Deoxyhaemoglobin as a biomarker of cerebral autoregulation

Abstract

Cerebral autoregulation (CA) maintains cerebral blood flow over a range of perfusion pressure. Continuous CA monitoring might define pressure targets minimising secondary brain injury, but application is limited by available monitoring modalities. Near-infrared spectroscopy (NIRS) is a noninvasive optical technique characterising aspects of CA. The NIRS-derived tissue oxygenation index (TOI) is correlated with blood pressure (BP) to produce an index of vascular reactivity (TOx) [1]. The contribution from extracerebral tissues, optical complexity of injured brain and complex physiology represented by NIRS are likely to limit agreement with other techniques. NIRS-measured deoxyhaemoglobin (HHb) may have advantages as its physiological confounds are less complicated and are predominantly in the cerebral venous circulation. This study compares HHb with established indices of reactivity - the mean velocity index (Mx) and oxygen reactivity index (ORx).