- Email: [email protected]
Complete recovery of the heart following exposure to profound hypothermia
J THoRAc
CARDIOVASC SURG
81:455-458, 1981
Complete recovery of the heart following exposure to profound hypothermia Cold injury has been suggested as a potential limitation to the use of temperatures below 10° to 15° C in clinical myocardial preservation. The isolated effects of profound hypothermia on myocardial function and energy metabolism were studied in the working rat heart preparation. Each heart was isolated and stabilized; then initial aortic flow, coronary flow, and heart rate were measured. The heart then was perfused in the Langendorf mode with oxygenated Krebs-Henseleit buffer for 20 minutes at 0.5°. 4°, 10°, ISo, or 20 0 C. After being rewarmed to 37" C. the heart was returned to the working mode for final functional measurements. In a control group. the perfusion temperature was kept at 37° C. Recovery of function in hearts exposed to hypothermic perfusion was not significantly different from that observed in the hearts kept at 37° C. When cold exposure time to 0.5 0 C perfusion was extended to 2 hours, heart function still returned to the same level as that of control hearts maintained at 37 0 C. and adenosine triphosphate (ATP) and glycogen levels were higher than those in the control group. Thus, under these conditions, cold exposure per se, even for 2 hours at temperatures near 0° C, has no deleterious effect upon myocardial function and energy metabolism.
B. William Shragge, M.D.,* Stanley B. Digerness, Ph.D., and Eugene H. Blackstone, M.D., Birmingham, Ala.
Hypothermia reduces the myocardial injury associated with ischemic arrest in cardiac operations. 1 Its protective effect is manifested in better mechanical function" and greater intracellular high-energy phosphate concentrations" than can be obtained with normothermic arrest. It is assumed that this protective effect is related to the reduction of biochemical reaction rates, and thus of energy utilization, as a function of temperature. Even so, energy utilization (as assessed by myocardial oxygen consumption) continues at a finite level even at temperatures near 0° C. 4 Thus, unless there are From the Department of Surgery, University of Alabama School of Medicine and Medical Center, Birmingham, Ala. Supported in part by the National Heart, Lung and Blood Institute (Specialized Center of Research for Ischemic Heart Disease, Contract No. 5P50HL 17667) of the National Institutes of Health. Presented at the Fifty-first Annual Scientific Session of the American Heart Association, Dallas, Texas, Nov. 14, 1978. Received for publication May 14, 1980. Accepted for publication July 16, 1980. Address for reprints: Stanley B. Digemess, Ph.D., Department of Surgery, University of Alabama Medical Center, University Station, Birmingham, Ala. 35294. *Presently in the Department of Surgery, McMaster University, Hamilton, Ontario, Canada.
detrimental effects, the lower the temperature the better should be the myocardial protection. Tyers and his colleagues" reported that exposure of the isolated rat heart to 60 minutes of ischemia following a brief intracoronary injection of 4° C buffer resulted in poorer functional recovery of cardiac output, coronary flow, and heart rate and lower levels of tissue adenosine triphosphate (ATP) and glycogen than exposure to ischemia following injection of 10° or 15° C buffer. They concluded that brief intracoronary infusions at temperatures less than 10° to 15° C injured the heart. We have designed a study to investigate further the question of possible damage to the heart by profound hypothermia, per se.
Materials and methods Experimental model. The isolated working rat heart, as described by Neeley and Rovetto," was used as the experimental model. Hearts were removed from anesthetized (300 mg sodium pentobarbitol intraperitoneal), heparinized (300 U intraperitoneal), fed, male Sprague-Dawley rats weighing 300 to 400 gm. The aorta was cannulated, and retrograde Langendorfperfusion was begun. Perfusion was continued until the left
0022-5223/811030455+04$00.4010 © 1981 The C. V. Mosby Co.
455
The Journal of Thoracic and Cardiovascular Surgery
456 Shragge, Digerness, Blackstone
Table I. Recovery of aortic flow after a 20 minute exposure to cold Percent recovery :±: SE 37 (control) 20 15 10 4 0.5
5 5 5
97 89 92 86 89 93
II
5 5
:±: 7.0 :±: 7.3 :±: 2.4
± 4.5 ± 4.0 ± 1.5
One-tailed p values
0.3 0.3 0.12 0.22 0.4
Legend: SE. Standard error. • Comparison with 37" C control group.
Table II. Recovery of aortic flow after a 20 minute exposure to cold (data corrected for heart rate) Percent recovery ± SE
Temperature (OC)
37 (control) 20 15 10
4 0.5
5 5 5 II
5 5
95 86 93 91 94 93
± :±: ± :±: ± ±
11.0 6.9 3.5 5.2 4.3 1.5
One-tailed p valuer
0.3 0.4 0.4 0.5 0.4
Legend: SE. Standard error.
• Comparison with 37" C control group.
atrium was cannulated, at which time the heart was allowed to pump against a 100 em fluid column, with the left atrial fluid reservoir positioned 10 em above the heart. An initial 20 minute working period allowed repeated baseline measurements of aortic flow, coronary flow, heart rate, and lactate dehydrogenase (LDH) release. Experiments were discontinued if these values were not stable. The perfusate was Krebs-Henseleit medium equilibrated with 95% oxygen and 5% carbon dioxide. Protocol. At the end of the initial working period, the heart was returned to the Langendorf mode but this time was perfused for 20 minutes or 2 hours with buffer that had been cooled to 20°, 15°, 10°,4°, or 0.5° C. At the same time, the chamber surrounding the heart was switched into the cooling circuit and filled with medium at the experimental temperature, so that the heart was cooled both internally and externally. At the end of the study period of hypothermic perfusion, the heart was warmed by retrograde (Langendorf) perfusion at 37° C for 10 minutes and then returned to a final working period for 20 minutes or I hour. Recovery of aortic and coronary flows was measured, as were heart rate and LDH release. At no time during the procedure was perfusion interrupted.
Experimental groups. Two experimental groups of hearts were studied and compared with two groups of control hearts. In the first experimental group, cold exposure was for 20 minutes, and the recovery working period was 20 minutes. At least six hearts were perfused at each of the five temperatures. In the second experimental group (three hearts), cold exposure was for 2 hours at 0.5° C, and the recovery working period was 1 hour. In the control groups, hearts were maintained at 37° C for either 20 minutes (first control group of five hearts) or 2 hours (second control group of three hearts), with 20 minute or 1 hour recovery working periods, respectively. All switching between Langendorf and working modes was done in these control groups as it was in the experimental groups. At the termination of each experiment in the second experimental group (hearts perfused at 0.5° C for 2 hours), the heart was quick-frozen with tongs cooled in dry ice, and the tissue was analyzed for ATP by the method of Adam 7 and for glycogen by the method of Hassid and Abraham. H Hearts in the second control group were treated similarly. In addition, in situ levels of ATP and glycogen were determined by quickfreezing the hearts of normal anesthetized rats immediately after opening the chest. Data analysis. The recovery of each heart from hypothermic perfusion was expressed as the ratio of final recovery value to initial value. The percent recovery of aortic flow, coronary flow, and heart rate from various levels of hypothermia was tested against that of the appropriate 37° C control group by means of an unpaired t test. A single-tailed p value was calculated for the null hypothesis that "cold does not injure the heart. ' , Results A 20 minute exposure to cold did not impair functional recovery. The percent recovery of aortic flow after a 20 minute exposure to temperatures between 0.5° and 20° C is shown in Table I. Average recovery ranged from 86% to 93%, compared with 97% in the control group at 37° C. When the aortic flow was corrected for heart rate, percent recovery was still similar to that in the control group (Table II). A 2 hour exposure to profound hypothermia did not impair functional recovery. Table III shows that recovery of both aortic flow and aortic flow per beat was similar in the group of hearts maintained at 0.5° C and in the control group maintained for 2 hours at 37° C. ATP and glycogen were well preserved during perfusion for 2 hours at 0.5° C. After 2 hours of perfusion at 0.5° C and I hour working period at 37° C, tissue
Volume 81 Number 3
457
Cardiac recovery after profound hypothermia
March. 1981
ATP and glycogen levels were depressed below those found in in situ hearts (Table IV). However, that the levels were not depressed below those found in the control group supports the hypothesis with a high degree of probability (one-tailed p value) that the cold did not injure the heart; in fact, the glycogen levels were significantly higher in the experimental group than in the control group (two-tailed p value = 0.01). Coronary flow recovered to initial levels (p > 0.2) following exposure to all temperatures, as did heart rate. LDH was not detected in the coronary effluent during recovery from perfusion at any temperature.
Discussion There are several confounding factors which make studies of the effects on the myocardium of cold infusions and maintenance of hypothermia difficult to design and interpret. First, in an attempt to mimic the clinical situation, a period of ischemia may be introduced." However, although altered function and energy metabolism are markers for assessing "cold injury," they also are a consequence of ischemia. 2. 3 Second, if biochemical tissue assays are made after a period which includes reperfusion, then the effects of the brief initial low temperature infusion may be masked by both ischemic injury and superimposed "reperfusion injury." Third, it is difficult to define precisely what is meant by "temperature." As the buffer enters the coronary arteries, its temperature will rise as heat is exchanged with the myocardium. Similarly, it is difficult to achieve and maintain a specific myocardial temperature. Infusion of a small volume of cold solution over a short period of time may not cool the heart uniformly nor remove a sufficient total number of calories to reduce myocardial temperature to that of the infused solution. When the infusion is stopped, provision must be made to continuously remove calories gained from the environment and continuing metabolism or else upward drift of temperature will result. The lower the specific myocardial temperature one wishes to maintain, the more important these considerations become. Finally, factors within the model itself may influence the results, including species differences and the nutritional state of the animal. Recognizing all of these pitfalls and the impracticality of separating the effects of these variables in one study, we have designed this study to test only the effects of the initial infusion at precisely controlled temperatures. We chose to prolong these infusions by 10 to 50 times the duration employed clinically to amplify the possible deleterious effects of this variable. We have studied a range of infusion temperatures
Table III. Recovery of cardiac performance after a 2 hour exposure to cold (percent recovery ± SE) 37 0 C: Control
One-tailed p value
(n = 3)
Aortic flow Aortic flow/beat
85 ± 3.8 81 ± 4.6
82 ± 3.5
0.3
84 ± 4.5
0.7
Legend: SE, Standard error.
Table IV. Adenosine triphosphate (AT?) and glycogen content* after 2 hour exposure to cold plus 1 hour of work (umole . gm dry : ! ± SE)
sr C:Control ATP Glycogen
(n = 3)
0.5 0 C cold (n = 3)
One-tailed p value
18.4 ± 1.31 14 ± 2.3
20.2 ± 1.42 49 ± 6.1
0.8 1.0
Legend: SE, Standard error.
• Normal in situ values: 27 ± 2.9 (n = 4) ATP; 91 ± 3.9 (n = 2) glycogen.
which have been employed clinically at various centers and have found no effect of a 20 minute exposure period on recovery of aortic flow, coronary flow, or heart rate. No increase in enzyme level was detected in the coronary effluent. We also have studied the effects of temperatures near 0° C, colder than those easily achieved in the clinical setting, and have found no obvious deleterious effects. Lengthening the time of exposure to profound hypothermia (0.5° C) to 2 hours did not cause poorer functional recovery than was exhibited by control hearts maintained at 37° C. ATP and glycogen levels, however, were significantly lower than the in situ values. Yet glycogen levels in the hypothermic group were higher than those in the corresponding normothermic control group. It appears that isolated perfused hearts lose energy stores with time but that hypothermia retards this loss." We have found no reason to interpret our data as indicating that profound hypothermia impairs glycolysis and anaerobic energy production or utilization. 10 Such an interpretation would ignore the preservation of ATP that we observed. In no case did we find evidence for myocardial necrosis as indicated by LDH release. Although this study was unable to provide any evidence for injury resulting from profound hypothermic infusion alone, there may be detrimental effects of hypothermia below 37° C, and particularly below about 27° C, resulting from the inactivation of many biochemical systems adapted to functioning at 37° C (for
45 8 Shragge, Digerness, Blackstone
example, cellular volume regulation). 11 However, these deleterious effects are probably time related and many are reversible. 12 This study has focused upon initial infusion temperature. It does not address the combination of hypothermia and ischemia, particularly when initial cooling is thorough and uniform and is maintained by external means during ischemia. Whether under such conditions greater injury would be observed with a 4° C infusion than with a 10° to 15° C one is unknown. Harlan and his colleagues 13 have demonstrated at least as good functional recovery when isolated rat hearts were maintained at 5° C as at 15° C with ischemic periods up to 240 minutes, and they reported superior recovery at 5° C when the ischemic period was extended to 300 minutes. However, these authors' studies did not include measurement of cellular glycogen or high-energy phosphate stores. Rosenfeldt and his colleages, 14 using a very different experimental model, also have found no deleterious effects of profound hypothermia (4° to 10° C). On the other hand, they did see injury when the perfusate temperature caused tissues to freeze (- 3° C). In summary, we find no reason to caution against using cardioplegic injectate temperatures near 0° C. REFERENCES Buckberg GO, Brazier JR, Nelson RL, Goldstein SM, McConnell DH, Cooper N: Studies of the effects of hypothermia on regional myocardial blood flow and metabolism during cardiopulmonary bypass. I. The adequately perfused beating, fibrillating, and arrested heart. J THoRAc CARDIOVASC SURG 73:87-94, 1977 2 Eckner FAa, Blackstone EH, Moulder PV: Tissue reaction to extracorporeal circulation and elective cardiac arrest. Virchows Arch 354: 105-135, 1971
The Journal of Thoracic and Cardiovascular Surgery
3 Hearse OJ, Stewart DA, Braimbridge MV: Hypothermic arrest and potassium arrest. Metabolic and myocardial protection during elective cardiac arrest. Circ Res 36: 481-489, 1975 4 Fuhrman GJ, Fuhrman FA, Field J: Metabolism of rat heart slices with special reference to effects of temperature and anoxia. Am J Physiol 163:642-647, 1950 5 Tyers GFO, Williams EH, Hughes HC, Todd GJ: Effect of perfusate temperature on myocardial protection from ischemia. J THoRAc CARDIOVASC SURG 73:766-771, 1977 6 Neely JR, Rovetto MJ: Methods in Enzymology, Vol 39, New York, 1975, Academic Press, Inc., pp 43-60 7 Adam H: Methods of Enzymatic Analysis, New York, 1965, Academic Press, Inc., pp 539-543 8 Hassid WZ, Abraham S: Methods in Enzymology, Vol 3, New York, 1957, Academic Press, Inc., pp 34-37 9 Aronson CE, Serlick ER: Effects of prolonged perfusion time on the isolated perfused rat heart. Toxicol Appl Pharmacol 38:479-488, 1976 10 Buckberg GO: A proposed "solution" to the cardioplegic controversy. J THoRAc CARDIOVASC SURG 77:803-815, 1979 I I Leaf A: On the mechanism of fluid exchange of tissues in vitro. Biochem J 62:241-248, 1956 12 Proctor E, Matthews G, Archibald J: Acute orthotopic transplantation of hearts stored for 72 hours. Thorax 26:99-102, 197 I 13 Harlan BJ, Ross DR, Macmanus Q, Knight R, Luber J, Starr A: Cardioplegic solutions for myocardial preservation. Analysis of hypothermic arrest, potassium arrest, and procaine arrest. Circulation 58:Suppl 1:114-118, 1978 14 Rosenfeldt FL, Arnold M, Fambiatos A, Stirling GR: Myocardial damage due to profound local hypothermia. Fact or fiction (abstr). Presented at the combined meeting of the Royal College of Surgeons and Physicians. Sidney, Australia, Feb. 24-29, 1980
CARDIOVASC SURG
81:455-458, 1981
Complete recovery of the heart following exposure to profound hypothermia Cold injury has been suggested as a potential limitation to the use of temperatures below 10° to 15° C in clinical myocardial preservation. The isolated effects of profound hypothermia on myocardial function and energy metabolism were studied in the working rat heart preparation. Each heart was isolated and stabilized; then initial aortic flow, coronary flow, and heart rate were measured. The heart then was perfused in the Langendorf mode with oxygenated Krebs-Henseleit buffer for 20 minutes at 0.5°. 4°, 10°, ISo, or 20 0 C. After being rewarmed to 37" C. the heart was returned to the working mode for final functional measurements. In a control group. the perfusion temperature was kept at 37° C. Recovery of function in hearts exposed to hypothermic perfusion was not significantly different from that observed in the hearts kept at 37° C. When cold exposure time to 0.5 0 C perfusion was extended to 2 hours, heart function still returned to the same level as that of control hearts maintained at 37 0 C. and adenosine triphosphate (ATP) and glycogen levels were higher than those in the control group. Thus, under these conditions, cold exposure per se, even for 2 hours at temperatures near 0° C, has no deleterious effect upon myocardial function and energy metabolism.
B. William Shragge, M.D.,* Stanley B. Digerness, Ph.D., and Eugene H. Blackstone, M.D., Birmingham, Ala.
Hypothermia reduces the myocardial injury associated with ischemic arrest in cardiac operations. 1 Its protective effect is manifested in better mechanical function" and greater intracellular high-energy phosphate concentrations" than can be obtained with normothermic arrest. It is assumed that this protective effect is related to the reduction of biochemical reaction rates, and thus of energy utilization, as a function of temperature. Even so, energy utilization (as assessed by myocardial oxygen consumption) continues at a finite level even at temperatures near 0° C. 4 Thus, unless there are From the Department of Surgery, University of Alabama School of Medicine and Medical Center, Birmingham, Ala. Supported in part by the National Heart, Lung and Blood Institute (Specialized Center of Research for Ischemic Heart Disease, Contract No. 5P50HL 17667) of the National Institutes of Health. Presented at the Fifty-first Annual Scientific Session of the American Heart Association, Dallas, Texas, Nov. 14, 1978. Received for publication May 14, 1980. Accepted for publication July 16, 1980. Address for reprints: Stanley B. Digemess, Ph.D., Department of Surgery, University of Alabama Medical Center, University Station, Birmingham, Ala. 35294. *Presently in the Department of Surgery, McMaster University, Hamilton, Ontario, Canada.
detrimental effects, the lower the temperature the better should be the myocardial protection. Tyers and his colleagues" reported that exposure of the isolated rat heart to 60 minutes of ischemia following a brief intracoronary injection of 4° C buffer resulted in poorer functional recovery of cardiac output, coronary flow, and heart rate and lower levels of tissue adenosine triphosphate (ATP) and glycogen than exposure to ischemia following injection of 10° or 15° C buffer. They concluded that brief intracoronary infusions at temperatures less than 10° to 15° C injured the heart. We have designed a study to investigate further the question of possible damage to the heart by profound hypothermia, per se.
Materials and methods Experimental model. The isolated working rat heart, as described by Neeley and Rovetto," was used as the experimental model. Hearts were removed from anesthetized (300 mg sodium pentobarbitol intraperitoneal), heparinized (300 U intraperitoneal), fed, male Sprague-Dawley rats weighing 300 to 400 gm. The aorta was cannulated, and retrograde Langendorfperfusion was begun. Perfusion was continued until the left
0022-5223/811030455+04$00.4010 © 1981 The C. V. Mosby Co.
455
The Journal of Thoracic and Cardiovascular Surgery
456 Shragge, Digerness, Blackstone
Table I. Recovery of aortic flow after a 20 minute exposure to cold Percent recovery :±: SE 37 (control) 20 15 10 4 0.5
5 5 5
97 89 92 86 89 93
II
5 5
:±: 7.0 :±: 7.3 :±: 2.4
± 4.5 ± 4.0 ± 1.5
One-tailed p values
0.3 0.3 0.12 0.22 0.4
Legend: SE. Standard error. • Comparison with 37" C control group.
Table II. Recovery of aortic flow after a 20 minute exposure to cold (data corrected for heart rate) Percent recovery ± SE
Temperature (OC)
37 (control) 20 15 10
4 0.5
5 5 5 II
5 5
95 86 93 91 94 93
± :±: ± :±: ± ±
11.0 6.9 3.5 5.2 4.3 1.5
One-tailed p valuer
0.3 0.4 0.4 0.5 0.4
Legend: SE. Standard error.
• Comparison with 37" C control group.
atrium was cannulated, at which time the heart was allowed to pump against a 100 em fluid column, with the left atrial fluid reservoir positioned 10 em above the heart. An initial 20 minute working period allowed repeated baseline measurements of aortic flow, coronary flow, heart rate, and lactate dehydrogenase (LDH) release. Experiments were discontinued if these values were not stable. The perfusate was Krebs-Henseleit medium equilibrated with 95% oxygen and 5% carbon dioxide. Protocol. At the end of the initial working period, the heart was returned to the Langendorf mode but this time was perfused for 20 minutes or 2 hours with buffer that had been cooled to 20°, 15°, 10°,4°, or 0.5° C. At the same time, the chamber surrounding the heart was switched into the cooling circuit and filled with medium at the experimental temperature, so that the heart was cooled both internally and externally. At the end of the study period of hypothermic perfusion, the heart was warmed by retrograde (Langendorf) perfusion at 37° C for 10 minutes and then returned to a final working period for 20 minutes or I hour. Recovery of aortic and coronary flows was measured, as were heart rate and LDH release. At no time during the procedure was perfusion interrupted.
Experimental groups. Two experimental groups of hearts were studied and compared with two groups of control hearts. In the first experimental group, cold exposure was for 20 minutes, and the recovery working period was 20 minutes. At least six hearts were perfused at each of the five temperatures. In the second experimental group (three hearts), cold exposure was for 2 hours at 0.5° C, and the recovery working period was 1 hour. In the control groups, hearts were maintained at 37° C for either 20 minutes (first control group of five hearts) or 2 hours (second control group of three hearts), with 20 minute or 1 hour recovery working periods, respectively. All switching between Langendorf and working modes was done in these control groups as it was in the experimental groups. At the termination of each experiment in the second experimental group (hearts perfused at 0.5° C for 2 hours), the heart was quick-frozen with tongs cooled in dry ice, and the tissue was analyzed for ATP by the method of Adam 7 and for glycogen by the method of Hassid and Abraham. H Hearts in the second control group were treated similarly. In addition, in situ levels of ATP and glycogen were determined by quickfreezing the hearts of normal anesthetized rats immediately after opening the chest. Data analysis. The recovery of each heart from hypothermic perfusion was expressed as the ratio of final recovery value to initial value. The percent recovery of aortic flow, coronary flow, and heart rate from various levels of hypothermia was tested against that of the appropriate 37° C control group by means of an unpaired t test. A single-tailed p value was calculated for the null hypothesis that "cold does not injure the heart. ' , Results A 20 minute exposure to cold did not impair functional recovery. The percent recovery of aortic flow after a 20 minute exposure to temperatures between 0.5° and 20° C is shown in Table I. Average recovery ranged from 86% to 93%, compared with 97% in the control group at 37° C. When the aortic flow was corrected for heart rate, percent recovery was still similar to that in the control group (Table II). A 2 hour exposure to profound hypothermia did not impair functional recovery. Table III shows that recovery of both aortic flow and aortic flow per beat was similar in the group of hearts maintained at 0.5° C and in the control group maintained for 2 hours at 37° C. ATP and glycogen were well preserved during perfusion for 2 hours at 0.5° C. After 2 hours of perfusion at 0.5° C and I hour working period at 37° C, tissue
Volume 81 Number 3
457
Cardiac recovery after profound hypothermia
March. 1981
ATP and glycogen levels were depressed below those found in in situ hearts (Table IV). However, that the levels were not depressed below those found in the control group supports the hypothesis with a high degree of probability (one-tailed p value) that the cold did not injure the heart; in fact, the glycogen levels were significantly higher in the experimental group than in the control group (two-tailed p value = 0.01). Coronary flow recovered to initial levels (p > 0.2) following exposure to all temperatures, as did heart rate. LDH was not detected in the coronary effluent during recovery from perfusion at any temperature.
Discussion There are several confounding factors which make studies of the effects on the myocardium of cold infusions and maintenance of hypothermia difficult to design and interpret. First, in an attempt to mimic the clinical situation, a period of ischemia may be introduced." However, although altered function and energy metabolism are markers for assessing "cold injury," they also are a consequence of ischemia. 2. 3 Second, if biochemical tissue assays are made after a period which includes reperfusion, then the effects of the brief initial low temperature infusion may be masked by both ischemic injury and superimposed "reperfusion injury." Third, it is difficult to define precisely what is meant by "temperature." As the buffer enters the coronary arteries, its temperature will rise as heat is exchanged with the myocardium. Similarly, it is difficult to achieve and maintain a specific myocardial temperature. Infusion of a small volume of cold solution over a short period of time may not cool the heart uniformly nor remove a sufficient total number of calories to reduce myocardial temperature to that of the infused solution. When the infusion is stopped, provision must be made to continuously remove calories gained from the environment and continuing metabolism or else upward drift of temperature will result. The lower the specific myocardial temperature one wishes to maintain, the more important these considerations become. Finally, factors within the model itself may influence the results, including species differences and the nutritional state of the animal. Recognizing all of these pitfalls and the impracticality of separating the effects of these variables in one study, we have designed this study to test only the effects of the initial infusion at precisely controlled temperatures. We chose to prolong these infusions by 10 to 50 times the duration employed clinically to amplify the possible deleterious effects of this variable. We have studied a range of infusion temperatures
Table III. Recovery of cardiac performance after a 2 hour exposure to cold (percent recovery ± SE) 37 0 C: Control
One-tailed p value
(n = 3)
Aortic flow Aortic flow/beat
85 ± 3.8 81 ± 4.6
82 ± 3.5
0.3
84 ± 4.5
0.7
Legend: SE, Standard error.
Table IV. Adenosine triphosphate (AT?) and glycogen content* after 2 hour exposure to cold plus 1 hour of work (umole . gm dry : ! ± SE)
sr C:Control ATP Glycogen
(n = 3)
0.5 0 C cold (n = 3)
One-tailed p value
18.4 ± 1.31 14 ± 2.3
20.2 ± 1.42 49 ± 6.1
0.8 1.0
Legend: SE, Standard error.
• Normal in situ values: 27 ± 2.9 (n = 4) ATP; 91 ± 3.9 (n = 2) glycogen.
which have been employed clinically at various centers and have found no effect of a 20 minute exposure period on recovery of aortic flow, coronary flow, or heart rate. No increase in enzyme level was detected in the coronary effluent. We also have studied the effects of temperatures near 0° C, colder than those easily achieved in the clinical setting, and have found no obvious deleterious effects. Lengthening the time of exposure to profound hypothermia (0.5° C) to 2 hours did not cause poorer functional recovery than was exhibited by control hearts maintained at 37° C. ATP and glycogen levels, however, were significantly lower than the in situ values. Yet glycogen levels in the hypothermic group were higher than those in the corresponding normothermic control group. It appears that isolated perfused hearts lose energy stores with time but that hypothermia retards this loss." We have found no reason to interpret our data as indicating that profound hypothermia impairs glycolysis and anaerobic energy production or utilization. 10 Such an interpretation would ignore the preservation of ATP that we observed. In no case did we find evidence for myocardial necrosis as indicated by LDH release. Although this study was unable to provide any evidence for injury resulting from profound hypothermic infusion alone, there may be detrimental effects of hypothermia below 37° C, and particularly below about 27° C, resulting from the inactivation of many biochemical systems adapted to functioning at 37° C (for
45 8 Shragge, Digerness, Blackstone
example, cellular volume regulation). 11 However, these deleterious effects are probably time related and many are reversible. 12 This study has focused upon initial infusion temperature. It does not address the combination of hypothermia and ischemia, particularly when initial cooling is thorough and uniform and is maintained by external means during ischemia. Whether under such conditions greater injury would be observed with a 4° C infusion than with a 10° to 15° C one is unknown. Harlan and his colleagues 13 have demonstrated at least as good functional recovery when isolated rat hearts were maintained at 5° C as at 15° C with ischemic periods up to 240 minutes, and they reported superior recovery at 5° C when the ischemic period was extended to 300 minutes. However, these authors' studies did not include measurement of cellular glycogen or high-energy phosphate stores. Rosenfeldt and his colleages, 14 using a very different experimental model, also have found no deleterious effects of profound hypothermia (4° to 10° C). On the other hand, they did see injury when the perfusate temperature caused tissues to freeze (- 3° C). In summary, we find no reason to caution against using cardioplegic injectate temperatures near 0° C. REFERENCES Buckberg GO, Brazier JR, Nelson RL, Goldstein SM, McConnell DH, Cooper N: Studies of the effects of hypothermia on regional myocardial blood flow and metabolism during cardiopulmonary bypass. I. The adequately perfused beating, fibrillating, and arrested heart. J THoRAc CARDIOVASC SURG 73:87-94, 1977 2 Eckner FAa, Blackstone EH, Moulder PV: Tissue reaction to extracorporeal circulation and elective cardiac arrest. Virchows Arch 354: 105-135, 1971
The Journal of Thoracic and Cardiovascular Surgery
3 Hearse OJ, Stewart DA, Braimbridge MV: Hypothermic arrest and potassium arrest. Metabolic and myocardial protection during elective cardiac arrest. Circ Res 36: 481-489, 1975 4 Fuhrman GJ, Fuhrman FA, Field J: Metabolism of rat heart slices with special reference to effects of temperature and anoxia. Am J Physiol 163:642-647, 1950 5 Tyers GFO, Williams EH, Hughes HC, Todd GJ: Effect of perfusate temperature on myocardial protection from ischemia. J THoRAc CARDIOVASC SURG 73:766-771, 1977 6 Neely JR, Rovetto MJ: Methods in Enzymology, Vol 39, New York, 1975, Academic Press, Inc., pp 43-60 7 Adam H: Methods of Enzymatic Analysis, New York, 1965, Academic Press, Inc., pp 539-543 8 Hassid WZ, Abraham S: Methods in Enzymology, Vol 3, New York, 1957, Academic Press, Inc., pp 34-37 9 Aronson CE, Serlick ER: Effects of prolonged perfusion time on the isolated perfused rat heart. Toxicol Appl Pharmacol 38:479-488, 1976 10 Buckberg GO: A proposed "solution" to the cardioplegic controversy. J THoRAc CARDIOVASC SURG 77:803-815, 1979 I I Leaf A: On the mechanism of fluid exchange of tissues in vitro. Biochem J 62:241-248, 1956 12 Proctor E, Matthews G, Archibald J: Acute orthotopic transplantation of hearts stored for 72 hours. Thorax 26:99-102, 197 I 13 Harlan BJ, Ross DR, Macmanus Q, Knight R, Luber J, Starr A: Cardioplegic solutions for myocardial preservation. Analysis of hypothermic arrest, potassium arrest, and procaine arrest. Circulation 58:Suppl 1:114-118, 1978 14 Rosenfeldt FL, Arnold M, Fambiatos A, Stirling GR: Myocardial damage due to profound local hypothermia. Fact or fiction (abstr). Presented at the combined meeting of the Royal College of Surgeons and Physicians. Sidney, Australia, Feb. 24-29, 1980