Central venous oxygen saturation during hypovolaemic shock in humans

We compared central venous oxygen saturation and central venous pressure (CVP) as indices of the effective blood volume during 50 degrees head-up tilt (anti-Trendelenburg's position) induced hypovolaemic shock in eight healthy subjects. Head-up tilt increased thoracic electrical impedance from 31 (28-36) (median and range) to 34 (30-40) Ohm, mean arterial pressure (MAP) from 79 (70-88) to 86 (80-99) mmHg, heart rate (HR) from 67 (56-71) to 99 (78-119) beats min-1 (p < 0.01) and total peripheral resistance (TPR) from 20 (16-32) to 36 (15-52) mmHg min l-1 (p < 0.03). Cardiac output decreased from 4.3 (3.0-4.8) to 2.7 (1.8-4.8) l min-1 (p < 0.03). After 29 (9-56) min presyncopal symptoms appeared, together with a decrease in MAP to 63 (43-79) mmHg, HR to 68 (30-112) beats min-1 and TPR to 22 (13-33) mmHg min l-1 (p < 0.02). During tilting CVP decreased from 3 (1-6) to 1 (-3-5) mmHg (p < 0.05) but thereafter remained stable. In contrast, central venous oxygen saturation showed a linear decrease with time from 0.75 (0.69-0.78) at rest to 0.60 (0.49-0.67) (p < 0.01) when presyncopal symptoms appeared. Central venous catheterization is a tool for measurement of central venous oxygen saturation in addition to recording of CVP. The results suggest that a reduced central blood volume is reflected more clearly in central venous oxygen saturation than in CVP.