Experimental evaluation of magnesium cardioplegia

J THoRAe

CARDIOVASC SURG

84:685-688, 1982

Experimental evaluation of magnesium cardioplegia The effectiveness of magnesium solution in protecting ischemic myocardium was evaluated in a blood-perfused, isolated rabbit papillary muscle preparation. Anoxic cardioplegia was induced by either a control solution containing no magnesium or a test solution containing a magnesium concentration of 160 mEq/L. The magnesium solution induced a very rapid cardiac arrest, less than 1 minute, in contrast to more than 10 minutes with the control solution. Restarting time was not affected by magnesium cardioplegia. Following 30 minutes of anoxic cardioplegia, the magnesium group showed significantly (p < 0.0l) better recovery of myocardial contractility, 91.3% ± 8.3% (n = 6), than the control group, 77.7% ± 4.1% (n = 6). These protective effects of magnesium were lost when the ischemic time was longer than 45 minutes.

Akio Wakabayashi, M.D., Tsunehiro Nishi, M.D. (by invitation), and J. Edward Guilmette, M.A. (by invitation), Orange, Calif.

MagneSium cardioplegia has never gained popularity in this country. In contrast, magnesium is a major active ingredient in all three representative European cardioplegic solutions (Table I). However, the magnesium concentration of these three solutions varies widely, 4 mEq/L in the Bretschneider solution, 1 32 mEq/L in the St. Thomas' Hospital solution," and 160mEq/L in the Kirsch solution. 3 Magnesium has been used in treating cardiac arrhythmias and digitalis intoxication," but its pharmacologic mechanism in arresting hearts is not well understood. The major physiological role of magnesium is the activation of membranebound adenosine triphosphatase." This enzyme system deals with energy production through oxidative phosphorylation and with distribution of sodium and potassium across the cell membrane." In addition, the magnesium complex with adenosine triphosphatase is the substrate for the enzymatic reactions that underlie the sliding filament mechanism of myofibrillar contraction and relaxation. 6 When the plasma magnesium concentration increases, the myocardial contractile force

gradually decreases and finally stops. This action may be due to the shortening of the duration of action potentials.? In order to stop cardiac contractions, however, a very high concentration of magnesium solution may be required. Doring and associates" reported that the magnesium concentration had to be raised to 100 mEq/L in order to stop heart action completely. The cardiac contractions of the hearts arrested in this manner in Doring's experiments could not be restarted with oxygenated blood perfusion. However, this is not a problem with the Kirsch solution," which contains a much higher concentration, 160 mEq/L, than the Doring solution. In previous publications by others;" 8-12 the protective effects of magnesium were studied in cardioplegic solutions containing several active ingredients; thus it was impossible to separate the effect of magnesium per se on the ischemic myocardium from that of the other components. The present study was undertaken to determine whether magnesium, used as the sole active ingredient in a cardioplegic solution, has any protective effect on the anoxic heart.

From the Department of Surgery, California College of Medicine, University of California Irvine Medical Center, Orange, Calif. Read at the Sixty-second Annual Meeting of The American Association for Thoracic Surgery, Phoenix, Ariz., May 3-5, 1982. Address for reprints: Akio Wakabayashi, M.D., Department of Surgery, California College of Medicine, University of California Irvine Medical Center, !OI City Drive South, Orange, Calif. 92668.

Methods

Isometric contractility of the posterior papillary muscle of an isolated, blood-perfused rabbit heart was used as an index of myocardial function. The details have been reported earlier. 6 In brief, the posterior papillary muscle of an excised rabbit heart with an incompetent mitral valve was suspended between a fixed point and

© 1982 The C. V. Mosby Co.

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0022-5223/82/I 10685+04$00.40/0

The Journal of

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Wakabayashi, Nishi. Guilmette

Thoracic and Cardiovascular Surgery

Table I. Active ingredients of European cardioplegic solutions

Table IV. Arrest time and restart time Procaine

Solution

(%)

10

Bretschneider No. 3 Kirsch St. Thomas' Hospital

0.2 0.3 0.027

4 160 32

o

16

Groups fA lB IIA liB IlIA IlIB

p Value 11.7 0.9 10.3 0.8 10.9 0.8

2.9 0.1 1.8 0.1 ± 1.5 } ± 0.1 ± ± ± ±

0.7 0.8 1.3 1.4 2.5 1.8

<0.01 <0.01 <0.01

± ± ± ± ± ±

} } }

0.2 0.1 0.5 0.7 0.5 0.8

NS NS NS

Table ll. Composition of test solution (in mEqlL) Solution *

I

A B 'pH

=

Na 135

o

0

Mg

o

5

o

160

I

Cl

I

50 4

o

140

o

160

7.40 at 37' C.

Table llI. Groups of experiments Groups IA lB IIA liB IlIA IlIB

A B A B A B

Anoxic time (min)

No. of experiments

30 30 45 45 60 60

6 6 6 6 6 6

a force I displacement transducer, which was mounted on a micrometer. The coronary arteries were perfused retrogradely by autogenous blood via a membrane oxygenator and a heat exchanger. Blood gases and serum potassium levels were monitored at intervals with a gas analyzer and flame photometer (Radiometer AIS, Copenhagen, Denmark). Blood was diluted with 5% glucose in lactated Ringer's solution, with the hematocrit value kept between 20% and 22%. Perfusion pressure was kept constant at 80 mm Hg by an overflow system. Temperature of the perfusate and hearts was kept at 37° C. The maximal net developed tension (TNmax) of the papillary muscle was determined at intervals. TNmax continued to rise to reach a plateau approximately 45 to 60 minutes after coronary perfusion was initiated. This TNmax was used as a baseline. Cardioplegia was then induced by infusing 10 ml of either the control solution (Solution A) or test solution (Solution B) (Table 11). The duration of normothermic anoxic cardioplegia was 30, 45, and 60 minutes. Two epicardial electrocardiographic electrodes were used to check electrical activity of the heart. The time required to

Table V. Percentage recovery of maximal net developed tension Groups fA lB IIA liB IlIA IlIB

I--p-e-rc-e-nt-ag-e-r-e-co-v-ery-77.7 91.3 65.1 55.1 4.5 18.6

± ± ± ± ± ±

p Value

4.1 8.3 16.0 27.1 5.1 11.1

<0.01 NS

<0.05

Table VI. Comparison of percent recovery of maximal net developed tension Anoxic time Method of cardioplegia

30 min

45 min

Magnesium

91 (78)* 90 (65) 91 (81)

65 (55) 43 (37) 63 (32)

Potassium" Procaine'?

'Figures in parentheses are percent recovery of control group.

make the electrocardiogram a straight line was referred to as an arrest time. Thirty-six experiments were equally divided into six groups (Table III). Following cardioplegia, the heart was reperfused with oxygenated blood for 60 minutes. A restarting time was defined as that required for the resumption of rhythmic contractions after reperfusion. The percent recovery of TNmax was determined at intervals until it reached a plateau. This value was then used as the percent recovery of myocardial contractility.

Results All hearts stopped very rapidly (0.8 ± 0.1 minutes) when infused with magnesium sulfate solution (Solution B) (Table IV). The arrest occurred significantly (p < 0.0 I) fasterthan with the control solution ( 10.9 ± 2.1 minutes). When reperfused, all hearts resumed

Volume 84

Magnesium cardioplegia

Number 5

68 7

November, 1982

rhythmic contractions spontaneously, Restarting time was slightly delayed when anoxic time was prolonged (Table IV), but there was no significant difference between the comparable control and magnesium groups (Table IV), The percent recovery of each group following cardioplegia is shown in Table V. When the anoxic time was 30 minutes, the magnesium group showed significantly (p < 0.01) better recovery (91.3% ± 8.3%) than the control group (77.7% ± 4.1%). When the anoxic time was extended to 45 minutes, however, there was no significant difference between the two groups (65.1% ± 16% versus 55.1% ± 27.1%, p < 0.01). When cardioplegia was continued for 60 minutes, the recovery of the magnesium group was again better (18.6% ± 11.1%) than that of the control group (4.5% ± 5.1%). The difference was statistically significant (p < 0.05), but both results were very poor. Discussion

The blood-perfused papillary muscle preparation'" used in this experiment has been successfully used in our laboratory since 1973 in evaluating various methods of myocardial protection. Its advantage is that the measurement of the myocardial contractility is precise, reproducible, and quantitative. The magnesium concentration of the Bretschneider solution is only 4 mEq/L, which is in the physiological range. Therefore, with the Bretschneider solution hearts will not be arrested without procaine and potassium. The magnesium concentration of the St. Thomas' Hospital solution.. 32 mEq/L, is strong enough to induce cardioplegia, but this solution also contains high concentrations of procaine and potassium. Therefore, it is not clear which component actually protects the ischemic myocardium. In addition, there are questions about possible antagonistic actions between magnesium and procaine and magnesium and potassium. Because our objective in this experiment was to evaluate the effectiveness of magnesium itself as a cardioplegic agent, we adopted a high concentration, 160 mEq/L, like the Kirsch solution, so that magnesium could induce cardioplegia without other active ingredients. With this high concentration of magnesium, no other cardioplegic agents such as procaine or potassium were necessary to stop heart actions quickly (Table IV). Rather than using aspartate solution, as advocated by Kirsch, we used sulfate solution because it was readily available. Also there is very little evidence that aspartate solution has any advantage over sulfate or chloride solution. 9 In our study, a very high concentration of magne-

sium in the cardioplegic solution stopped hearts very rapidly but did not delay restarting time, as a high concentration (0.3%) of procaine solution did. 14 The recovery of the heart was significantly improved by magnesium cardioplegia, but this protective effect was time limited, as with procaine':' or potassium" cardioplegia. This study showed that magnesium has protective effects on the ischemic myocardium, and the magnitude of its effectiveness is comparable to that of potassium or procaine (Table VI). Recently Pemot and associates-" reported a beneficial effect of magnesium-rich, calcium-poor cardioplegic solution on the preservation of myocardial adenosine triphosphates and cardiac function of rat hearts during anoxic arrest. They stated that tissue enzymes and cardiac function were best preserved with magnesium-rich, calcium-poor solution and an addition of potassium to this cardioplegic solution did not enhance the protective effect of the magnesium. Antoni, Engstfeld, and Fleckenstein 7 observed that cardioplegia induced by magnesium solution could be reversed by potassium, procaine, epinephrine, or quinidine by increasing the action potential duration that was shortened by magnesium and thereby restoring contractions of the myocardium. Their study indicates that an addition of potassium to the magnesium cardioplegic solution may reduce the effectiveness of magnesium in protecting anoxic hearts. In conclusion, a high concentration of magnesium solution has a protective effect on the ischemic myocardium. However, its effect diminishes as the ischemic time exceeds 30 minutes at normothermia.

2

3

4 5 6 7

REFERENCES Sondergaard T, Berg E, Staffeldt I, Szczepanski K: Cardioplegic cardiac arrest in aortic surgery. J Cardiovasc Surg 16:288-290, 1975 Hearse DJ, Stewart DA, Braimbridge MY: Cellular protection during myocardial ischemia. The development and characterization of a procedure for the induction of reversible ischemic arrest. Circulation 54: 193-202, 1976 Kirsch Y, Rodewald G, Kalmar P: Induced ischemic arrest. Clinical experience with cardioplegia in open-heart surgery. J THoRAc CARDIOVASC SURG 63: 121-130, 1972 Iseri LT, Freed J, Bures AR: Magnesium deficiency and cardiac disorders. Am J Med 58:837-846, 1975 Wacker WEC: The biochemistry of magnesium. Ann NY Acad Sci 162:717-726, 1969 Polimeni P, Page E: Magnesium in heart muscle. Circ Res 33:367-374, 1973 Antoni H, Engstfeld G, Fleckenstein A: Die [Mg++]Lahmung des isloierten Froschmyokards. Ein Beitrag zur

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The Journal of Thoracic and Cardiovascular Surgery

Wakabayashi, Nishi, Guilmette

Frage der Beziehung zwischen Aktionspotential und Kontraktion. Pflugers Arch 275:507-525, 1962 Doring V, Baumgarten HG, Pokar H, Gercken G: Metabolism and fine structure of the [Mgttj-procainearrested perfused heart. Bas Res Cardiol 70: 186-197, 1975 Bretschneider Hl , Hubner G, Knoll D, Lohn B, Nordbeck H, Spieckermann PG: Myocardial resistence and tolerance to ischemia. Physiological and biochemical basis. 1 Cardiovasc Surg 16:241-260, 1975 Hoelscher B: Studies by electron microscopy on the effects of magnesium chloride-procaine amide or potassium citrate on the myocardium in induced cardiac arrest. 1 Cardiovasc Surg 8: 103-166, 1967 Hoelscher P, DOring B, Baumgarten HG, Dokar H, Gerchen G: Metabolism and fine structure of the [Mgttj-procaine-arrested perfused heart. Bas Res Cardiol 70: 180197, 1975 Hearse Dl, Stewart DA, Braimbridge MV: Myocardial protection during ischemic cardiac arrest. 1 THoRAc CARDIOVASC SURG 75:877-885, 1978 Ino T, Wakabayashi A, Guilmette MD, Shinto RA, Connolly JE: Effect of hypothermic anoxic cardioplegia on myocardial contractility. Ann Thorac Surg 22:424-428, 1976 Nishi T, Guilmette JE, Wakabayashi A: Experimental evaluation of myocardial preservation techniques. V. A membrane stabilizing agent, procaine hydrochloride. Ann Thorac Surg 30:349-355, 1980 Wakabayashi A, Ito Y, Nishi T, Guilmette lE, Connolly lE: Experimental evaluation of myocardial preservation techniques. IV. Potassium cardioplegia. Am 1 Surg 138: 154-161, 1979 Pernot AC, Ingwall lS, Menasche P, Grousset C, Cercot M, Mollet M, Piwnica A, Fossel ET: Limitations of po-

tassium cardioplegia during cardiac ischemic arrest. A phosphorus 31 nuclear magnetic resonance study. Ann Thorac Surg 32:536-545, 1981

Discussion DR. GEORG W. RODEWALD Hamburg, Federal Republic of Germany

I would like to comment on the use of magnesium in high concentrations. We abandoned the Kirsch solution, introduced in Hamburg in 1969, almost completely after 1975 for several reasons. Not the least of our reasons was its interference with thrombocytes and, therefore, pulmonary function disturbances. Moreover, myocardial protection with the Kirsch solution in combination with hypothermia was clinically safe for only 40 minutes. Despite this published experience, the solution seems to have a long life. For our group, however, it is a historical reminiscence, and we do not wish to be identified with the Kirsch solution any longer. MR. MARK BRAIMBRIDGE London, England

I would like to make two points about this interesting paper. First, magnesium alone, while it will stop the heart, does not do so as quickly as potassium and therefore will be less effective in minimizing adenosine triphosphate consumption in continued electromechanical activity. Second, the main function of magnesium in this context is to enhance the protective effects of potassium, reducing calcium influx during ischemia. In the rat heart, this action is maximal near the optimal concentration of 16 mmoles/L (32 mEq/L). Too high or too low a concentration will significantly reduce its effect, so that it is necessary in assessing any study of magnesium to take the specific concentration used into account.