Effect of perfusate temperature on myocardial protection from ischemia

Commentary on Selected Aspects of Cardioprotection

Three aspects of cardioprotection are discussed in this article. The first is myocyte death as a function of the duration and severity of ischemia in experimental acute myocardial infarction in the dog heart. The short period of time during which reperfusion with arterial blood will salvage myocytes is demonstrated along with data showing that this period diminishes significantly if collateral flow is very low or absent. The second topic is a discussion of potential mechanisms underlying postconditioning. It begins with a review of the changes that lead to irreversible injury during acute ischemia in the dog heart along with a discussion of the genesis of contraction band necrosis and no reflow when myocardium is salvaged by unrestricted reperfusion with arterial blood in order to provide a basis to discuss the potential mechanisms underlying postconditioning, a situation in which reflow is intermittent and restricted. Postconditioning is reported to achieve greater myocyte salvage than unrestricted reflow. Potential explanations for this beneficial effect include: first, sufficient sarcolemmal repair occurring during the intermittent reflow (reoxygenation) to prevent cell death by explosive cell swelling, and second, prevention of the opening of the mitochondrial permeability transition pore, thereby preventing mitochondrial failure and cell death in the reperfused tissue. Since there is no way available to identify and specifically study the myocytes that would have died if not protected by postconditioning, direct demonstration of mechanisms is difficult or impossible. Finally, the third topic in this commentary is an analysis of the obstacles faced by investigators using small rodent hearts to establish cardioprotective mechanisms. Such studies provide valid data but the relationship of the changes and the proposed mechanisms underlying these changes are not necessarily directly transferable to ischemic large animal hearts including the heart of man.

The influence of temperature on metabolic and cellular protection of the heart during long-term ischemia: a study using P-31 magnetic resonance spectroscopy and biochemical analyses

We have compared the influence of two different cold temperatures (below 10 degreesC) for cardiac ischemia by measuring a large variety of hemodynamic and metabolic parameters during ischemia and reflow. Isolated isovolumic rat hearts were arrested with a preservation solution which was developed in our laboratory and then submitted to 5 h of cold storage (4 degreesC, group I; and 7.5 degreesC, group II) in the same solution. After an additional period of 50 min of ischemia at 15 degreesC with intermittent cardioplegic infusion, hearts were reperfused for 60 min at 37 degreesC. Function was assessed during the control period and reflow. High-energy phosphates and intracellular pH were followed by 31P magnetic resonance spectroscopy. Analyses of metabolites and enzymes were performed by biochemical assays and HPLC in coronary effluents and in freeze-clamped hearts to assess cellular integrity. The energetic pool was better preserved at 4 degreesC during ischemia (ATP at the end of 4 degreesC ischemia, 59 +/- 7% in group I vs 31 +/- 5% in group II, P < 0.01) and reflow (P < 0.05) but membrane protection was higher when increasing the temperature to 7.5 degreesC (reduction of creatine kinase leakage, 89 +/- 16 IU/min in group I vs 51 +/- 5 IU/min in group II, P < 0.05). As a result, functional recovery, represented by the rate pressure product, was higher in hearts preserved at 7.5 degreesC (52 +/- 6% recovery in group I vs 77 +/- 7% in group II at the end of reflow, P < 0.05). Altogether, cold storage at 7.5 degreesC provides a better protection than storage at 4 degreesC.

Temperature threshold and preservation of signaling for mitochondrial membrane proteins during ischemia in rabbit heart

Temperature modulates both myocardial energy requirements and production. We have previously demonstrated that myocardial protection induced by hypothermic adaptation preserves expression of genes regulating heat shock protein and the nuclear-encoded mitochondrial proteins, the adenine nucleotide translocator isoform 1 (ANT1), and the beta subunit of F1-ATPase (beta F1-ATPase). This preservation is associated with a reduction in ATP depletion similar to that noted in cardioplegic arrested hearts preserved at a critical temperature (30 degrees C) or below. We tested the hypothesis that expression of these genes may also be subject to this temperature threshold phenomenon. Isolated perfused rabbit hearts were subjected to ischemic cardioplegic arrest at 4, 30, or 34 degrees C for 120 min. Cardiac function indices and steady-state mRNA levels for ANT1, beta F1-ATPase, and HSP70-1 were measured prior to ischemia (B) and after 45 min of reperfusion. Cardiac function was significantly depressed in the 34 degrees C group. Ischemia at 34 degrees C reduced steady-state mRNA levels for ANT1 and beta F1-ATPase from B, but these levels were similarly preserved at 4 and 30 degrees C. HSP70-1 levels were mildly elevated (fourfold) above B to similar levels at all three temperatures. These results indicate that mRNA expression for ANT1 and beta F1-ATPase is specifically preserved in a pattern consistent with the temperature threshold phenomenon. HSP70-1 expression is not influenced by ischemic temperature. Preservation of gene expression for these mitochondrial proteins implies that signaling for mitochondrial biogenesis or resynthesis is maintained after ischemic insult.

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.

Ischemia affects cardiac proteins in healthy animals less severely than in human patients

BACKGROUND

Recently, our group showed that in human hearts proteins are extremely sensitive to ischemic injury. The purpose of this investigation was to evaluate the effects of ischemia on contractile and cytoskeletal proteins in rabbit and pig hearts and to compare these findings with those obtained in humans.

METHODS

Rabbit hearts were arrested by perfusion with Euro-Collins solution at different temperatures. Hearts perfused with buffer served as controls. Tissue samples were incubated for varying time intervals and processed for immunohistochemistry and electron microscopy. Porcine hearts were treated in the same manner. Changes in the localization of myosin, desmin, and tropomyosin antibodies were evaluated and the degree of ischemic injury was determined by electron microscopy.

RESULTS

Healthy animal hearts tolerate ischemia better than human hearts. Cardiac proteins are more sensitive to ischemia than the ultrastructural cellular organelles. Temperatures as low as 0 degree C produce more cell damage than 4 degrees C and should therefore be avoided. The Euro-Collins solution protects the myocardium better than buffer.

CONCLUSIONS

We conclude that healthy animal hearts are more resistant to ischemia than diseased human hearts and that results from experimental studies should be interpreted with caution with regard to the human situation.

Post-ischemia ERG recovery is influenced by temperature

In experimental work on retinal ischemia and its medical management, we have been disturbed by the large variations in the electroetinogram (ERG) recovery of rabbit eyes subjected to similar degrees of ischemia. We investigated whether body temperature might be one of the critical factors. We studied pigmented Dutch rabbits that had been subjected to 60 min of ocular ischemia followed by 4 h of reperfusion at different body temperatures. Ischemia was produced by cannulating the anterior chamber and raising the intraocular pressure. Scotopic ERGs and rectal temperatures were recorded at regular intervals. Rabbits with a subnormal mean temperature of 35.5-37.7 degrees C throughout the experiment showed mean a- and b-wave recoveries of 131.52% and 107.68% of preischemic values after 4 h reperfusion. At temperatures between 37.8-38.9 degrees C, the a- and b-waves only recovered to 88.43% and 32.0% respectively. Even small degrees of cooling greatly enhanced post-ischemic ERG recovery. This may explain some of the variations in ERG recovery that have been reported in the literature. We suggest that body temperature should be tightly controlled between 37.8 and 38.9 degrees C during ischemia experiments to stabilize results and minimize errors in judging therapeutic effects. Retinal cooling may be a means to protect the retina against ischemic damage in clinical situations.

Determination and comparison of heat exchange efficiency of four commercially available blood cardioplegia sets

An in vitro comparison of the heat exchange properties of the Electromedics D1081A, Sarns MP-4 'Conducer Coil' 165720, Bentley HE-100 and the Shiley BCD Plus 4: 1 blood cardioplegia sets was performed. The efficiency was calculated for each heat exchanger and post-heat exchange blood path temperatures were compared. The calculated efficiency for each heat exchanger is as follows: Electromedics D1081A at 18 l/minute coil water flow (0.95); Sarns MP-4 165720 at 9 l/minute coil water flow (0.93); Bentley HE-100 at 10 l/minute coil water flow (0.91); Shiley BCD Plus at 15 l/minute coil water flow (0.90). Blood path, precoil temperatures for each unit were compared statistically with no differences found (p > 0.05). Blood side, postcoil temperatures were then compared. The average postcoil blood path temperature of the Electromedics D1081A was 1.6 degrees +/- 0.1 degree C, of the Sarns MP-4 165720 it was 2.1 degrees +/- 0.1 degree C, of the Bentley HE-100 it was 2.7 degrees +/- 0.3 degree C, and the Shiley BCD Plus 3.0 degrees +/- 0.4 degree C. The results of this experiment indicate that the average postcoil temperature of the Electromedics D1081A was lower than the Sarns MP-4 165720 (p < 0.05) which was lower than that of the Bentley HE-100 (p < 0.05) and Shiley BCD Plus (p < 0.05). No statistical difference was found between the average postcoil temperature of the Bentley HE-100 and the Shiley BDC Plus.

The effects of a temperature below 15 degrees C on the myocardial calcium and ultrastructure in donor heart preservation in a canine model

The effects of 6-h hypothermic cardioplegic arrest on myocardial biochemical, morphologic, and functional recovery were investigated in two groups of dogs. Group 1 (n = 6) was subjected to hypothermia of 15 degrees C and group 2 (n = 6) was subjected hypothermia of 5 degrees C. Although the myocardial calcium (Ca) concentration was significantly higher at the end of reperfusion in group 2 compared to group 1, the MB-fraction of creatine kinase, mitochondrial aspartate aminotransferase, recovery of left ventricular systolic function, and mitochondrial morphologic integrity were better in group 2 than in group 1. These findings suggest that hypothermia of 5 degrees C in 6-h cardioplegia is not necessarily coupled with interference in myocardial contractility, despite the Ca overload that occurs during reperfusion.

Optimal level of hypothermia for prolonged myocardial protection assessed by 31P nuclear magnetic resonance

The optimal level of hypothermia during myocardial preservation for cardiac transplantation is not known. Phosphorus 31 nuclear magnetic resonance spectroscopy was used to assess the effect of different preservation temperatures (15 degrees C in group 1, 4 degrees C in group 2) on the myocardial high-energy phosphate profiles during prolonged global ischemia and subsequent reperfusion of isolated rat hearts. Adenosine triphosphate depletion during ischemia was more gradual in group 2, leading to significant differences in myocardial adenosine triphosphate concentrations between the two groups after 3 hours of ischemia. The fall in intracellular pH during ischemia was significantly less pronounced in hearts preserved at 4 degrees C as compared with those at 15 degrees C. The postischemic recovery of both the left ventricular peak systolic pressure and the maximum rate of increase of left ventricular pressure was enhanced in group 2, although the ischemic period was 3 hours longer than in group 1. Hypothermia at 4 degrees C as compared with 15 degrees C appears to prolong myocardial protection with respect to adenosine triphosphate preservation, prevention of the fall in intracellular pH, and the enhancement of postischemic hemodynamic recovery.

Adverse effect of prearrest hypothermia in immature hearts: rate versus duration of cooling

It has been suggested that rapid cooling before the induction of arrest may be harmful to the newborn myocardium. The objective of this study was twofold: (1) to evaluate whether prearrest rapid cooling is indeed detrimental to myocardial recovery and (2) if so, to evaluate whether the adverse effect of prearrest hypothermia is dependent on the rate of cooling or the total duration of cold perfusion. After an initial stabilization period isolated Langendorff hearts (n = 5 per group) from neonatal piglets (5 to 7 days old) were randomized to four groups: group 1, 5 minutes of rapid cooling to 15 degrees C; group 2, 20 minutes of slow cooling to 15 degrees C; group 3 and group 4, rapid and slow cooling, respectively, with the addition of St. Thomas cardioplegic solution. All groups were then subjected to 2 hours of ischemia at 15 degrees C followed by 30 minutes of reperfusion at 38.5 degrees C. Post-ischemic recovery of left ventricular developed pressure was significantly greater in group 1 versus group 2 (80% +/- 3% versus 61% +/- 2%; p less than 0.05) and in the presence of cardioplegia, group 3 versus group 4 (72% +/- 3% versus 57% +/- 3%; p less than 0.05). The increase in left ventricular end-diastolic pressure was significantly less in group 1 versus group 2 (8% +/- 5% versus 33% +/- 7%; p less than 0.01). Myocardial adenosine triphosphate content recovery correlated with ventricular recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Epicardial damage induced by topical cooling during paediatric cardiac surgery

OBJECTIVE

To study electrocardiographic changes in infants and children in whom topical cooling was used during surgical repair of congenital heart defects.

DESIGN

A retrospective study of all patients who had surgical repair of congenital heart disease during cold blood cardioplegia and topical cooling from January to August 1990. Eleven patients (group 1) had topical cooling with ice and 15 (group 2) with cold saline.

PATIENTS

All 36 paediatric patients operated on during this period. All the available electrocardiographic records were analysed. Ten patients in whom reliable records were not available were excluded. Twenty six patients entered in this retrospective study.

INTERVENTIONS

Topical cooling with ice or with a slush of cold saline.

MAIN OUTCOME MEASURE

Recordings from all the precordial leads were examined and scored as the sum of the maximum ST elevation (mV) in each precordial lead. The score obtained for each electrocardiogram was recorded together with the timing of the electrocardiogram (preoperative, arrival in intensive care unit immediately after surgery, postoperatively in the intensive care unit, and at discharge).

RESULTS

There were no differences between the two groups in terms of demographic data, diagnosis, duration of ischaemia, and postoperative myocardial performance. There was temporary ST elevation during the first 48 postoperative hours in all the children in group 1 but in only seven of the 15 children in group 2 (Fisher's test, p less than 0.005). The mean (SD) score for maximum ST elevation was 1.34 (0.83) mV in group 1 and 0.52 (0.64) mV in group 2 (Student's t test, p less than 0.01).

CONCLUSIONS

These temporary electrocardiographic changes in the presence of adequate myocardial performance were attributed to epicardial damage induced by hypothermicosmotic injury. The use of ice for topical cooling may damage the epicardium in children.

Phrenic nerve injury. A prospective study

In cardiac surgery, topical iced saline solution slush has become an important adjunct in maintaining myocardial hypothermia during cardioplegic arrest. One complication of this technique is phrenic nerve injury (PNI). In an attempt to reduce the incidence of PNI, a prospective study was undertaken to evaluate the impact of phrenic nerve insulation on PNI during cardiac surgery. Seventy-six consecutive patients who underwent coronary revascularization constituted the control group (CG) and were compared with 76 patients who underwent a similar procedure with the addition of phrenic nerve insulation. In the intervention group (IG), a foam insulation pad was placed between the heart and the pericardium in an effort to reduce exposure of the phrenic nerve to iced saline solution slush. There was no difference in major demographic descriptors or operative variables between the CG and the IG, except that the internal mammary artery was used more frequently in the IG (64 percent vs 36 percent, p = 0.0006). The in-hospital mortality was similar between the groups (CG, 0.0 percent; IG, 1.3 percent; p = 1.0); however, the incidence of roentgenographically diagnosed PNI was much greater in the CG (14/76 patients [18 percent] vs 0/76 patients [0 percent]; p = 0.0006). Patients with and without PNI were similar with regard to age, gender, aortic cross clamp time, cardiopulmonary bypass time, and number of grafts (p greater than 0.05). All unilateral PNI occurred on the left. Three patients with bilateral PNI required tracheostomy and prolonged mechanical ventilation. In-hospital mortality was similar for patients with and without PNI (0 percent vs 0.7 percent), but mean postoperative hospital stay for patients with clinically diagnosed PNI was longer than for those without PNI (32 vs 11 days, p = 0.04). This prospective study demonstrates that the incidence of PNI can be significantly reduced by the routine use of phrenic nerve insulation.

Percutaneous cooling of ischemic myocardium by hypothermic retroperfusion of autologous arterial blood: effects on regional myocardial temperature distribution and infarct size

The effects of synchronized coronary venous retroperfusion of cooled autologous arterial blood on regional myocardial temperature distribution and infarct size were studied in open chest dogs with 3.5 h of left anterior descending coronary artery occlusion. After 30 min of occlusion, the dogs were randomly assigned to one of three groups: 1) untreated control group (n = 5), 2) normothermic retroperfusion group (infusion temperature 32 degrees C) (n = 7), and 3) hypothermic retroperfusion group (infusion temperature 15 degrees C) (n = 7). Regional myocardial temperatures were measured by using needle-tipped thermistors stabbed in the 1) anterior wall distal to the occlusion site, 2) anterior wall proximal to the occlusion site, 3) left lateral wall, 4) posterior wall, and 5) right ventricular free wall. Rectal and pulmonary artery temperatures were also measured. In the hypothermic retroperfusion group, the anterior wall temperature decreased rapidly by 5 degrees C at 15 min of retroperfusion (p less than 0.05 vs. normothermic retroperfusion or untreated control groups), whereas the temperature at other sites decreased with a linear trend over time. Myocardial temperatures in the ischemic area (distal anterior wall) were generally lower than those in the other sites during the first 60 min of hypothermic retroperfusion and the largest intramyocardial temperature difference (3.6 degrees C) was found at 15 min after retroperfusion. Infarct size expressed as a percent of the risk area was significantly smaller in the hypothermic retroperfusion group (6.2 +/- 3.3%) than in the control (64.9 +/- 14%) or normothermic retroperfusion groups (24.1 +/- 6.7%).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypothermic ischemia and reperfusion on calcium transport by myocardial sarcolemma and sarcoplasmic reticulum

The effects of hypothermic ischemia and reperfusion on sarcolemma and sarcoplasmic reticulum Ca2+ transport were studied in vesicles isolated from rabbit hearts. Hypothermic global ischemia was produced by immersing hearts in saline at 4 degrees C for 3 h. Following hypothermic ischemia, reperfusion was carried out for 40 min using a Langendorff perfusion system for the working heart. Na+,K(+)-ATPase activity of sarcolemmal vesicles (SL), was not depressed by hypothermic ischemia nor by ischemia and reperfusion. The initial rate of Na(+)-Ca2+ exchange in SL vesicles was not depressed, but the maximum amount of Ca2+ uptake was increased both after hypothermic ischemia and after reperfusion. Ca2+ uptake activity of sarcoplasmic reticulum vesicles (SR) isolated from hearts subjected to hypothermic ischemia was slightly lower than that of control, and was further reduced following reperfusion. Ca(2+)-ATPase activity of SR was unaffected by hypothermic ischemia, while it was markedly lowered after reperfusion. Although the phosphoenzyme level in SR vesicles was slightly decreased, the turnover rate was reduced after reperfusion. Reperfusion injury thus took place mainly in SR while SL appeared to be tolerant to ischemia and reperfusion.

Optimizing myocardial hypothermia: I. Temperature probe design and clinical inferences

Myocardial hypothermia is an essential component of myocardial preservation for most cardiac operations. Because of multiple causes of rewarming, it is necessary to monitor temperatures at specific sites (right and left ventricular epicardium and endocardium or cavity). Thus, plastic temperature probes have been designed and fabricated to facilitate temperature monitoring at these sites. Using a bare thermocouple as a standard, in vitro comparison of metallic probes and plastic probes revealed differences of 4.0 degrees +/- 0.9 degrees C and 0.7 degrees +/- 0.6 degrees C, respectively (p less than 0.005). Consequently, metallic probes do not have sufficient accuracy to detect transmural temperature gradients because of "stem effect." Using the plastic probes to evaluate temperature changes in porcine hearts after cardioplegia-induced hypothermia revealed a temperature rise of 1 degree C/min at all sites if control of systemic and venous return and local myocardial cooling are not provided. The use of temperature monitoring at multiple sites permits identification and prevention of various causes of myocardial rewarming and is facilitated by the use of plastic probes described herein which contain dual thermocouples.

Perioperative assessment of the quality of myocardial protection

Perioperative myocardial protection constitutes one of the major advances of modern cardiac surgery, but perioperative assessment of its quality is difficult in clinical practice. This review details our approach to myocardial protection in specific situations, as well as the techniques we use to assess it in three different settings: Is distribution of antegrade cardioplegia homogeneous in patients presenting coronary artery stenosis? The problem was to determine if cardioplegic solutions used antegradely or retrogradely could represent a reliable echographic contrast for myocardial imaging in coronary artery disease patients and normal coronary artery patients, and further if significant differences of myocardial contrast exist between the two methods. Does warm reperfusion cardioplegia improve postoperative myocardial function in patients with an ejection fraction (EF) less than 35%? Pre- and postoperative hemodynamic workup with construction of Starling curves were used to answer this question. Does reperfusion of ischemic hearts induce free radical formation in humans? Deleterious effects of reperfusion are usually determined by studying the effects of free radicals on the myocardium, by determining the concentrations of substances that reflect the degree of membrane phospholipid destruction. However, the specificity and the sensitivity of such assays are low. We recently described white cell chromosomal aberrations secondary to free radical formation, after ischemic and reperfusion injury. Values obtained are expressed as the percentage of the total number of chromosomes. Although specialized laboratory techniques are required, this test is highly accurate.

Cardiac transplantation: the ideal myocardial temperature for graft transport

The ideal preservation method and cooling temperature for transport of donor hearts are not known. Serious derangements in myocardial relaxation are well described with different methods of cooling. To assess this problem, human right atrial trabeculae contracting isometrically at 34 degrees C in vitro were subjected to hypothermic arrest at 1, 4, 12, and 20 degrees C for 1, 2, 4, 24, and 48 hours. Control conditions were resumed, and myocardial mechanical recovery was assessed over 1 hour. Contraction was 50% depressed after a 1- to 2-hour exposure to 1 degree C and was almost completely arrested following a 4-hour exposure. Muscles cooled to 4 degrees C recovered poorly, whereas those cooled to 12 and 20 degrees C did well. In the latter 2 groups, force development increased rapidly on rewarming and exceeded the precooling contraction force (p less than 0.05). A 100% increase in relative resting force was seen in muscles cooled to 1 and 4 degrees C (p less than 0.05). This finding suggests a failure of calcium homeostasis at very low temperatures. We conclude that atrial preservation is optimal at about 12 degrees C.

The effect of pericardial insulation on hypothermic phrenic nerve injury during open-heart surgery

Phrenic nerve injury was evaluated prospectively in 133 patients undergoing open-heart surgery using iced saline slush for topical hypothermia. In the control group of 70 patients no attempt was made to shield the phrenic nerves from direct exposure to ice. Phrenic nerve damage occurred in 73% of these patients, as assessed by persistent diaphragm paralysis evident on inspiratory chest roentgenogram. In 2 patients the paralysis was bilateral. In the second group of 63 patients a pericardial insulation pad was used to prevent contact of the iced slush to the phrenic nerve. Diaphragm paralysis was observed in 17% of these patients. This difference was highly significant (p less than .001). Diaphragm paralysis in the control group was clinically significant; life-threatening respiratory complications developed in 7 patients (14%), frequently resulting in multiple reintubations, tracheostomy, and prolonged mechanical ventilation. In addition, 4 patients with phrenic nerve injury exhibited a clinical syndrome consistent with gastric ileus, which may possibly represent hypothermic injury to the thoracic vagi. The likelihood of phrenic nerve injury when iced saline slush is used for topical myocardial cooling and the possibility of developing serious respiratory disability would support the routine use of pericardial insulation when this method of hypothermia is used.

Myocardial energy metabolism in the induction phase of cardioplegia in relation to myocardial temperature during open heart surgery

Changes in myocardial high energy phosphate and lactate concentrations during initiation of cardioplegia for open heart surgery were studied in 21 patients and related to their individual myocardial temperatures. Left ventricular myocardial biopsies were taken 10 min post aortic cross-clamping and were analysed for ATP, creatine phosphate (CP), creatine (C) and lactate. The patients were divided into three groups according to the achieved myocardial temperature: 6-10 degrees C, 11-15 degrees C and 16-24 degrees C. The results indicated that optimal myocardial protection during the induction phase of the cardioplegia was obtained in the 11-15 degrees C group in which the highest ATP concentration and simultaneously lowest lactate concentration was maintained.

Preservation of myocardial high-energy phosphates in open-heart surgery with deep general hypothermia and multidose crystalloid cardioplegia

Myocardial energy metabolism during deep general hypothermia (20 degrees C) and multidose crystalloid cardioplegia, and also during subsequent reperfusion, was studied in eight patients undergoing isolated aortic valve replacement. Six serial transmural biopsy samples from the left ventricular apex were analyzed for high-energy phosphates and their degradation products. Reductions in ATP, total adenine nucleotide content and energy charge were insignificant during cardioplegia, as were changes in adenosine and uric acid concentrations. During reperfusion, however, there was slight but significant reduction in total adenine nucleotide content, despite adequate oxygenation as indicated by reversal of lactate accumulation. These observations suggest that the reperfusion phase is accompanied by metabolic aberrations which are not overcome by good oxygenation in relation to the metabolic rate.

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)

Interactions between pharmacological cardioplegia and hypothermia for intraoperative myocardial protection

Cold cardioplegia is currently the method of choice for providing myocardial protection during open-heart surgical procedures. Two components of protection, perfusion cooling and pharmacological cardiac arrest, were investigated in the guinea pig heart-lung model. The effects of two cardioplegic solutions, the University of Alabama Hospital solution and the St. Thomas' Hospital solution, and a control perfusate were compared. The results confirmed the efficacy of hypothermia as a protective agent and the additional protection afforded by pharmacological cardioplegia. Infusion temperature critically influenced the cardioprotective action of the Alabama solution: Striking protection was afforded only under hypothermic conditions, whereas myocardial damage was exacerbated by the infusion at 37 degrees C. The St. Thomas' Hospital solution provided substantial protection independent of infusion temperature. Thus, the safety margin of the Alabama solution was narrower than that of the St. Thomas' solution. It is suggested that the difference between the two cardioplegic solutions partially depends on their coronary vasoactivity, since the administration of the Alabama solution at 37 degrees C increased coronary perfusion pressure. It would seem worthwhile to use a temperature-independent cardioplegic solution devoid of coronary vasoconstricting action.

Relationship between myocardial adenine nucleotide catabolism and tissue blood flow rate in experimental ischaemia

A method was developed for tissue preservation and evaluation of the adenine nucleotide metabolism in small samples of myocardium after 45 min of ischaemia. Ischaemia was produced by coronary artery occlusion in anaesthetized cats. Adenine nucleotides and their metabolites were measured by isocratic liquid chromatographic systems which allow quantitative analysis of the nucleotides and their metabolites inosine, hypoxanthine and xanthine in biopsies of 5-20 mg tissue. Regional myocardial blood flow was measured in the tissue surrounding the biopsies by the distribution of 15 micron radiolabelled microspheres. In central ischaemic regions the ATP level was approximately 1 mumol/g wet weight, whereas in normally perfused myocardium the ATP level was approximately 5 mumol/g tissue. In tissue with intermediate flow values, intermediate ATP levels were found. Energy charge, which summarizes all adenine nucleotide concentrations, was reduced from 0.88 to 0.50, and the molar concentrations of inosine, hypoxanthine and xanthine increased in ischaemic tissue. We conclude that this method provides reliable characterization of the local cellular energy status in cat hearts with ischaemic regions.

The optimal temperature for preservation of the myocardium during global ischemia

To determine the myocardial temperature that provides maximal preservation of the heart during global ischemic arrest, five groups of dogs were studied (6 per group). In all animals, the aorta was cross-clamped for 120 minutes. Serial biopsies were done for determination of adenosine triphosphate and creatine phosphate, and study by electron microscopy. Starling curves were derived prior to cardiopulmonary bypass and 60 minutes after bypass. Mitochondrial changes were graded on a scale of 0 to 4. In the control group (Group 1), the aorta was clamped when the rectal temperature reached 25 degrees C (myocardial temperature, 18 degrees to 22 degrees C). In Groups 2, 3, 4, and 5, myocardial temperature was maintained at 6 degrees C, 10 degrees C, 14 degrees C, and 18 degrees C (all +/- 2 degrees C), respectively, by the use of systemic and topical hypothermia and repeated injections of cold cardioplegic solution into the aortic root. All groups showed a depression of left ventricular stroke work index, particularly Group 1 (no survivors), Group 2, and Group 3. The high-energy phosphate stores were well preserved in all groups except Group 1. The mitochondrial ultrastructure showed significant changes in all groups, especially Groups 1 and 5. These data indicate that satisfactory preservation of mitochondrial ultrastructure and high-energy phosphates was achieved at myocardial temperatures lower than 18 degrees C. Extreme hypothermia (Groups 2 and 3) was associated with significant reduction in ventricular function under the experimental conditions employed.

Myocardial blood flow and oxygen consumption in the empty-beating, fibrillating, and potassium-arrested hypertrophied canine heart

Myocardial oxygen consumption and blood flow distribution were examined in severely hypertrophied canine hearts in the empty-beating, fibrillating, and pharmacologically arrested states. Hypertrophy was produced using a subcoronary valvular aortic stenosis model that mimics the clinical situation of aortic valvular stenosis. Oxygen content of the total coronary sinus collection was compared with a large volume arterial sample using a Lex-O2-Con-TL analyzer, which had been validated by the Van Slyke-Neill method. Transmural blood flow was measured in each state using microspheres, and perfusion pressure was maintained at 80 mm Hg. Oxygen consumption in the empty-beating hypertrophied heart was found to be the same as that previously reported for normal hearts. Blood flow was evenly distributed in the empty-beating heart, with an endocardial/epicardial ratio of 0.99 +/- 0.15 (SEM) milliliters per minute per gram of left ventricular weight. Oxygen consumption failed to increase significantly with fibrillation; however, blood flow distribution favored the subepicardium, suggesting that oxygen consumption determinations in the fibrillating hypertrophied heart may not accurately reflect metabolic demand. Basal oxygen consumption of the hypertrophied heart as determined by the potassium-arrested, blood-perfused model was the same as that previously described for normal hearts. Blood flow during potassium arrest favored the subendocardium (endocardial/epicardial ratio = 1.14 +/- 0.27 ml/min/gm LV weight).

Myocardial protection in infant open heart surgery

Myocardial protection was evaluated in 2 groups of 5 infants each undergoing correction of either tetralogy of Fallot (TOF) or subcristal ventricular septal defect (VSD). In group A, profound hypothermia and total circulatory arrest (PHTCA) was utilized. In group B, profound hypothermia and total circulatory arrest combined with potassium cardioplegia (PHTCA + K) was the method of protection used. The analysis was carried out by sequential measurements of clinical, electrocardiographic, enzymatic (CK-MB) and ultrastructural parameters. There were no operative deaths. One infant had a second operation for recurrent VSD. The average anoxic time was 35.4 min in group A (PHTCA) and 32.6 min in group B (PHTCA + K). Analysis of our data demonstrated that when potassium cardioplegia was added to PHTCA, there was less intraoperative myocardial damage according to physiological, ultrastructural and biochemical parameters than when profound hypothermia and total circulatory arrest was applied alone.

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.

Superior action of magnesium-lidocaine-1-aspartate cardioplegia to glucose-insulin-potassium cardioplegia in experimental myocardial protection

The effect of 2 hours of hypothermic Mg-lidocaine cardioplegia upon left ventricular function, myocardial high-energy stores, edema, and ultrastructure was studied as compared to glucose-insulin-potassium (GIK) cardioplegia in 12 mongrel dogs. The myocardial temperature recorded in the ventricular septum was kept at 20 degrees C during the cardioplegia. The heart was re-warmed up to 37 degrees C by the support of cardiopulmonary bypass, then, observations were made during a 60 minutes reperfusion. Left ventricular function was preserved at a more physiological level in cases of Mg-lidocaine cardioplegia. Myocardial ATP as preserved at significantly higher levels following Mg-lidocaine cardioplegia than in cases of GIK cardioplegia (p < 0.05). However, content of myocardial creatine phosphate was higher in the GIK cardioplegia group than that in Mg-lidocaine group in the subendocardium and the ventricular septum. Myocardial edema was significantly suppressed following Mg-lidocaine cardioplegia, and such was significantly lower than in cases of GIK cardioplegia (p < 0.05). The myocardial ultrastructure was protected from ischemic insult in the Mg-lidocaine cardioplegia group. These data suggest that Mg-lidocaine-1-aspartate solution is superior to GIK solution as a cardioplegic solution, and that such will feasibly provide myocardial protection for 2 hours of hypothermic cardiac arrest, in an experimental reperfused model.

Perioperative protection of the myocardium in patients with impaired ventricular function

Seventeen patients with poor ventricular function and severe coronary artery obstruction were operated on employing hypothermic potassium cardioplegic solution for myocardial preservation. Preoperatively and postoperatively, serial hemodynamics, electrocardiograms (ECG), MB-CPK studies, and technetium pyrophosphate scans were obtained for all patients. All ECGs and scans were negative for perioperative infarction. Peak MB-CPK levels were 40 +/- 25 units per liter. Two patients had MB-CPK levels suggestive of perioperative myocardial infarction. The preoperative cardiac index was 2.8 +/- 0.8 L/min/m2 and remained the same in the perioperative period. Stroke work index and total peripheral resistance were within normal range and remained constant throughout the period of study. Three patients required epinephrine (0.5 micrograms per minute) during the first 6 hours postoperatively, and in 2 patients an intraaortic balloon was inserted prophylactically and removed on the second postoperative day. Good myocardial preservation can be achieved in patients with severe coronary artery obstruction and preexisting left ventricular dysfunction using hypothermic potassium cardioplegic solution.