Peripheral venous oxygen saturation during head-up tilt induced hypovolaemic shock in humans

We followed central, median cubital and dorsal metacarpal venous oxygen saturations (SvO2) during 50 degrees head-up tilt (anti-Trendelenburg's position) induced central hypovolaemia in eight males. Head-up tilt resulted in slight tachycardia of 101 (60-120) beats min-1 (median with range) and a stable mean arterial pressure (MAP) of 100 (88-114) mmHg. After 13 (6-23) min presyncopal symptoms appeared, accompanied by decreases in heart rate to 75 (51-97) beats min-1 and in MAP to 59 (49-76) mmHg (p < 0.01). Cardiac output decreased 0.9 (0.3-1.6) 1 min-1 while thoracic electrical impedance increased 3.4 (-1.2-5.9) Ohm (p < 0.01). Tilt-up decreased central venous pressure, but during sustained tilt it remained unchanged. Arterial oxygen saturation did not change. Head-up tilt decreased central SvO2 by 12 (5-24)% (p < 0.01). Median cubital SvO2 decreased 8 (-5-25)% (p < 0.02) during tilting, and it remained at this level during sustained tilt. Only five of eight samples from the dorsal metacarpal vein could be obtained. In these samples SvO2 was lowered by 15 (7-26)% (p = 0.01) at the onset of presyncopal symptoms. The results indicate that loss of central blood volume is reflected in central as well as peripheral SvO2. However, for reliable monitoring of blood volume changes, central SvO2 is the most useful variable, as this SvO2 changed consistently with the central blood volume, and blood samples could be obtained readily from the central venous catheter.