Effect of multiple-dose potassium cardioplegia on myocardial ischemia, return of ventricular function, and ultrastructural preservation

J

THORAC CARDIOVASC SURG

80: 102-110, 1980

Effect of multiple-dose potassium cardioplegia on myocardial ischemia, return of ventricular function, and ultrastructural preservation To evaluate the myocardial protection afforded by multiple-dose versus single-dose administration of potassium cardioplegic solution. we studied 24 isolated feline hearts before. during. and after I hour of ischemic arrest. Intramyocardial gas tensions. ventricular function. histologic preservation. and postischemic myocardial edema were compared in hearts maintained at 27° C during the ischemic period. Equal groups of hearts received no infusion of cardiople gic solution. a single dose of potassium solution at the onset of ischemia. or multiple infusions of the cardioplegic solution throughout the arrest period. During ischemia. single-dose cardioplegic administration resulted in less accumulation of myocardial carbon dioxide (Pm m , ) than did hypothermia alone. reflecting a reduction in metabolic activity during ischemia. The fact that multiple-dose cardioplegia further reduced Pm m , accumulation suggests an intermittent washout of metabolic end products. During reperfusion, hearts protected by multidose cardioplegia demonstrated superior preservation of ventricular performance compared to hearts protected by single-dose cardioplegia or hypothermia alone. In addition. multiple infusions of the cardioplegic solution resulted in optimal structural preservation in both light and electron microscopic studies.

Scott K. Lucas, M.D., Edward B. Elmer, B.A., John T. Flaherty, M.D., Chadwick C. Prodromos, M.D., Bernadine H. Bulkley, M.D., Vincent L. Gott, M.D., and Timothy J. Gardner, M.D., Baltimore, Md.

Chemical cardioplegia and hypothermia have become widely accepted clinical techniques to protect the myocardium during global ischemia. By reducing rates of metabolic activity, hypothermia results in improved preservation of high-energy phosphate levels and prolongs the safe cross-clamp period.":" Potassium cardioplegia has been shown both experimentally'" !" and clinically 14-19 to provide additive protection to myocardial cooling by inducing immediate cardiac standstill and thereby obviating utilization of myocardial energy stores for useless electromechanical activity. The current focus of experimental investigation centers on methods to obtain optimum benefit from carFrom the Departments of Surgery, Medicine, and Pathology, The Johns Hopkins University School of Medicine, Baltimore, Md. Supported by U.S. Public Health Service Grant I ROI HL 194140 from the National Heart, Lung and Blood Institute and the Walter Cardiovascular Surgical Research Fund. Received for publication Sept. 5, 1979. Accepted for publication Dec. 14, 1979. Address for correspondence: Timothy J. Gardner, M.D., Division of Cardiac Surgery, The Johns Hopkins Hospital, 601 N. Broadway, Baltimore, Md. 21205.

102

dioplegic techniques. The present study compares the effects of multiple-dose versus single-dose administration of potassium cardioplegia on the time course of myocardial gas tensions during ischemic arrest and reperfusion, the recovery of postarrest ventricular function, the severity of myocardial edema, and myocardial preservation at the cellular and subcellular levels.

Methods Twenty-four isolated feline hearts were studied utilizing a modified Langendorff''" perfusion column. After induction of intraperitoneal anesthesia with sodium pentobarbital (30 mg/kg), median sternotomy and cardiectomy were performed, and the excised heart was immersed briefly in chilled (4° C) Krebs-Ringer-bicarbonate dextran solution" (KRB-dextran) (Table I). Within 60 seconds, the heart was transferred to a cannula at the base of an overflow column through which linear retrograde aortic perfusion was accomplished with oxygenated 37° C KRB-dextran solution at a constant aortic root pressure of 80 mm Hg (Fig. I). The perfusate was filtered with a 40 JL polyester filter and not recirculated. Hearts which did not spontaneously

0022-5223/80/070102+09$00.90/0 © 1980 The C. V. Mosby Co.

Volume 80 Number 1 July, 1980

defibrillate within 2 minutes of the onset of perfusion were electrically converted to sinus rhythm with a single 2 watt-second direct-current shock (Medrad Cardioverter Model 72103-A 2). With care taken to avoid the circumflex coronary artery, the left atrium was opened and the mitral valve and chordae tendineae excised, effectively venting the left ventricle and allowing insertion of a latex balloon into the left ventricular cavity. The balloon was attached to a plastic button via a 15 gauge needle and the apparatus sutured to the mitral anulus. Oversized holes in the plastic button effectively vented thebesian flow, and a flange prevented balloon herniation through the aortic valve during ventricular systole. The balloon cannula was connected by a short rigid tube to a lowvolume pressure transducer (Statham P23Db), and the balloon was filled with a sufficient volume of saline to produce a left ventricular end-diastolic pressure (EDP) of 10 mm Hg. Isovolumic left ventricular pressure and its first time derivative were recorded with a Honeywell HM 1508 Visicorder, an Accudata MI04 amplifier, and an Accudata 132 differentiator. The frequency response of the system was linear from 0 to 30 Hz. Developed pressure (systolic minus diastolic pressure) and maximum positive dP/dt were used as indices of left ventricular function. After the sinus node had been crushed, the hearts were paced at 110 beats/min with a Medtronic 5880 pacemaker at a pulse amplitude 15% above threshold. The superior and inferior venae cavae were ligated and the pulmonary artery was cannulated to allow volumetric measurement of coronary flow. An electrode in the right ventricular myocardium allowed the recording of myocardial electrograms throughout the study. A 22 gauge Teflon-coated gas probe was inserted at a midmyocardial depth in the posterolateral wall of the left ventricle through which a mass spectrometer (Scientific Research Instruments, Medspec MS-8) continuously monitored myocardial oxygen (Pmo2) and carbon dioxide (Pm C02) tensions. Details of this method have been previously described. 22 - 24 A needle thermistor (Yellow Springs Instrument Company, Yellow Springs, Ohio) was used to measure myocardial temperature in the interventricular septum. All mass spectrometer readings were corrected for temperature according to the method of Holness and Brock. 25 After an initial 10 minute period of normothermic stabilization, control measurements of gas tensions and left ventricular function were obtained. Global ischemia was then produced by interrupting aortic root perfusion. In hearts receiving a single dose of cardioplegic solution, a 10 ml bolus of 4° C potassium solution

Multiple-dose potassium cardioplegia

I03

Perfusale

_"over--ftow

110

em

H2 0

Aorta __ Bullon sewn in ,,/ ,,------mur a; -armotus . .' -.To fluid filled 'r-ansJucer

Fig. I. Diagram of isolated heart preparation.

Table I. Composition of solutions for perfusion and cardioplegia Biochemical composition

KRB-dextran

Sodium (mEq/L) Potassium (mEq/L) Calcium (mEq/L) Magnesium (mEq/L) D-glucose (gm/L) Dextran-70 (ml/L) Osmolality (mOsm) pH (with NaHCO,,)

140.0 5.0 2.5 1.0 3.0 167.0 300.0 7.4

Potassium cardioplegic solution

90.0 37.0

o

o

17.6

o

290.0 7.4

Legend: KRB, Krebs-Ringer-bicarbonate.

(Table I) was injected into the aortic inflow cannula immediately following interruption of myocardial perfusion. Hearts in the multiple-dose cardioplegia group received two additional 10 cc doses of potassium solution (27° C) at 20 and 40 minutes of ischemia. During the ensuing 60 minute arrest period, the latex balloon was emptied to simulate the operative conditions of a nondistended left ventricle. All hearts were maintained at 27° C by topical cooling throughout the arrest period. Group II and III hearts, receiving initial infusions of cold cardioplegia solution, attained 27° C slightly more quickly than the Group I hearts. All hearts were 27° C within 4 minutes of the onset of ischemia. Ischemia was terminated after I hour, following which there was a 45 minute period of reperfusion with oxygenated KRB-dextran solution at 37° C. Those few hearts which did not spontaneously defibrillate were

The Journal of Thoracic and Cardiovascular Surgery

I 04 Lucas et al .

Fig. 2. Left. Transverse histologic section through both ventricles at the mid ventricular level. Ischemic damage was assessed by reviewing the entire section of ventricle at low and high power for evidence of contraction band injury (right). (Hematoxylin and eosin stain . Left. x4; right . x425 .)

electrically converted within 2 minutes of the onset of reperfusion. With the intraventricular balloon deflated, the hearts were maintained in a beating, nonworking state during the first 15 minutes of reperfusion . Following reinflation of the balloon with the same volume which had been used previously for control measurements, left ventricular function was measured at 15 minute intervals and myocardial gas tensions and temperature were recorded continuously during the reperfusion period . At the completion of reperfusion, the position of the gas probe was confirmed. The weight of a left ventricular biopsy was determined before and after dessication for 48 hours, and myocardial water content was calculated by the following formula: (

I - Dry we~ght) Wet weight

x 100 = % HO 2

Four additional cat hearts, which were excised but neither perfused nor subjected to prolonged ischemia, were used to determine control myocardial water content. Additional transverse sections of myocardium were fixed for light microscopy in buffered 10% formalin, embedded in paraffin, sectioned, and stained with hematoxylin and eosin and phosphotungstic acid-hematoxylin stains. Electron microscopic samples were fixed in cold 3% glutaraldehyde with O.IM phosphate buffer, washed with several changes of 0 .1M phosphate buffer (pH 7.4), postfixed for l'h hours with osmium tetroxide in sucrose-phosphate buffer, dehydrated in a graded series of alcohols and acetone, and embedded in epoxy resin . Semithin (1 IJ-) sections of epoxy resin-embed-

ded tissue were stained with toluidine blue and examined by light microscopy . Ultrathin sections were stained with lead citrate and uranyl acetate and examined with the electron microscope (AE!). Ischemic damage was estimated by the presence and extent of contraction band injury across the entire transmural section of ventricle at the level of the papillary muscle (Fig . 2). The amount of injury was assessed quantitatively by grading the slides at 100 x magnification, which resulted in visualization of approximately 10% of the entire field. A 6 by 6 grid was placed in the eyepiece, and foci of contraction band injury falling within a box in the grid were counted in each of 10 randomly selected fields. The grading was done without knowledge of the identity of the histologic section, and on two separate occasions. From each slide, the scores of 10 fields were summed, and for each study group a mean score was determined. Electron micrographs were evaluated qualitatively for the presence and severity of mitochondrial swelling, disruption and mineralization, contraction bands, and disorganization of sarcomeres as well as clumping of nuclear chromatin, all of which were interpreted as signs of ischemic injury. Studies were performed on three groups of eight hearts, with all groups maintained at 27° C during the ischemic period. Group I hearts (control) received no cardioplegic solution and underwent simple hypothermic arrest, while Group II hearts received a single infusion of the 4° C potassium cardioplegic solution . Group III hearts received a potassium cardioplegic solution immediately following aortic cross-clamping and

Volume 80 Number 1 July, 1980

Multiple-dose potassium cardioplegia

1 05

220 200 180 160 140 PmC02 (mmHQl

120

,,/' .J"

100

~

u\

}

\

Y "" u \ r-__..L.----r 27° //.. .-f - ~ \\. 127° KCL Multidose

80 60

" /It\

-l . . 1 ,+

1 -'

lL

r

oC·l--'--"'l27°KCL

Single Dose

40 20

REPERFUSION ISCHEMIC ARREST

0'--_----JC---'_--1._--'-_-L_-'-_...I...-_ _

CONTROL 15

30

45

60

.15

.30 .45

TIME (Minutes)

Fig. 3. Myocardial carbondioxide tension (Pm co,) in Groups I, II, and III, recorded prior to and during I hour of ischemic arrest and during the 45 minute reperfusion period. (*p < 0.001 versus 27° C; **p < 0.01 versus single-dose potassium cardioplegia.)

at 20 minute intervals thereafter. Subsequent doses of cardioplegic solution were infused at 27° C to maintain a constant myocardial temperature. Composition of the cardioplegic solution is listed in Table I. The results are expressed as the mean ± one standard error of the mean. Paired and unpaired Student's t tests were used for statistical analysis. Differences were considered significant when p was 0.05 or less. Results Following the injection of the potassium cardioplegic solution, ventricular electrical activity, monitored by a myocardial electrode, ceased immediately in Group II and III hearts. In the Group I hearts which did not receive the cardioplegic solution, ventricular bradycardia and fibrillation were recorded for variable periods up to 8 minutes after the interruption of aortic root perfusion. Myocardial gas tensions. The mean preischemic Pm co, in the three groups ranged from 63 ± 3 to 71 ± 4 mm Hg (Fig. 3). Following the onset of ischemia in hearts protected by 27° C hypothermia alone (Group I), Proco• ·rose steadily to 180 ± 12 mm Hg by the end of the 60 minute ischemic period. In hearts receiving a single infusion of the potassium solution

(Group II), Proco. rose less rapidly to a value of 115 ± 6 mm Hg at the end of the ischemic period (p < 0.001 versus Group I). Group III hearts, in which multiple infusions of cardioplegic solution were administered, demonstrated no significant increase in Pmco. during ischemia, with a peak level of 71 ± 4 mm Hg reached after 60 minutes of ischemic (p < 0.02 versus Group II). Furthermore, each administration of the cardioplegic solution in the Group III hearts resulted in a transient cessation of Proco• rise and a flattening of the Pm co• time-course curve (Fig. 4). After release of the aortic cross-clamp, Pmco. briefly rose in all groups, reaching a peak level within 3 minutes of the onset of reperfusion. In Group I hearts, Pmco, peaked at 186 ± 8 mm Hg in comparison to the peak level of 123 ± 5 mm Hg seen in Group II hearts (p < 0.001). Multiple infusions of the potassium cardioplegic solution (Group III) resulted in an even lower peak level of Pmco. (89 ± 5 mm Hg) when compared to that of the single infusion hearts (Group II) (p < 0.001). After the initial peak, Pmco. in all groups rapidly fell to control values, and no significant differences were noted among the groups after 15 minutes of reperfusion . There were no significant differences in Proo•

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1 06 Lucas et at.

Thoracic and Cardiovascular Surgery

90

...

120

CardiOplegia

I

80

1270

110

Cardioplegia 70

I

100

il

60 PmC02 (mmHg)

-.1270

50

DP (% Controt)

70

30

60

20

50

1

o

IC

TIME

20

30

4C

50

40

60

O~"'-<

OF ISCHEMIC ARREST (Minutes)

--,"=___-~---:':=___--I

CONTROL

Fig. 4. Time course of myocardial carbon dioxide tension

'30

'15

I

+45

TIME OF REPERFUSION (Mlnutesl

(Pm co,) during ischemic arrest in six of eight hearts receiving

multiple-dose potassium cardioplegia (Group III).

KCL Single Dose

80

40

0

KCL Mullidose

Fig. 6. Recovery of developed pressure (DP) in Groups I, II, and III, recorded at 15, 30, and 45 minutes of reperfusion. (*p < 0.01 versus 27° C; **p < 0.05 versus single-dose potassium cardioplegia.) 140 130 I

120

I

....

I

110 100 ':'"

o

/

\~....

REPERFUSION ISCHEMIC ARREST CONTROL 15

30

45

60

.15

.30

.45

90 Max dP/dt (%Cant,all

80

"

'':"

'~~,

"

70

changes among the three groups (Fig. 5). Generally, Pm.; fell from a control range of 31 to 44 mm Hg to low levels (I to 6 mm Hg) in all groups during the initial period of ischemia and remained at this low level until the onset of reperfusion. During early reperfusion, a transient rise of Pm.; levels above control was noted in all hearts with a subsequent rapid return to control values. Ventricular performance. Control developed pressure, recorded prior to the induction of ischemia, was 118 ± 4 mm Hg in Group I hearts, 116 ± 8 mm Hg in Group II hearts, and 112 ± 7 mm Hg in Group III hearts. These values are not significantly different.

60

I

;' til /

r

/ .......... ./ . . . . . . . . . 27° KCL Single Dose

I/o

\\

TIME (Minutes)

Fig. 5. Myocardial oxygen tension (Pmo,) in Groups 1, II, and III, recorded prior to and during I hour of ischemic arrest and during the reperfusion period.

.I /. '1 .

I

27° KCL Mullidose

\,

,

27°

,,

50

oL~,,-----,----~---

CONTROL

,15

I

.30

.45

TIME OF REPERFUSION (Minutes)

Fig. 7. Recovery of maximum positive dP/dt in Groups I, II, and III, recorded at 15, 30, and 45 minutes of reperfusion. (*p < 0.05 versus 27° C; **p < 0.001 versus 27° C.)

Control maximum positive dP/dt values were also statistically similar for the three groups-Group I 1,240 ± 66 mm Hg/sec, Group II 1,180 ± 81 mm Hg/sec, and the Group III 1,126 ± 73 mm Hg/sec. Following 60 minutes of ischemia and 45 minutes of reperfusion, developed pressure in Group I hearts was 57% ± 7% of control and maximum positive dP/dt

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Multiple -dose potassium cardioplegia

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July, 1980

Fig. 8. Electron micrograph from a heart exposed to multiple-dose cardioplegia , showing virtually no evidence of ischemic injury . The mitochondria (m) are not swollen and the sarcomeres (s) are in register, tightly packed , and not contracted. nucl, Nucleus . (X 12,000 .)

was 72% ± 7% of control (Figs. 6 and 7). The use of a single dose of cardioplegic solution (Group II) resulted in improved functional recovery to 86% ± 6% control developed pressure (p < 0 .01 versus Group I) and 95% ± 7% of control maximum dP/dt (p < 0.05 versus Group I). Hearts receiving multiple doses of the potassium solution (Group III) demonstrated further improvement in . recovery, regaining 119% ± 13% prearrest dP/dt and 108% ± 7% prearrest developed pressure (p < 0.05 versus Group I).

Left ventricular end-diastolic pressure (EDP) was elevated in all groups after ischemic arrest and reperfusion. As measured by the isovolumic method, the control EDP was 10.1 ± 0.2 mm Hg for all hearts. Although hearts receiving multidose cardioplegia (Group III) demonstrated slightly less elevation of EDP (12.4 ± 1.5 mm Hg) than hearts receiving a single dose of potassium (13.5 ± 1.3 mm Hg) or hypothermia alone (16.6 ± 2.9 mm Hg), differences among the groups were not statisticalIy significant.

The Journal of Thoracic and Cardiovascular Surgery

I a 8 Lucas et at.

Table II. Myocardial water content Group Group I (n = 8) Group II (n = 8) Group III (n = 8) Control (n = 4)

.s:':

L--p-e-rc-e-n-, 86.32 86.64 85.32 79.00

± ± ± ±

0.45 0.56 0.52 0.11

Ultrastructural changes. Histologic evaluation among the three study groups demonstrated differences in the injury score between the groups. Group I (hypothermia without cardioplegia) had a score of 31 .6 ± 5.9; Group II (single-dose cardioplegia), a score of 22.0 ± 3.0; and Group III (multidose cardioplegia), a score of 11.2 ± 1.4. Differences between Groups II and I and Groups II and III were significant at the p < 0.01 level. Electron microscopic evaluation of the tissue showed a range of mild-to-moderate injury with evidence of significant myocardial protection in all groups (Fig. 8). Myocardial water content. Myocardial water content was increased in all groups when compared to control hearts, which were neither perfused nor made ischemic (p < 0.001) (Table II). Although Group III hearts tended to show somewhat less edema formation, no significant differences were noted among experimental groups. Discussion By all parameters measured in this study, multipledose administration of potassium cardioplegic solutions provided superior myocardial protection during ischemic arrest. Possible mechanisms responsible for the improved myocardial protection provided by multipledose cardioplegia administration include: (1) the delivery of additional buffer to decrease the severity of intracellular acidosis, (2) washout of metabolic end products such as lactate and hydrogen ions, and (3) the periodic replenishment of metabolic substrate. Previous studies in this laboratory and by others have described the use of mass spectrometry for the measurement of myocardial carbon dioxide tensions as an index of metabolic activity during ischemia. 9. 26-29 After the onset of global ischemia, Pm.; falls as tissue oxygen is rapidly depleted. Under these anaerobic conditions, myocardial glycogen is utilized as substrate for glycolysis in order to generate high-energy phosphates. As glycogen stores are depleted, adenosine triphosphate (ATP) utilization will exceed production. Since ATP hydrolysis is associated with the generation of hydrogen ions, which are subsequently buffered by the bicarbonate buffer system, carbon dioxide is produced as ATP is hydrolyzed. Without coronary flow,

no washout is present and the concentration of carbon dioxide as well as other metabolic end products, such as lactate, would be expected to rise. The rate of rise of Pmco, thus reflects the rate of metabolic activity under ischemic conditions and reflects the rate of ATP depletion. The induction of immediate diastolic arrest by the administration of potassium cardioplegia stops the utilization of energy stores for useless electrical or mechanical activity and therefore reduces myocardial metabolic activity at the onset of global ischemia. Hypothermia also reduces metabolic activity during ischemia.!" However, since a similar degree of hypothermia was maintained in all groups, the decreased accumulation of Pmco, seen in Group II hearts must reflect the reduction in metabolic activity occurring with the induction of total cardiac arrest by the cardioplegic agent. The further reduction in Pm m, levels seen when multidose cardioplegia (Group III) is compared with single-dose potassium administration (Group II) cannot be explained by differences in metabolic activity alone, since both groups maintained cardioplegia and were cooled to a similar temperature during arrest. Differences in Pmco, accumulation may be explained by three possible mechanisms. First, the delivery of additional bicarbonate buffer by multidose administration of cardioplegia would be expected to reduce the severity of intracellular acidosis resulting from ischemia. A decrease in the degree of acidosis would lessen the inhibition of the glycolytic enzymes glyceraldehyde-3phosphate dehydrogenase and phosphofructokinase. 30 Second, the accumulated carbon dioxide and lactate would be washed out by intermittent perfusion of the coronary circulation by the cardioplegic solution. The time course of Pm co, during ischemia in Group III suggests that this intermittent washout mechanism is playing an important role. The third potential benefit of multidose administration is periodic replenishment of metabolic substrate. The glucose contained in the cardioplegic solution may be metabolized via the glycolytic pathway. Reduction of the severity of acidosis would facilitate the utilization of this glucose and thereby allow for the production of ATP by anaerobic pathways. Recent studies from this laboratory have utilized 31-phosphorus nuclear magnetic resonance to serially monitor intracellular pH and ATP levels in the intact, functioning heart. Lower levels of Pmco,, as measured in the present study, can be seen to correlate with improved preservation of myocardial ATP and less severe degrees of acidosis during global ischemia.t" With a similar protocol used in isolated rabbit hearts, mul-

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Multiple-dose potassium cardioplegia

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July. 1980

tidose administration of a potassium cardioplegic solution resulted in superior preservation of ATP, a reduction in the fall of intracellular pH during ischemic arrest, and improved recovery of postischernic ventricular function when compared to single-dose cardioplegia or hypothermia alone. 32 Although the exact relationship between ATP levels and the recovery of postischemic ventricular performance is not clear, 33 reduced ATP levels have been shown by others to be associated with decreased postarrest ventricular performance." 3. 22 Since ATP depletion appears to be related to carbon dioxide production, then higher peaks of PmClh levels during ischemia would be expected to be associated with poor return of ventricular function following reperfusion, as was shown in the current experiment. Maintenance of higher levels of ATP during ischemia would be expected to correlate with improved structural preservation, as high-energy phosphate stores remain at levels sufficient for maintenance of cell membrane integrity and cell viability following ischemia and reperfusion. Superior preservation of myocardial structure is reflected by the fact that the lowest cumulative injury score in this study was seen in hearts receiving multidose cardioplegia. Several previous studies reported by other investigators have also demonstrated the value of multidose cardioplegia. Engelman and associates;" using an in vivo pig preparation, demonstrated improved myocardial contractility and less severe compliance changes following ischemic arrest with multiple infusions of either potassium or magnesium-procaine solutions. Using intermittent infusion of potassium cardioplegia in a dog model, Nelson and co-workers'" noted normal postischemic coronary blood flow and distribution patterns, no change in compliance, and return to prearrest ventricular function. Similar results were obtained by others using various combinations of multidose sanguineous or asanguineous cardioplegic solutions. 36-38 Additional benefits of multidose cardioplegic infusions applicable to the clinical situation, but not to the present isolated heart model, include improved maintenance of myocardial hypothermia as well as maintenance of adequate extracellular potassium concentration for continued cardioplegia despite the dilutional effect of noncoronary collateral flow. 39 Weisel and colleagues'" demonstrated improved postarrest myocardial performance curves and decreased release of lactate and myocardium-specific creatine kinase in patients receiving multiple-dose rather than single-dose cardioplegia. In summary, the results of this study demonstrate a reduction in the severity of the ischemic insult incurred

during a period of cardiac arrest and an associated improvement in the return of ventricular performance during reperfusion when multiple-dose administration of potassium cardioplegia is compared to single-dose cardioplegia or hypothermia alone. In addition, multiple-dose cardioplegia resulted in superior structural preservation in both light and electron microscopic studies. REFERENCES Gott VL, Bartlett M, Johnson JA, Long OM, Lillehei CW: High energy phosphate levels in the human heart during potassium citrate arrest and selective hypothermic arrest. Surg Forum 10:544-546, 1959 2 Gott VL, Bartlett M, Long OM, Lillehei CW, Johnson JA: Myocardial energy substances in the dog heart during potassium and hypothermic arrest. J Appl Physiol 17: 815-819, 1962 3 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 4 Fuhrman GJ, Fuhrman FA, FieldJ: Metabolism of rat heart slices with special reference to effects of temperature and anoxia. Am J Physiol 163:642-647, 1950 5 Flaherty JT, Schaff HV, Goldman RA, Gott VL: Metabolic effects and functional consequences of progressive degrees of hypothermia during global ischemia. Am J Physiol 5:H839-H845, 1979 6 Griepp RB, Stinson EB, Shumway NE: Profound local hypothermia for myocardial protection during open-heart surgery. J THORAC CARDIOVASC SURG 66:731-741,1973 7 Hearse OJ, Stewart OA, Braimbridge MV: Hypothermic arrest and potassium arrest. Metabolic and myocardial protection during elective cardiac arrest. Circ Res 36: 481-489, 1975 8 Gay WA, Ebert PA: Functional, metabolic, and morphologic effects of potassium-induced cardioplegia. Surgery 74:284-290, 1973 9 Schaff HV, Dombroff R, Flaherty JT, Bulkley BH, Hutchins GM, Goldman RA, Gott VL: Effect of potassium cardioplegia on myocardial ischemia and post arrest ventricular function. Circulation 58:240-249, 1978 10 Kay HR, Levine FH, Fallon IT, Grone GJ, Butchart EG, Rao S, McEnany MT, Austen WG, Buckley MJ: Effect of cross-clamp time, temperature, and cardioplegic agents on myocardial function after induced arrest. J THORAC CARDIOVASC SURG 76:590-603, 1978 11 Behrendt OM, Jochim KE: Effect of temperature of cardioplegic solution. J THORAC CARDIOVASC SURG 76: 353-357, 1978 12 Fey KH, Follette OM, Livesay JJ, Nelson RL, Bugyi H, Deland EC, Buckberg GO: Effect of membrane stabilization on the safety of hypothermic arrest after aortic cross-clamping. Circulation 56:Suppl 2:143-149, 1977 13 Todd GJ, Tyers GFO: Amelioration of the effects of is-

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chemic cardiac arrest by the intracoronary administration of cardioplegic solutions. Circulation 52: 1111-1117, 1975 Tyers GFO, Manley NJ, Williams EH, Shaffer CW, Williams DR, Kurusz N: Preliminary clinical experience with isotonic hypothermic potassium-induced arrest. J THORAC CARDIOVASC SURG 74:674-681, 1977 Roe BB, Hutchinson JC, Fishman NH, Ullyot OJ, Smith DL: Myocardial protection with cold ischemic, potassium-induced cardioplegia. J THoRAc CARDIOVASC SURG 73:366-373, 1977 Engelman RM, Levitsky S, O'Donoghue MJ, Auvil J: Cardioplegia and myocardial preservation during cardiopulmonary bypass. Circulation 58:Suppl 1:107-115, 1978 Weisel RD, Lipton IH, Lyall RN, Baird RJ: Cardiac metabolism and performance following cold potassium cardioplegia. Circulation 58:Suppl 1:217-226, 1978 Follette DM, Mulder DG, Maloney lV, Buckberg GD: Advantages of blood cardioplegia over continuous coronary perfusion or intermittent ischemia. 1 THoRAc CARDIOVASC SURG 76:604-619, 1978 Conti VR, Bertranou EG, Blackstone EH, Kirklin JW, Digerness SB: Cold cardioplegia versus hypothermia for myocardial protection. Randomized clinical study. 1 THORAC CARDIOVASC SURG 76:577-589, 1978 Langendorff 0, Siebert G: Studien uber die innervation der athembewegungen, Arch Anat Physiol 241, 1881 Weisfeldt ML, Shock NW: Effect of perfusion pressure on coronary flow and oxygen usage of the non-working heart. Am 1 Physiol 218:95-99, 1970 Brantigan lW, Gott VL, Martz MN: A Teflon membrane for measurement of blood and intramyocardial gas tensions by mass spectroscopy. 1 Appl Physiol 32:276-281, 1972 Khuri SF, O'Riordan 1, Flaherty IT, Brawley RK, Donahoo lS, Gott VL: Mass spectrometry for the measurement of intramyocardial gas tensions. Methodology and application to the study of myocardial ischemia, Recent Advances in Studies on Cardiac Structure and Metabolism, Vol 10, Metabolism of Contraction, TE Roy, G Rona, eds. Baltimore, 1975, University Park Press Brantigan lW, Perna AM, Gardner TJ, Gott VL: Intramyocardial gas tensions in the canine heart during anoxic cardiac arrest. Surg Gynecol Obstet 134:67-72, 1972 Holness DE, Brock Al: Temperature dependent characteristics of Teflon membranes used in mass spectrometry . Med Instrum 9:23-25, 1975 Khuri SF, Flaherty JT, O'Riordan JB, Pitt B, Brawley RK, Donahoo lS, Gott VL: Changes in intramyocardial ST segment voltage and gas tensions with regional myocardial ischemia in the dog. Circ Res 37:455-463, 1975 Bixler TJ, Gardner TJ, Flaherty IT, Goldman RA, Gott VL: Effects of procaine-induced cardioplegia on myocardial ischemia, myocardial edema, and postarrest ven-

The Journal of Thoracic and Cardiovascular Surgery

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