Right ventricular-vascular interaction in congestive heart failure. Importance of low-frequency impedance

Vasodilator agents are widely used in congestive heart failure. These agents may have important effects on the pulsatile aspects of right ventricular hydraulic load.

Fifteen patients with severe congestive heart failure were studied during cardiac catheterization by use of high-fidelity pressure transducers and a catheter-mounted flow velocity probe. Three graded doses of nitroprusside were infused as pulmonary artery (PA) pressure and flow were continuously recorded. From Fourier transforms of signal-averaged waves, PA impedance, hydraulic power, and wave reflection indices were derived. At the highest dose of nitroprusside (66 +/- 41 micrograms/min), cardiac output was significantly improved, whereas PA mean and wedge pressure, resistance, impedance at the first harmonic, characteristic impedance, and wave reflection amplitude were all reduced. At the dose (32 +/- 20 micrograms/min) at which cardiac output first showed improvement, only PA mean pressure and first-harmonic impedance were significantly reduced. Hydraulic power cost per unit of forward flow was also lowered at this dose, despite lack of significant change in pulmonary vascular resistance. At the lowest dose of nitroprusside (11 +/- 4 micrograms/min), six patients experienced a decrease in stroke volume, whereas the other nine were either unchanged (n = 1) or showed an increase (n = 8). Multiple regression revealed that only the change in first-harmonic impedance correlated with this effect, increasing when stroke volume decreased and decreasing when stroke volume increased (P = .02). The change in first-harmonic impedance at this dose appeared to be caused by alterations in the amplitude of PA wave reflections. At higher doses, changes in mean PA pressure (but not in pulmonary vascular resistance) correlated with changes in stroke volume.

Nitroprusside vasodilation at low doses alters PA hemodynamics in congestive heart failure primarily through changes in low-frequency impedance. In some patients, this effect is associated with decreased stroke output. At higher doses, favorable alterations in resistance, low- and high-frequency impedance, and wave reflections all contribute to increased forward flow and decreased power requirement per unit forward flow. These findings show that ventricular-vascular interaction is importantly affected by pulmonary vasodilation and that appreciation of pulsatile properties is required to understand the effects of pulmonary vasodilation on cardiac output.