Effect of nitroprusside on wave reflections in patients with heart failure

The relationship between wave reflections and ventricular-vascular coupling has been the subject of considerable speculation. Since we have previously shown that low-dose nitroprusside infusion improved ventricular-vascular coupling (as evidenced by increases in cardiac output and in aortic and pulmonary arterial total external power) in patients with severe left ventricular failure and secondary pulmonary hypertension, we chose to examine the changes in their aortic and pulmonary arterial wave reflections in this study. Wave reflection indexes examined included calculated backward and forward pressure waves and the ratio of their magnitudes (reflection factor), the reflection coefficient spectrum obtained by taking the ratios of the corresponding Fourier harmonics of the backward and forward waves, two terminal reflection coefficients calculated as gamma t = (R - Zc)/(R + Zc), where Z is characteristic impedance and R is either total resistance or vascular resistance, and the difference between the maximum and minimum impedance moduli for frequencies of 4 to 15 Hz. In the systemic vasculature, nitroprusside produced large reductions in the elevated vascular resistances and decreased aortic reflections as indexed by the reflection factor and by both terminal reflection coefficients. In contrast, however, no significant changes were found in the pulmonary artery wave reflection indexes despite large reductions in the pulmonary resistances.