EDITORIALS
Aspects of Myocardial Protection Paul A. Ebert, M.D. The attention paid to myocardial protection is clearly improving overall operative results and appears to be providing patients with better cardiac performance after open-heart surgery. Three articles in this issue deal with different aspects of myocardial protection, and certain observations can be made. The research work of Scott and associates (p 507) emphasizes that there are limits to myocardial protection and that, as one approaches the safe limits, individual variations in cardiac reserve make it difficult to predict what can be anticipated from any technique of protection. One of the difficulties in evaluating myocardial protection and the benefits or disadvantages of chemical cardioplegia is the inconsistent role that temperature has been given in many methods and studies. For instance, in the work by Scott's group, hearts and solutions were maintained at 30°C, which in itself provides a degree of myocardial protection; but that protocol cannot clearly demonstrate whether potassium supplements or further temperature reduction alone led to the improved results. Yet it is clear that the benefit was greater when potassium was added to the solutions than when simple aortic cross-clamping at a similar temperature was used. The work of Lolley and associates (p 515) clearly shows that when one pays more attention to myocardial protection, one is able to reduce both the incidence of perioperative infarction and the degree of myocardial insult that occurs. Here again, one has to question whether exogenous anaerobic substrate enhancement is the benefit or whether continuous cold perfusion with solution at 4°C achieves better overall myocardial protection. It would seem that the difference in groups in this study was also a result of temperature differential as well as substrate provision. Although continuous perfusion that produces From the Department of Surgery, University of California, San Francisco, School of Medicine, San Francisco, CA 94143.
495 0003-497517810026-0601$01.25 @ 1978 by Paul A. Ebert
an anaerobic substrate, coronary washout, and elution of lactic acidosis is important, the concept of uniform global hypothermia and maintenance of low temperature during aortic cross-clamping may be the most important aspect of these results. Different techniques of myocardial protection are evaluated by Behrendt and associates (p 499), who compared the use of cold cardioplegic solution versus the technique of intermittent cross-clamping at 28°C. In other groups, continuous coronary perfusion with and without the heart beating has not appeared to give significantly different results. In the assessment by Behrendt's group it is clear that patients who received injections of cold cardioplegic solution into the aortic root had less change in their maximum contractile element velocity after the aorta was unclamped compared with the intermittent cross-clamp group. Thus, although many newer solutions and techniques are being proposed, certain facts seem to be emerging as fairly constant. The concentration of potassium or other metabolic components of the cardioplegia solution appears to be less important than proper application of techniques and correct maintenance of hypothermia. It seems clear that the main role of potassium is to stop the heart rapidly before its metabolic stores are depleted; that of hypothermia is to maintain the heart with very low energy requirements during anoxia. From our own experience, it seems that as long as the potassium concentration of the solution is less than 40 mEq per liter, the likelihood that the potassium concentration per se will produce any myocardial damage is small. In fact, raising the potassium concentration above 25 mEq per liter does not stop the heart more quickly. It is unclear whether continued perfusion with potassium is supplementary or not, but if cardiac electrical activity returns, then additional potassium perfusion is probably beneficial. The longer the heart is anoxic before a cardioplegic solution is introduced, the more the
496 The Annals of Thoracic Surgery Vol 26
No 6 December 1978
myocardial metabolic storehouse is depleted. Thus, hypothermia applied at this time may be effective in lowering temperature, but it may be reducing metabolic requirements in myocardium already depleted of energy substrate. Therefore the techniques applied during the operation are of importance in cardioplegia. I consider it extremely important to administer the cardioplegic solution as soon after aortic cross-clamping as possible. Thus, the heart is arrested very quickly with the cold potassium solution, and is not allowed to beat without coronary perfusion and thus deplete its metabolic reserves. The epicardial temperature obtained with either surface or perfusion cooling may vary little. However, differential temperature measurements across the myocardium clearly show that the advantage of perfusing the coronary arteries with a cold solution is that a more rapid and even distribution of cooling across all layers of the myocardium is achieved. Once the heart has been cooled by perfusion techniques, application of cold to its surface frequently helps to maintain myocardial temperature and will keep the heart at the same level through all layers. However, when surface cooling alone is used, the endocardium undergoes considerable metabolic depletion before total arrest is achieved. Monitoring myocardial temperature with a small probe placed through the wall into the septum is of value, and one finds that the myocardial temperature can be maintained at 20" to 22°C. Sometimes differential cooling of the heart-meaning that the atrium is warmer than the ventricle-is accomplished, so that atrial activity is resumed even though ventricular septa1 temperature is low. This obviously indicates metabolic activity of both conduction and myocardial tissue in the atrium, and additional cold potassium solution should be applied to reduce atrial activity. Myocardial temperature monitoring is of value, but when electrical activity resumes, one must assume that areas of the myocardium not being
monitored are warming; thus, it is necessary to reinfuse the cold cardioplegic solution both to stop electrical activity and to reduce metabolic rate. Another point emphasized in the article by Behrendt and associates, which previously appeared in the literature and should be reiterated here, is that the heart seems to tolerate one period of cardiac arrest with hypothermia better than intermittent periods of aortic occlusion even if the total duration of single arrest exceeds the summation of the periods of intermittent arrest. Thus, the findings that reinfusion of blood to the dilated, "possibly unprotected," vascular beds causes injury and that three or four such insults are more detrimental than a single one after a prolonged period of arrest seem to be emerging as facts. Continued interest and work in cardioplegia will certainly improve the results of cardiac surgery and leave more patients with better postoperative myocardial performance than in the past. It is presently unclear what the safe period of myocardial protection during cardiac arrest actually is and where the safety point ends for each individual heart. The large number of solutions and components recommended need to be separately evaluated since many are probably closer to personal preferences than established entities. Cold blood may still prove to be an excellent solution and to accomplish as much protection as many of the specialized solutions. It may be, though, that none of these measures is as important as the techniques of their application, and thus the safe period of arrest probably varies with each surgeon. However, the principles of stopping the heart rapidly before anoxia occurs, reducing the temperature by perfusion cooling with solutions in the range of 4" to 1O"C, and maintaining cardiac temperature in the range of 20" to 23°C during arrest appear to be valid and of more importance than the specific ionic concentration of the perfusate.
Aspects of Myocardial Protection Paul A. Ebert, M.D. The attention paid to myocardial protection is clearly improving overall operative results and appears to be providing patients with better cardiac performance after open-heart surgery. Three articles in this issue deal with different aspects of myocardial protection, and certain observations can be made. The research work of Scott and associates (p 507) emphasizes that there are limits to myocardial protection and that, as one approaches the safe limits, individual variations in cardiac reserve make it difficult to predict what can be anticipated from any technique of protection. One of the difficulties in evaluating myocardial protection and the benefits or disadvantages of chemical cardioplegia is the inconsistent role that temperature has been given in many methods and studies. For instance, in the work by Scott's group, hearts and solutions were maintained at 30°C, which in itself provides a degree of myocardial protection; but that protocol cannot clearly demonstrate whether potassium supplements or further temperature reduction alone led to the improved results. Yet it is clear that the benefit was greater when potassium was added to the solutions than when simple aortic cross-clamping at a similar temperature was used. The work of Lolley and associates (p 515) clearly shows that when one pays more attention to myocardial protection, one is able to reduce both the incidence of perioperative infarction and the degree of myocardial insult that occurs. Here again, one has to question whether exogenous anaerobic substrate enhancement is the benefit or whether continuous cold perfusion with solution at 4°C achieves better overall myocardial protection. It would seem that the difference in groups in this study was also a result of temperature differential as well as substrate provision. Although continuous perfusion that produces From the Department of Surgery, University of California, San Francisco, School of Medicine, San Francisco, CA 94143.
495 0003-497517810026-0601$01.25 @ 1978 by Paul A. Ebert
an anaerobic substrate, coronary washout, and elution of lactic acidosis is important, the concept of uniform global hypothermia and maintenance of low temperature during aortic cross-clamping may be the most important aspect of these results. Different techniques of myocardial protection are evaluated by Behrendt and associates (p 499), who compared the use of cold cardioplegic solution versus the technique of intermittent cross-clamping at 28°C. In other groups, continuous coronary perfusion with and without the heart beating has not appeared to give significantly different results. In the assessment by Behrendt's group it is clear that patients who received injections of cold cardioplegic solution into the aortic root had less change in their maximum contractile element velocity after the aorta was unclamped compared with the intermittent cross-clamp group. Thus, although many newer solutions and techniques are being proposed, certain facts seem to be emerging as fairly constant. The concentration of potassium or other metabolic components of the cardioplegia solution appears to be less important than proper application of techniques and correct maintenance of hypothermia. It seems clear that the main role of potassium is to stop the heart rapidly before its metabolic stores are depleted; that of hypothermia is to maintain the heart with very low energy requirements during anoxia. From our own experience, it seems that as long as the potassium concentration of the solution is less than 40 mEq per liter, the likelihood that the potassium concentration per se will produce any myocardial damage is small. In fact, raising the potassium concentration above 25 mEq per liter does not stop the heart more quickly. It is unclear whether continued perfusion with potassium is supplementary or not, but if cardiac electrical activity returns, then additional potassium perfusion is probably beneficial. The longer the heart is anoxic before a cardioplegic solution is introduced, the more the
496 The Annals of Thoracic Surgery Vol 26
No 6 December 1978
myocardial metabolic storehouse is depleted. Thus, hypothermia applied at this time may be effective in lowering temperature, but it may be reducing metabolic requirements in myocardium already depleted of energy substrate. Therefore the techniques applied during the operation are of importance in cardioplegia. I consider it extremely important to administer the cardioplegic solution as soon after aortic cross-clamping as possible. Thus, the heart is arrested very quickly with the cold potassium solution, and is not allowed to beat without coronary perfusion and thus deplete its metabolic reserves. The epicardial temperature obtained with either surface or perfusion cooling may vary little. However, differential temperature measurements across the myocardium clearly show that the advantage of perfusing the coronary arteries with a cold solution is that a more rapid and even distribution of cooling across all layers of the myocardium is achieved. Once the heart has been cooled by perfusion techniques, application of cold to its surface frequently helps to maintain myocardial temperature and will keep the heart at the same level through all layers. However, when surface cooling alone is used, the endocardium undergoes considerable metabolic depletion before total arrest is achieved. Monitoring myocardial temperature with a small probe placed through the wall into the septum is of value, and one finds that the myocardial temperature can be maintained at 20" to 22°C. Sometimes differential cooling of the heart-meaning that the atrium is warmer than the ventricle-is accomplished, so that atrial activity is resumed even though ventricular septa1 temperature is low. This obviously indicates metabolic activity of both conduction and myocardial tissue in the atrium, and additional cold potassium solution should be applied to reduce atrial activity. Myocardial temperature monitoring is of value, but when electrical activity resumes, one must assume that areas of the myocardium not being
monitored are warming; thus, it is necessary to reinfuse the cold cardioplegic solution both to stop electrical activity and to reduce metabolic rate. Another point emphasized in the article by Behrendt and associates, which previously appeared in the literature and should be reiterated here, is that the heart seems to tolerate one period of cardiac arrest with hypothermia better than intermittent periods of aortic occlusion even if the total duration of single arrest exceeds the summation of the periods of intermittent arrest. Thus, the findings that reinfusion of blood to the dilated, "possibly unprotected," vascular beds causes injury and that three or four such insults are more detrimental than a single one after a prolonged period of arrest seem to be emerging as facts. Continued interest and work in cardioplegia will certainly improve the results of cardiac surgery and leave more patients with better postoperative myocardial performance than in the past. It is presently unclear what the safe period of myocardial protection during cardiac arrest actually is and where the safety point ends for each individual heart. The large number of solutions and components recommended need to be separately evaluated since many are probably closer to personal preferences than established entities. Cold blood may still prove to be an excellent solution and to accomplish as much protection as many of the specialized solutions. It may be, though, that none of these measures is as important as the techniques of their application, and thus the safe period of arrest probably varies with each surgeon. However, the principles of stopping the heart rapidly before anoxia occurs, reducing the temperature by perfusion cooling with solutions in the range of 4" to 1O"C, and maintaining cardiac temperature in the range of 20" to 23°C during arrest appear to be valid and of more importance than the specific ionic concentration of the perfusate.
