Subcutaneous oxygen and carbon dioxide tensions during head-up tilt-induced central hypovolaemia in humans

The effect of hypovolaemic shock on subcutaneous oxygen and carbon dioxide tensions was studied in man. Subcutaneous oxygen (Psc,O₂) and carbon dioxide (Psc,CO₂) tensions were monitored, during 50° head-up tilt (anti-Trendelenburg's position)-induced central hypovolaemia, in two females and eight males, using a silicone tonometer on the lateral upper aspect of the right arm. All cardiovascular variables remained stable at rest. Incremental tilting to 50° increased heart rate (HR) and mean arterial pressure (MAP) (p<0.01), while stroke volume (SV), cardiac output (CO) and central venous saturation (SvO₂) decreased (p<0.05). Presyncopal symptoms appeared after 28 (8-48) min (mean and range) as HR decreased from 82 (63-108) to 52 (36-70) beats min^-1 (p<0.05), MAP from 88 (61-106) to 46 (37-54) mmHg and SvO₂ from 0.68 (0.56-0.76) to 0.58 (0.39-0.70) (p<0.01). On return of the tilt table to the horizontal position HR, MAP, SV, CO and SvO₂ immediately re-established resting values. The Psc,O₂ was 83 (72-102) mmHg at rest and during tilting it increased to 89 (82-111)mmHg followed by a decrease to 72 (58-97)mmHg (p<0.01) at the appearance of presyncopal symptoms. Psc,O₂ returned to the pretilt level over 45 (30-60) min of recovery. Arterial oxygen, carbon dioxide and Psc,CO₂ did not change significantly. Subcutaneous oxygen tension decreases during marked central hypovolaemia and it returns slowly to the resting level during recovery. Psc,O₂ is a more sensitive marker of impaired tissue oxygenation than arterial oxygen pressure.