Effects of phlebotomy on haemodynamic characteristics during exercise in Standardbred trotters with red cell hypervolaemia

Five Standardbred trotters with red cell hypervolaemia (RCHV) were compared before and after removal of approximately 22% (36 ml/kg bwt) of the total blood volume in order to evaluate the haemodynamic responses, haemorheologi cal alterations and oxygen transport during exercise to fatigue. Data were recorded during submaximal exercise at 4 different speeds on a treadmill an d then during continued running at the highest speed step until fatigue, Ox ygen uptake (V-O2), pulmonary artery pressure (PAP), systemic artery pressu re (SAP), heart rate (HR), haematocrit and haemoglobin concentrations (Hb) were measured. Arteriovenous O-2 content difference (C(a-(v) over bar )O-2) , pulmonary vascular resistance (PVR) and total systemic resistance (TSR) w ere calculated. Whole blood and plasma viscosity and erythrocyte aggregatio n tendency were determined with a rotational viscometer, Endoscopy was perf ormed after exercise. ANOVA was used for statistical analysis. Phlebotomy r esulted in a decrease in haematocrit and Db during the course of exercise. Blood and plasma viscosity were lower and erythrocyte aggregation tendency was higher after phlebotomy, Throughout exercise, including submaximal work and continued running to fatigue, PAP, SAP, PVR, TSR and C(a-(v) over bar )O-2 were lower after phlebotomy, HR was higher after phlebotomy during sub maximal exercise. Oxygen delivery and V-O2 were lower after phlebotomy in t he period from submaximal exercise to fatigue, Run time to fatigue was shor ter after phlebotomy, Four horses showed exercise-induced pulmonary haemorr hage (EIPH) before phlebotomy and the degree of bleeding was diminished but not abolished after phlebotomy, The reductions in PVR, TSR, PAP and SAP af ter phlebotomy were probably a result of reduced blood viscosity In conclus ion, although a 22% reduction in blood volume improved the haemodynamic and haemorheological parameters and the degree of EIPH, it was found that RCHV trotters have to rely on high oxygen delivery to the working muscles for m aintenance of maximal performance.