Effects of morphine on cerebral blood flow autoregulation CO2-reactivity in experimental subarachnoid hemorrhage

Previous reports show that naloxone improves ischemic deficits and clinical conditions in patients after subarachnoid hemorrhage (SAH). These observations have raised concern about the routine use of morphine in the treatment of severe headache after SAH. The present study was carried out to investigate the effects of morphine on cerebral vasoreactivity after experimental SAH. Cerebral blood flow (CBF) autoregulation was studied in two groups of eight rats each with experimental SAH. A bolus intravenous injection of morphine, 1 mg/kg, was administered in one group and the other was used as a control group. During eucapnia, CBF was measured by the intracarotid 133Xenon method during decreasing mean arterial blood pressure (MABP). CO2-reactivity was investigated in two corresponding groups where CBF was measured at decreasing PaCO2 levels during constant MABP. Morphine decreased mean baseline CBF by 34% and 26% in the study of autoregulation and CO2-reactivity, respectively. Cerebral blood flow autoregulation was found impaired in both controls and the morphine group. However, the mean slope of the linear regression lines of CBF/MABP was 0.49 +/- 0.32 ml/100g/min/mm Hg in the morphine group, which was significantly lower than 1.24 +/- 0.59 ml/100g/min/mm Hg in the controls (p < 0.05). Also the mean CO2-reactivity was significantly lower, 0.64 +/- 0.53 %/0.1kPa, in the morphine group, compared to 2.36 +/- 0.87 %/0.1kPa in the controls (p < 0.001). The results show that in rats with SAH, morphine partially restores CBF autoregulation but attenuates CO2-reactivity.