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Keywords:

  • horse;
  • V̇O2max;
  • hypoxaemia;
  • muscle O2 diffusing capacity

Summary

A major factor contributing to the athletic ability of the horse is exercise induced splenic contraction that augments both circulating blood volume (by about 12 litres) and haemoglobin concentration [Hb] (by a factor of 1.5–2). We examined separately the consequences of 1) splenectomy and 2) subsequent transfusion of blood at the lower post splenectomy [Hb] on pulmonary and muscle O2 transport and pulmonary haemodynamics, to elucidate the separate influences of blood volume and [Hb] augmentation on these processes. We studied 6 Thoroughbreds (bwt 472 kg), (3 splenectomised, 3 controls). Splenectomy reduced V̇O2max by 31 ± 3% (s.e.); transfusion of 11.7 litres whole blood at 42% haematocrit restored V̇O2max to 94 ± 7% of original values. Splenectomy reduced maximal cardiac output by 15 ± 6%; transfusion increased it to 106 ± 10% of pre-splenectomy values. The combination of splenectomy and transfusion reduced PaCO2 by 14 ± 0.5 Torr, increased PaO2 by 17 ± 0.4 Torr and arterial saturation by 5 ± 3% at V̇O2max. Splenectomy reduced maximal pulmonary artery pressure from 90.4 ± 1.4 to 42.4 ± 4.8 Torr; transfusion partially restored this to 71.5 ± 5.5 mmHg, despite the lower haematocrit. Splenectomy reduced muscle O2 diffusional conductance (DO2) in direct proportion to the reduced maximal [Hb] while transfusion only slightly increased DO2. On the other hand, DO2 was poorly related to changes in cardiac output.

Although based on a small number of horses, this study suggests that 1) increased [Hb] from splenic contraction is not essential for achieving V̇O2max; a normal circulating blood volume at a moderate [Hb] is sufficient, 2) [Hb] plays an important role in overall muscle O2 diffusional conductance and also affects pulmonary artery pressure and 3) splenectomy produced substantial hyperventilation (in relation to V̇O2), permitting eucapnia and increased arterial oxygenation at V̇O2max. This was still observed after transfusion despite restoration of exercise capacity and V̇O2max.