Response of vertebral and carotid blood flow to isocapnic changes in end-tidal oxygen tension

The response of the vertebral and carotid blood flow to isocapnic hypoxia was measured in 9 cats anaesthetized with chloralose-urethane using perivascular electromagnetic flow probes. The carotid flow was already significantly increased when going from hyperoxia (PETO2 55 kPa) to normoxia. For the vertebral blood flow a significant increase compared to hyperoxia was observed at a moderate level of hypoxia (PETO2 9 kPa). The time course of the response of the blood flow to isocapnic step-like changes in PETO2 was fitted with a first order model. The mean time constant (+/- SD) for steps into hypoxia for the carotid flow was 35 +/- 38 sec(8 cats) and for the vertebral flow, 44 +/- 37 sec (5 cats). The mean time constant (+/- SD) for steps out of hypoxia was significantly smaller and found to be 23 +/- 22 sec (8 cats) and 19 +/- 18 sec (4 cats), respectively. We argue that a major part of the changes in vertebral and carotid blood flow to steps into hypoxia goes to brain tissue.