Quantifying cerebral hypoxia by near-infrared spectroscopy tissue oximetry: the role of arterial-to-venous blood volume ratio

Abstract

© 2017 The Authors. Tissue oxygenation estimated by near-infrared spectroscopy (NIRS) is a volume-weighted mean of the arterial and venous hemoglobin oxygenation. In vivo validation assumes a fixed arterial-to-venous volume-ratio (AV-ratio). Regulatory cerebro-vascular mechanisms may change the AV-ratio. We used hypotension to investigate the influence of blood volume distribution on cerebral NIRS in a newborn piglet model. Hypotension was induced gradually by inflating a balloon-catheter in the inferior vena cava and the regional tissue oxygenation from NIRS (rStO2,NIRS) was then compared to a reference (rStO2,COX) calculated from superior sagittal sinus and aortic blood sample co-oximetry with a fixed AV-ratio. Apparent changes in the AV-ratio and cerebral blood volume (CBV) were also calculated. The mean arterial blood pressure (MABP) range was 14 to 82 mmHg. PaCO2 and SaO2 were stable during measurements. rStO2,NIRS mirrored only 25% (95% Cl: 21% to 28%, p<0.001) of changes in rStO2,COX. Calculated AV-ratio increased with decreasing MABP (slope:-0.007·mmHg-1, p<0.001). NIRS estimates that CBV decreased with decreasing MABP (slope: 0.008ml/100g/mmHg, p<0.001). Thus, cerebral NIRS oximetry responded poorly to changes in tissue oxygenation during hypotension induced by decreased preload. An increase in the AV-ratio during hypotension due to arterial vasodilation and, possibly, cerebral venous collapse may be a part of the explanation.