Ascending aortic impedance patterns in the kangaroo: their explanation and relation to pressure waveforms.

This study seeks to explain mechanisms responsible for the peculiar ascending aortic pressure waveform and impedance spectral pattern in kangaroos. Pulsatile pressure and blood flow velocity were measured and input impedance calculated in the ascending aorta, descending thoracic aorta, and brachiocephalic artery of 15 rock kangaroos. Pressure and velocity waveforms and impedance spectral patterns were interpreted with the aid of an asymmetric uniform T-tube model of the systemic arterial tree. The ascending aortic pressure waveform displayed a very large secondary wave that began in late systole or early diastole and continued throughout most of diastole. The peak of this secondary wave (which almost always occurred in diastole) was often greater than peak systolic pressure and results from apparently intense wave reflections from peripheral vascular beds. This contention is supported by the configuration of the impedance spectral pattern that is explained on the basis of a single (or dominant) functionally discrete reflecting site in the lower part of the body. These findings are explicable on the basis of body size and shape and the extreme eccentric location of the heart within the body. Wave reflections from the diminutive upper body are so small that they are totally dominated by intensive wave reflections from the large muscular lower body. These conclusions are supported by results obtained from the asymmetric T-tube model.