Nimodipine does not increase cerebral blood flow during hypocapnia in the rat

Nimodipine is a calcium entry blocker that shows promise in the therapy for cerebral ischemia. This study was undertaken to examine the interaction of nimodipine with the use of hypocapnia to control cerebral blood volume by reduction in cerebral blood flow (CBF), as might be applicable to patients undergoing anesthesia for neurosurgical procedures. Male adult Sprague-Dawley rats were anesthetized with pentobarbital 50 mg/kg i.p. and were mechanically ventilated at either normocapnia (Paco2 35-40 mm Hg) or hypocapnia (Paco2 23-25 mm Hg). Animals were randomized into four experimental groups in a 2 x 2 factorial design, employing Paco2 level and drug group as between-group factors: vehicle plus normocapnia (group 1, n = 6), nimodipine plus normocapnia (group 2, n = 7), vehicle plus hypocapnia (group 3, n = 6), and nimodipine plus hypocapnia (group 4, n = 6). Nimodipine (1 mug/kg/min) or vehicle was administered i.v. for a period of 45 min. CBF was then measured using [C]iodoantipyrine autoradiography. Hypocapnia decreased global CBF (p <0.0001) and nimodipine increased CBF (p <0.05). Although nimodipine increased global CBF (p <0.05) during normocapnia (89 +/- 8 versus 61 +/- 4 ml/100 g/min), during hypocapnia there was no significant difference between nimodipine and vehicle (45 +/- 3 versus 40 +/- 2 ml/100 g/min). Hypocapnia decreased local CBF in all structures examined, whereas nimodipine increased local CBF in some, but not all structures. In this model, prior institution of hypocapnia prevented nimodipine-induced global CBF increases.