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Summary This study investigated changes in packed cell volume (PCV), pulmonary artery and aortic pressures, and the interaction between oesophageal pressure and pulmonary artery and aortic pressures during strenuous exercise in the horse. It was hypothesised that oesophageal pressure changes summate with pulmonary artery and aortic pressures during exercise and contribute to exercise‐induced pulmonary haemorrhage (EIPH). Acute treadmill exercise (10 m/sec, 3° incline) produced increases in heart rate (HR) from 50 to 202 beats/min; mean pulmonary artery pressure (PAP) from 28 to 80 mmHg; mean aortic pressure (AP) from 108 to 157 mmHg; and PCV from 0.35 to 0.52 litres/litre. EIPH was observed in three of seven horses after treadmill exercise, but no differences in the above variables were observed between the two groups of horses. Electronic subtraction of the oesophageal pressure signal from PAP and AP signals indicated peak transmural pressures of approximately 150 mmHg pulmonary and 175 mmHg aortic pressure. The elevated PAP associated with exercise appeared related more to increased HR and less to PCV (blood viscosity) or AP (bronchial). Both pulmonary artery and aortic peak transmural vascular pressures were substantially influenced by oesophageal pressure changes; peak and mean pulmonary artery and aortic pressures were significantly higher than resting pressures, and may conceivably contribute to EIPH.

Summary Intravenous frusemide (1.0 mg/kg bwt) or phentolamine (0.33 mg/kg bwt) was given to 7 horses 1 h before exercise and their effects on pulmonary artery and aortic pressure changes during strenuous exercise were examined. Short‐term near‐maximal treadmill exercise (10 m/sec, 3° incline) produced increases in heart rate, mean pulmonary artery pressure (PAP), mean aortic pressure (AP), and packed cell volume (PCV). Frusemide did not affect heart rate, PAP or PCV during exercise. Frusemide significantly decreased mean AP by 10 to 15 mmHg during exercise. Phentolamine produced an increase in heart rate relative to control only early in exercise but not during later, more strenuous, exercise. Phentolamine had no statistically significant effect on AP, PAP or PCV, but a significant reduction was observed between 180 and 230 sec of exercise when PAP and AP were standardised against heart rate. Frusemide did not prevent horses from haemorrhaging during exercise in this study. Treatment with phentolamine did not sufficiently reduce the PAP and AP to test our hypothesis that a reduction in PAP and AP would eliminate EIPH.

Résumé Une surcharge expérimentale en hydrates de carbone fut provoquée sur 15 chevaux; elle engendra des changements hématologiques importants durant l'apparition d'un syndrome d'hypertension et de fourbure. Des changements significatifs dans l'hématocrite, dans la formule leucocytaire, dans la glycémie et la proteinémie se produisirent avant l'apparition d'une boite rie (de valeur 3 Obel) et avant l'apparition d'un état circulatoire hypercinétique. Les plaquettes sanguines diminuèrent 8 heures après l'installation de la boiterie. Les constatations de cette étude indiquent une hémoconcentration due à un déplacement des liquides de l'organisme et une élèvation leucocytaire importante avec un accroissemen des adrenoglucocorticoides et/ou des catécholamines en circulation. Zusammenfassung Die experimentelle Kohlenhydratüberfütterung von 15 Pferden bewirkte regelmässige haematologische Veränderungen während der Entwicklung von Hufrehe/Hochdruck. Signifikante Veränderungen des Haematokrits, des Differentialblutbilds, der Serum‐Glucose‐ und Protein‐konzentrationen liessen sich beobachten vor dem Auftreten einer Lahmheit dritten Grades (Obel) und des hyperkinetischen zirkulatorischen Zustands. Die Thrombocytenzahlen sanken 8 Stunden nach dem Auftreten der schweren Lahmheit signifikant ab. Die Resultate dieser Studie weisen auf eine Haemokonzentration hin, die auf Flüssigkeitsverschiebungen innerhalb der Kompartimente beruht, und auf eine leukocytäre Stressantwort in Uebereinstimmung mit erhöhten zirkulierenden Adreno‐glucocorticoiden und/oder Katecholaminen. Summary Experimental carbohydrate overload of 15 horses resulted in consistent haematological changes during development of laminitis hypertension. Significant alterations in packed cell volume, leucocyte differential count, serum glucose levels and protein values occurred before onset of Obel grade 3 lameness and the hyperkinetic circulatory state. Blood platelets were significantly decreased 8 h after the onset of severe lameness. Findings in this study were indicative of haemoconcentration due to compartmental fluid shifts and a leucocytic stress response consistent with increased circulating adrenogluco‐corticoids and/or catecholamines.