Potentiation by calcium channel blockade of hypoxic myocardial depression in the neonate.

The objectives of this study were to examine the independent and combined effects of beta blockade (practolol) and calcium channel blockade (verapamil) on cardiac responses to hypoxia in the neonate. Lambs were anaesthetized with pentobarbital (20 mg/kg) and were prepared for measurements of left ventricular (LV) performance under controlled hemodynamic conditions. Force generation was assessed from curves relating LV systolic pressure (SP) to end-diastolic pressure (LVEDP) over a broad range of afterloading. Velocity was determined from simultaneous measurements of LV dP/dtmax. Values obtained at LVEDP 10 cm H2O were used to compare interventions. Practolol (P) caused no significant reduction in SP10, but dP/dt10 fell from 51 to 37 (X 10(2] mm Hg/sec (p less than 0.05). Verapamil (V), 2 micrograms/min/kg, reduced measures of contractility (p less than 0.01). Doubling the dose of V further reduced SP10 to 79% of control. Hypoxemia (PaO2, 32 torr) increased SP10 from 172 to 192 mm Hg, and dP/dt10 from 51 to 85 (X 10(2] mm Hg/s (p less than 0.001). After P, the same degree of hypoxia elicited no changes in LV function. During infusion of V (4 micrograms/min/kg), hypoxia reduced SP10 from 138 to 122 mm Hg (p less than 0.01) and dP/dt10 from 29 to 24 (X 10(2] mm Hg/sec (p less than 0.05). It is concluded that in the absence of adrenergic support, hypoxia significantly depresses both force and velocity parameters of contractility in hearts with calcium channel blockade.