Nitric-Oxide-Induced Reoxygenation Injury in the Cyanotic Immature Heart Is Prevented by Controlling Oxygen Content During Initial Reoxygenation

Abstract

Reintroduction of high levels of molecular oxygen after a hypoxic period is followed by a burst of nitric oxide (NO), peroxynitrite, and oxygen free radicals (OFR), which are highly cytotoxic. This study indicates that hyperoxic reoxygenation of cyanotic immature hearts on cardiopulmonary bypass (CPB) induces a reoxygenation injury and that, by reducing NO and OFR production during institution of CPB with subsequent reoxygena tion under blood cardioplegic arrest, this oxygen-related damage can be avoided and biochemical and functional status improved.

Of 25 immature piglets (3-5 kg, two to three weeks old), 6 underwent one hour of CPB including thirty minutes of aortic clamping with substrate-enriched modified blood cardioplegia (hypocalcemic, alkalotic, and hyperosmolar; warm induction-cold replen ishment-warm reperfusion) without preceding hypoxia (controls). Nineteen others were made hypoxic (arterial [PO₂] 20-30 mmHg) for up to two hours by lowering the fraction of inspired oxygen (FIO₂) on ventilator. These hypoxic piglets were then reoxygenated on CPB at different PO₂ levels (hyperoxic, normoxic, or hypoxic) for five minutes, followed by the aforementioned blood cardioplegic (BCP) arrest regimen.

Myocardial conjugated diene (CD) production as a marker of lipid peroxidation, and NO production, determined as its spontaneous oxidation products, nitrite (NO₂ ^-) and nitrate (NO₃ ^-), were assessed during blood cardioplegic induction, and antioxidant reserve capacity was determined by incubating myocardium in the oxidant t-butylhy droperoxide (t-BHP). Myocardial function was evaluated from end-systolic elastance (Ees, conductance catheter). Blood cardioplegic arrest caused no functional or biochem ical changes in normoxic control immature piglets. In contrast, brief reoxygenation at PO₂ > 400 mmHg, followed by BCP-arrest (hyperoxic) resulted in marked CD production (42 ±4 vs 3 ±1 A₂₃₃ nm/minute/100 g; P<0.05), and NO production (4500 ±500 vs 450 ±32 mmol/minute/100 g; P<0.05) during blood cardioplegic induction, reduced antiox idant reserve capacity (malondialdehyde [MDA] at 4.0 mM of t-BHP: 1342 ±59 vs 958 ± 50 nM/g protein; P<0.05), and caused profound myocardial dysfunction; Ees recovered only 21 ±2% (vs 104 ±7; P<0.05), despite the blood cardioplegic regimen shown to be cardioprotective in control normoxic piglets. Conversely, controlling initial PO₂ to normoxic (100 mmHg) or hypoxic (20-30 mmHg) levels reduced lipid peroxida tion (CD production 16 ±2*, 2 ±1*^† A₂₃₃nm/minute/100 g) and NO production (1264 ±736*, 270 ± 182*^† mmol/minute/100 g), restored antioxidant reserve capacity (MDA at 4.0mM of t-BHP: 940 ±95*, 982 ±88* nM/g protein), and allowed significant func tional recovery (58 ±11%* and 83 ±8%*^†), in a PO₂-dependent fashion.

The authors conclude that reoxygenation of hypoxemic immature hearts by initiating hyperoxic CPB causes oxidant-related damage characterized by lipid peroxidation, enhanced NO production, and reduced antioxidants, leading to functional depression that nullifies the cardioprotective effects of blood cardioplegia. These detrimental effects can be reduced in a PO₂-dependent fashion by controlling initial PO₂ on CPB and subse quent reoxygenation during blood cardioplegic arrest.

* = P<0.05 vs hyperoxic,^† = P<0.05 vs normoxic (data are mean ± SE, ANOVA).

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