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

Abstract

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.

Key Words:

• blood pressure
• central venous pressure
• heart rate
• thoracic electrical impedance