Aortic input impedance in heart failure.

The input impedance of the systemic circulation was calculated from recordings of pulsatile pressure and flow in the ascending aorta of 20 patients. Ten patients had clinical and hemodynamic evidence of heart failure. The other 10 subjects had no clinical evidence of heart failure and were used as a control group. In the heart failure patients, both input resistance and characteristic impedance (index of aortic distensibility) were significantly increased compared to pressure- and age-matched control subjects. Oscillations of impedance moduli, represented by the difference between maximum and minimum moduli, were also significantly increased in the heart failure patients compared with the control subjects. The increased characteristic input impedance in these heart failure patients suggests that the human aorta is stiffer in heart failure, and the larger oscillations in the impedance spectrum indicate an increase in pressure and flow wave reflections. From reflected wave theory in elastic tubes, reflected pressure waves add to the amplitude of incident pressure waves at the entrance of the system, whereas reflected flow waves subtract from the magnitude of the forward flow. Thus, changes in aortic distensibility could have an important influence on the pulsatile function of the failing left ventricle.