Prevention of reoxygenation injury in hypoxaemic immature hearts by priming the extracorporeal circuit with antioxidants

Abstract

This study tests the hypothesis that abrupt reoxygenation of cyanotic immature hearts when starting cardiopulmonary bypass produces an unintended reoxygenation injury that: (i) nullifies the cardioprotective effects of blood cardioplegia; and (ii) is avoidable by adding the antioxidants, N-(2-mercaptopropionyl)-glycine (MPG) plus catalase to the cardiopulmonary bypass prime. Twenty immature piglets (aged 2-3 weeks) underwent 30 min of blood cardioplegic arrest (BCP) with standard clinical blood cardioplegia (hypocalcaemic, alkalotic, hyperosmolar, substrate-enriched). Six piglets remained normoxaemic (BCP). Fourteen others were made hypoxic (PO2 20-30 mmHg) for up to 2 h by lowering ventilator FiO2 (5-7%) before undergoing reoxygenation on cardiopulmonary bypass at PO2 400 mmHg. In eight animals, the pump prime was not supplemented with antioxidants (Reox + BCP), whereas MPG (80 mg/kg) and catalase (CAT; 5 mg/kg) were added to the pump prime in the other six (MPG/CAT). Myocardial function (end-systolic elastance, conductance catheter), oxidant damage (myocardial conjugated diene production), oxygen consumption and antioxidant reserve capacity were evaluated. Blood cardioplegic arrest caused no functional or biochemical changes in controls without preceding hypoxia. In contrast, hypoxia and reoxygenation in animals undergoing the same blood cardioplegic protocol (Reox + BCP) caused profound myocardial dysfunction, as end-systolic elastance recovered only to 21(2)% (P < 0.05 versus control) of baseline values. Additionally, it reduced antioxidant reserve capacity (malondialdehyde, MDA at 4.0 mM of t-BHP: 1342(59) (P < 0.05 versus control) versus 788(53) mmol/g protein), and led to significantly greater production of conjugated dienes during warm induction (42(4.4) (P < 0.05 versus control) versus 3.3(1.4) A233 nm/100 g per min) and reperfusion (22(2.7) (P < 0.005 versus control) versus 2(0.6) A233 nm/100 g per min). Conversely, supplementation of MPG plus catalase to the pump prime reduced lipid peroxidation (conjugated diene production during warm induction: 22.3(7) A233 nm/100 g per min P < 0.05 versus Reox + BCP), restored antioxidant reserve capacity (MDA at 4.0 M of t-BHP: 975(139) mmol/g protein P < 0.05 versus Reox + BCP) and allowed almost complete functional recovery (80(8)%). Abrupt reoxygenation of hypoxaemic immature hearts on cardiopulmonary bypass causes oxidant damage, nullifies the cardioprotective effects of blood cardioplegia, and leads to reduced myocardial contractility. Antioxidant supplementation of the cardiopulmonary bypass prime avoids these detrimental effects, and results in improved biochemical and functional status.