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 degrees 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 degrees 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.