Evaluation of myocardial preservation using 31P NMR

The purpose of this study was (1) to monitor myocardial high-energy phosphate content and recovery of left ventricular (LV) contractile function following normothermic graded cardiac ischemia and single-dose hypothermic potassium cardioplegia, and (2) to assess the temporal limits of LV functional recovery during single-dose cardioplegia maintained at 17 degrees C. Rabbit hearts (30) were perfused, equipped with an LV balloon, paced at 240 beats/min, and placed in a nuclear magnetic resonance (NMR) magnet. Hearts underwent either graded, global normothermic ischemia or potassium cardioplegia arrest maintained at 17 degrees C for 1 hr. Myocardial high-energy phosphate level, LV contractility, and temperature were monitored continuously. Phosphocreatine (PCr) fell to 10 +/- 2, 2 +/- 1, and 0% of control and ATP to 70 +/- 3, 19 +/- 7, and 0% of control at 10, 40, and 60 min of 37 degrees C ischemia. After 1 hr of reperfusion, regression analysis of final developed pressure (DP) on end ischemic ATP (EIATP) content revealed: DP = 1.02 EIATP + 18 (r = 0.95). Following single-dose cardioplegia, maintained at 17 degrees C, PCr fell to 16 +/- 3% of control at 60 min while ATP fell only to 92 +/- 5% control. With reperfusion, recovery of DP was 100%. It was concluded that (1) PCr serves as an energy buffer for ATP, (2) EIATP predicts recovery of LV function, (3) single-dose cardioplegia maintained at 17 degrees C provides complete myocardial preservation for up to 60 min.     

Recovery of left ventricular function after graded cardiac ischemia as predicted by myocardial P-31 nuclear magnetic resonance

The purpose of this study was to determine noninvasively some critical level of high-energy phosphate stores that relates to the recovery of ventricular contractile function after graded cardiac ischemia. Rabbit hearts (n = 30) were equipped with an intraventricular balloon to monitor developed pressure and +/- dp/dt and placed in a nuclear magnetic resonance magnet (Bruker, 4.7 Tesla). Each heart underwent 10, 20, 40, or 60 minutes of global ischemia followed by 1 hour of reperfusion. The pH as determined by nuclear magnetic resonance dropped from 7.14 +/- 0.04 to 7.07 +/- 0.07 (p less than 0.02) at 1 minute and to 6.19 +/- 0.08 at 30 minutes of ischemia; pH ceased to fall thereafter. Phosphocreatine was depleted to 10% +/- 7% of its preischemic control in 10 minutes. Adenosine triphosphate (ATP) concentrations were 71% +/- 14% and 1% +/- 2% at 10 and 60 minutes. Regression analysis of recovered developed pressure on end-ischemic ATP (EIATP) revealed: developed pressure = 0.93 (EIATP) + 23 (r2 = 0.99). We conclude that: anaerobic metabolism as evidenced by a fall in pH appears to be active for 30 minutes after normothermic ischemia and then ceases; phosphocreatine buffers the fall in ATP during early ischemia; there is a tight correlation between EIATP and recovery of left ventricular contractile function with a threshold content of approximately 80% below which recovery of function will not be complete.     

Backtable heat-enhanced preconditioning: a simple and effective means of improving function of heart transplants

BACKGROUND: Cardiac harvest teams are usually committed to immediately transfer the explanted donor heart into its cold storage solution. We tested the opposite hypothesis that a brief prestorage episode of heat-enhanced ischemic preconditioning could be protective. METHODS: Fifty-three isolated isovolumic rat hearts underwent 4 hours of cold (4 degrees C) storage in the Celsior preservation solution and 2 hours of reperfusion. Control hearts were immediately immersed after arrest. In the 3 treated groups, 2 customized thermal probes were first applied onto the left ventricular free wall of the explanted heart at 22 degrees C, 37 degrees C or 42.5 degrees C for 15 minutes before immersion. Each of the selected temperatures were monitored at the probe-tissue interface by a thermocouple. RESULTS: Whereas base line end-diastolic pressure was set at = 8 mm Hg in all groups, it increased during reperfusion (mean +/- SEM) to 28+/-3, 27+/-3, 17+/-1, and 18+/-2 mm Hg in control, 22 degrees C, 37 degrees C and 42.5 degrees C-heated hearts, respectively (37 degrees C and 42.5 degrees C: p < 0.05 versus controls and 22 degrees C). Slopes of pressure-volume curves featured similar patterns. Likewise, reperfusion dP/dT (mm Hg/s(-1)) was significantly lower in control and 22 degrees C hearts (1,119+/-114 and 1,076+/-125, respectively) than in those undergoing prestorage heating to 37 degrees C and 42.5 degrees C (1,545+/-109 and 1,719+/-111, p < 0.05 and p < 0.01 versus controls and 22 degrees C, respectively). Western blot analysis of LV samples did not demonstrate any upregulation of HSP 72 in either group. Conversely, the involvement of preconditioning was evidenced by the loss of protection in the 42.5 degrees C-heated hearts when, in 2 additional groups, the storage solution was supplemented with either the protein kinase C and tyrosine kinase inhibitors chelerythrine (5 micromol/L) and genistein (50 micromol/L) or the mitochondrial K(ATP) channel inhibitor 5-hydroxydecanoate (200 micromol/L). CONCLUSIONS: A brief period of postexplant ischemia with enhancement by topical heating ("backtable preconditioning") could be a simple and effective means of improving the functional recovery of heart transplants.     

Effects of heat stress on metabolism of high-energy phosphates. Comparison of normothermic and hypothermic ischemia.

BACKGROUND: Alterations in metabolic pathways may contribute to the cardioprotective effects of heat stress (HS). We investigated the effects of HS on ATP and phosphocreatine (PCr) levels in the ischemic rat myocardium, after both normothermic and hypothermic ischemia. METHODS: Two protocols were used: (1) normothermic ischemia (20 min at 37 degrees C) with no myocardial protection (n=6 HS; n=6 control); (2) hypothermic ischemia (4 hrs at 4 degrees C) after cardioplegic arrest (n=6 HS; n=6 control). ATP and PCr levels in the heart were measured using 31P nuclear magnetic resonance spectroscopy. RESULTS: At the end of normothermic ischemia, ATP levels were better maintained in HS hearts (C vs HS: 4.51+/-0.66 vs 7.81+/-1.06 micromol/g dry wt+/-SEM, p=0.04). A trend for higher ATP content in HS hearts was observed after 40 min of reperfusion (C vs HS: 11.7+/-1.5 vs 16.9+/-2.0 micromol/g dry wt+/-SEM, p=0.09). PCr content was also higher at the end of 40 minutes of reperfusion in HS hearts (C vs HS: 46.4+/-2.9 vs 56.9+/-3.0 micromol/g dry wt+/-SEM, p=0.03). After prolonged hypothermic ischemia under cardioplegic arrest, heat stress again led to better preservation of ATP levels at the end of ischemia (C vs HS: 5.71+/-0.88 vs 9.23+/-1.38 micromol/g dry wt+/-SEM, p=0.05) and after 40 minutes of reperfusion (C vs HS: 16.8+/-1.4 vs 24.6+/-2.8 micromol/g dry wt+/-SEM, p=0.03). PCr levels were also better maintained at the end of ischemia (C vs HS: 4.87+/-0.77 vs 12.4+/-3.0 micromol/g dry wt+/-SEM, p=0.03) and after 40 minutes of reperfusion in HS hearts (C vs HS: 55.1+/-7.0.vs 79.8+/-7.3 micromol/g dry wt+/-SEM, p=0.03). CONCLUSIONS: Heat stress induces changes in the energy profile of the heart which results in better preservation of ATP and phosphocreatine levels. These changes could be observed after brief normothermic ischemia and also after prolonged hypothermic ischemia under cardioplegic arrest, mimicking conditions of preservation for cardiac transplantation.     

ATP precursor depletion and postischemic myocardial recovery

Although cardioplegia reduces myocardial metabolism during ischemia, adenosine triphosphate (ATP) depletion occurs, which may contribute to poor functional recovery after reperfusion. Augmenting myocardial adenosine during ischemia is successful in improving ATP repletion and myocardial recovery following ischemia. If adenosine is an important determinant of ischemic tolerance, then depletion or elimination of myocardial adenosine should lead to poor functional and metabolic recovery after ischemia. To test this hypothesis, isolated, perfused rabbit hearts were subjected to 120 min of 34 degrees C ischemia. Hearts received St. Thomas cardioplegia alone or cardioplegia containing 200 microM adenosine, or cardioplegia containing 15, 5, 2.5, or 0.025 micrograms/ml adenosine deaminase (ADA), which catalyzes the breakdown of adenosine to inosine, making adenosine unavailable as an ATP precursor. Functional recovery was determined and myocardial nucleotide levels were measured before, during, and after ischemia. Following ischemia and reperfusion, control hearts recovered to 51 +/- 3% of preischemic developed pressure (DP). There was significantly better recovery in adenosine-augmented hearts (68 +/- 7%), while ADA hearts had significantly worse recovery. Hearts treated with 0.025 microgram/ml ADA recovered to only 29 +/- 5% of DP and higher dose ADA hearts failed to demonstrate any recovery of systolic function. Furthermore, adenosine enhanced metabolic recovery, whereas ADA resulted in greatly depleted ATP and precursor reserves. Postischemic developed pressure closely paralleled the availability of myocardial adenosine, consistent with the hypothesis that myocardial adenosine levels at end ischemia and early reperfusion are important determinants of functional recovery after global ischemia.     

Transportation or noise is associated with tolerance to myocardial ischemia and reperfusion injury

Myocardial stress can result in myocellular phenotypic changes including enhanced activity of antioxidant enzyme systems. Accordingly, endogenous tissue antioxidant enzyme activity has been associated with resistance to cardiac ischemia and reperfusion injury. The present study was designed to determine if environmental perturbations could alter myocardial antioxidant enzyme (catalase) activity and function after ischemia. Isolated perfused rat hearts (Langendorff apparatus, 37 degrees C) were subjected to 20 min global ischemia (37 degrees C) and 40 min reperfusion. Rats studied immediately following shipment had increased myocardial catalase activity (1330 +/- 3.5 U/g, P < 0.05 vs quarantined control) and increased resistance to ischemia and reperfusion injury (end reperfusion developed pressure, DP 55 +/- 4.0 mm Hg, P < 0.05 vs quarantined control). However, control rats that were quarantined for 4 weeks exhibited a progressive decrease in catalase activity (760 +/- 10 U/g) for 3 weeks of quarantine. There was a concurrent decrease in resistance to myocardial ischemia and reperfusion injury (DP 40 +/- 3.6 mm Hg). Similarly, quarantined rats subjected to construction-related noise levels in excess of 90 dB (A scale) had increased myocardial catalase activity (1140 +/- 3.3 U/g, P < 0.05) and functional tolerance to ischemia and reperfusion (DP 66 +/- 3.3 mm Hg, P < 0.05). Finally, rats experiencing 90-dB noise levels for 2 days exhibited increased myocardial catalase activity (1125 +/- 30 U/g, P < 0.05) and myocardial ischemia and reperfusion injury tolerance (DP 62 +/- 1.7 mm Hg, P < 0.05). We conclude that variations in environmental conditions can relate to changes in antioxidant defense mechanisms and tolerance to myocardial ischemia and reperfusion injury in the rat.     

Recovery of the neonatal heart after normothermic ischemia. Effect of oxygen and catalase

The objective of this study was to determine the effect of oxygen and the oxygen radical-scavenging enzyme catalase on the neonatal rabbit heart exposed to global ischemia. The experiments were performed with an isolated neonatal (7 to 10 days of age) working heart model in which normothermic (37 degrees C) ischemia was produced for 60 minutes. Left ventricular developed pressure, ratio of change of ventricular pressure to change in time, and aortic flow were measured before ischemia and 30 minutes after reperfusing the hearts with physiologic saline solution. In the control group (ischemia only), developed pressure and ratio of change of ventricular pressure to change in time recovered to 27% +/- 3% (mean +/- standard error of the mean) and 24% +/- 7% of baseline; the hearts were incapable of ejecting (aortic flow = 0). Treatment of hearts before and after ischemia with catalase (150 units/ml of perfusate) was studied in a second group (control plus catalase), but functional recovery (developed pressure = 32% +/- 1%; ratio of change of ventricular pressure to change in time = 24% +/- 2%, and aortic flow = 0) was not significantly different from the control group. The effect of washout midway through the ischemic period with a low oxygen (oxygen concentration less than 35 mm Hg) solution was measured in a third group (hypoxic physiologic saline solution). Functional recovery (developed pressure = 13% +/- 3%; ratio of change of ventricular su pressure to change in time = 13% + 2%; aortic flow = 0) was not significantly different from the control and control plus catalase groups. In marked contrast were the effects of washout with an oxygenated (oxygen concentration greater than 500 mm Hg) solution (oxygenated physiologic saline solution) in which functional recovery (developed pressure = 78% +/- 3%; ratio of change of ventricular pressure to change in time = 80% +/- 3%; aortic flow = 39% +/- 9%) was significantly better than in the control, control plus catalase, and hypoxic physiologic saline solution groups. Use of modified St. Thomas' Hospital cardioplegic solution (cardioplegic solution group) during the ischemic period also resulted in substantial functional recovery (developed pressure = 80% +/- 3%; ratio of change of ventricular pressure to change in time = 78% +/- 5%; aortic flow = 64% +/- 7%) that did not differ significantly from that in the oxygenated physiologic saline solution group.(ABSTRACT TRUNCATED AT 400 WORDS)     

Bradykinin pretreatment improves ischemia tolerance of the rabbit heart by tyrosine kinase mediated pathways

BACKGROUND: Depressed myocardial performance is an important clinical problem after open-heart surgery. We hypothesized that: (1) pretreating the heart with bradykinin improves postischemic performance, and (2) bradykinin activates protein tyrosine kinase (TK). METHODS: Twenty-seven adult rabbit hearts underwent retrograde perfusion with Krebs-Henseleit buffer (KHB) followed by 50 min of 37 degrees C cardioplegic ischemia with St. Thomas' cardioplegia solution (StTCP). Ten control hearts received no pretreatment. Ten bradykinin-pretreated hearts received a 10-minute infusion of 0.1 microM bradykinin-enriched KHB and cardioplegic arrest with 0.1 microM bradykinin-enriched StTCP. Seven others received 40 microM Genistein (Research Biochemicals, Natick, MA), a selective inhibitor of TK, added to both the 0.1-microM bradykinin-enriched KHB and 0.1-microM bradykinin-enriched StTCP solutions. RESULTS: Bradykinin pretreatment significantly improved postischemic myocardial performance and coronary flow (CF) compared with control (left ventricular developed pressure: 53 +/- 5 vs 27 +/- 4 mm Hg; +dP/dt(max): 1,025 +/- 93 vs 507 +/- 85 mm Hg/s; CF: 31 +/- 3 vs 22 +/- 2 mL/min; p < 0.05). Inhibition of TK with Genistein prevented this improvement in myocardial function, resulting in recovery equivalent to untreated controls. CONCLUSIONS: Bradykinin pretreatment may be an important new strategy for improving myocardial protection during heart surgery. The molecular mechanism of action may be similar to those activated by ischemic preconditioning.     

Modulation of mitochondrial adenosine triphosphate-sensitive potassium channels and sodium-hydrogen exchange provide additive protection from severe ischemia-reperfusion injury

BACKGROUND: Preconditioning and inhibition of sodium-proton exchange attenuate myocardial ischemia-reperfusion injury by means of independent mechanisms that might act additively when used together. The hypothesis of this study is that treatment with a sodium-proton exchange inhibitor and a mitochondrial adenosine triphosphate-sensitive potassium channel opener produces superior functional recovery and a greater decrease in left ventricular infarct size compared with treatment with either drug alone in a model of severe global ischemia. METHODS: Isolated crystalloid-perfused rat hearts (n = 8 hearts per group) were administered vehicle (control, 0.04% dimethyl sulfoxide), diazoxide (100 micromol/L in 0.04% dimethyl sulfoxide), cariporide (10 micromol /L in 0.04% dimethyl sulfoxide), or diazoxide and cariporide before 40 minutes of ischemia at 35.5 degrees C to 36.5 degrees C and 30 minutes of reperfusion. RESULTS: The combination group had superior postischemic systolic function compared with that seen in the cariporide, diazoxide, and control groups (recovery of developed pressure: 91% +/- 7% vs 26% +/- 5%, 35% +/- 6%, and 16% +/- 3%, respectively; P <.05). Postischemic diastolic function in the combination group was superior compared with that seen in the other groups (change(pre-post) diastolic pressure of 67 +/- 4 mm Hg with control, 49 +/- 11 mm Hg with diazoxide, 59 +/- 10 mm Hg with cariporide, and 3 +/- 3 mm Hg with diazoxide and cariporide combination; P <.05). The left ventricular infarct area was less in the combination group compared with that in the cariporide, diazoxide, and control groups (6% +/- 2% vs 35% +/- 7%, 25% +/- 3%, and 37% +/- 9%, respectively; P <.05). CONCLUSIONS: Combining a selective mitochondrial adenosine triphosphate-sensitive potassium channel opener with a selective reversible inhibitor of sarcolemmal sodium-proton exchange improves recovery of contractile function from severe global ischemia in the isolated buffer-perfused rat heart. The putative mechanism for this benefit is superior protection of mitochondrial function.     

Prolonged preservation of the blood-perfused canine heart with glycolysis-promoting solution.

BACKGROUND: Prolonged ischemia and inadequate myocardial preservation remain significant perioperative risk factors in cardiac transplantation. Long-term preservation techniques that have been effective in small rodent hearts have not been as effective in larger animal models or in clinical studies. We developed a cardioplegia solution formulated to promote high-energy phosphate production from glycolysis and determined its efficacy in a blood perfused canine heart model subjected to 24 hours of ischemia. METHODS: Hearts harvested from adult dogs (n = 6 per group) were flushed with a histidine-buffered cardioplegia solution containing glucose or University of Wisconsin solution. The hearts were maintained at 4 degrees C for 24 hours then reperfused with autologous blood. After reperfusion, left ventricular pressures were measured with an intracavitary balloon at varying balloon volumes and compared with control nonischemic hearts. Predicted stroke volume and ejection fraction were calculated at an end-systolic pressure of 70 mm Hg and end-diastolic pressure of 15 mm Hg. RESULTS: Developed pressure was better preserved in the hearts that received histidine-buffered solution (93+/-9 versus 38+/-7 mm Hg, p<0.05), along with a higher end-diastolic volume at 15 mm Hg (31+/-3 versus 22+/-2 mL histidine-buffered versus University of Wisconsin solutions, respectively, p<0.05). Stroke volume and ejection fraction were also higher in the histidine group (17+/-2.5 versus 2.3+/-1.2 mL and 50%+/-3.5% versus 9% +/-4.5%, respectively) in the presence of dobutamine. CONCLUSIONS: The highly buffered glycolysis-promoting cardioplegia solution provided effective preservation of the blood perfused canine heart with superior recovery of pump performance after 24 hours of hypothermic ischemia compared with University of Wisconsin solution in this model.     

The effect of hypothermic ischemia on recovery of left ventricular function and preload reserve in the neonatal heart

Neonatal and adult myocardium respond differently to ischemia. In addition, the neonatal heart possesses a limited preload reserve. The effect of uninterrupted hypothermic ischemia on recovery of left ventricular function and preload reserve was studied in two groups of isolated rabbit hearts: group 1 (neonates, n = 8), 7 to 10 days old; group 2 (adults, n = 15), 6 to 12 months old. Peak left ventricular systolic pressure, the first derivative of left ventricular systolic pressure, and heart rate were measured at left ventricular pressures of 0, 5, 10, and 15 mm Hg before and after 120 minutes of global ischemia at 27 degrees C. Before ischemia, left ventricular systolic pressure increased significantly at each increment of left ventricular end-diastolic pressure for both groups of hearts. After hypothermic ischemia, recovery of left ventricular systolic pressure was significantly reduced at each level of left ventricular end-diastolic pressure among neonatal hearts (range 75% to 79% of control values). The postischemic recovery of left ventricular systolic pressure in the adult hearts was markedly reduced from baseline values (range 43% to 53% of control values) and was significantly worse than that of neonatal hearts at each level of left ventricular end-diastolic pressure (p less than 0.001). Both groups were able to respond to increasing preload after ischemia. The slope of the curve describing the relationship between left ventricular end-diastolic pressure and percent recovery of left ventricular systolic pressure was not different from zero for neonatal hearts but was significantly greater than zero among the adults (0.22 +/- 0.21 versus 0.73 +/- 0.07, p = 0.0056). After ischemia, the first derivative of left ventricular systolic pressure fell significantly from control values among neonatal hearts (71% of control values). The reduction was considerably greater, however, among the adult hearts (54% of control values). These data indicate that the neonatal heart recovers systolic function better than the adult heart after global ischemia with moderate hypothermia.     

Safety of prolonged aortic clamping with blood cardioplegia. III. Aspartate enrichment of glutamate-blood cardioplegia in energy-depleted hearts after ischemic and reperfusion injury

This study tests the hypothesis that aspartate enrichment of glutamate-blood cardioplegia improves metabolic and functional recovery after ischemic and reperfusion damage. Ischemic and reperfusion damage were produced in 15 dogs by 45 minutes of aortic clamping at 37 degrees C and 5 minutes of blood reperfusion, before 2 more hours of aortic clamping (simulated operation). Six received multidose blood cardioplegia at 4 degrees C. In nine others, the cardioplegic solution was infused at 37 degrees C for the first 5 minutes, followed by multidose infusions at 4 degrees C. Four received 26 mmol glutamate-enriched cardioplegic solution. In five, the glutamate (13 mmol) cardioplegic solution was enriched with aspartate (13 mmol). Oxygen uptake and ventricular function (stroke work index, left atrial pressure) were measured. These data suggest aspartate enrichment produced the highest oxygen uptake (32 +/- 4 versus 17 +/- 2 ml/100 gm for glutamate and 7 +/- 1 ml/100 gm for 4 degrees C blood cardioplegia). Complete functional recovery occurred in aspartate/glutamate-treated hearts (stroke work index 90% +/- 4%, left atrial pressure 12 +/- 2 mm Hg), whereas recovery was incomplete with both glutamate alone (stroke work index 66% +/- 14%, left atrial pressure 20 +/- 3 mm Hg) and 4 degrees C blood cardioplegia at low cardiac outputs. Eight of 10 hearts not receiving aspartate failed at high cardiac outputs. Aspartate enrichment of glutamate-blood cardioplegia improves recovery after severe ischemic/reperfusion damage by improving oxidative metabolism during cardioplegic infusion and during postischemic work.     

Protection of the immature heart. Temperature-dependent beneficial or detrimental effects of multidose crystalloid cardioplegia in the neonatal rabbit heart

There are conflicting reports of the detrimental or beneficial effects of hypothermic cardioplegia in the immature heart. We therefore investigated the temperature-dependence of myocardial protection and the ability of single-dose and multidose infusions of cardioplegic solution to protect the immature heart during hypothermic ischemia. Isolated, working hearts (n = 6 per group) from neonatal rabbits (aged 7 to 10 days) were perfused aerobically (37.0 degrees C) for 20 minutes before infusion (2 minutes) with either perfusion fluid (noncardioplegia control) or St. Thomas' Hospital cardioplegic solution and ischemic arrest (for 4, 6, and 18 hours) at various temperatures between 10.0 degrees and 30.0 degrees C. Hearts arrested with cardioplegic solution received either one preischemic infusion only (single-dose cardioplegia) or repeated infusions at intervals of 60 or 180 minutes (multidose cardioplegia). Ischemic arrest with single-dose cardioplegia for 4 hours at 10.0 degrees, 20.0 degrees, 22.5 degrees, 25.0 degrees, 27.5 degrees, and 30.0 degrees C resulted in 96.0% +/- 4.3%, 96.6 +/- 2.5%, 87.0% +/- 3.8%, 71.8% +/- 10.0% (p less than 0.05 versus 10.0 degrees C group), 35.1% +/- 10.3% (p less than 0.01 versus 10.0 degrees C group), and 3.0% +/- 1.9% (p less than 0.04 versus 10.0 degrees C group) recovery of preischemic cardiac output, respectively. With 6 hours of ischemia at 20.0 degrees C, single-dose cardioplegia significantly (p less than 0.01) increased the recovery of cardiac output from 20.9% +/- 13.1% (control) to 76.4% +/- 4.4%, whereas multidose cardioplegia (infusion every 60 minutes) further increased recovery to 97.8% +/- 3.8% (p less than 0.01 versus control and single-dose cardioplegia). In contrast, after 6 hours of ischemia at 10.0 degrees C, cardiac output recovered to 93.4% +/- 1.2% (control) and 92.3% +/- 3.1% (single-dose cardioplegia), whereas multidose cardioplegia reduced recovery to 76.9% +/- 2.2% (p less than 0.01 versus both groups). This effect was confirmed after 18 hours of ischemia at 10.0 degrees C; single-dose cardioplegia significantly increased the recovery of cardiac output from 24.5% +/- 10.9% (control) to 62.9% +/- 13.3% (p less than 0.05), whereas multidose cardioplegia reduced recovery to 0.8% +/- 0.4% (p less than 0.01 versus single-dose cardioplegia) and elevated coronary vascular resistance from 8.90 +/- 0.56 mm Hg.min/ml (control) to 47.83 +/- 9.85 mm Hg.min/ml (p less than 0.01). This effect was not reduced by lowering the infusion frequency (from every 60 to every 180 minutes).(ABSTRACT TRUNCATED AT 400 WORDS)     

Opioids confer myocardial tolerance to ischemia: interaction of delta opioid agonists and antagonists

BACKGROUND: Mammalian hibernation biology is now known to be mediated by delta opioids. The altered myocellular physiology of hibernation closely parallels that of hypothermic ischemia used to protect the heart for cardiac surgery. METHODS AND RESULTS: The present study examined the interaction of delta opioid agonists and antagonists on myocardial tolerance to ischemia. By means of a nonhibernating isolated rabbit heart model, functional and metabolic myocardial parameters were assessed during nonischemic baseline and postischemic recovery periods. Control hearts with standard cardioplegic protection alone were compared with those with cardioplegia plus preperfusion with a delta opioid agonist, a delta opioid antagonist, or both. All hearts were then subjected to 2 hours of global ischemia. Compared with cardioplegia alone, postischemic left ventricular developed pressure, coronary flows, and myocardial oxygen consumption were all increased with administration of delta opioid agonists and decreased below baseline with delta opioid antagonists. Functional recovery of left ventricular developed pressure was improved with opioids (control hearts: 36 +/- 3 mm Hg vs hearts with cardioplegia plus delta opioid agonist: 65 +/- 5 mm Hg, P <.01) and inhibited with antagonists (control hearts: 36 +/- 3 mm Hg vs hearts with cardioplegia plus delta opioid antagonist: 17 +/- 5 mm Hg, P <.05), and true to form, the protective opioid effect was negated when combined with an antagonist (control hearts: 36 +/- 3 mm Hg vs hearts with cardioplegia plus delta opioid agonist and delta opioid antagonist: 42 +/- 4 mm Hg, P = not significant). CONCLUSIONS: This study demonstrates that cardiac tolerance to ischemia may be mediated by delta opioids.     

The effect of intramyocardial pH on functional recovery in neonatal hearts receiving St. Thomas' Hospital cardioplegic solution during global ischemia.

The experiments in the present study were designed to address two issues: Is it possible to manipulate intramyocardial pH in neonatal hearts with different buffers in cardioplegic solution and, if so, do differences in intramyocardial pH during ischemia influence functional recovery? Isolated working hearts from 7- to 10-day-old rabbits underwent 60 minutes of cardioplegic arrest at 37 degrees C with cardioplegic washouts at the onset of ischemia and at 30 minutes. Hearts were reperfused with oxygenated physiologic saline solution (pH = 7.4), returned to the working mode for 30 minutes, and hemodynamic measurements were obtained to compare with baseline values. Intramyocardial pH was held constant during the ischemic interval by infusing cardioplegic solution containing different buffers: histidine (pK 6.0 at 37 degrees C), bicarbonate (pK 6.4), or tromethamine (pK 8.1). The intramyocardial pH was measured continuously with a Khuri glass electrode system (Vascular Technology, Inc., North Chelmsford, Mass.). Cardioplegic solutions buffered to pH values of 6.0 (histidine), 7.4 (bicarbonate), and 8.0 (tromethamine) were associated with ischemic intramyocardial pH values of 6.3 +/- 0.03, 7.02 +/- 0.05, and 7.88 +/- 0.06, respectively. Functional recovery was best in the acidic (histidine) and worst in the basic (tromethamine) groups. Recoveries of developed pressure, the rate of rise of pressure over time, and aortic flow were significantly better for each parameter in the bicarbonate-treated compared with the tromethamine-treated hearts (p less than 0.005). Recovery in the histidine group, however, was superior to that in both the bicarbonate-treated and the tromethamine-treated hearts (p less than 0.005). Regression analysis demonstrated that a significant inverse relationship existed between functional recovery and intramyocardial pH, supporting the conclusions that intramyocardial pH is an important determinant of functional recovery in the neonatal heart and that acidic conditions during normothermic ischemia optimize preservation of myocardial function.     

Myocardial protection in cyanotic neonatal lambs

To investigate the susceptibility of cyanotic neonatal myocardium to ischemia and the effectiveness of cardioplegia for protection, we induced cyanosis in 2- to 5-day-old lambs (n = 16) by connecting the left atrial appendage to the main pulmonary artery with a 4 mm polytetrafluoroethylene graft, which produced an arterial oxygen tension of 34.1 +/- 1.2 torr. Seven to 10 days after creation of the model, isolated perfused hearts from cyanotic animals were subjected to 2 hours of ischemia with topical cooling or crystalloid cardioplegia (K = 30 mEq/L) for myocardial protection (both at 15 degrees C). Identical studies were performed on hearts from 16 normoxemic neonatal lambs 5 to 14 days old. The overall effect of cyanosis was to produce a significant impairment in recovery of maximum developed pressure (p less than 0.05) after ischemia. The overall effect of cardioplegia was to produce a significant improvement in recovery of maximum developed pressure, developed pressure at V10 (the balloon volume to produce an end-diastolic pressure of 10 mm Hg during the preischemic period), and peak rate of pressure rise at V10 (p less than 0.05). The protective effect of cardioplegia was more prominent in cyanotic hearts than in normoxemic hearts for recovery of maximum of peak rate of pressure rise and peak rate of pressure rise at V10 (p less than 0.05). End-diastolic pressure at V10 and the diastolic stiffness constant at 10 and 20 mm Hg were all significantly higher after ischemia in the cyanotic hearts than in the normoxemic hearts (p less than 0.05). We conclude that in neonatal hearts cyanosis may increase the vulnerability to ischemia and cardioplegia appears to enhance the recovery of systolic but not diastolic function in these hearts.     

Effect of PCO2-adjusted pH on the neonatal heart during hypothermic perfusion and ischemia

The effect of pH regulation on the function of the isolated neonatal heart during continuous hypothermic perfusion and arrest was tested in 3- to 6-day-old piglet hearts. Three groups of hearts were perfused from an adult support pig, with pH varied by a carbon dioxide/oxygen gas exchanger and temperature controlled by a heat exchanger. After control function studies were obtained at normothermia with a pH of 7.4, the hearts were cooled over 15 minutes to 10 degrees C. Hypothermic perfusion was maintained for 1 hour, followed by rewarming to 37 degrees C. In group 1 (n = 5), the alpha stat (neutral) model, the blood perfusate was maintained at a pH of 7.4 (calculated at 37 degrees C). In group 2, the alkaline model, the pH was maintained at 7.9, and in group 3, the pH-stat (acid) model, the pH was maintained at 7.0. In addition, the effect of 1 hour of hypothermic ischemia after hypothermic perfusion at a pH of 7.0 was evaluated in five hearts (group 4). After rewarming no significant difference was noted in functional recovery (group 1 = 93% +/- 5%, group 2 = 92% +/- 6%, group 3 = 96% +/- 5%, and group 4 = 95% +/- 2%), oxygen consumption, coronary resistance, lactate extraction, and myocardial extravascular water content. We conclude that neonatal heart function is resistant within the range of this study to changes in pH caused by changes in carbon dioxide tension during hypothermic perfusion and ischemia.     

Myocardial protection with oxygenated esmolol cardioplegia during prolonged normothermic ischemia in the rat

OBJECTIVE: We previously showed that arrest with multidose infusions of high-dose (1 mmol/L) esmolol (an ultra-short-acting beta-blocker) in oxygenated Krebs-Henseleit buffer (esmolol cardioplegia) provided complete myocardial protection after 40 minutes of normothermic (37 degrees C) global ischemia in isolated rat hearts. In this study we investigated the importance of oxygenation for protection with esmolol cardioplegia, compared it with that of St Thomas' Hospital cardioplegia, and determined the protective efficacy of multidose esmolol cardioplegia for extended ischemic durations. METHODS: Isolated rat hearts (n = 6/group) were perfused in the Langendorff mode at constant pressure (75 mm Hg) with oxygenated Krebs-Henseleit bicarbonate buffer at 37 degrees C. The first part of the first study had four groups: (i) multidose (every 15 minutes) oxygenated (95% oxygen/5% carbon dioxide) Krebs-Henseleit buffer during 60 minutes of global ischemia, (ii) multidose deoxygenated (95% nitrogen/5% carbon dioxide) Krebs-Henseleit buffer during 60 minutes of global ischemia, (iii) multidose oxygenated esmolol cardioplegia during 60 minutes of global ischemia, and (iv) multidose deoxygenated esmolol cardioplegia during 60 minutes of global ischemia. The second part of the first study had three groups: (v) multidose St Thomas' Hospital solution during 60 minutes of global ischemia, (vi) multidose oxygenated St Thomas' Hospital solution during 60 minutes of global ischemia, and (vii) multidose oxygenated esmolol cardioplegia during 60 minutes of global ischemia. In the second study, hearts were randomly assigned to 60, 75, 90, or 120 minutes of global ischemia and at each ischemic duration were subjected to multidose oxygenated constant flow or constant pressure infusion of (i) Krebs-Henseleit buffer (constant flow), (ii) Krebs-Henseleit buffer (constant pressure), (iii) esmolol cardioplegia (constant flow), or (iv) esmolol cardioplegia (constant pressure). All hearts were reperfused for 60 minutes, and recovery of function was measured. RESULTS: Multidose infusion of oxygenated esmolol cardioplegia completely protected the hearts (97% +/- 5%) after 60 minutes of 37 degrees C global ischemia. Deoxygenated esmolol cardioplegia was significantly less protective (45% +/- 8%). Oxygenation of St Thomas' Hospital solution did not alter its protective efficacy in this study (70% +/- 4% vs 69% +/- 7%). Infusion of esmolol cardioplegia at constant pressure provided complete protection for 60, 75, and 90 minutes (104% +/- 5%, 95% +/- 5%, and 95% +/- 3%, respectively), whereas protection with constant-flow esmolol cardioplegic infusion was significantly decreased at ischemic durations longer than 60 minutes. This decrease in efficacy of constant-flow esmolol cardioplegia was associated with increasing coronary perfusion pressure leading to myocardial injury. CONCLUSIONS: Oxygenation of esmolol cardioplegia (Krebs-Henseleit buffer plus 1.0 mmol/L esmolol) was essential for optimal myocardial protection. Multidose infusion of oxygenated esmolol cardioplegia provided good myocardial protection during extended periods of normothermic ischemia. Esmolol cardioplegia may provide an efficacious alternative to hyperkalemia.     

Continuous measurement of intramyocardial pH: relative importance of hypothermia and cardioplegic perfusion pressure and temperature

The continuous measurement of intramyocardial pH was used to follow the progression of ischemia and permit correlation to functional recovery. Adequacy of myocardial preservation following 38 degrees C or 25 degrees C global ischemia alone or with the administration of one or two doses of 38 degrees C, 25 degrees C, or 1 degree C crystalloid cardioplegia at aortic root perfusion pressures of 90 mm Hg or 130 mm Hg was assessed. A new miniature myocardial transducer incorporating fiberoptic technology and dual pH and temperature-sensing capability was placed into the left ventricular free wall and septum of 44 sheep undergoing ischemic arrest during cardiopulmonary bypass. All groups underwent global ischemia until myocardial pH was 6.8. An intramyocardial pH level of 6.8 reliably correlated to similar levels of functional recovery in each group. Aortic root perfusion pressure of 130 mm Hg provided enhanced myocardial protection by increasing the total ischemic time (5 to 10 minutes) with one (p less than 0.01) or two (p less than 0.001) doses of cardioplegic solution until a given functional level of recovery was attained. Aortic root perfusion pressure of 90 mm Hg provided no added benefit in total ischemic time, rate of change of pH, or degree of recovery of function. Hypothermic (25 degrees C) global ischemia alone enhanced myocardial protection by providing increased time (p less than 0.01) until a given functional level of recovery was attained with a slower rate of change of pH (p less than 0.01) compared with normothermic (38 degrees C) global ischemia alone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Optimal myocardial protection with fluosol cardioplegia

An oxygenated perfluorocarbon cardioplegic solution was examined, utilizing a blood-perfused canine model. Twenty-one animals were divided into three equal groups, and each animal received Fluosol cardioplegia at one of three infusion temperatures: 20 degrees C, or 4 degrees C. All hearts underwent 90 minutes of ischemia, during which time 150 ml of the cardioplegic solution was infused every 30 minutes. Myocardial oxygen and carbon dioxide tensions (PmO2 and PmCO2) were monitored continually using mass spectrometry, and myocardial oxygen consumption was calculated with each cardioplegic injection. The mean increase in PmO2 was 7.1 +/- 0.9 mm Hg with 20 degrees C Fluosol infusions, 31.1 +/- 4.7 mm Hg with 10 degrees C Fluosol injections, and 22.2 +/- 4.7 mm Hg with infusions of 4 degrees C Fluosol. Average myocardial oxygen consumptioN, expressed as cubic centimeters of oxygen per 100 gm of left ventricle (wet weight), was 21.2 +/- 0.5 with 20 degrees C Fluosol, 22.8 +/- 1.3 for 10 degrees C Fluosol, and 19.6 +/- 1.0 for 4 degrees C Fluosol. Mean myocardial temperatures with infusions of 20 degrees C, 10 degrees C, and 4 degrees C solutions were 21.4 +/- 0.1 degree C, 16.9 +/- 0.4 degree C, and 15.9 +/- 0.5 degree C, respectively. After 45 minutes of reperfusion, maximum rate of rise of left ventricular pressure, expressed as percentage of preischemic control, was 70.9 +/- 3.9% for 20 degrees C Fluosol, 90.9 +/- 3.2% for 10 degrees C Fluosol, and 90.4 +/- 2.3% for 4 degrees C Fluosol (p less than 0.005, 20 degrees C versus 10 degrees C, 4 degrees C Fluosol). In addition, the 10 degrees C and 4 degrees C Fluosol hearts had essentially normal structure by light and electron microscopy. These data demonstrate tht Fluosol cardioplegia results in near optimal myocardial protection when infused at cold temperatures (4 degrees C to 10 degree C). The increases intramyocardial oxygen and myocardial oxygen consumption with each injection demonstrate that there is enhanced oxygen delivery and utilization, which may account for the improved functional recovery observed in these hearts.