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A previous report' showed that a vasorelaxant (isoxsuprine hydrochloride) prevented the hyperventilation build-up response in the human electroencephalogram, providing additional evidence that the response is due to cerebral vasoconstrictor reaction to hypocapnia, and that cerebral vasoconstriction can be blocked by such drugs.The availability of two other marketed vasorelaxants suggested a comparative study employing human volunteers. METHOD Ten healthy, young, normotensive individuals aged 21 to 32 years (mean age 27.9) participated in the study (5 male and 5 female). The drugs used were cyclandelate, isoxsuprine, and nylidrin. A resting EEG recording was obtained for each subject and another following voluntary forced hyperventilation, by which presence of build-up was demonstrated. Medication was then given by mouth and the subjects hyperventilated 4 to 10 minutes every 30 min. for 3 hours. Two trials were made with twice the tablet dosage provided by manufacturers, and a third trial was made at double this dosage if no blocking was obtained.Isoxsuprine and nylidrin were also given intramuscular. Subjects hyperventilated every 10 min. until blocking was observed, and thereafter every 15 min. until build-up reappeared.Three to five days were allowed between each trial. In determining duration of blocking, any period was classified as blocked if no build-up appeared upon its termination.

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