August 1, 1979Open Access
Aortic Input Impedance during Nitroprusside Infusion
Structured PICO
Does nitroprusside infusion reduce vascular load and improve ventricular function in patients with heart failure?
P
Population
12 patients with heart failure (cardiac index <2.0 liters/min per m(2) and left ventricular end diastolic pressure >20 mm Hg)
I
Intervention
Nitroprusside infusion at varying doses (9-19, 20-38, and 39-77 μg/min)
C
Comparator
Baseline (pre-infusion)
O
Outcome
Total aortic input impedance spectrum (mean and pulsatile components), left ventricular end diastolic pressure, and stroke volumesurrogate
Nitroprusside acutely improves ventricular function in heart failure by reducing both mean and pulsatile components of vascular load, an effect that may be independent of aortic pressure reduction.
Abstract
Beneficial effects of nitroprusside infusion in heart failure are purportedly a result of decreased afterload through "impedance" reduction. To study the effect of nitroprusside on vascular factors that determine the total load opposing left ventricular ejection, the total aortic input impedance spectrum was examined in 12 patients with heart failure (cardiac index 20 mm Hg). This input impedance spectrum expresses both mean flow (resistance) and pulsatile flow (compliance and wave reflections) components of vascular load. Aortic root blood flow velocity and pressure were recorded continuously with a catheter-tip electromagnetic velocity probe in addition to left ventricular pressure. Small doses of nitroprusside (9-19 mug/min) altered the total aortic input impedance spectrum as significant (P < 0.05) reductions in both mean and pulsatile components were observed within 60-90 s. With these acute changes in vascular load, left ventricular end diastolic pressure declined (44%) and stroke volume increased (20%, both P < 0.05). Larger nitroprusside doses (20-38 mug/min) caused additional alteration in the aortic input impedance spectrum with further reduction in left ventricular end diastolic pressure and increase in stroke volume but no additional changes in the impedance spectrum or stroke volume occurred with 39-77 mug/min. Improved ventricular function persisted when aortic pressure was restored to control values with simultaneous phenylephrine infusion in three patients. These data indicate that nitroprusside acutely alters both the mean and pulsatile components of vascular load to effect improvement in ventricular function in patients with heart failure. The evidence presented suggests that it may be possible to reduce vascular load and improve ventricular function independent of aortic pressure reduction.