A comparison of the monitors INVOS 3100 and NIRO 500 in detecting changes in cerebral oxygenation

Measurements of cerebral haemoglobin oxygenation of 2 near-infrared spectroscopy devices (INVOS 3100 and NIRO 500) were compared during and after hypocapnia.

Fifteen awake, healthy volunteers, who hyperventilated to obtain end-tidal CO2 (EtCO2) values of approximately 20 mmHg, were studied. During hyperventilation and 8 min thereafter, EtCO2, INVOS 3100 (RSO2 = regional cerebral oxygenation) and NIRO 500 recordings (HbO2 = oxyhaemoglobin, Hb = deoxyhaemoglobin, Hb-diff = HbO2-Hb, CtO2 = oxidised cytochrome oxidase aa3) were analysed.

Hyperventilation induced a significant decline in EtCO2 from 30.5 to 14.7 mmHg (P < 0.001) and RSO2 from 67.1% to 62.7% (P = 0.025). At hypocapnia, only Hb (+1.61 +/- 0.48 mumol/L; P < 0.001) and Hb-diff (-3.01 +/- 2.0 mumol/L; P < 0.001) indicated a decline in cerebral haemoglobin oxygenation. Within 8 min after hyperventilation, both EtCO2 and RSO2 normalised to values insignificantly different from baseline. In contrast, Hb and Hb-diff remained significantly different (Hb: +2.52 +/- 1.28 mumol/l; P < 0.001, Hb-diff: -4.31 +/- 4.0 mumol/L; P < 0.001). A correlation with EtCO2 was found for RSO2 (R = 0.35; P < 0.001) and CtO2 (R = 0.42; P < 0.001). All volunteers were continuously awake and none presented clinical symptoms of cerebral hypoxia.

Changes in cerebral haemoglobin oxygenation state were reflected more accurately by INVOS 3100 than NIRO 500. The cause may be the different technology of the monitors, since INVOS 3100 eliminates the contribution of extracranial oxygenation.