Maximum oxygen transport and utilisation before and after splenectomy

Abstract

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 O 2 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 V̇O 2max by 31 ± 3% (s.e.); transfusion of 11.7 litres whole blood at 42% haematocrit restored V̇O 2max 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 PaCO 2 by 14 ± 0.5 Torr, increased PaO 2 by 17 ± 0.4 Torr and arterial saturation by 5 ± 3% at V̇O 2max . 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 O 2 diffusional conductance (DO 2 ) in direct proportion to the reduced maximal [Hb] while transfusion only slightly increased DO 2 . On the other hand, DO 2 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 V̇O 2max ; a normal circulating blood volume at a moderate [Hb] is sufficient, 2) [Hb] plays an important role in overall muscle O 2 diffusional conductance and also affects pulmonary artery pressure and 3) splenectomy produced substantial hyperventilation (in relation to V̇O 2 ), permitting eucapnia and increased arterial oxygenation at V̇O 2max . This was still observed after transfusion despite restoration of exercise capacity and V̇O 2max .