Significance of systemic vascular resistance in determining the hemodynamic effects of hydralazine on large ventricular septal defects.

The hemodynamic effects of hydralazine were studied in 17 infants and young children with ventricular septal defects to clarify the significance of systemic vascular resistance (SVR) in determining these effects. Patients with peak pulmonary arterial pressures greater than 75% of systemic pressure were placed in group I, which was further divided into group Ia (n = 6), consisting of those with a control SVR of 20 U.m2 or higher, and group Ib (n = 8), consisting of those with a lower SVR. Group II consisted of three patients with lower pulmonary arterial pressures. Intravenous injection of hydralazine (0.3 mg/kg) reduced SVR in all but two patients. The magnitude of reduction correlated with prehydralazine (control) SVR (r = .66, p less than .01). Systemic blood flow (Qs) increased from 3.7 +/- 0.7 to 5.0 +/- 0.8 l/min/m2 (p less than .005). The mean systemic arterial pressure for all patients decreased from 69 +/- 2 to 65 +/- 2 mm Hg (p less than .01) and the mean pulmonary arterial pressure decreased from the control value by 9 +/- 4% (p less than .01) in group I and by 17 +/- 1% in group II. Pulmonary blood flow (Qp) did not change significantly in either group. The Qp/Qs ratio was reduced from 3.6 +/- 0.4 to 2.4 +/- 0.2 (p less than .02) in group Ia. In sharp contrast, however, it increased from 2.6 +/- 0.3 to 3.3 +/- 0.5 (p = .06) in group Ib. The posthydralazine Qp/Qs ratio, expressed as percent of the control value, inversely correlated with the control SVR (r = -.61, p = .02) in group I. The response was not different in the group II patients. Thus, we conclude that control SVR is important for prediction of the hemodynamic effects of afterload reduction by hydralazine in infants and young children with large ventricular septal defects, and that this drug may be beneficial in patients with high control SVRs since a high SVR brings about a decrease in the Qp/Qs ratio.