Cerebral blood flow in rabbits during the nasopharyngeal reflex elicited by inhalation of noxious vapor

We used chronically implanted Doppler ultrasonic flow probes to measure internal carotid and vertebral blood flow during the nasopharyngeal reflex elicited by inhalation of formaldehyde vapor in conscious rabbits. Internal carotid flow gradually increased to 157 ± 5% of baseline and vertebral artery increased to 123 ± 9% of baseline, with maximum values reached approximately 20-40 s after administration of vapor, at a time when arterial pO₂ had decreased from 80 ± 3 to 53 ± 4 mmHg. Increases in flow were associated with increases in vascular conductance. The delayed increases in cerebral blood flow contrasted with rapid decreases in ear and distal aortic news, measured at the same time. Our results indicate that forebrain vascular conductance increases in response to inhalation of noxious vapor, possibly reflecting cerebrovascular events associated with hypoxemia.