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° 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 pre-syncopal 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.