Influence of respiration and changes in expiratory pressure on cerebral haemoglobin concentration measured by near infrared spectroscopy

Near infrared spectroscopy (NIRS) was used to measure the changes in concentration of cerebral oxy-and deoxygenated haemoglobin ([HbO(2)] and [Hb]) in six healthy adult volunteers spontaneously breathing against increased expiratory pressures (IEPs) between 0 and 20 cm H2O. During expiration, an increase in [HbO(2)] was recorded, accompanied by a smaller decrease in [Hb], producing a small increase in total cerebral haemoglobin concentration ([Hb(sum)]). The mean +/- SD change in [Hb(sum)] at the maximum IEP of 20 cm H2O was 1.2 +/- 0.7 mu mol L(-1) (equivalent to 1.4%). Changes in [Hb(sum)] correlated with IEP level (r = 0.95) and changes in MABP (r = 0.96). The results suggest that homeostatic mechanisms do not maintain cerebral blood volume or flow constant over the period of a single breath in normal adults.