The effect of hypocapnia on skeletal muscle microcirculatory blood flow, oxygenation and pH

Hypocapnia is known to decrease blood flow and tissue oxygen tension in the brain and the splanchnic organs, but there are few and contradictory results in skeletal muscle. The aim of the present study was to investigate the effect of hypocapnia on microcirculatory blood flow, local skeletal muscle PO2 (PtO2) and pH (pHt). Blood flow was measured using laser-Doppler flowmetry (LDF) and a multichannel antimony microelectrode served for PtO2 and pHt measurements. These parameters were studied in six anesthetized and mechanically ventilated rabbits during normocapnia (arterial PCO2 4.4 kPa), hypocapnia (PCO2 2.3 kPa) and finally a second period of normocapnia (PCO2 4.5 kPa). During normocapnia the relative LDF flux was 100 +/- 53% (mean +/- SD), decreased to 80 +/- 48% (p < 0.01) during hypocapnia and returned to 103 +/- 51% during the second period of normocapnia (N.S.). The PtO2 was 3.2 +/- 1.2 kPa, 2.9 +/- 0.8 kPa (p < 0.05) and 3.2 +/- 1.0 kPa (N.S.), respectively. pHt remained unchanged, although there was a significant arterial alkalosis (pH increased 0.10 units, p < 0.01). These results show that hypocapnia induces a 20% decrease in LDF flow and a 9% reduction in PtO2 with an unchanged pHt level. In this skeletal muscle model, the decrease in microcirculatory blood flow due to vasoconstriction leads to a reduction in both tissue oxygenation and in the removal of acid metabolites, which counteract a developing tissue alkalosis.