, at 26 minutes of exercise, vasoconstriction resulted in a significant reduction in blood flow to all cerebral and brain-stem regions. In the cerebellum, the gray matter blood flow and vascular resistance remained near control values even at 26 minutes of exercise. Vasoconstriction in various regions of the cerebrum and brainstem at 26 minutes of exertion may have occurred in response to exercise-induced hypocapnia, arterial hypertension, and/or sympathetic neural activation.

Free access
in American Journal of Veterinary Research