Effects of cold and warm blood cardioplegia assessed by myocardial pH and release of metabolic markers

[Therapeutic hypothermia and management of sudden death]

Due to its protective effects on the brain and potentially the myocardium, cooling therapy is clearly part of the standard of care of any sudden death especially in the setting of myocardial infarction. Recent guidelines recommend cooling therapy (32 to 34 °C) for 12 to 24 hours in unconscious patients with spontaneous circulation after resuscitated sudden death. We provide here a review of clinical evidence, cooling techniques and potential adverse effects of cooling therapy.

Prolonged warm ischemia for transfusion-free arterial switch and ventricular septal defect surgery in a 4.5-Kg baby

Blood-free pediatric surgery is increasingly used for surgical correction of simple cardiopathies. Herein, we describe a complex cardiopathy, arterial switch operation and ventricular septal defect, with pre-operative thrombocytopenia in a 4.5 Kg baby treated with warm surgery and intermittent warm blood microplegia without any blood product. Bypass time was 89 min and aortic cross-clamp time 61 min. The maximal length of warm ischemia (time between microplegia injections) was 42 minutes. The postoperative course was uneventful. The patient was weaned off the ventilator after 7 hours, was discharged from the ICU on day 2 and was discharged from the hospital on day 7. The two main factors involved in this result were high pre-operative hemoglobin level and bypass technique with small prime volume, microplegia and warm perfusion. However, the success of this challenging case is also the result of teamwork and of rigorous patient care.

Beating-heart valve surgery: is the introduction of lung perfusion/ventilation the next step?

Myocardial and pulmonary ischemia during cardiopulmonary bypass has been associated with postoperative cardiac and pulmonary dysfunction, as well as poor outcomes. Beating-heart valve surgery utilizing continuous coronary perfusion with warm oxygenated blood via the antegrade/retrograde routes, is a novel strategy for myocardial protection. Conceptually, it is proposed that maintenance of pulmonary perfusion and ventilation during the cardiopulmonary bypass period also might be advantageous. The most current evidence regarding these evolving techniques and further areas of research are discussed in this article.

Intra-arrest rapid head cooling improves postresuscitation myocardial function in comparison with delayed postresuscitation surface cooling

OBJECTIVE

To compare resuscitation outcomes and myocardial function among intra-arrest head cooling, delayed surface cooling, and uncooled controls.

DESIGN

Prospective animal study.

SETTING

University-affiliated animal research laboratory.

SUBJECTS

Twenty-four male domestic pigs.

INTERVENTIONS

Ventricular fibrillation remained untreated for 10 mins after which animals were assigned into three groups: 1) intra-arrest head cooling, 2) postresuscitation surface cooling, and 3) uncooled controls. Head cooling by evaporative perfluorochemical began coincident with the start of cardiopulmonary resuscitation and continued for a total of 4 hrs. Surface cooling using a cooling blanket began at 2 hrs after return of spontaneous circulation and continued for 8 hrs. Control animals were treated identically with the exception for cooling.

MEASUREMENTS AND MAIN RESULTS

Return of spontaneous circulation was achieved in eight of eight head-cooled animals, in seven of eight surface-cooled animals, and in seven of eight of controls. Myocardial functions measured by transthoracic echocardiography were significantly better in the head-cooled animals than in surface-cooled and controls. All head-cooled animals survived for more than 96 hrs. This contrasted with six of eight survivors after surface cooling, and only two of eight among controls.

CONCLUSIONS

Both intra-arrest head cooling and delayed surface cooling improved postresuscitation myocardial dysfunction. The beneficial effects were greatest with head cooling initiated with cardiopulmonary resuscitation.

Promising though not yet proven: Emerging strategies to promote myocardial salvage

Remarkable advances in our ability to achieve early and sustained culprit vessel patency in acute myocardial infarction have been satisfying, but our enthusiasm must be tempered by the knowledge that the overall treatment strategy often leaves an inadequate long term clinical result. Early success of percutaneous therapy as judged at angiography does not ensure recovery of normal left ventricular function, the most important determinant of survival in acute myocardial infarction. That congestive heart failure and death still complicate apparently successful percutaneous procedures underscores the need to develop novel therapies which salvage jeopardized myocardium, limit infarct size and preserve left ventricular function. An ever-increasing body of data demonstrates a multifactorial mechanism of myocyte injury and microvascular collapse and also demonstrates that these injuries seem to have a profound impact on long-term outcomes. Given these findings, microvascular protection during the acute event has become the focus of a variety of emerging technologies. The goal of these mechanical and pharmacologic therapies is the restoration of normal metabolic function at the myocyte level. The acute pathologic mechanisms which contribute to sustained left ventricular dysfunction despite angiographically successful revascularization will be reviewed as will be several strategies being developed to counter these pathologic mechanisms.

Multiparameter Fiber Optic Sensor for the Assessment of Intramyocardial Perfusion

OBJECTIVES

The objective of this study was to characterize a multiparameter fiber optic sensor for detection of changes in intramyocardial perfusion and to demonstrate a method of determining critical values for pH, PCO2, and PO2 to indicate onset of anaerobic metabolism.

METHODS

Six swine underwent a 20-minute occlusion of the left anterior descending coronary artery (LAD). Myocardial pH, PCO2, and PO2 were measured continuously in the LAD and left circumflex coronary artery (CFX) territories. Critical values for each parameter were calculated from these data.

RESULTS

During occlusion LAD myocardial pH declined from 7.36 +/- 0.04 to 6.85 +/- 0.04; PCO2 rose from 57.0 +/- 2.9 to 154.0 +/- 18.0 torr, PO2 fell from 78 +/- 20 to 6 +/- 5 torr. No myocardial pH or PCO2 changes were observed in the CFX region, however, CFX PO2 was affected in some animals during LAD occlusion and release. Methods for determining the ischemic threshold from these sensor data are presented.

CONCLUSIONS

Multiparameter fiber optic sensors reliably respond to coronary occlusion and thus have the potential to help guide myocardial protection strategies for both on- and off-pump cardiac surgery.

Therapeutic hypothermia

Hypothermia is common during anaesthesia and surgery owing to anaesthetic-induced inhibition of thermoregulatory control. Perioperative hypothermia is associated with numerous complications. However, for certain patient populations, and under specific clinical conditions, hypothermia can provide substantial benefits. Lowering core temperature to 32-34 degrees C may reduce cell injury by suppressing excitotoxins and oxygen radicals, stabilizing cell membranes, and reducing the number of abnormal electrical depolarizations. Evidence from animal studies indicates that even mild hypothermia provides substantial protection against cerebral ischaemia and myocardial infarction. Mild hypothermia has been shown to improve outcome after cardiac arrest in humans. Randomized trials are in progress to evaluate the potential benefits of mild hypothermia during aneurysm clipping and after stroke or acute myocardial infraction. However, as hypothermia can cause unwanted side-effects, further research is needed to better quantify the risks and benefits of therapeutic hypothermia.

Induction of mild systemic hypothermia with endovascular cooling during primary percutaneous coronary intervention for acute myocardial infarction

OBJECTIVES

The purpose of this study was to evaluate the safety and feasibility of endovascular cooling during primary percutaneous coronary intervention (PCI) for acute myocardial infarction (AMI).

BACKGROUND

In experimental models of AMI, mild systemic hypothermia has been shown to reduce metabolic demand and limit infarct size.

METHODS

In a multi-center study, 42 patients with AMI (<6 h from symptom onset) were randomized to primary PCI with or without endovascular cooling (target core temperature 33 degrees C). Cooling was maintained for 3 h after reperfusion. Skin warming, oral buspirone, and intravenous meperidine were used to reduce the shivering threshold. The primary end point was major adverse cardiac events at 30 days. Infarct size at 30 days was measured using (99m)Tc-sestamibi SPECT imaging.

RESULTS

Endovascular cooling was performed successfully in 20 patients (95%). All achieved a core temperature below 34 degrees C (mean target temperature 33.2 +/- 0.9 degrees C). The mean temperature at reperfusion was 34.7 +/- 0.9 degrees C. Cooling was well tolerated, with no hemodynamic instability or increase in arrhythmia. Nine patients experienced mild episodic shivering. Major adverse cardiac events occurred in 0% vs. 10% (p = NS) of treated versus control patients. The median infarct size was non-significantly smaller in patients who received cooling compared with the control group (2% vs. 8% of the left ventricle, p = 0.80).

CONCLUSIONS

Endovascular cooling can be performed safely as an adjunct to primary PCI for AMI. Further clinical trials are required to determine whether induction of mild systemic hypothermia with endovascular cooling will limit infarct size in patients undergoing reperfusion therapy.

Myocardial hypothermia: a potential therapeutic technique for acute regional myocardial ischemia

The importance of temperature in the development of necrosis after myocardial ischemia in the beating heart is becoming apparent. Recent studies have shown that the proportion of the ischemic risk zone that becomes necrotic is directly correlated with temperature. This fact suggests the potential therapeutic benefits of reducing myocardial temperature after coronary artery occlusion. We have shown in a number of experimental protocols in the rabbit model of myocardial infarction that topical regional hypothermia reduces infarct size even when instituted after coronary artery occlusion. The reduction in myocardial temperature required to obtain this benefit is modest ( 30 degrees C to 34 degrees C). Topical regional hypothermia allows targeted cooling of a zone of the heart. Myocardial cooling can also be achieved by perfusing the pericardial sac with a chilled fluid by using a closed-circuit catheter system that does not cause cardiac tamponade. This technique also protects myocardium during ischemia. Myocardial hypothermia might be a useful technique to limit ischemic damage during infarction or as adjunctive therapy during minimally invasive cardiac surgery.

Biochemical markers of myocardial injury during cardiac operations

The evaluation of myocardial damage in relation to cardiac operation from a clinical and a research perspective is of great importance, particularly for the evaluation of different cardioprotective strategies. Although measurements of serum biochemical markers have often been used, their value has been limited by their lack of sensitivity and specificity in the presence of skeletal muscle damage. A newer range of markers are now available that may reliably indicate both perioperative myocardial infarction, as well as more subtle degrees of subclinical myocyte injury. In this review, the application of biochemical markers for clinical and research purposes during cardiac operation is considered.

Effect of the potassium-channel opener nicorandil as an adjunct to cardioplegia on myocardial preservation in isolated rabbit hearts

We studied the effect of nicorandil on the hemodynamic, biochemical, and ultrastructural changes in rabbit hearts (n = 50) rendered cardioplegic with a single injection of Bretschneider's HTK solution over 30 min or 60 min at 37 degrees C or 15 degrees C, followed by reperfusion at 37 degrees C for 60 min. Particular attention was focused on the aspects of dose-response relationship, temperature sensitivity, and ischemic tolerance. Isolated hearts were prepared for modified Langendorff circulation using modified Krebs-Henseleit bicarbonate solution bubbled with a 95% O2(-5)% CO2 gas mixture, to which nicorandil (0, 0.1, 1, and 5 mM) was added. The optimal concentration of nicorandil was 1mM, which increased the recovery of left ventricular (LV) function, affecting coronary flow and the myocardial cyclic adenosine monophosphate, but not the myocardial concentrations of adenine nucleotide compounds or total calcium. These effects were abolished by the addition of glibenclamide to the HTK, but they were not diminished by a high potassium (K+) concentration of 20mM. The addition of nicorandil 1mM to the HTK at 15 degrees C did not improve the recovery of LV function. Our result suggested that nicorandil used adjunctly prevents LV functional depression after 30min, and possibly 60min of cardioplegia at 37 degrees C, and that this effect is not disturbed by a high K+ concentration up to 20 mM. However, nicorandil has temperature sensitivity whereby it loses its efficacy at 15 degrees C.

Continuous versus intermittent warm blood cardioplegia: functional and energetics changes

BACKGROUND

The aim of this study was to compare the protective effects of continuous warm blood cardioplegia (CWBC) and intermittent warm blood cardioplegia (IWBC) in an experimental model of blood-perfused, isolated rabbit heart.

METHODS

In the CWBC group, cardiac arrest was induced by continuous infusion of blood cardioplegia (10 mEq/L KCl) followed by 30 minutes of reperfusion with blood. In the IWBC group, after 5 minutes of perfusion with blood cardioplegia (10 mEq/L KCl), coronary flow was abolished for 10 minutes, followed by reperfusion with blood cardioplegia for 5 minutes. This sequence was repeated three times for a total period of 45 minutes. Finally the hearts were reperfused for 30 minutes with blood.

RESULTS

Infusion of potassium induced a marked increase in coronary perfusion pressure (from 50 +/- 3 to 98 +/- 1 mm Hg; p < 0.01), which remained elevated throughout in the CWBC group, whereas in the IWBC group, it dropped to 0 during each no-flow period. In both groups, cardioplegia resulted in a significant reduction in oxygen consumption (from 5.5 +/- 0.2 to 0.6 +/- 0.03 mL O2.min-1.100 g-1 wet wt; p < 0.01). During CWBC, glucose extraction was significantly reduced (from 152 +/- 10 to 64 +/- 18 micrograms.min-1.g-1 wet wt; p < 0.01). Free fatty acid uptake and creatine kinase and lactate release were not affected. During IWBC, in contrast, a transient but significant release of creatine kinase (from 643 +/- 254 to 2,234 +/- 296 mU.min-1.g-1 wet wt; p < 0.01) and lactate (from 63 +/- 22 to 374 +/- 32 micrograms.min-1.g-1 wet wt; p < 0.01) occurred after each period of ischemia. Despite these metabolic differences, both cardioplegic procedures allowed a prompt and complete recovery of mechanical function and tissue content of high-energy phosphates.

CONCLUSIONS

Both CWBC and IWBC exert optimal protection in the isolated blood perfused rabbit heart. Thus, IWBC can be safely used to improve visualization of the surgical field.

Effect of L-arginine on metabolic recovery of the ischemic myocardium

BACKGROUND

The release of nitric oxide is decreased after myocardial ischemia and reperfusion. Whereas the precursor L-arginine can stimulate the release of nitric oxide, its effect on metabolic recovery after myocardial ischemia is unknown.

METHODS

To study the effect of L-arginine on metabolic recovery after myocardial ischemia, cardioplegia infusion, and reperfusion, 33 dogs were placed on cardiopulmonary bypass and subjected to a sequence of 30 minutes of normothermic global ischemia, 30 minutes of warm blood cardioplegic arrest, and 30 minutes of reperfusion. A pH probe was inserted in the anterior wall of the left ventricle, and tissue pH was measured throughout the experiment. Coronary blood flow in the left anterior descending coronary artery and the circumflex coronary artery was measured. Blood samples from the coronary sinus were taken to measure blood pH and levels of lactate, creatine kinase, and troponin T.

RESULTS

In the control group of 9 dogs, tissue pH averaged 6.4 +/- 0.1, 6.5 +/- 0.1, and 6.8 +/- 0.1 after the end of global ischemia, cardioplegia, and reperfusion, respectively. Tissue pH averaged 6.4 +/- 0.1, 6.6 +/- 0.1, and 6.9 +/- 0.1, respectively, in the experimental group of 9 animals with 2 mmol/L of L-arginine added to the cardioplegic solution. Tissue pH averaged 6.2 +/- 0.1, 6.7 +/- 0.1, 7.1 +/- 0.1, respectively, in the third group of 9 animals that received an additional infusion of L-arginine (10 mg.kg-1.min-1) during reperfusion. Tissue pH recovered faster in groups with L-arginine (p = 0.00001). A hyperemic response of coronary blood flow was shown at reperfusion in animals in the control group only. In 6 dogs, L-NAME (N-nitroarginine methyl ester), an inhibitor of nitric oxide synthesis, was injected and resulted in a slower pH recovery on reperfusion compared with that of animals that received L-arginine.

CONCLUSIONS

The addition of L-arginine to the cardioplegic solution and the systemic circulation during reperfusion resulted in a significant increase in coronary blood flow during cardioplegia infusion and in a faster recovery of myocardial tissue pH, possibly by increasing coronary blood flow through the release of nitric oxide.

Intermittent warm blood cardioplegia. Warm Heart Investigators

BACKGROUND

Warm heart surgery implies continuous perfusion with normothermic blood cardioplegia. Interruption of cardioplegia, however, facilitates construction of distal coronary anastomoses and is the method practiced by many surgeons. To determine whether intermittency is harmful, we present results from 720 coronary bypass patients, protected with intermittent antegrade warm blood cardioplegia, that were derived from a previous study of normothermic versus hypothermic cardioplegia.

METHODS AND RESULTS

Mean +/- SD age was 60.8 +/- 9.0 years; 27% of cases were urgent; 16% of patients had > 50% left main stenosis, and 19% had grade III or IV ventricles. A mean of 3.2 +/- 0.9 grafts was constructed. The average aortic cross-clamp time was 61.8 +/- 22.2 minutes. The longest single time off cardioplegia (LTOC) averaged 11.4 +/- 4.0 minutes per patient. The cumulative time off cardioplegia as a percentage of the cross-clamp time (PTOC) was 48.2 +/- 18.6% per patient. LTOC and PTOC were divided into quartiles (LTOC, < 10, 10 to 11, 12 to 13, and > 13 minutes; PTOC, < 36%, 36% to 49%, 50% to 62%, and > 62%) and related to the prespecified composite outcome of mortality, myocardial infarction according to serial CK-MB sampling, and low-output syndrome (LOS). Longer LTOC was harmful (event rates per quartile, 13.5%, 10.3%, 10.9%, and 19.0%; P = .046), whereas longer PTOC was protective (16.1%, 17.2%, 9.4%, and 10.6%; P = .07). Stepwise logistic regression was performed, controlling for demographic and angiographic predictors. In the multivariate models, LTOC remained detrimental (P = .07) and PTOC remained beneficial (P = .053). Additional modeling after entering surgeon identity (P < .001) into the risk equation eliminated the PTOC effect, whereas LTOC remained predictive of adverse outcomes (P = .053; odds ratio, 1.06; 95% CI, 1.00, 1.13).

CONCLUSIONS

The data indicate that a reasonable margin of safety exists with intermittent, antegrade warm blood cardioplegia. Repeated interruptions of warm blood cardioplegia are unlikely to lead to adverse clinical results if single interruptions are < or = 13 minutes.

Intermittent antegrade warm versus cold blood cardioplegia: a prospective, randomized study

A prospective, randomized study was performed in 200 patients undergoing coronary artery bypass grafting to compare the myocardial protection obtained with intermittent antegrade warm versus cold blood cardioplegia. Preoperative and surgical characteristics of the two cohorts were similar. Intermittent antegrade infusion of warm blood cardioplegia failed to achieve sustained electromechanical arrest of the heart in 13%. The only difference in clinical outcomes was the more frequent spontaneous return to sinus rhythm after the unclamping of the aorta in the warm group (88% versus 70%, p = 0.002). Mortality (1% each) and myocardial infarction (2% and 4%) rates were similar. Rates of increase in serum activity of the isoenzyme of creatine kinase (CK-MB), CK-MB mass concentration, and cardiac troponin-T level as well as total release of troponin T were significantly lower in the warm group, and fewer patients in this group had a clinically significant increase in serum CK-MB mass (20% versus 39%, p = 0.005) and troponin T (20% versus 56%, p = 0.00001). Thus, intermittent antegrade warm blood cardioplegia is appropriate and clinically safe; the lower release of biochemical markers of myocardial damage suggests improved protection during first-time coronary artery bypass grafting.