Continuous and non-invasive study of brain oxygenation in the calf by near infrared spectroscopy

Near infrared spectroscopy (NIRS) was used to detect changes in brain oxygenation in five tranquilised calves that were placed on a hypoxic gas mixture (10 per cent O2/90 per cent N2) and hyperoxic gas mixture (30 per cent O2/70 per cent N2) for five minutes at each concentration. A NIRO 500 (Hamamatsu, Japan) was used for the NIRS, with the incident light source and separate detector (optodes) placed on shaved skin on the most dorsal surface of the frontal bone. Sequential arterial blood gas sample analyses provided confirmation of the appropriate change in systemic oxygenation status. By the end of the five-minute-period of breathing 10 per cent oxygen, NIRS of the calf head detected highly significant changes in haemoglobin oxygenation reflective of hypoxaemia, with oxyhaemoglobin decreasing by 23.5 units (P<0.01) and deoxyhaemoglobin increasing by 45.6 units, (P<0.01) from the baseline of breathing room air. Total haemoglobin (oxyhaemoglobin + deoxyhaemoglobin) showed a significant increase of 22.1 units (P<0.05) but there was no significant change in NIRS determined cytochrome aa3 oxygenation. Concomitant blood gas alterations included significant decreases in PaO2 (-27.8 mmHg, P<0.01), haemoglobin saturation (-29.0 per cent, P<0.05), and PaCO2 (-7.8 mmHg, P<0.05) and significantly increased blood pH (0.059, P<0.05). At the end of the five minutes of breathing 30 per cent oxygen NIRS of the calf head detected significantly increased oxyhaemoglobin (13.1 units, P<0.01) and decreased deoxyhaemoglobin (-13.7 units, P<0.05) when compared with baseline breathing of room aim. Total haemoglobin and cytochrome aa3 were unchanged from baseline. The accompanying arterial blood gas changes included significant increases in PaO2 (30.9 mmHg, P<0.05), arterial O2 saturation (11.7 per cent, P<0.05), and significantly decreased pH (-0.026, P<0.05). This study showed that NIRS can be used to continuously and non-invasively detect cerebral oxygenation changes in the live calf in response to both increased and decreased systemic arterial oxygen. Additionally, despite induction of profound hypoxaemia, cytochrome aa3 in the brain did not appear to become reduced.