Temperature-response studies of the detrimental effects of multidose versus single-dose cardioplegic solution in the rabbit heart

Cardioprotection of neonatal heart using normothermic hyperkalaemia: the importance of delivery and terminal cardioplegia

Cardioprotection of immature hearts remains controversial and largely based on the use of hypothermic cardioplegia. Recent clinical trials in pediatric open-heart surgery suggest that normothermic cardioplegic arrest is also cardioprotective. However, the advantages of using normothermic cardioplegia delivered as single- or multi-dose with or without terminal cardioplegia are unknown. This work investigates the efficacy of these techniques and the mechanism(s) underlying their protective effect. Neonatal (7-10 days) rabbit hearts in a working mode were exposed to normothermic global ischemia (60 or 90 min) protected with one of the following cardioplegic (hyperkalaemic buffer) protocols: single-dose, multi-dose infused every 30 min, single-dose or multi-dose with terminal cardioplegia. The extent of functional recovery (e.g., aortic and coronary flow), ischemic stress (e.g., myocardial ATP, lactate) and reperfusion injury (lactate dehydrogenase (LDH) release) were assessed. Recovery following 60 min global ischemia was improved (p < 0.05) by single-dose and multi-dose cardioplegic delivery (from 5% to 60% and 80%, respectively). Improved recovery was augmented by 2 min terminal cardioplegia (to 90% and 97% for single-dose and multi-dose, respectively). Extending ischemia to 90 min with single-dose resulted in 0% recovery that was not improved by 2 min terminal cardioplegia. However, 5 min (not 10 min) terminal cardioplegia significantly improved recovery (32%). Multi-dose followed by 5 min terminal cardioplegia resulted in full recovery. Cardioprotective interventions were associated with a reduction in LDH release and attenuated changes in myocardial metabolites. During normothermic cardioplegic arrest of neonatal heart: (i) multi-dose is superior to single-dose; (ii) terminal cardioplegia confers additional protection to single-dose and multi-dose; and (iii) protection is likely to be due to metabolic preservation.

Proper timing of blood cardioplegia in infant lambs: superiority of a multiple-dose regimen

BACKGROUND

In the pediatric and infant age groups, it is unclear whether repeated infusions of blood cardioplegia solution during ischemic arrest are beneficial or detrimental when compared with a single-dose regimen.

METHODS

Twenty lambs (aged 6 to 7 weeks) were placed on cardiopulmonary bypass. A miniature glass-tip electrode measured myocardial pH and hydrogen ion concentration, [H+], in the anterior wall. The aorta was clamped for 2 hours. Group S (n = 10) received a single dose of blood cardioplegia solution. Group M (n = 10) received multiple doses of blood cardioplegia solution at 20-minute intervals.

RESULTS

The amount of [H+] generated during the cross-clamp period was greater in group S than in group M (39.2 +/- 10.1 nmol/L versus 0.4 +/- 1.4 nmol/L, p < 0.008). The percent increase in the time constant, tau, an index of diastolic relaxation, was more prolonged after cardiopulmonary bypass in group S when compared with group M (51.4% +/- 2.8% versus 6.4% +/- 3.0%, p < 0.0001). Similarly, the percent decrease in end systolic elastance, a measure of systolic contractility, was greater in group S after cardiopulmonary bypass when compared with group M (29.5% +/- 1.4% versus 7.3% +/- 1.3%, p < 0.0001).

CONCLUSIONS

In this infant lamb model, multiple doses of blood cardioplegia solution provided superior metabolic preservation and hemodynamic support after 2 hours of aortic clamping when compared with a single-dose regimen.

Improved myocardial preservation with short hyperthermia prior to cold cardioplegic ischemia in immature rabbit hearts

OBJECTIVE

Recent observations have been shown that the induction and accumulation of heat shock proteins (HSPs) by short exposure to nonlethal whole-body hyperthermia with normothermic recovery are closely associated with transient resistance to subsequent ischemia-reperfusion challanges. Here, this study was performed to investigate whether a shortly heat shock pretreatment affects the left ventricular (LV) function after cold cardioplegic ischemia in reperfused neonatal rabbit hearts.

METHODS

Hearts from neonatal New Zealand White rabbits were isolated perfused (working heart preparation) and exposed to 2 h of cold cardioplegic ischemia followed by reperfusion for 60 min. To induce the heat shock response neonatal rabbits (n=5, HT-group) were subjected to whole-body hyperthermia at 42.0-42.5 degrees C for 15 min, followed by a normothermic recovery period of 60 min, before harvesting and the onset of global hypothermic cardioplegic arrest. Another set of hearts (n=5, control group) without a heat treatment underwent a similar perfusion and ischemia protocol served as control. The postischemic recovery was assessed by measuring several parameters of LV function. LV biopsies from all control and heat treated animals were taken before ischemia and at the end of reperfusion to examine myocardial HSP levels by Western blot analysis.

RESULTS

At 60 min of reperfusion the HT-group showed significant better recovery of ventricular function such as LV developed pressure (DP) (74.6+/-10 vs. 52.1+/-8.5%, P<0.05), LV positive dP/dt (910+/-170 vs. 530+/-58 mmHg/s, P<0.01) and LV end-diastolic pressure (LVEDP) (8+/-2 vs. 18.4+/-5 mmHg, P<0.05) than control. Myocardial oxygen consumption (MVO(2)) was significantly higher in the HT-group compared with control (0.054+/-0.006 vs. 0.041+/-0.002 ml/g per min, P<0.05). Significant postreperfusion lower level in lactate production was observed in the HT-group (0.83+/-0.11 vs. 1.67+/-0.8 mmol/l, P<0.05). Also, the recovery of hemodynamic parameters such as aortic flow, coronary flow and cardiac output was significantly superior (P<0.05) in the HT-group. Furthermore, high expression of HSP72(+)/73(+) were detected in the myocardial tissue samples of heat-treated rabbits by immunoblotting, appearing even at 60 min of normothermic recovery after heat stress.

CONCLUSIONS

These data in the immature rabbit heart indicate that previous shortly heat treatment with high level expression of heat shock proteins (HSP72(+)/73(+)) before hypothermic cardioplegic ischemia provides transient tolerance against myocardial injury and could be an improvement for the postischemic functional recovery of neonatal hearts.

Temperature threshold and modulation of energy metabolism in the cardioplegic arrested rabbit heart

Hypothermia protects ischemic tissues by reducing ATP utilization and accumulation of harmful metabolites. However, it also reduces ATP production, which might cause deterioration in the energy supply/demand ratio. Modulation of energy supply/demand according to temperature has not been previously studied in detail. In this study, isolated, perfused rabbit hearts (n = 60) were used to determine the effects of various temperatures on myocardial energy metabolism and function during cardioplegic arrest. Ischemia was induced by crystalloid cardioplegic solution at 4, 18, 30, and 34 degrees C for 120 min, respectively. At each temperature, the hearts were divided into a glucose-treated group which contained 22 mM glucose in cardioplegic solution as the only substrate and a control group which contained 22 mM mannitol to keep same osmolarity. Following 15 min reperfusion, recovery of left ventricular developed pressure (DP), +/- dP/dtmax, and the product of heart rate and DP were significantly higher in 30, 18, and 4 degrees C groups than those in 34 degrees C control group. The functional recovery was also significantly higher in the 34 degrees C glucose-treated group than that in the 34 degrees C control group, but there was no difference between those groups at 30 degrees C and the temperature below 30 degrees C. Myocardial ATP concentration was significantly lower in 34 degrees C control group than those in other groups. There is a close relationship between myocardial ATP concentration and functional recovery (R2 = 0.90). The accumulations of lactate and CO2 were significantly higher at 34 degrees C in glucose-treated group than those in the control group. However, there was no significant difference between these two groups at 30 degrees C and the temperature below 30 degrees C. These results indicate that under these study conditions: (1) a marked decrease in energy supply/demand occurs above 30 degrees C, implying that a temperature threshold exists; and (2) this can be ameliorated by provision of glucose as substrate in cardioplegia solution.

The contribution of Na+/H+ exchange to ischemia-reperfusion injury after hypothermic cardioplegic arrest

BACKGROUND

Na+/H+ exchange has been reported to be one of the key mechanisms in myocardial ischemia-reperfusion injury. However, the effect of temperature on Na+/H+ exchange is not fully understood.

METHODS

Sodium-propionate-induced cell swelling, an indicator of the function of the Na+/H+ exchanger, was measured in rat thymic lymphocytes. A Langendorff perfused rat heart model was also employed to investigate the effect of the pharmacologic inhibition of Na+/H+ exchange on the recovery of cardiac function after hypothermic ischemia. This was done using FR168888, an inhibitor of Na+/H+ exchange.

RESULTS

In the in vitro study, rat lymphocytes were observed to swell at 17 degrees, 22 degrees, and 27 degrees C, indicating that the Na+/H+ exchanger remains functional even under hypothermic conditions. FR168888 was found to significantly inhibit Na+/H+ exchange-induced cell swelling, even at 17 degrees C. In the in vivo study, pretreatment with FR168888 was found to prevent the deterioration of ventricular function, even after 5 hours of hypothermic cardioplegic arrest. This was associated with a decrease in the reperfusion-induced elevation in resting tension.

CONCLUSIONS

These results suggest that Na+/H+ exchange in the heart still occurs, even under hypothermic conditions, and contributes to reperfusion injury, even after hypothermic cardioplegic arrest.

Neonatal myocardial oxygen consumption during ventricular fibrillation, hypothermia, and potassium arrest

BACKGROUND

Many investigators have examined oxygen consumption in adult heats under conditions that simulate those encountered during cardiac operations and those that approximate basal metabolism. Few studies, however, have addressed this issue in neonatal myocardium.

METHODS

Hearts from 3- to 9-day-old piglets were studied in a blood-perfused isolated heart preparation in working, empty beating, fibrillating, potassium chloride-arrested (at 37 degree C and 15 degree C), and hypothermic (15 degree C) states.

RESULTS

Oxygen consumption (expressed in milliliters of O2 per 100 g of ventricular tissue per minute; mean +/- standard deviation) was 6.69 +/- 1.91 for working hearts and fell to 3.19 +/- 1.08 for empty-beating hearts, 3.72 +/- 0.84 for fibrillating hearts, 1.30 +/- 0.34 for potassium-arrested hearts at 37 degree C, 0.37 +/- 0.18 for hypothermic (15 degree C) hearts, and 0.32 +/- 0.10 for potassium-arrested hearts at 15 degree C. All values were significantly different except the two obtained at 15 degree C.

CONCLUSIONS

Vented fibrillating hearts used more oxygen than empty beating hearts. The addition of an arresting concentration of KCl did not lower oxygen consumption below that observed with hypothermia alone at 15 degree C. If potassium-based cardioplegia is incrementally beneficial in neonatal myocardial protection over that afforded by hypothermia alone, its effects cannot be explained by reduction in oxygen demand.

Long-term hypothermic preservation of cardiac myocytes isolated from the neonatal rat ventricle: a comparison of various crystalloid solutions

In this study, the functional and biochemical effects of crystalloid solutions on immature cardiac myocytes incubated under hypothermic conditions were evaluated. Cardiac myocytes were isolated from neonatal rat ventricles and cultured for 4 days, following which 12.5 x 10(5) myocytes per flask were incubated at 4 degrees C for 3, 6, 12, and 18 h in five types of crystalloid solutions: lactated Ringer's (LR), St. Thomas' Hospital (ST), University of Wisconsin (UW), 5% glucose-based potassium (GK), and normal saline (NS). The levels of creatine phosphokinase (CPK) and lactate dehydrogenase (LDH) in the solutions were measured after each hypothermic incubation, following which the myocytes were cultured for an additional 24 h at 37 degrees C to evaluate the recovery of the myocyte beating rate. In the LR, UW, and NS groups, the recovery ratios of the myocyte beating rate were over 95% of the control (the beating rate prior to hypothermic incubation) at 3 h, but decreased to 20.3, 15.1, and 0%, respectively, at 18 h. The ST and GK groups had significantly lower recovery ratios than the other three groups (72.9% and 63.4%, respectively) at 3 h. The release of CPK and LDH in the LR, UW, and NS groups was significantly suppressed compared to the ST and GK groups, with the greatest suppression observed in the LR group. Moreover, the ST and GK groups had the highest CPK and LDH levels, respectively. Thus, LR solution had the least cytotoxic effects, indicating that it could be the most suitable basic solution of the various cardioplegic or preservation solutions during the neonatal period.

Calcium and cardioplegic protection of the ischemic immature heart: impact of hypoxemia from birth

Because many infants who require cardiac operation have cyanotic heart disease, we determined whether the existing calcium content of St. Thomas' II solution (1.2 mmol/L) is optimal to protect the immature rabbit heart hypoxemic from birth during subsequent ischemia. Modified hypothermic St. Thomas' II solutions (calcium content, 0 to 2.4 mmol/L) were compared with hypothermic Krebs bicarbonate buffer in protecting chronically hypoxemic (PaO2 = 34 +/- 11 mmHg, SaO2 = 63% +/- 3%) versus normoxemic (PaO2 = 76 +/- 11 mmHg, SaO2 = 92% +/- 3%) immature hearts (7 to 12 days old) during ischemia. Hearts (n = 6 per group) underwent aerobic 'working' perfusion with Krebs bicarbonate buffer and cardiac function was measured. The hearts were then arrested with a 3 minute infusion of either cold (14 degrees C) Krebs buffer (1.8 mmol calcium/L) as hypothermia alone or modified St. Thomas' II solution before 6 hours of hypothermic (14 degrees C) global ischemia. Hearts were reperfused and postischemic enzyme leakage and recovery of function were measured. A bell-shaped dose-response profile was observed for recovery of postischemic aortic flow but not for postischemic creatine kinase leakage, with improved protection occurring at lower calcium concentrations. Optimal myocardial protection occurred at a calcium content of 0.4 mmol/L, which was significantly better than with hypothermia alone or standard St. Thomas' II solution. We conclude that the existing calcium concentration of St. Thomas' II solution is responsible, in part, for its inadequate protection of immature myocardium hypoxemic from birth during ischemia.