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StO₂ Proof of Concept

Anaesthesia induced changes in tissue saturation in response to vascular occlusion test

Ranjan S, Thomson S, Tuccillo ML, Addei A, Hagger R, Rahman T, Al-Subaie N, Hamilton M. Int Care Med. 2009;35(Suppl 1):S157.

Introduction: General anaesthesia is associated with changes in the microcirculation and tissue oxygen uptake

Objectives: This study looks into muscle tissue saturation using near infrared spectroscopy (NIRS), both in healthy volunteers and in patients under inhalational general anaesthesia.

Methods: This was a prospective observational study performed on awake subjects and anaesthetised patients. A standardised 3 min vascular occlusion test (VOT) was performed by rapid inflation of a forearm cuff to 50 mmHg above systolic pressure. Muscle tissue oxygen saturation was measured using a Hutchinson Technology^TM InSpectra^TM StO₂ tissue oxygenation monitor, with a 15 mm probe applied to the right hand thenar eminence.

This test was carried out on awake healthy volunteers with no concurrent illnesses under reproducible conditions. In the anaesthetised group a VOT was performed 10 minutes after a standardised intravenous anaesthetic induction, whilst general anaesthesia was maintained with oxygen, air and isoflurane. Data were described as median (Interquartile range) and Mann–Whitney test was performed to assess statistical significance.

Results: A total of 25 healthy volunteers and 22 anaesthetised patients were enrolled. There were statistically significant differences between the two groups as illustrated in Table 1. Baseline StO₂ was significantly higher in the anaesthetised group [84.00 (7.50) vs. 79.00 (7.00)] %, p = 0.004. After cuff inflation the down slope StO₂ was lower in the anaesthetised group [-8.15 (3.22) vs. -10.92 (3.13)] %/minute, p = 0.001. The subsequent upslope, on release of the cuff pressure, was also lower in the anaesthetised group [147.75 (86.87) vs. 218.57 (71.04)] %/minute, p = 0.009. The minimum StO₂, at 3 min ischemia, was significantly higher in the anaesthetised group [59.50 (13.25) vs. 45.00 (12.5)] %, p = 0.001 in addition to the area within the occlusion curve [35.45 (15.77) vs. 52.2 (11.85)] %.min, p<0.001. Maximum StO₂ was also higher in the anaesthetised group [96.50 (3.25) vs. 94.00 (3.5)] %, p = 0.004. The difference between baseline StO₂ and minimum StO₂ was found to be lower in the anaesthetised group [24 (9) vs. 33 (9)] %, p<0.001.

TABLE 1 VASCULAR OCCLUSION TEST FINDINGS

Conclusion: This study shows differences in tissue oxygen saturation during a VOT between awake and anaesthetised patients. This maybe due to anaesthetic related micro circulatory effects or changes in cellular uptake of oxygen or both.