Comparison of muscle tissue oxygenation response curves to two time-based vascular occlusion tests: evidence of diminishing returns?

Introduction

Dynamic testing of muscle tissue oxygenation (StO₂) with near-infrared spectroscopy and vascular occlusion (VOT) has been used to study pathophysiological states, but there is a paucity of data for standardised techniques in normal subjects. Three-minute VOT is frequently described. We have collected StO₂ data for this technique and compared them with a shorter 2-minute test.

Methods

Twenty-five subjects were studied using an InSpectra 650 monitor and a 15 mm probe. VOT was applied at systolic blood pressure + 50 mmHg on opposite arms for 3-minute and 2-minute time periods. StO₂ data are reported for baseline, downslope (DS) and upslope (US)%/minute, overshoot%, area over the ischaemic curve (AOC Isch) and area under the recovery curve (AUC Rec)%/minute and recovery time (Rec Time) in minutes. Differences were analysed using paired t tests. Individual differences were then examined in relation to a predicted magnitude of response based on the ischaemic load and occlusion time.

Results

Three-minute VOT versus 2 minutes: AOC Isch (-52.99 vs. -25.79%/min, P < 0.001), overshoot (14.96 vs. 13.2%, P = 0.004), AUC Rec (18.89 vs. 15.33%/min, P = 0.01), Rec Time (2.37 vs. 2.05 min, P = 0.004). DS (-11.18 vs. -12.35%/min, P = 0.08) and US (212.99 vs. 201.24%/min, P = 0.365) were not significantly different. The 2-minute ischaemic load was 0.5 times that of 3 minutes (AOC Isch -25.79/-52.99). The ratios of response parameters were compared with this value using one-sample t tests and were significantly greater: overshoot (0.89 ± 0.16, P < 0.001), AUC Rec (0.86 ± 0.35, P < 0.001), Rec Time (0.88 ± 0.21, P < 0.001).

Conclusion

StO₂ VOT is a promising tool that is likely to advance our knowledge of microvascular function under ischaemic stress. To standardise data interpretation it is imperative that similar techniques are used. We have reported data for two reproducible VOTs in a healthy control group. Understandably, there are differences in hyperaemic response parameters that are dependent on the ischaemic time; however, the relationship is no linear. Our data suggest that although 2-minute VOT represents a halving of the ischaemic load versus 3 minutes, subjects demonstrate almost 90% comparable hyperaemic responses. This suggests a diminishing return in hyperaemia with longer occlusive time. Further work is required to evaluate the efficacy and practicality of different VOTs.