Input impedance of the canine vertebral artery

In experiments on six anaesthetized dogs it was found that the vertebral artery contributed 69.2 +/- 1.7% of cerebral and 41.5 +/- 2.5% of cephalic flow. The input characteristics were that the vertebral artery had a flow of 0.71 +/- 0.05 ml X s-1 into an input impedance of 23.7 +/- 1.9 GN X m-5 X s. This compared to external and internal carotid flows of 0.69 +/- 0.05 and 0.31 +/- 0.02 ml X s-1 into input impedances of 23.9 +/- 2.1 and 52.1 +/- 2.7 GN X m-5 X s respectively. Standardized per 100 g of tissue the impedances of the vertebral and internal carotid arteries were similar at 18.6 +/- 2.0 and 19.1 +/- 1.5 GN X m-5 X s X 100 g-1 respectively, while that for the external carotid was higher at 129.9 +/- 10.2 GN X m-5 X s X 100 g-1. The impedance spectrum for the vertebral artery was of low impedance type, settling about a characteristic impedance of 2.01 +/- 0.28 GN X m-5 X s without significant oscillation, associated with the phase only becoming slightly positive above 10 Hz and reflexion coefficients of less than 0.25 by a frequency of 6 Hz. This was similar to the spectrum for the internal carotid artery and contrasted with the high impedance spectrum with major reflexion found for the external carotid artery.