NEAR-INFRARED SPECTROSCOPY DETECTS CEREBRAL-ISCHEMIA DURING HYPOTENSION IN PIGLETS

We have previously reported concordant changes in cerebral intravascul ar oxygenation measured by near infrared spectroscopy (NIRS) and mean arterial blood pressure (MAP) in premature infants. We hypothesized th at the cerebral oxygenation changes are caused by MAP-induced alterati ons in cerebral blood flow (CBF) and studied these parameters in neona tal piglets (n = 6). Changes in cerebral intravascular oxygenation wer e measured by NIRS from the hemoglobin difference (HbD) signal (oxyhem oglobin-deoxyhemoglobin). CBF was measured by the radioactive microsph ere technique. The cerebral circulation was also monitored by Doppler determinations of CBF velocity (time average mean velocity) in the ant erior cerebral artery. Hypotension to <50% of baseline MAP was achieve d by a ligature around the ascending aorta. Arterial oxygenation was m aintained constant by mechanical ventilation. As observed in our studi es of premature infants, cerebral HbD and MAP showed concordant change s. Hypotension was accompanied by significant decreases both in CBF (4 2.8 +/- 12.5% of baseline p < 0.01) and HbD (-65.0 +/- 22.0 mu mol/L.d pf, p < 0.01). HbD was significantly correlated with MAP (p < 0.05) an d time average mean velocity (p = 0.01). Importantly, decreases in cer ebral total hemoglobin (HbT), a measure of cerebral blood volume, did not correlate significantly with decreases in MAP. We conclude that 1) decreases in cerebral intravascular oxygenation, as assessed by NIRS, observed with decreases in MAP reflect a decline in CBF, and hence ox ygen delivery, 2) the HbD signal is more sensitive to changes in CBF t han the HbT signal, and 3) NIRS recordings may have clinical utility i n detecting cerebral ischemia.