Near-infrared spectroscopy of the thenar eminence to estimate forearm blood flow

Background: Near-infrared spectroscopy of the thenar eminence (NIRSth) can be used at the bedside to assess tissue oxygenation (StO2), the reperfusion response to ischaemia and the tissue haemoglobin index (THI). Its ability to estimate forearm blood flow (FBF) has not previously been assessed. Objectives: We aimed to test whether short-lived venous occlusion-induced changes in NIRSth-derived THI (delta THI/ minute) correlate with strain gauge plethysmography (SGP) measurements. Methods: We measured FBF in nine volunteers with SGP by venous occlusion, while estimating delta THI. Measurements were obtained in two forearm positions (elevated and horizontal) at baseline and during induced hyperaemia. Results: We performed 246 paired measurements at rest and after occlusion-induced hyperaemia. At rest, mean SGP-estimated FBF was 3.5-3.6 mL/dL/minute at baseline, compared with 12.9-13.6 mL/dL/minute during hyperaemia. At rest, delta THI was 6.1-8.2/minute, compared with 29.7-32.5/minute during hyperaemia. delta THI was a significant predictor of SGP FBF (P < 0.01), with stronger correlation during hyperaemia (P < 0.01). An equation was developed to convert delta THI/minute into FBF at mL/dL/minute (FBF = 0.362 x delta THI/minute + 0.864). Conclusions: NIRSth can be used to estimate FBF. Given its portability and its ability to also measure StO2 and vascular reactivity, NIRSth can assist in providing a comprehensive bedside assessment of the forearm circulation in critically ill patients.