HYPOCAPNIA REVERSES THE FENTANYL-INDUCED INCREASE IN CEREBRAL BLOOD-FLOW VELOCITY IN AWAKE HUMANS

Investigations on the effects of opioids on cerebrovascular dynamics h ave repeatedly demonstrated mild to moderate increases in cerebral blo od flow velocity in the middle cerebral artery (CBFVMCA), cerebral blo od flow, and cerebrospinal fluid pressure in humans and animals. Howev er, the influence of hypocapnia on these fentanyl effects has not been investigated. We compared mean CBFVMCA during normo-and hypocapnia be fore and after administration of fentanyl (2.5 mu g/kg i.v.) in 20 awa ke humans. During normocapnia (end-tidal carbon dioxide [ETCO2] 40 mmH g) fentanyl significantly increased mean CBFVMCA (60 +/- 10 cm/s vs. 8 1 +/- 12 cm/s [mean +/- SD]; p < 0.01), whereas during hypocapnia (ETC O2 25 mmHg) mean CBFVMCA values were identical (40 +/- 7 cm/s vs. 40 /- 7 cm/s) before and after fentanyl administration. These results con firm previous findings that administration of fentanyl increases CBFVM CA, but, more importantly, clearly indicate that hypocapnia reverses t his potentially undesirable effect.