Cerebral microvascular reactivity to carbon dioxide during isoflurane anesthesia as assessed by laser-Doppler flowmetry.

BACKGROUND: To determine the effects of carbon dioxide on the cerebral microcirculation during isoflurane anesthesia. METHODS: We studied 10 rabbits using laser Doppler flowmetry on the parietal cortex. After surgical preparation, the end-tidal concentration of isoflurane was maintained at one MAC (2.05%). Mean arterial pressure, rectal temperature and hematocrit were kept constant. The arterial carbon dioxide tension (PaCO2) was randomly adjusted to three levels: 20-25 mmHg (hypocapnia); 35-40 mmHg (normocapnia); and 45-50 mmHg (hypercapnia). Cerebral blood flow (CBF) was measured after target PaCO2 had been achieved. RESULTS: CBF at normocapnia was 27.4 +/- 13.5 (mean +/- SD) ml/100 gm/min, which increased to 42.4 +/- 14.3 ml/100 g/min (p < 0.01) and decreased to 15.3 +/- 7.5 ml/100 g/min (p < 0.05) at hypercapnia and hypocapnia respectively. The slopes of CBF versus PaCO2 were 1.60 +/- 0.63 ml/100 g/min/mmHg from normocapnia to hypercapnia and -0.79 +/- 0.51 ml/100 g/min/mmHg from normocapnia to hypocapnia. The corresponding percentage of alteration in CBF on the respective change of PaCO2 was 7.2 +/- 5.1 and -3.0 +/- 1.2%/mmHg. CONCLUSIONS: These data indicate that the changes of the cerebral microvascular blood flow reflect the CO2 effects during isoflurane anesthesia.