Laboratory Evaluation of In t raoperative Mvocardial Protection d
The Need f o r Appropriate A?iiviial Models Bruce A. Reitz, M.D.
ABSTRACT The investigative laboratory is the appropriate place to evaluate methods of intraoperative myocardial protection. It is becoming apparent that diseased myocardium is more susceptible to intraoperative injury than is normal heart muscle, and so animal models that simulate human cardiac disease states such as chronic left ventricular hypertrophy should be used to assess various techniques. Long-term studies on animals surviving operation are necessary in order to evaluate the chronic effects of various methods of protection.
I
n o r d e r to be successful, an open-heart operation must correct a specific anatomical defect and at the same time should not cause any impairment of preexisting cardiac function. T h e best method of protecting the heart during any particular operation has not yet been proved, and most of the techniques presently used have been developed in a clinical situation. Similarly, almost all long-term studies following cardiopulmonary bypass have been performed in patients, with the inherent limitations present in any clinical assessment of cardiac function and morphology. New methods of myocardial protection are currently being developed, and the investigative laboratory is the appropriate location for these innovations to be evaluated. The best model for assessing intraoperative myocardial protection varies between laboratories and depends on available facilities, veterinary support, the expertise of a particular investigator, and financial resources. Although many of the great advances in cardiac physiology and biochemistry have been made using small animal preparations, large animals (usually dogs but occasionally calves, pigs, o r primates) are necessary to provide a model upon which cardiac surgical procedures similar to those performed clinically can be carried out.
Ventricular Hypertrophy One of the most important considerations in evaluating injury to heart muscle duringan operation is the state of the muscle before the procedure. Hypertrophic myocardium has been demonstrated to be much more susceptible than normal myocardium to the detrimental effects of anoxia, ischemia, and ventricular fibrillation. From the Clinic of Surgery, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. Present address: Division of Cardiovascular Surgery, Stanford University Medical Center, Palo Alto, Calif. 94305. Address reprint requests to the Clinic of Surgery, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014. VOL. 20, NO. 1, JULY, 1975
7
REITZ Levii:sky and colleagues [lo] compared normal dogs with dogs in which supravalvular aortic constriction had been created. In short-term right heart bypass studies with ischemic arrest for 60 minutes, they noted marked diminution of function in all animals but a 20% greater depression in the dogs with chronic left ventricular outflow obstruction. Hottenrott and associates [7] presented a careful study of normal dogs and dogs in which left ventricular hypertrophy had been previously created by supravalvular aortic banding. They showed that in spite of adequate coronary artery perfusion, normal and hypertrophied hearts behave differently during ventricular fibrillation. In contrast to normal dog hearts, oxygen consumption in the hypertrophied hearts failed to increase, vascular resistance rose progressively, and biochemical evidence of severe ischemia developed. This was due to relative underperfusion of the subendocardium in the thicker hearts. These studies suggest that intraoperative methods of protection should first be assessed in animal hearts that simulate various disease states in man.
Models Vari’Dusmethods for creating left ventricular hypertrophy in animals have been proposed, including techniques in which a pressure load is imposed by obstructing left ventricular outflow or in which a volume load is created. Supravalvular aortic constriction is one of the simplest and most widely used means of inducing ventricular hypertrophy [5, 7, 8, 10, 111. It has the advantage of not violating the pericardial sac, but erosion of the band through the aorta is a common complication, especially in dogs, and the dynamics of coronary blood flow are probably different than in valvular aortic stenosis. Using inflow occlusion techniques, metal clips can be placed on the commissures of the aortic valve to create valvular stenosis [4];this produces a predictable course of ventricular hypertrophy but is technically more difficult than supravalvular banding. In some laboratories, dogs with congenital left ventricular outflow obstruction are available. Ventricular hypertrophy and dilatation are produced by volume-loading techniques, including the creation of mitral regurgitation by tearing chordae [6], perforation of one or more cusps of the aortic valve to produce chronic regurgitation [9, 121, and creation of a large peripheral arteriovenous fistula [13]. Right ventricu1,ar hypertrophy can be produced by pulmonary artery banding with o r without creation of tricuspid regurgitation [ 11. The same variations in subendocardial perfusion occurring on the left side of the heart can be seen in the hypertrophied right ventricle. A laboratory model of irreversible left ventricular rigor (the “stone heart” [3]) has also been reported [2]. Proof of the quality of protection afforded by any particular method will depend upon long-term observations. Evaluation should include ventricular function as well as histological appearance. An ongoing study in our laboratory has demonstrated marked differences in cardiac function and histology six months after anoxic arrest as compared with hearts protected by topical hypothermia or coronary artery perfusion [2al. 8
T H E ANNALS OF THORACIC SURGERY
Laboratory Evaluation
It is clear that damage secondary to anoxia or ischemia, which is seen in normal hearts, will be accentuated in hypertrophic hearts. We suggest therefore that methods of protecting the myocardium during operation should be evaluated on models that mimic human disease states as closely as possible. In this way, laboratory results can be more specifically applied to clinical situations.
Discussion (Drs. Buckberg, Behrendt, Levitsky, Archie, Reitz, Guyton, Morgan, Feinberg, and Morrow) Volume- and pressure-loaded ventricles appear to behave differently after valve replacement. For example, in mitral replacement for mitral regurgitation, a high afterload is imposed that was not present previously. After valve replacement this dilated, hypertrophied heart is not similar hemodynamically to the purely hypertrophic left ventricle seen in aortic stenosis. There are practical problems of cost and time associated with producing these models at most laboratories. A good model should mimic a particular disease, allow the majority of animals to survive, and have a predictable outcome. Adult dogs subjected to aortic constriction for a short period to achieve a gradient over 30 mm Hg have a difficult time surviving (Behrendt).A puppy is a better animal to band than an adult dog because it will “grow into” the stenosis and attain greater hypertrophy (Buckberg). In adult dogs (average 20 kg), an abdominal aorta-toinferior vena cava fistula 10 to 12 mm in size will produce moderate hypertrophy in six to eight weeks if the high-output heart failure is tolerated. This technique obviates the need for thoracotomy, and the fistula can be repaired easily (Reitz). The simplest method of assessing the degree of hypertrophy is to compare left or right ventricular weight, or both, to total body weight, remembering that a specific insult may change heart weight by producing cellular edema. A note of caution should be introduced about the term hypertrophy. This lesion evolves over a period of time, and a variety of mechanical and biochemical changes occur during its various stages of development. For example, there is an initial period of improvement in mitochondria1 function that later levels off (Feinberg). It is important to try to evaluate hearts at a comparable state of hypertrophy, and therefore it is imprecise to state that “hypertrophied hearts” are studied. The duration and degree of hypertrophy must be specified. Another area of study should be the chronically ischemic heart seen in advanced coronary artery disease. This heart, although not hypertrophied, is also subject to injury during operation, and suitable studies should be carried out with laboratory models. From a metabolic point of view the terms anoxia and bchemia are not synonymous and it is important that a distinction be made between the two. Reasonable coronary flow can be maintained in small animals with no oxygen (anoxia) or reduced oxygen (hypoxia) delivery. In these situations, metabolic products are washed out and a heart can be maintained with reasonable function for quite a long time. If substrate is provided, adenosine triphosphate (ATP) will be formed. Ischemia means a reduction in flow and, with it, corresponding oxygen VOL. 20, NO. 1 , JULY, 1975
9
REITZ deficiency. From a biochemical standpoint this is quite a different set of circumstances, because the products of glycolysis accumulate and the ability to make ATP is reduced (Morgan). Several investigators have found that oxygen consumption per gram of tissue is greater in hypertrophied hearts than in normal ones. This could partially account for the increased vulnerability to ischemia in hypertrophy (Buckberg). The mechanism of increased oxygen consumption is unknown but might be due to stresses induced by catecholamine activity (Feinberg) or to mechanical factors such as increased ventricular wall tension, which requires more ATP (Morgan).
Reference S 1 . Archie, J . P., Fixler, D. E., and Ullyot, D. J. Regional myocardial blood flow in lambs with concentric right ventricular hypertrophy. Circ Res 34: 143, 1974. 2. Armstrong, R. G., Stanford, W., Cline, R. E., and Guillebeau, J. T h e stone heart: Development of a canine model, with ultrastructural survey and metabolic manipulation. A m Thorac Surg 16:480, 1973. 2a. Brody, W. R., Reitz, B. A., Andrews, M. J., Roberts, W. C., and Michaelis, L. L. Long-term morphologic and hemodynamic evaluation of the left ventricle after cardiopulinonary bypass: A comparison of normothermic anoxic arrest, coronary artery perfusion, and profound topical hypothermia. J Thorac Cardiovasc Surg. (In press.) 3. Cooley, D. A., Reul, G. J., and Wukasch, D. C. Ischemic contracture of the heart: “Stone heart.” A m J Cardiol 29:575, 1972. 4. Copeland, J. G., Maron, B. J., Luka, N. L., Ferrans, V. J., and Michaelis, L. L. Experimental production of aortic valvular stenosis: Short term and long term studies in dogs. J Thorac Cardiovasc Surg 67:37 1 , 1974. 5. Geha, A. S., Malik, A. B., Abe, T., and O’Kane, H. 0. Response of the hypertrophied heart ts3 stress. Surgery 74:276, 1973. 6. Haller, J . A., and Morrow, A. G. Experimental mitral insufficiency: An operative method with chronic survival. Ann Surg 142:37, 1955. 7. Hottenrott, C. E., Towers, B., Kurkji, H. J., Maloney, J. V., and Buckberg, G. T h e hazard o f ventricular fibrillation in hypertrophied ventricles during cardiopulmonary bypass. J Thorac Cardiovasc Surg 66:742, 1973. 8. Iyengar, S. R. K., Ramchand, S., Charrette, E. J. P., Iyengar, C. K. S., and Lynn, R. B. Anoxic cardiac arrest: An experimental and clinical study of its effects.J Thorac Cardiotasc Surg 66:722, 1973. 9. Karp, I
The Need f o r Appropriate A?iiviial Models Bruce A. Reitz, M.D.
ABSTRACT The investigative laboratory is the appropriate place to evaluate methods of intraoperative myocardial protection. It is becoming apparent that diseased myocardium is more susceptible to intraoperative injury than is normal heart muscle, and so animal models that simulate human cardiac disease states such as chronic left ventricular hypertrophy should be used to assess various techniques. Long-term studies on animals surviving operation are necessary in order to evaluate the chronic effects of various methods of protection.
I
n o r d e r to be successful, an open-heart operation must correct a specific anatomical defect and at the same time should not cause any impairment of preexisting cardiac function. T h e best method of protecting the heart during any particular operation has not yet been proved, and most of the techniques presently used have been developed in a clinical situation. Similarly, almost all long-term studies following cardiopulmonary bypass have been performed in patients, with the inherent limitations present in any clinical assessment of cardiac function and morphology. New methods of myocardial protection are currently being developed, and the investigative laboratory is the appropriate location for these innovations to be evaluated. The best model for assessing intraoperative myocardial protection varies between laboratories and depends on available facilities, veterinary support, the expertise of a particular investigator, and financial resources. Although many of the great advances in cardiac physiology and biochemistry have been made using small animal preparations, large animals (usually dogs but occasionally calves, pigs, o r primates) are necessary to provide a model upon which cardiac surgical procedures similar to those performed clinically can be carried out.
Ventricular Hypertrophy One of the most important considerations in evaluating injury to heart muscle duringan operation is the state of the muscle before the procedure. Hypertrophic myocardium has been demonstrated to be much more susceptible than normal myocardium to the detrimental effects of anoxia, ischemia, and ventricular fibrillation. From the Clinic of Surgery, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. Present address: Division of Cardiovascular Surgery, Stanford University Medical Center, Palo Alto, Calif. 94305. Address reprint requests to the Clinic of Surgery, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014. VOL. 20, NO. 1, JULY, 1975
7
REITZ Levii:sky and colleagues [lo] compared normal dogs with dogs in which supravalvular aortic constriction had been created. In short-term right heart bypass studies with ischemic arrest for 60 minutes, they noted marked diminution of function in all animals but a 20% greater depression in the dogs with chronic left ventricular outflow obstruction. Hottenrott and associates [7] presented a careful study of normal dogs and dogs in which left ventricular hypertrophy had been previously created by supravalvular aortic banding. They showed that in spite of adequate coronary artery perfusion, normal and hypertrophied hearts behave differently during ventricular fibrillation. In contrast to normal dog hearts, oxygen consumption in the hypertrophied hearts failed to increase, vascular resistance rose progressively, and biochemical evidence of severe ischemia developed. This was due to relative underperfusion of the subendocardium in the thicker hearts. These studies suggest that intraoperative methods of protection should first be assessed in animal hearts that simulate various disease states in man.
Models Vari’Dusmethods for creating left ventricular hypertrophy in animals have been proposed, including techniques in which a pressure load is imposed by obstructing left ventricular outflow or in which a volume load is created. Supravalvular aortic constriction is one of the simplest and most widely used means of inducing ventricular hypertrophy [5, 7, 8, 10, 111. It has the advantage of not violating the pericardial sac, but erosion of the band through the aorta is a common complication, especially in dogs, and the dynamics of coronary blood flow are probably different than in valvular aortic stenosis. Using inflow occlusion techniques, metal clips can be placed on the commissures of the aortic valve to create valvular stenosis [4];this produces a predictable course of ventricular hypertrophy but is technically more difficult than supravalvular banding. In some laboratories, dogs with congenital left ventricular outflow obstruction are available. Ventricular hypertrophy and dilatation are produced by volume-loading techniques, including the creation of mitral regurgitation by tearing chordae [6], perforation of one or more cusps of the aortic valve to produce chronic regurgitation [9, 121, and creation of a large peripheral arteriovenous fistula [13]. Right ventricu1,ar hypertrophy can be produced by pulmonary artery banding with o r without creation of tricuspid regurgitation [ 11. The same variations in subendocardial perfusion occurring on the left side of the heart can be seen in the hypertrophied right ventricle. A laboratory model of irreversible left ventricular rigor (the “stone heart” [3]) has also been reported [2]. Proof of the quality of protection afforded by any particular method will depend upon long-term observations. Evaluation should include ventricular function as well as histological appearance. An ongoing study in our laboratory has demonstrated marked differences in cardiac function and histology six months after anoxic arrest as compared with hearts protected by topical hypothermia or coronary artery perfusion [2al. 8
T H E ANNALS OF THORACIC SURGERY
Laboratory Evaluation
It is clear that damage secondary to anoxia or ischemia, which is seen in normal hearts, will be accentuated in hypertrophic hearts. We suggest therefore that methods of protecting the myocardium during operation should be evaluated on models that mimic human disease states as closely as possible. In this way, laboratory results can be more specifically applied to clinical situations.
Discussion (Drs. Buckberg, Behrendt, Levitsky, Archie, Reitz, Guyton, Morgan, Feinberg, and Morrow) Volume- and pressure-loaded ventricles appear to behave differently after valve replacement. For example, in mitral replacement for mitral regurgitation, a high afterload is imposed that was not present previously. After valve replacement this dilated, hypertrophied heart is not similar hemodynamically to the purely hypertrophic left ventricle seen in aortic stenosis. There are practical problems of cost and time associated with producing these models at most laboratories. A good model should mimic a particular disease, allow the majority of animals to survive, and have a predictable outcome. Adult dogs subjected to aortic constriction for a short period to achieve a gradient over 30 mm Hg have a difficult time surviving (Behrendt).A puppy is a better animal to band than an adult dog because it will “grow into” the stenosis and attain greater hypertrophy (Buckberg). In adult dogs (average 20 kg), an abdominal aorta-toinferior vena cava fistula 10 to 12 mm in size will produce moderate hypertrophy in six to eight weeks if the high-output heart failure is tolerated. This technique obviates the need for thoracotomy, and the fistula can be repaired easily (Reitz). The simplest method of assessing the degree of hypertrophy is to compare left or right ventricular weight, or both, to total body weight, remembering that a specific insult may change heart weight by producing cellular edema. A note of caution should be introduced about the term hypertrophy. This lesion evolves over a period of time, and a variety of mechanical and biochemical changes occur during its various stages of development. For example, there is an initial period of improvement in mitochondria1 function that later levels off (Feinberg). It is important to try to evaluate hearts at a comparable state of hypertrophy, and therefore it is imprecise to state that “hypertrophied hearts” are studied. The duration and degree of hypertrophy must be specified. Another area of study should be the chronically ischemic heart seen in advanced coronary artery disease. This heart, although not hypertrophied, is also subject to injury during operation, and suitable studies should be carried out with laboratory models. From a metabolic point of view the terms anoxia and bchemia are not synonymous and it is important that a distinction be made between the two. Reasonable coronary flow can be maintained in small animals with no oxygen (anoxia) or reduced oxygen (hypoxia) delivery. In these situations, metabolic products are washed out and a heart can be maintained with reasonable function for quite a long time. If substrate is provided, adenosine triphosphate (ATP) will be formed. Ischemia means a reduction in flow and, with it, corresponding oxygen VOL. 20, NO. 1 , JULY, 1975
9
REITZ deficiency. From a biochemical standpoint this is quite a different set of circumstances, because the products of glycolysis accumulate and the ability to make ATP is reduced (Morgan). Several investigators have found that oxygen consumption per gram of tissue is greater in hypertrophied hearts than in normal ones. This could partially account for the increased vulnerability to ischemia in hypertrophy (Buckberg). The mechanism of increased oxygen consumption is unknown but might be due to stresses induced by catecholamine activity (Feinberg) or to mechanical factors such as increased ventricular wall tension, which requires more ATP (Morgan).
Reference S 1 . Archie, J . P., Fixler, D. E., and Ullyot, D. J. Regional myocardial blood flow in lambs with concentric right ventricular hypertrophy. Circ Res 34: 143, 1974. 2. Armstrong, R. G., Stanford, W., Cline, R. E., and Guillebeau, J. T h e stone heart: Development of a canine model, with ultrastructural survey and metabolic manipulation. A m Thorac Surg 16:480, 1973. 2a. Brody, W. R., Reitz, B. A., Andrews, M. J., Roberts, W. C., and Michaelis, L. L. Long-term morphologic and hemodynamic evaluation of the left ventricle after cardiopulinonary bypass: A comparison of normothermic anoxic arrest, coronary artery perfusion, and profound topical hypothermia. J Thorac Cardiovasc Surg. (In press.) 3. Cooley, D. A., Reul, G. J., and Wukasch, D. C. Ischemic contracture of the heart: “Stone heart.” A m J Cardiol 29:575, 1972. 4. Copeland, J. G., Maron, B. J., Luka, N. L., Ferrans, V. J., and Michaelis, L. L. Experimental production of aortic valvular stenosis: Short term and long term studies in dogs. J Thorac Cardiovasc Surg 67:37 1 , 1974. 5. Geha, A. S., Malik, A. B., Abe, T., and O’Kane, H. 0. Response of the hypertrophied heart ts3 stress. Surgery 74:276, 1973. 6. Haller, J . A., and Morrow, A. G. Experimental mitral insufficiency: An operative method with chronic survival. Ann Surg 142:37, 1955. 7. Hottenrott, C. E., Towers, B., Kurkji, H. J., Maloney, J. V., and Buckberg, G. T h e hazard o f ventricular fibrillation in hypertrophied ventricles during cardiopulmonary bypass. J Thorac Cardiovasc Surg 66:742, 1973. 8. Iyengar, S. R. K., Ramchand, S., Charrette, E. J. P., Iyengar, C. K. S., and Lynn, R. B. Anoxic cardiac arrest: An experimental and clinical study of its effects.J Thorac Cardiotasc Surg 66:722, 1973. 9. Karp, I
