Ascending aorta input impedance during hemorrhagic shock and after hyperosmotic solution infusion.

Aortic input impedance was determined in four dogs anesthetized with chloralose-urethane. Flow and pressure were recorded in the ascending aorta during random heart pacing. Signals were processed on a HP 3882A spectrum analyzer utilizing the Fast Fourier Transform. Measurements were made under control conditions, during hemorrhagic shock, and after infusion of a hyperosmotic solution (HS). Results show that HS decreases peripheral resistance, input impedance, frequency of minimal input impedance, and frequency of zero degree phase crossing. The last two indicate a slower pressure wave velocity. These data, together with a reduction in characteristic impedance, strongly suggest a decrease in arterial vascular tonus.