Near infrared spectroscopy-measured changes in cerebral blood volume and cytochrome aa3 in newborn lambs exposed to hypoxia and hypercapnia, and ischemia: a comparison with changes in brain perfusion and O2 metabolism.

OBJECTIVES: Validation of near infrared spectroscopy (NIRS)-measured changes in cerebral blood volume (deltaCBV) and cytochrome aa3 (deltaCytaa3) as estimators of changes in brain perfusion and oxygenation in the newborn lamb during hypoxia and hypercarbia, and additional hypotension. METHODS AND MATERIALS: In 33 newborn lambs brain perfusion assessed by carotid artery blood flow (deltaQcar: ml/min)and cerebral metabolic rate of oxygen (deltaCMRO2: ml O2/min) were related to NIRS-derived deltaCBV (ml/100 g) and deltaCytaa3 (microM) during combined hypoxia and hypercarbia and additional hypotension. Combined hypoxia and hypercapnia was induced by ventilation with 6-8% of O2 and 10% of CO2 during 30 min, and additional hypotension ( < 35 mmHg for 5 min) was induced by careful withdrawal of blood. RESULTS: CBV increased during hypoxia and hypercarbia, decreased during additional hypotension and was related to deltaQcar: (0.009 ml/100 g change per ml/min Qcar: P < 0.0001). Cytaa3 increased during hypoxia and hypercarbia, decreased during subsequent additional hypotension andshowed a reverse relationship with deltaCMRO2 (-1.65 microM change per ml O2/min CMRO2: P <0.0001). Cytaa3 remained above baseline during reperfusion. CONCLUSIONS: deltaCBV estimates changes in brain perfusion, but overestimates brain perfusion during hypotension. The pattern of deltaCytaa3 suggests less oxygen utilisation by brain tissue during hypoxia and subsequent reperfusion.