Muscle perfusion and oxygen consumption by near-infrared spectroscopy in septic-shock and non-septic-shock patients

To measure muscle blood flow (Qtis) and oxygen consumption (VO₂tis) in septic and non-septic critically ill patients by near-infrared spectroscopy (NIRS).

Surgical intensive care unit of a university hospital.

Four patients with septic shock, eight post-surgical critically ill patients and ten healthy volunteers.

Oxyhaemoglobin (HbO₂) and deoxyhaemoglobin (HbH) variations after venous occlusion were measured by NIRS in the brachioradialis muscle. We calculated Qtis by the rate of HbO₂ and HbH increase in the first 30 s of venous occlusion divided by haemoglobin blood concentration. VO₂tis was calculated by subtraction of the arterial HbH from the initial increase of HbH after venous occlusion extrapolated to 1 min. Tissue oxygenation index [TOI = HbO₂/(HbO₂+HbH)] was also measured before venous occlusion. Two measurements in patients with septic shock, and one measurement in non-septic-shock patients and healthy subjects, were obtained. Of the measurements, 35% were repeated because of low-quality NIRS signal. VO₂tis and Qtis were two times larger (P<0.05) in patients with septic shock than in patients without and in healthy subjects. The TOI was very similar among the three groups.

In septic-shock patients the increase in VO₂tis was associated with an equivalent increase in Qtis. Therefore, tissue O₂ supply does not seem to be a limiting factor for muscle O₂ consumption. NIRS combined with venous occlusion allows a rapid, non-invasive and simultaneous assessment of regional perfusion and oxygen consumption. In case of microcirculatory shunt occurrence, the TOI should be cautiously used to assess tissue oxygenation state.