Cardioplegia for the immature myocardium

J

THORAC CARDIOVASC SURG 1990;100:910-3

Cardioplegia for the immature myocardium A comparative study in the neonatal rabbit This study examined the effect of hypothermia (15 0 C) alone or combined with variOiti cardioplegic solutiOiti on flDlctional recovery of the neonatal heart after 120 minutes of global ischemia in an isolated working rabbit heart modeL Control hearts were preserved with hypothermia alone, and groups 1 to 6 were given different hyperkalemic crystalloid cardioplegic solutiOiti. Each cardioplegic solution differed in Na+ and Ca++ content. Aortic flow, coronary flow, cardiac output, heart rate, peak systolic pressure, and stroke work were measured before ischemia and after 35 and 45 minutes of reperfusion. There were no statistical differences in hemodynamic recovery in the six groups in which cardioplegia was used. However, hearts preserved with multidose hyperkalemic cardioplegia showed significantly better recovery of cardiac output (86% versus 75%; p < 0.05), coronary flow (88% versus 72%; p < 0.05), and stroke work (86% versus 75%; p < 0.05) than those preserved with hypothermia alone. These results suggest that hypothermic hyperkalemic cardioplegia improves preservation of the neonatal rabbit heart but that variations in Ca++ and Na+ content appear not to provide further myocardial protection.

Maurizio Diaco, MD, Verdi J. DiSesa, MD, Shu-Ching Sun, MD, Rita Laurence, BS, and Lawrence H. Cohn, MD, Boston, Mass.

Optimal myocardial protection during pediatric cardiac operations is still controversial. Although the use of cold cardioplegia with mild hypothermia is a well-established method of myocardial protection in adults, 1,2 in infants this is not uniformly accepted. Indeed, when total circulatory arrest with profound hypothermia is used in neonates, some surgeons believe that no further myocardial protection is needed. In reviewing 400 pediatric cases, for example, Bull, Cooper, and Stark! were unable to show a clear superiority of cardioplegia over intermittent crossclamping. Furthermore, current experimental data suggest a greater tolerance to hypothermict" and normothermic ischemia in neonatal hearts compared with older hearts.. On

From the Department of Surgery, Harvard Medical School and Brigham and Women's Hospital, Boston, Mass. Presented in part at the Surgical Forum,American Collegeof Surgeons, October 1988. Received for publication July 24,1989. Accepted for publication Jan. 25, 1990. Address for reprints: Verdi J. DiSesa, MD, Department of Surgery, Hospital of the University of Pennsylvania, 3400 Spruce St., Philadelphia, PA 19104.

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the other hand, Crawford, Barnes, and Heath 7 showed significantly superior myocardial protection when cold crystalloid potassium cardioplegia was compared with historic control studies. Kirklin and colleagues" showed that in infants less than 3 months of age the use of cardioplegia significantly decreased the probability of death from acute cardiac failure if the ischemic time exceeded 40 minutes. Current cardioplegic techniques, largely based on experience with adult patients, may be inadequate for the neonatal myocardium." Numerous investigators have reported structural.F"!' metabolic.lf'? and functional I2, IS, 16 differences between neonatal and adult hearts. Particularly evident in the neonatal heart are the underdevelopment ofthe sarcoplasmic reticulum, 12, 16 the direct control by the sarcolemma of Ca++ influx.l" and the greater dependence of the myocardial cells on extracellular Ca++ for excitation-contraction coupling.'? These differences may require adjustment of ionic concentration in the cardioplegic solution for optimal protection during ischemia. Our study was designed to compare preservation of the neonatal rabbit heart with hypothermia alone or combined with various hyperkalemic cardioplegic solutions that differed in Na+ and Ca"" content.

Volume 100 Number 6 December 1990

Cardioplegia for immature myocardium 9 1 1

Table II. Cardioplegic Na+icc;: content

Table I. Basic cardioplegic solution Glucose (gm/L) K+ (mEq/L) HC03 (mEq/L)

pH

5 28 14 7.4

Material and methods Hearts from immature (12 to 14 days old) New Zealand white rabbits were studied with a modified Langendorff apparatus. All animals received humane care in compliance with National Institutes of Health guidelines. The animals were anesthetized with ether, and before cardiectomy heparin (150 IV jkg) was injected through the inferior vena cava. The heart was excised and placed in physiologic saline solution at 4 0 C. The aorta was cannulated, and retrograde Langendorff perfusion with Krebs- Henseleit solution was initiated at a pressure of 60 cm H 20. The perfusion medium was equilibrated with 95% oxygen and 5% carbon dioxide at 37 0 C. During Langendorff perfusion the left atrium was cannulated. After an initial period of retrograde perfusion, the hearts were converted to the working mode by filling the left atrium with oxygenated KrebsHenseleit solution at a constant preload of 15 em H 20. The left ventricle ejected the perfusate through a pressure chamber into the aortic ejection line against an afterload of 60 em H 20. Aortic and right heart effluent were recirculated through a common reservoir, filtered, and oxygenated before reentering the left atrium. Aortic and coronary flow were measured by timed collection from the aortic ejection line and the right ventricle, respectively. Heart rate was recorded by a strip chart, and aortic pressure was constantly assessed with a pressure transducer (Statham; Spectramed Inc., Critical Care Division, Oxnard, Calif.) connected via a sidearm to the aortic ejection line. Cardiac output was derived from the sum of the aortic and coronary flow. Stroke work (dynes per centimeter) was calculated from the formulal'': Aortic pressure (cmH20) X Cardiac output (mljmin) . S tro ke work =

Heart rate (beatsjrrun)

Experimental protocol. After aortic cannulation the hearts were perfused in the nonworking mode for 10 minutes and then converted to the working mode for 20 minutes. Baseline measurements were then obtained. The preservation protocol was designed to simulate clinical cardioplegic techniques. The hearts were arrested with cold infusion (4 0 C) of either Krebs-Henseleit solution or cardioplegic solutions of six different Na+ and Ca++ concentrations (Tables I and II). Electrolyte concentrations were obtained by adding appropriate amounts of sodium and calcium salts, as is the usual clinical practice. Each heart then underwent 120 minutes of global ischemia at 150 C, by immersion in a cold saline bath with continuous measurement of ambient temperature. Because these hearts are small, there is rapid equilibration of myocardial temperature with ambient temperature, although only the latter was measured. In the hypothermia alone group (control), cold was the only preservation procedure. In the cardioplegia groups additional 5 ml doses of cardioplegic solution were infused every 30 minutes. At the end of 120 minutes of global ischemia, the hearts were reperfused in the nonworking mode for 10 minutes and finally

Experimental group

Na+ (mEq/L) Ca++ (rnmol/L)

77 0

2

3

77

77

1.2

2.4

4

5

6

144

144 1.2

144 2.4

0

reconverted to the working mode for an additional 35 minutes. At 35 and 45 minutes of reperfusion, postischemic measurements were made. Hemodynamic data at 35 and 45 minutes of reperfusion were averaged and expressed as a percentage of the prearrest control values. Mean and standard error of the mean were calculated for each group. One-way analysis of variance was used to compare the six groups in which cardioplegia was used. Student's t test was applied to compare hearts preserved with hypothermia alone with those preserved with hypothermia and cardioplegia. A p value of less than 0.05 was considered significant.

Results Baseline measurements were comparable in all groups. Percent recovery after global ischemia for each group is shown in Table III. Although all cardioplegic solutions achieved significant preservation of cardiac output (82% to 92%), coronary flow (79% to 95%), peak systolic pressure (89% to 96%), and stroke work (82% to 91%), there was no statistical difference in hemodynamic recovery among the six groups. In the control group (preserved with hypothermia alone), overall hemodynamic recovery was satisfactory (cardiac output = 75%; coronary flow = 72%; heart rate = 98%; peak systolic pressure = 93%; stroke work = 75%). However, hearts preserved with hypothermia and multidose hyperkalemic cardioplegia showed significantly better recovery of cardiac output (86% versus 75%, p < 0.05), coronary flow (88% versus 72%, p < 0.05), and stroke work (86% versus 75%, p < 0.05) (Table IV).

Discussion Whether the inclusion of Ca++ in cardioplegic solutions is a benefit or a hazard remains controversial. Many investigators propose that the absence of Ca"" in cardioplegic solutions avoids the danger of Ca++ influx during ischemia. 19, 20 Other studies suggest that Ca++-free solutions cause sarcolemmal membrane injury (calcium paradoxj.U In the neonatal myocardial cell with its greater dependence on extracellular Ca++ for excitationcontraction coupling, the presence of Ca++ in the extracellular space during ischemia might aggravate Ca++ influx and the reduction of adenosine triphosphatase stores. On the other hand, increased extracellular sodium concentration might facilitate Ca++ efflux through the

The Journal of Thoracic and Cardiovascular Surgery

Diaco et al.

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Table III. Percent functional recovery duringreperfusion ofrabbit hearts after arrest with hypothermia alone or with hypothermia and hyperkalemiccardioplegic solutioncontaining different concentrations of Na+ and Ca++

CF CO HR

psp

SW

3

I (n = 6)

(n = 7)

(n = 6)

4 (n = 6)

(n = 7)

6 (n = 6)

72 75 98 93 75

92 82 95 94 83

95 92 101 94 89

92 87 94 94 91

79 86 103 89 82

83 84 93 96 89

90 83 98 92 82

± ± ± ± ±

8 5 3 2 5

± ± ± ± ±

4 4 2 1 3

2

± ± ± ± ±

9 6 2 1 5

± ± ± ± ±

Values are mean ± standard error of the mean. CF. Coronary flow; CO. cardiac output;

Table IV. Percent functional recovery after arrest with hypothermia aloneand hypothermia plus hyperkalemiccardioplegic solutions (all concentrations) Control (hypothermia alone) CF CO HR

psp

SW

72 75 98 93 75

± ± ± ± ±

5

Control

8 5 3 2 5

Groups J to 6 (hypothermia + cardioplegia) 88 86 97 93 86

± ± ± ± ±

3 2 1 1 2

p <0.05 p < 0.05

NS NS

p <0.05

Values are mean ± standard error of the mean. For explanation of acronyms see Table III. NS, Not significant

Na+ICa++ exchange system and produce a more relaxed heart in arrest. Alternatively, high Na+ concentration might exacerbate the Ca"" paradox by entering slow channels during arrest and depolarizing the membrane, leading to accumulation of intracellular Ca++.2,22 This may not apply in the neonatal heart, which has a decreased density of slow channels.P Our study was designed to assess these questions by varying the sodium and calcium content of solutions used to preserve the immature heart. The cardioplegic solutions were prepared and administered according to standard clinical protocols so that the results would be clinically relevant. Our data suggest that cold potassium cardioplegia improves protection of the neonatal heart and that variations of Ca"" and Na+ content in the cardioplegic solution appear to have no independent effects. These findings confirm other studies showing that the most important components of a cardioplegic solution for the neonatal heart are cold and elevated potassium concentration.e'r-? The lack of effect ofdifferent Na+ and Ca++ contents in the cardioplegic solutions may be due to the immature transport structure in the membrane of the neonatal myocardial cells. In several studies a decreased density ofT-tubules and Ca++ pumps in neonatal myocardium has been reported.P: 14 Furthermore,

cell

6 5 4 2 3

± ± ± ± ±

6 3 4 3 5

± ± ± ± ±

7 4 4 1 4

± ± ± ± ±

7 5 4 1 4

HR. heart rate; PSP, peak systolic pressure; SW, stroke work.

Pridjian and associates 10showed unchanged intracellular concentration ofNa+ and Ca++ in neonatal hearts after 40 minutes of ischemia and reperfusion. These investigators attributed the results to age-dependent alterations in membrane ionic pumps and channels. In more recent invesrigations," the same group has reported greater susceptibility to Na+ and Ca"" accumulation after ischemia and reperfusion in up to 7-day-old hearts. The effects of ischemia in hearts from animals of this age may be distinct from those in older (12 to 14 days) but still immature hearts. Further studies are needed to define the role ofCa++ and Na+ in cardioplegia for the neonatal heart and to determine how subtle age differences may affect intramembrane ionic transport and tolerance to ischemia in immature myocardium. Our study confirms the importance of cold and potassium in optimal preservation of the immature heart. Left open is the question of whether manipulation of sodium and calcium content may be of further benefit, perhaps in distinct subsets of pediatric patients. REFERENCES I. Engelman RM, Levitsky S. A textbook of clinical cardioplegia. 1st ed. Mount Kisco, New York: Futura, 1982. 2. Hearse DJ, Braimbridge MY, Jynge P. Protection of the ischemic myocardium: cardioplegia. 1st ed. New York: Raven Press, 1981. 3. Bull C, Cooper J, Stark J. Cardioplegic protection of the child's heart. J THORAC CARDIOVASC SURG 1984;88:28793. 4. Grice WN, Konishi T, Apstein CS. Resistance of neonatal myocardium to injury during normothermic and hypothermic ischemic arrest and reperfusion. Circulation 1987; 76(Pt 2):YI5Q-5. 5. Yano Y, Braimbridge MY, Hearse DJ. Protection of the pediatric myocardium: differential susceptibility to ischemic injury of the neonatal rat heart. J THORAC CARDIOVASC SURG 1987;94:887-96. 6. Bove E, Stammers A. Recovery of left ventricular function after hypothermic global ischemia. J THORAC CARDlOVASC SURG 1986;91:115-22.

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7. Crawford FA, Barnes TY, Heath BJ. Potassium-induced cardioplegia in patients undergoing correction of congenital heart defects. Chest 1980;78:316-20. 8. Kirklin JK, Blackstone EH, Kirklin JW, McKay R, Pacifico AD, Bargeron LM Jr. Intracardiac surgery in infants under age 3 months: incremental risk factors for hospital mortality. Am J CardioI1981;48:50Q-6. 9. Sawa Y, Matsuda H, Shimazaki Y, et al. Ultrastructural assessment of the infant myocardium receiving crystalloid cardioplegia. Circulation 1987;76(Pt 2):V141-5. 10. Pridjian A, Levitsky S, Krukenkamp I, et al. Developmental changes in reperfusion injury: a comparison of intracellular cation accumulation in the newborn, neonatal, and adult heart. J THORAC CARDIOVASC SURG 1987;93:42833. II. Nakanishi T, Jarmakani JM. Developmental changes in myocardial mechanical and subcellular organelles. Am J Physiol 1984;246:H615-25. 12. Boland R, Martonosi A, Tillack TW. Developmental changes in the composition and function of the sarcoplasmic reticulum. J Bioi Chem 1974;249:612-23. 13. Lodge NJ, Golband H. Calcium exchange in the adult and neonatal canine myocardium. Pediatr Cardiol 1984;5:253. 14. Mahony L, D'Anniballe L. Increased calcium channel density in cardiac sarcoplasmic reticulum from fetal sheep. Pediatr Cardiol 1984;5:521. 15. Nayler WG, Fassold E. Calcium accumulation and ATPase activity of cardiac sarcoplasmic reticulum before and after birth. Cardiol Res 1977;11:231-7. 16. Bers DM, Philipson KD, Langer GA. Cardiac contractility and sarcolemmal calcium binding in several muscle preparations. Am J Physiol 1981;240:H576-83. 17. George BL, Nakanishi T, Shimizu T, Nishioka K, Jarmakani JM. Effect of verapamil on mechanical function of the neonatal rabbit heart. Pediatr Res 1981;15:463. 18. Katz AM. Physiology of the heart. 1st ed. New York: Raven Press, 1977:209-27.

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19. Boggs BR, Torchiana DF, Geffin GA, et al. Optimal myocardial preservation with an acalcemic crystalloid cardioplegic solution. J THORAC CARDIOVASC SURG 1987;93: 838-46. 20. Reuter H. Exchange of calcium ions in the myocardium. Circ Res 1974;34:599-605. 21. Zimmerman ANE, Hulsmann We. Paradoxical influence of calcium ions on the permeability of the cell membranes of the isolated rat heart. Nature 1966;211:646-7. 22. Alto LE, Dhalla NS. Myocardial cation contents during induction of calcium paradox. Am J Physiol 1979;237: H13-9. 23. Boucek RJ, Shelton ME, Artman M, Landon E. Myocellular calcium regulation by the sarcolemmal membrane in the adult and immature rabbit heart. Basic Res Cardiol 1985;80:316-25. 24. Avkiran M, Hearse DJ. Protection of the myocardium during global ischemia. Is crystalloid cardioplegia effective in the immature myocardium? J THORAC CARDIOVASC SURG 1989;97:220-8. 25. Ganzel BL, Katzrnark SL, Mavroudis e. Myocardial preservation in the neonate: beneficial effects of cardioplegia and systemic hypothermia on piglets undergoing cardiopulmonary bypass and myocardial ischemia. J THORAC CARDIOVASC SURG 1988;96:414-22. 26. Bove EL, Stammers AH, Gallagher KP. Protection of the neonatal myocardium during hypothermic ischemia: effect of cardioplegia on left ventricular function in the rabbit. J THORAC CARDIOVASC SURG 1987;94:115-23. 27. Schachner A, Vladutiu A, Montes M, et al. Myocardial protection in infant open heart surgery. Scand J Thorac Cardiovasc Surg 1983;17:101-7. 28. Pridjian AK, Levitsky S, Krukenkamp I. Developmental changes in reperfusion injury: Comparison of intracellular ion accumulation in ischemic and cardioplegic arrest. J THORAC CARDIOVASC SURG 1988;96:577-81.