Developmental changes in reperfusion injury

Development of cardiac sensitivity to oxygen deficiency: comparative and ontogenetic aspects

Hypoxic states of the cardiovascular system are undoubtedly associated with the most frequent diseases of modern times. They originate as a result of disproportion between the amount of oxygen supplied to the cardiac cell and the amount actually required by the cell. The degree of hypoxic injury depends not only on the intensity and duration of the hypoxic stimulus, but also on the level of cardiac tolerance to oxygen deprivation. This variable changes significantly during phylogenetic and ontogenetic development. The heart of an adult poikilotherm is significantly more resistant as compared with that of the homeotherms. Similarly, the immature homeothermic heart is more resistant than the adult, possibly as a consequence of its greater capability for anaerobic glycolysis. Tolerance of the adult myocardium to oxygen deprivation may be increased by pharmacological intervention, adaptation to chronic hypoxia, or preconditioning. Because the immature heart is significantly more dependent on transsarcolemmal calcium entry to support contraction, the pharmacological protection achieved with drugs that interfere with calcium handling is markedly altered. Developing hearts demonstrated a greater sensitivity to calcium channel antagonists; a dose that induces only a small negative inotropic effect in adult rats stops the neonatal heart completely. Adaptation to chronic hypoxia results in similarly enhanced cardiac resistance in animals exposed to hypoxia either immediately after birth or in adulthood. Moreover, decreasing tolerance to ischemia during early postnatal life is counteracted by the development of endogenous protection; preconditioning failed to improve ischemic tolerance just after birth, but it developed during the early postnatal period. Basic knowledge of the possible improvements of immature heart tolerance to oxygen deprivation may contribute to the design of therapeutic strategies for both pediatric cardiology and cardiac surgery.

Studies of hypoxemic/reoxygenation injury: without aortic clamping. II. Evidence for reoxygenation damage

This study tested the hypothesis that the developing heart is susceptible to oxygen-mediated damage after reintroduction of molecular oxygen and that this "unintended" reoxygenation injury causes lipid peroxidation and functional depression that may contribute to perioperative cardiac dysfunction. Among 49 Duroc-Yorkshire piglets (2 to 3 weeks old, 3 to 5 kg) 15 control studies were done without hypoxemia to test the effects of the surgical preparation (n = 10) and 60 minutes of cardiopulmonary bypass (n = 5). Twenty-nine piglets underwent up to 2 hours of ventilator hypoxemia (with inspired oxygen fraction reduced to 6% to 7%) to lower arterial oxygen tension to approximately 25 mm Hg. Five piglets did not undergo reoxygenation to determine alterations caused by hypoxemia alone. Twenty-four others received reoxygenation by either raising ventilator inspired oxygen fraction to 1.0 (n = 12) or instituting cardiopulmonary bypass at oxygen tension 400 mm Hg (n = 12). Ventilator hypoxemia produced sufficient hemodynamic compromise and metabolic acidosis that 18 piglets required premature reoxygenation (78 +/- 12 minutes). To avoid the influence of acidosis and hemodynamic deterioration during ventilator hypoxemia, five others underwent 30 minutes of hypoxemia during cardiopulmonary bypass (circuit primed with blood at oxygen tension 25 mm Hg) and 30 minutes of reoxygenation (oxygen tension 400 mm Hg) during cardiopulmonary bypass. Biochemical markers of oxidant damage included measurement of coronary sinus and myocardial conjugated dienes to determine lipid peroxidation and antioxidant reserve capacity assessed by incubating myocardial tissue in the oxidant t-butylhydroperoxide. Functional recovery was determined by inscribing pressure volume loops to determine end-systolic elastance and Starling curves by volume infusion. No biochemical or functional changes occurred in control piglets. Hypoxemia without reoxygenation did not change plasma levels of conjugated dienes, but lowered antioxidant reserve capacity 24%. Reoxygenation by ventilator caused refractory ventricular arrhythmias in two piglets (17% mortality), raised levels of conjugated dienes 45%, and reduced antioxidant reserve capacity 40% with recovery of 39% of mechanical function in the survivors. Comparable biochemical and functional changes occurred in piglets undergoing ventilator hypoxemia and/or cardiopulmonary bypass hypoxemia and reoxygenation on cardiopulmonary bypass. We conclude that hypoxemia increases vulnerability to reoxygenation damage by reducing antioxidant reserve capacity and that reoxygenation by either ventilator or cardiopulmonary bypass produces oxidant damage with resultant functional depression that is not a result of cardiopulmonary bypass. These findings suggest that initiation of cardiopulmonary bypass in cyanotic immature subjects causes an unintended reoxygenation injury, which may increase vulnerability to subsequent ischemia during surgical repair.

The contraction state of myofibrils during global ischemia and after reperfusion following different forms of cardiac arrest. Correlation with metabolic parameters in the canine heart

This study was undertaken in order to obtain information on the mode of reaction of the contractile apparatus after different forms of cardiac arrest, global ischemia and reperfusion, as well as on possible correlations between the contraction state of myofibrils and biochemical parameters. During the survival time, before the level of 3 mumol/gww creatine phosphate (CP) is reached, the contraction state shows only minor changes. During the revival time in which ATP tissue concentrations decay to 4 mumol/gww, the contribution of ATP, lactate, anorganic phosphate (Pa) and acidosis to the degree of relaxation depends on the method of cardiac arrest. At defined biochemical values, the degree of relaxation is comparable after aortic cross clamping (ACC) and St. Thomas perfusion, but significantly different compared to HTK perfusion. Thus, during the revival time, the relaxation of sarcomeres depends predominantly on the composition of the solutions used for cardiac arrest. The re-entry of contraction below 3 mumol/gww ATP is correlated with the ATP concentration, independent of the form of cardiac arrest. Reperfusion after HTK or St. Thomas cardioplegia and reversible ischemia leads to the focal formation of contraction bands, which do not occur during ischemia. This contraction state is significantly more pronounced after reperfusion of St. Thomas arrested hearts. Thus, the contraction state of myofibrils is influenced not only by alterations in metabolite concentrations, but also by the composition of cardioplegic solutions and by the characteristic conditions (sufficient energy, oxygen and Calcium) during reperfusion.

The concentration of calcium in neonatal cardioplegia

The optimal calcium concentration in cardioplegia for the newborn has not been determined. Therefore, the effect of 0, 0.6, 1.2, 1.8, and 2.4 mmol/L calcium in modified St. Thomas cardioplegia was evaluated in isolated working hearts of 7- to 10-day-old rabbits. Functional recovery was determined by comparing aortic flow, developed pressure, and first derivative of left ventricular pressure (dP/dt) before and after 1 hour of normothermic (37 degrees C) ischemia. As percentages of baseline values, recovery of developed pressure and dP/dt averaged 10% +/- 1% (mean +/- standard error of the mean) and 10% +/- 1% with 0 mmol/L, 46% +/- 7% and 44% +/- 8% with 0.6 mmol/L, 79% +/- 2% and 76% +/- 2% with 1.2 mmol/L, 67% +/- 2% and 61% +/- 5% with 1.8 mmol/L, and 65% +/- 5% and 65% +/- 7% with 2.4 mmol/L calcium, respectively. Significant improvement in recovery of developed pressure and dP/dt was detected when the calcium concentration was increased from 0 to 0.6 mmol/L and from 0.6 to 1.2 mmol/L, but the groups with 1.2, 1.8, and 2.4 mmol/L did not differ from one another significantly in terms of developed pressure and dP/dt recovery. There was no recovery of aortic flow when 0 mmol/L calcium was used; at calcium concentrations of 0.6, 1.2, 1.8, and 2.4 mmol/L, recovery of aortic flow averaged 16% +/- 7%, 63% +/- 10%, 23% +/- 10%, and 36% +/- 11% of baseline values, respectively. Recovery of aortic flow with 1.2 mmol/L calcium was significantly higher than at concentrations of 0.6 and 1.8 mmol/L.(ABSTRACT TRUNCATED AT 250 WORDS)