Depressed Function in Remote Myocardium After Myocardial Infarction: Influence of Orotic Acid

Depressed Function in Remote Myocardium After Myocardial Infarction: Influence of Orotic Acid Andrew D. Cochrane, FRACS, Shobhna Pathik, MBBS,Joseph J. Smolich, MBBS,PhD, Robert A. J. Conyers, FRCPA, and Franklin L. Rosenfeldt, FRACS CardiacSurgicalResearchUnit, BakerMedicalResearchInstitute,Prahran,Instituteof Reproductionand Development,Monash MedicalCentre, Clayton,and Departmentof Biochemistry,AlfredHospital,Prahran,Victoria,Australia

Background. We have previously shown that infarction impairs recovery of global function after subsequent cardioplegic arrest and that therapy with erotic acid improves recovery. The aim of this study was to measure the effect of infarction on regional and global left ventricular function and to determine whether erotic acid exerts a beneficial effect exclusive of the effects of cardioplegia. Methods. Acute myocardial infarction was produced in dogs. They then received either erotic acid or placebo (control) orally (n = 12 per group). Fractional radial shortening and systolic wall thickening were measured by two-dimensional echocardiography before and 1 and 3 days after infarction with and without fl-adrenergic blockade, and in 6 dogs up to 9 days after infarction. Global function was measured under anesthesia 4 days after infarction. Results. In control animals, fractional radial shortening in the infarct decreased from 20.6°/0* 5.17. before infarction to s.o~. * 2.27. at day 1 and to l.!)~. k 1.!)O/.at day 3 (p < 0.01). In the border zone radial shortening declined from 21.90/. * 3.7y0 to 11.00/o* 2.30/.at day 1 and 9.30/.*

c

linical experience with cardiac operations early after myocardial infarction suggests a vulnerable period beginning 6 hours after infarction and lasting for 24 hours [1] or more, when mortality from surgical reperfusion is increased. In previous experimental studies in the dog [2] and in the rat [3, 4] models, we found that in the first 1 to 4 days after infarction the heart has reduced tolerance to cardioplegic arrest, which may explain the vulnerable period for operation. We also showed that treatment with the pyrimidine precursor erotic acid for 2 to 4 days after infarction resulted in a twofold improvement in recovery from a subsequent episode of cardioplegic arrest [2-4]. There was also evidence of a beneficial effect of erotic acid on global function after infarction itself regardless of the effects of cardioplegia [2]. Similarly Yeh and coworkers [5] showed that erotic acid can improve recovery from global ischemia when given over a 4-day period. We have also shown that recent infarction reduces high-energy phosphates in the remainin~ noninfarcted Accepted for publication June 21, 1996. Address reprint requests to Dr Rosenfeldt, Baker Medical Research Institute, Commercial Rd, Prahran, 3181, Victoria, Australia.

by The Societyof ThoracicSurgeons Publishedby ElsevierScience Inc

01996

Z.8°/0 at day 3 (p c 0.05). In the noninfarcted myocardium radial shortening also declined from 27.lYo * l.g~o before infarction to 18.3% & 2.3y0 on day 1 (p < 0.05) and to 16.07. * 2.89’oon day 3 after infarction (p < 0.05) with recove~ to preinfarct levels by 9 days after infarction. These findings were confirmed by measurements of systolic thickening. Before infarction P-receptor blockade decreased fractional shortening in all regions of the left ventricle, but this effect was absent on day 3 after infarction, implying that the myocardium had become less responsive to &adrenergic stimulation. Measurements of global function 4 days after infarction showed marked depression of stroke work. There was no effect of erotic acid treatment on regional or global function. Conclusions. Myocardial infarction causes reversible depression of resting function and ~-adrenergic responsiveness in the remote and border zone areas, which is not prevented by metabolic therapy with erotic acid. This finding may explain the adverse response of the infarcted heart to cardioplegic arrest. (Ann Thorac Surg 1996;62:1765-72)

myocardium and that erotic acid therapy prevents this reduction [6], providing an explanation both for the sensitivity of the recently infarcted heart to cardioplegic arrest and for the beneficial effects of erotic acid. These findings encouraged us to believe that metabolic therapy with erotic acid might improve cardiac function after myocardial infarction, independent of any effect of cardioplegia, which would have important implications not only in the setting of postinfarct operation but also for the management of myocardial infarction in general. Indeed, several clinical reports from the Soviet Union in the 1970s showed dramatic improvements in cardiac function with erotic acid therapy after myocardial infarction [7, 8]. However, the true magnitude of the erotic acid effect and the site of action in the infarcted heart are essentially unknown. Therefore, in this study we aimed to investigate the effect of acute myocardial infarction on regional and global function and the effect of erotic acid therapy in the early postinfarct period. We used &adrenoreceptor blockade to attenuate the effect of sympathetic influences on the heart to measure intrinsic cardiac performance. 0003-4975/96/$15.00 PII SOO03-4975(96)O0673-X

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1996;62:1765-72

group, indicating no effect of treatment on the incidence of arrhythmias. A total of 31 dogs underwent echocmdiographic study up to day 3, but in 7 of these dogs the quality of the study was unsatisfactory, most com$ntdy because of persistent ventricular arrhythmias. Therefore, 24 dogs(12in the placeboand 12 in the eroticacid~oup)

F

INSTRUMENTATION LADLIGATION

/\

/’-% PLACEBO ~ ECHO-DAYIf+

OROTIC :ID n = 12

n = 12

I ECHO-DAY3*

\ n=3

I

FINALSTUDY /’ UNDER ANESTHESIA n=3 ECHO-DAYSAND9

Assessmentof RegionalLeftVentricularFunction

I

Fig 1. The expm”mental protocol.(LAD= lejianteriordescending cmwaryartery;MI = myocardial infarct.)

Material and Methods

Inductionof MyocardialInfarction Greyhounddogs,mean bodyweight27 kg (range,23 to 33 kg), were anesthetizedwith intravenousketamine (3 mglkg)and xylazine(0.6 mg/kg),intubatedand ventilated with oxygen,nitrousoxide,and halothane.Under sterile conditions,the heart was exposedthrougha left thoracotomy,catheterswere insertedinto the left internal mamma~ artery and the left atrium for pressure monitorin~ and pacing wires were sutured to the left atrialappendage.All visibleepicardialanastomosesbetween the left anteriordescendingcorona~ artery and branches of the circumflexand right coronary arteries werethenligated,andthe leftanteriordescendingartery distalto itsfirstdiagonalbranchwasoccludedbyligature in two stages over 20 minutes,to producea moderatesize~ anteroseptaltransmuralinfarct [2]. To minimize infarct-relatedarrhythmias,lignocainewas given as a 2 mglkg bolus before ligation and ccmtinuedas an infusionof4 mghninuntiltheendoftheprocedure.After exteriorizationof cathetersandpacingwiresbetweenthe scapulas,the thoracotomywas closedand the dogswere allowedto recover. Immediatelyafterinfarction,dogswererandomizedto receive either oral erotic acid (50 mglkg per day) or placebo (lactose), given three times a day in blinded fashion,commencing2 hours after infarction(Fig1). To confirmabsorptionof the erotic acid, urinaryeroticacid levelswere measuredusinga spectrophotometricassay [9].Theurinespecimenswere frozenaftercollection,and analyzedaftercompletionof the study.Elevatedurina~ levels of erotic acid were demonstratedin all erotic acid-treateddogs. Of 44 dogs undergoingthe coronary ligationprocedure,a totalof 19 (43°L)diedof ventriculararrhythmias, 13 duringthe first3-dayperiodof the echocardiographic study,and a further6 dogsbeforethe open-cheststudy on day 4 after infarction.Amongthe dogs that did 9 were in the erotic acid group and 10 in the untreated

I

were suitablefor the echocardiomarddcanalvsisbefore and after infarction.At day 4, Ii ~ogs (n =“8 in each group) underwentopen chest study of global cardiac function,whereas6 dogsunderwenti%nal studyat 9 days afterinfarction. All dogs receivedhumanetreatmentaccordingto the Code of Practice of the NationalHealth and Medical ResearchCouncilof Australia.

II

Regionalleft ventzictdar(LV) functionwas assessed by measuringfractionalradial shorteningand perce@a~e systolicwallthickenin~fromcross-sectionaltwo-dimensional echocardiographic(2-D echo) images. To assess the effect of erotic acid therapy on regional function, studies were performedin the conscious, nonsedated stateat three timepoints:2 daysbeforeinfarctio~and 1 and 3 days afterinfarction.As intrinsicchangesin myocardial performance may be confoundedby the increased circulatingcatecholaminesand increasedsympathetic activitythat accompanymyocardialinfarction [10],the preinfarctionand day 3 postinfarction2-R echo studieswere repeated after fl-adrenoreceptorblockade withpropranolol.&Blockadewas not attemptedon the first day after infarctionbecause of the potentialproblemsof hypotensionand modificationof infarctsize [11]. To assessthe longerterm effectsof myocardialinfarctionanderoticacidtherapyon regionalfunctio~a subaet of 6 dogs,3 in the eroticacid groupand 3 in the placebo group, were studied echocardiographicallyuntil day 9 afterinfarction. Duringthe 2-D echo studies,the dogs lay recumbent on a padded table, right side down in a quiet and darkenedroom.Duringthe studybeforeinfarcti~ lead II of the electrocardiogramand the heart rate were recorded,and afterinfarctionthe arterialbloodpressure and left atrial pressure were also recorded on a chart recorder.Heartswere imagedwitha 77500beta scanner (Hewlett-Packard,Andover,MA) coupledto a 3.5-MHz transducerpositionedbeneaththe right side of the thorax. Short-axisviewsofthe leftventriclewereobtainedat the midpapillarylevel [12]. Images were recorded on a videorecorder(PanasonicNV-8200,MatsushitaEledric, Japan) and gated with the electrocardiogramto display the end-diastolicand end-systolicframes+ All2-D echostudieswereperformedat a constantrate of 150 beats/rein.Preliminarystudiesindicatedthat this rate wasthe slowestthatwouldregularlypreventbreakthrough arrhythmiasafter infarction.Before infarction, heart rate was increasedto this target rate by administrationofintravenousatropine,0.6 to 1.0 mg afterinfarction atrial pacing was used. Baselinebefore and after infarctionheartratescouldnotbe matchedin 9 dogsdue to postinfarctiontachycardiaor persistentarrhythmias(5 in the placeboand 4 in the erotic acid group);therefore,

I

Ann Thorac Surg 1996;62:176.5-72

these measurements were discarded. However, after /3-adrenoreceptor blockade, rates were matched before and after infarction in all dogs. To control ventricular arrhythmias during the echocardiographic study, intravenous lignocaine was administered (2 mg/kg followed by an infusion of 4 mg/min). Measurements of fractional shortening made before (18.OYOt 3.lYo) and during the Iignocaine administration (17.5% f 1..5?’.) in 5 dogs showed no significant effect on contractility (p = 0.9). For the ~-adrenoreceptor blockade studies, 2-D echo images were also recorded 10 minutes after the administration of intravenous propranolol, 0.5 mg/kg.The completeness of the &adrenoreceptor blockade achieved was confirmed in 6 dogs before infarction using an isoprenaline dose-response test. To standardize the loading conditions under which the 2-D echo studies were performed after myocardial infarction, the mean left atrial pressure was maintained at 5 mm Hg above the level of the thoracic spine, by intravenous infusion of isotonic saline solution and 3.5”/. polygeline (Haemaccel). Echocardiographic Analysis The 2-D echo images were analyzed with a computerized analysis system (Cardio 500; Kontron Bildanalyse, Munich, Germany). After manual definition of the endocardial contours of the end-diastolic and end-systolic frames on the computer screen, the outlines were automatically digitized by the analysis program to provide appropriate regional measurements. To assess regional contractile function, the fractional radial shortening (as a percentage) of the ventricular cavity was calculated over 36 radiants at 10-degree intervals as 100 X [(EDR – ESR)/ EDR], where EDR = end-diastolic radius (in millimeters) and ESR = end-systolic radius (in millimeters) [13, 14]. The zero reference of the radiants was defined as the mid-point between the anterior and posterior papillary muscles [14]. A fixed rather than a floating centroid was used to avoid spurious increases in shorte~ing within the infarct zone [13]. The radiants corresponding to the infarct, border zone, and remote (noninfarcted) regions of the echocardiographic image were defined by comparison of the 2-D echo images with the postmortem midpapillary slice of the formalin-fixed heart. To evaluate the contractile function of the infarcted area, five radiants encompassing the center of the infarct were selected, and the mean fractional radial shortening calculated (Fig 2). For the border zone, three radiants on either side of the infarct in the adjacent noninfarcted myocardium were identified, and an average fractional radial shortening calculated. The choice of three radiants adjacent to the infarct zone was based on the observation that the functional border zone extends circumferentially for approximately 30 degrees beyond the interface of infarcted noninfarcted myocardium [15]. Mean fractional radial shortening for the remote myocardium was obtained using the five radiants directly opposite the infarct. Three to five beats were analyzed from each recording. Repeated analysis of the same recordings indicated that the measurement technique was highly reproducible (coefficient of variation, 8?’0).

COCHRANE ET AL

OROTICACID AFTER MYOCARDIAL INFARCTION

1767

SEPTUM

~

POSTERIOR

ANTERIOR LV

LV

BORDER zONE

20 ~

INFARCT

I

24

;\ BORDER ZONE

16=1 , * 8

@ 32

4/

NONINFARCTED

36

RADIANT

NUMBER

Fig 2. Cross-sectionalview of the lefi ventricle (LV) at mid-papillary level. (A) Area of infarctionon the anatomicsection. (B) Computer-derivedoutputof the end-diastolicand end-systoliccontours, and the 36 radiantsused to measurefunction. The radiantsusedfor the infarct,remotezone, and borderzone are indicated. (C) Graphic representationof the radial shortening, (LV = left ventricle.)

A potential drawback of fractional radial shortening is that it may be influenced by changes in the diastolic dimensions [13], a feature not shared by the percentage systolic wall thickening [16]. Therefore, to provide an assessment of regional function that was unaffected by infarct-related changes in ventricular dimensions, the systolic wall thickening of the remote and infarct zones was obtained from the same 2-D echo systolic and diastolic frames used for the measurement of radial shortening. Systolic wall thickening (as a percentage) was calculated as 100 X [(EST – EDT)/EDTl, where EST = end-systolic wall thickness (in millimeters) and EDT = end-diastolic wall thickness (in millimeters). Assessment of Global Le$ Ventricular Function On the fourth day after infarction, global LV function was measured in 8 placebo and 8 erotic acid-treated dogs. The animals were anesthetized with intravenous ketamine (3 mg/kg)and xylazine (0.3 mg/kg), intubated, and mechanically ventilated. Anesthesia was maintained with an infusion of alpha-chloralose (25 mg. kg–l oh–l)

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COCHRANE ET AL OROTIC ACID AFTER MYOCARDIAL INFARCTION

and ketamine (1 mgs kg-l” h-l). Electrocardiogram, femoralarterypressure,leftatrialpressure,arterialblood gases, end-tidalpartialpressure of carbon dioxide,and rectal temperaturewere monitored.The chest incision was reopened.A thermodilutioncardiacoutputcatheter (EdwardsLaboratories,Irving CA) was inserted into the pulmonary artery through a jugular vein. To minimize reflex autonomic stimulation of the heart during changes in loading conditions, the ansa subclavia and cervical vagosympathetictrunks were sectioned bilaterally [17] and /3-adrenoreceptors were blocked with propranolol (loading dose of 0.5 mglkg followed by an infusion of 40 p “kg-l “rein-l). Heart rate was maintained at 150 beats/ min with atrial pacing. The dogs were heparinized (3 mg/kg) and a cannula, connected to a reservoir containing a mixture of Haemaccel and donor dog blood, was inserted into the left atrial appendage to control left atrial filling pressure, For comparison with normal values, the same procedure was carried out in 6 noninfarcted dogs. To assess global LV function, cardiac output was measured during stepwise increases in left atrial pressure to achieve a maximum left atrial pressure of 15 mm Hg. The LV function curves were generated for the relation between the stroke work index (SWI) and mean left atrial pressure. SWI (g-m/kg) was calculated as [CO X (MAP– LAP) X 1.36]/[HR X BW X 100], where CO = cardiac output (in milliliters per minute), MAP = mean arterial pressure (in mm Hg),LAP = mean left atrial pressure (in mm Hg), HR = heart rate (in beats per minute), and BW = body weight (kilograms).

InfarctDelineation At the end of the experiment, 250 mL of 2% triphenyltetrazolium chloride was infused into the left atrium to define the infarcted area [18]. The dogs were then killed with phenobarbitone sodium. The heart was excised and fixed in formalin for 7 days. The atria and right ventricular free wall were removed, and the left ventricle was cut perpendicular to its long axis into slices 7 mm in thickness. The infarcted area on each slice, defined as the area not stained by triphenyltetrazolium chloride [18], was traced onto acetate transparericies and planimetered on a digitizing tablet. The proportion of infarcted myocardium in each slice was calculated, each slice was weighecL and the infarcted areas of all slices summed to yield the percentage of infarct for the entire left ventricle.

Statistical Analysis Longitudinal changes in regional function before and after infarction and the effect of erotic acid were analyzed with repeated measures analysis of variance. The effect of &adrenoreceptor blockade on fractional radial shortening was compared before infarction and on day 3 after infarction using paired t tests. Global function in the placebo and erotic acid-treated groups was compared using analysis of variance and the Newman-Keuls nmdtiple comparison procedure. Values are reported as mean f standard error, and a p value less than 0.05 was considered significant.

1S96*:1765-72

Results

InfarctCharacteristics The boundaries of the infarcts were sharply demarcated and the infarct affected the entire thickness of the LV wall, apart from a thin rim of overlying epicardium. The infarct size was not significantly dMerent between the two groups, comprising 17.4% * 1.2% of the LV mass in the control animals and 20.6% ~ 1.8”/O in the erotic aad group (p = 0.20).

Hemodynamics The resting heart rate in the control dogs increased from

98 ~ 6 beats/rein before infarction to 157 ~ 9 bea@/min

on day 1 after infarction (p < 0.01), and was similar on day 3 after infarction at 139 t 5 beats/rein. In the treated animals heart rate was 89 k 5 beats/rein before infarction, 149 ? 5 beatslmin on day 1, and 123 k 8 beatshnin

at day 3, whichwas not significantlydMerentfrom the controlanimals. After infarction,there was no diference in the mean arterial pressure or left atrial pressures at day 1 and day 3 between the control and treated animals. After control of the heart rate at 150 beats/miw elevation of the left atrial pressure to 5 mm H& and administration of lignocaine (on day 1 only), the mean arterial pressure on day 1 was 107 t 6 mm Hg in the control and 97 f 3 mm Hg in the treated dogs, and on day 3 was 108 t 3 mm Hg in the control and 100 x 5 mm Hg in the treated dogs (p = not significant).

E~ectof Infarctionon RegionalFunti”on In the control animals, in the infarct zone the fractional radial shortening decreased markedly on day 1 and on day 3 (p < 0.01) (Table 1 and Fig 3). In both the border zone and the remote myocardium the fractional radial shortening in the control animals declined on day 1 and on day 3 (p <0.05 in both areas). in the infarct zone of the control animals the systolic thickening decreased from 32.7% t 4.4% before infarction to –0.8’%0? z.s~. on day 1, and to –2.5% ? 3.1% on day 3 after infarction (both p < 0.01). The systoIic thickening in the remote myocardium was 43.6% t 4.070 before infarction, and decreased to 33.7% f 5.9% on day 1 after infarction (p = not significant), and to 24.lYo t 4.6Y0on day 3 after infarction (p < 0.05).

Eflectof &Blockadeon RadialShortening In the control group before infarction (n = 12), @-blockade produced a significant decline in fractional radial shortening in all three zones (Fig 4). In the future infarct zone the radial shortening declined from 20.6°/0t 5.1°L to 10.5% * 2.8% (p < 0.001), in the prospective border zone from zl.g~. t 3.7~0to 14.5% A 2.0% (p C 0.02), and in the remote myocardium the radial shortening declined from 27.1% 2 1.9% to 21.7% ? 1.6% (p < 0.01). In the erotic acid-treated group before infarction, p-blockade produced similar declines in fractional shortening in all zones. However, 3 days after infarction, j+bkwkade in the control animals had no significant effect on radial shortening in either the infarct zone (1.9% & 1.9% before versus –2.4Y0 ~ 2.5~0after blockade, p = 0.14) (see Fig 4),

COCHRANE ET AL

Ann Thorac Surg 1996:62:1765-72

OROTICACIDAFTERMYOCARDL4L INFARCTION

Table 1. FractionalRadial Shortening (%) in the Remote Myocardium,Infarct Zone, and Border Zone, Before Infarctionand at Dav 1 and Dav 3 After Infarction Regionand Group Infarctzone Control Treated Borderzone Control Treated Remote zone Control Treated

No. of Dogs

Pre-MI

Day1

Day3

7 8

20.6 f 5.1 20.4 t 4.9

3.0>2.2. _I).3 ~ 4@

1.9 ~ 1.9. 0.3 ~ l,9b

7 8

21.9 * 3.7 23.5 f 2.3

11.0 t 2.3’ 9,3 ~ 2.8. 14.3 * 2.1C 10.9 ~ 2.6”

7 8

27.1 ~ 1.9 28.6 * 1.4

18.3 ~ 2.3C 16.0 t 2.8C 20.4 t 1.9’ 18.2f 3.8’

a The 95% confidence limits for the difference between the control and treatment groups are: infarct zone: –3.67 to 6.87, border zone: –5.89 to b p < 0.01 versus before 9.o9, and remote zone: –7.05 to 11.45. d p <0.05 versus c p <0.001 versus before infarction; infarction; before infarction.

1769

1.9% before infarction to 8.7% ? 2.30% at day 1 and 8.7%

t 1.73Y0 at day 3, and then improved significantly to 17.57. ? 2.3% at day 9 (p <0.01 for day 9 versus day 3). In the remote myocardium the radial shortening declined from 26.4% t 1.8Y0before infarction to 19.r)~ot 1.8Y0at day 1 and 21.4Y0 ? 2.3Y0 at day 3, before recovering to 29.47. f 1.8Y0at day 9 (p z 0.01 for day 1 versus day 9 and p = 0.05 for day 3 versus day 9). Global LejlVentricular Function In control and treated dogs with myocardial infarction the maximum stroke work was markedly depressed below that in normal dogs (p < 0.001) (Fig. 6). There was no significant difference between the maximum stroke work attainable in the control infarcted dogs before P-blockade 2.2 ? 0.17 g-mlkg and the treated group 1.73 ? 0.24 g-m/kg (p = 0.25) or after &blockade, 1.8 * 0.33 g-m/kg in the control dogs and 1.6 % 0,37 g-mlkg in the treated animals (p = 0.7).

Comment the border zone (9.370 ? 2.87. before versus 8.2Y0 ? 2.8Y0 after blockade, p = 0.77), or in the remote myocardium (16.0% t 2.8% before versus 15.1% ~ 1.8% after blockade, p = 0.77). Comparing the preinfarction and day 3 postinfarction values after /3-blockade,radial shortening was reduced in the remote myocardium (p < 0.05) and in the infarct zone (P <0.000, and not statistically different in the border zone. In the erotic acid-treated group (n = 12) the effects of blockade were similar to the control group in all instances.

There were three findingsin this study: (1) there was a depression of function in areas of myocardium remote INFARCT 30

1

25

❑ Control ■ OATraded

TT

20-

15lo-

M

5-

* **

0

Hr/I

T

I

E~ect of Orotic Acidon RegionalFunctionAfler

1

Infarction In the treated animals, in both the infarct zone and the border zone, there were significant reductions in fractional shortening on both day 1 and day 3 compared with measurements obtained before infarction, but these were not different from the control group (Table 1, Fig 3). In the remote myocardium the fractional radial shortening decreased on day 1 (p < 0.05) and on day 3, and was not significantly different from the control group (p = 0.8). After /3-blockade (day 3 only) there was no significant difference between the control and treated groups in fractional shortening in any of the three regions.

BORDERZONE 2520-

The 6 dogs studied up to 9 days after infarction (3 control and 3 erotic acid treated) showed a similar pattern of reduced radial shortening at day 1 and 3 followed by progressive recovery. Because there was no effect of erotic acid discernible in either the early (days 1 and 3) or late (days 5 and 9) measurements, the results of both groups were combined to study the delayed recove~ of regional function (Fig 5). In these 6 dogs, the radial shortening in the infarct declined significantly from 19.9% ? 7.2% to 1.8% ? 4.7% on day 1 (p c 0.001), remained stable at day 3 and 5, and then showed a small, nonsignificant increase by day 9. In the border zone the radial shortening declined significantly from 22.1% f

*

*

105-

0REMOTE 3cr-

Rep”onalFunctionAjlerProlongedRecovery

* *

15-

L *

252015-

lo5-

0 Pre-Ml

Day 1

Day 3

Fig 3. Fra&’onalradial shorteningin the infarct, border zone, and remotezones before infarction (Pre-MI)and on day 1 and day 3 afferinfarction.Therewasno dij%rencebetweenthe controland treatedgroups. Significantlydiferentfrom before infarction: *p < 0.05; “p < 0.01; ***p<0,001. (OA = erotic acid.)

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Ano ThoracSurg 1996;S2176S-72

COCHRANEET AL OROTICACIDAITERMYOCARDIAL INFARCTION

30

INFARCT u canffd

Za

■ Seta-blackade

20 15k

-L

.5J

BORDERZONE 30, 25 20

Ml *

Ubik

drial structuraldegeneration[24].Areas of focalnecrosis (microinfarction)associatedwith abnormallactate metabolismhavebeenobbervedin the reroutemyc)caidium [26].The decreasein high-energyphosphatelevelsdemonstratedin dogs by Gubdjamasonaiid colleagues{23] was presentbetween1 and 4 days after infarction, consistent with the time-related changesin functionthat we obsewed. The functionaland metabolicchanges in the remote zone Suggestan imbalancebetween energy utilization and production,and that the remote noninfarctedmyocardiumis understiess,resultingin a reducedfunctional reserve. The remainingmyocardiumincurs an obvious functionalstress as the reduced mass of viablemuscle must work harder to maintain a satisfactorycardiac output.It also sutlersa mechanicaldisadvantageattributable to the presence of the akinetic or dyskinetic infarct,because eneqjy expendedin the motion of this area is wasted.Furthermore,the rem&ing myocardium is underneurohumoralstressfromthe increasedsympathetic drive and elevated levels of circulating catecholamines[10,271. 30 25 20

2015-

15

lo-

10

5-

5

0 Pre-Ml

Dcty3

Fig4. Ejfectof ~-blockade onfractionalradialshdtteningin the infarct,borderzone,andremotemyoc~rdium for thecontrolanimals (n = 12), beforeinfarction(Pre-MUandon day3 aj%rinfarction. Significant differencefiom pre-~-blockadevalue: ‘p < 0.05; **p< 0.01; -p ~ 0.001.

0 .5J

‘1

BORDER ZONE

-r

20 15

from a fidl-thidmess myocardial i~~ct at 1 to 3 days aft= infarction, recovering 9 days after infarction; (2) in the remote and border zone rnyocardium, there was a 10SSof the negative inotropic effett of adrenergic blockade 3 days after infarction, consistent with reduced adrenergic responsiveness; and (3) there was no beneficial effect of metabolic therapy with erotic acid on contractile function after myocardial infarction.

10 5 REMOTE

35

1 30

DepressedFunctionin RemoteNoninfarcted Myocardium

2s

Previous stxidiesof the remote, noninfarctedrnyocardiurnafter myocardialinfarctionhavqreportedthat the

20

contractile function may be augmented, unaltered [1921], or reduced [22]. Reduced function accords with the morphologic and biochemical abnormalities that have been demonstrated in, this region, includingreduced

15

levelsof high-energyphosphates[23, 6], reducedglycogen [24, 21, and decreasedactivityof mitochondrialenzymes [25],as wellas intracellularedemaand mitochon-

Befor6

1 I_

3

5

9

Duysctffer&#c#cnon J

Fig5. Fractionalradialshortening intheinfarct,borderzone,and renwternyocardium in6 @s (3 treatedand3 untreated)studiedjbr 9 daysafierinfarti”on.

Ann Thorac Surg 1996;62:1765-72

COCHRANEETAL

OROTICACIDAFTERMYOCARDIAL INFARCTION

3 Normal

1

Ml MI+OA

o~ 0

5

10

15

LeftAtrialPressure (mmHg) ventricularstrokework indexed for body weight plottedagainstleftatrialpressu~e for th; erotic acid@S~, treated(MI + 0~ n = 8), untreated(M& n = 8), and noninfarcted (NonnaLn = 6) groups with (3-blockade.Mm’mum strokework in infarctedgroups is significantlyless than in noninfarcteddogs (p < O.-WI).Therewasno differencebetweentreatedand control&oups (p = O.-25). Fig 6. I@

The fact that the remaining myocardium possesses a reduced functional reserve helps explain (1) why the recently infarcted heart responds poorly to the additional insult of global ischemia associated with cardioplegic arrest [2-4] and (2) why the recently infarcted heart responds favorably to myocardial protective methods during operation that minimize global ischemia [22]. Changes in /3-Adrenerg”c Responsiveness Before infarction we found, as would be expected, that /3-receptor blockade produced a decrease in fractional shortening in all regions of the left ventricle. However, on day 3 after infarction, &receptor blockade failed to produce a significant change in fractional shortening in any region. This implies that before infarction removal of the effects of circulating catecholamines reduced contractility, whereas after infarction removal of these effects produced no change in function. The most likely explanation is that as a result of persistent catecholamine stimulation after infarction the myocardium had become less responsive to p-adrenergic activity. This is consistent with previous studies of the remote myocardium [27] that demonstrated loss of response to isoproterenol stimulation at 3 days after infarction, with a reduced number of /3-receptor binding sites and reduced ailinity of the remaining /3-receptors. Orotic Acid Eflect Orotic acid is an important intermediate in the synthetic pathway for the pyrimidine nucleotides uridine and cytidine. Uridine nucleotides are important components of glycogen and RNA,whereas cytidine is an important constituent of the phospholipid component of cell membranes. After myocardial infarction there is a reduction in high-energy phosphates and glycogen in the remaining myocardium [23], and we have shown that erotic acid can prevent this [2, 6]. However, in the present study we were unable to demonstrate a significant effect of erotic acid on function in any zone, up to 9 days after infarction. This

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finding of a lack of erotic acid effect after infarction alone is consistent with our previous findings in rats [3, 4, 6] of no major effect of erotic acid administration on global LV function after infarction alone, but a significant and powerful effect of erotic acid after the added stress of cardioplegic arrest. Increased energy stores may improve tolerance to cardioplegic arrest but may not necessarily improve resting cardiac function. The previous clinical studies from Russia in the 1960s and 1970s [7, 8], showing an impressive benefit of erotic acid therapy after myocardial infarction, were not blinded or placebo controlled; however, it is possible that the hearts in these studies were more stressed than in the present study and hence could show an effect of erotic acid. It is probable that the role of erotic acid is to provide a form of “metabolic preconditioning” to improve the metabolism of the stressed heart. Thus, treatment with erotic acid might provide no detectable improvement in baseline function of the heart after infarction but, atler the additional ischemic stress of cardioplegic arrest, the treated myocardial cells might be better able to maintain cellular function. Limitations of the Study The relatively small size of the study population is a problem in that a negative result may not reflect the true absence of a beneficial effect of treatment, but reflect the small sample size. This is clearly a problem with studies involving animals that are a scarce resource and in particular in this study because there was considerable attrition from fatal ventricular arrhythmias. For the comparison of radial shortening at days 1 and 3, full and satisfactory data were collected in 7 control and 8 treated animals. On the basis of the standard errors of the data presented, and assuming a p value of less than 0.05 (two-tailed) and power of 0.8, to detect a significant ditTerence in the infarct zone between the two groups would require an absolute difference in the radial shortening of 4~0. Similarly, in the border zone the same criteria would require an absolute difference of 6Y0. Although it is possible that a small benefit of erotic acid therapy on regional wall motion has not been detected due to the small sample size, there is little evidence of a clinically important trend in the data. Clinical Significance It is generally accepted that after a large myocardial infarction there is a vulnerable period beginning at 6 hours and lasting up to a week when mortality and morbidity from operation is significantly increased. Despite recent improvements in operation and myocardial preservation, patients with recent infarcts or mechanical complications of infarction still have a higher mortality from surgical revascularization [28, 29]. Previously, we have postulated that this is due to a reduced tolerance of the noninfarcted myocardium to ischemia, because of the mechanical and metabolic stress placed on it by the infarct [2], The present study, by demonstrating both impairment of global function and impairment of regional function in all zones of the left ventricle, confirms that the reduced function after infarction is attributable

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COCHRANE ETAL

Ann Thoracslug 1*6.2176$72

OROTICACIDAFI13RMYOCARDIAL INFARCTION

to the combinedeffectof both the noncorttractileinfarct and the impairedfunctionin the remote’zone. Furthermore, the time course of depressed function in the remotemyocardiumthat we have observedis similarto the period of higher surgical risk in clinicalpractice. Althoughin this studywe did not determinethe precise reasons for this functionalimpairmen~these observationsare in accordwithstudiesin our ownlaboratory[6, 2] and elsewhere[23-25]that deacribemajorbiochemical derangementsin the noninfarctedmne. In turn, these disturbancesare likely to be exacerbatedby chronic p-adrenoreceptoroverstimulation. We concludethat in the recentlyinfarctedheart, the remote and border zone myocardiummay exhibitdepressed function.Consequently,in patientswith recent infarctionundergoingoperation,techniquesof myocardialpreservationshouldbe usedthat minimizeischernic damageto the alreadystressedmyocardium.Metabolic therapywitheroticacidwouldbe expectedto produceno major benefit in the patient with an uncomplicated infarct.Nevertheless,on the basisof our previouswork [2-4] we believethat erotic acid therapyis beneficialin the periodafterinfarctionin patientswithcomplications that may necessitatecardiac operationwithin1 weekof infarctionand use it routinelyin these patientsin our clinicalpractice. We gratefullyacknowledgethe technicalassistanceof Christine Egan and LealeyLangleyin the animalstudies,the technical assistanceofDrJim Cameronin the computerizedanalysis,and the adviceof JohnWilliamsof the AustralianNationalUniversity.

References 1.DeWoodM& SporesJ, Berg R Jr, et al. Acute myocardial infarction:a decadeof experiencewithsurgicalreperfusion in 701 patients.Circulation1983;68(Suppl2):t%16. 2. NewmanM& ChenXZ,RabinovM,WilliamsJF, Rosenfeldt FL, Sensitivityofthe recentlyinfarctedheartto cardioplegic arrest. Beneficialeffect of pretreatmentwith erotic acid. J ThoracCardiovascSurg1989;97:593-604. 3. MunschC, WilliamsJF, RosenfeldtFL. The impairedtoleranceoftherecentlyinfarctedrat heartto cardioplegicarrest: theprotectiveeilectoferoticacid.J MolCellCardiol1989;21: 751:4. 4. Munsch CM, RosenfeldtFL, O’Halloran~ LangleyLH, ConversRA, WilliamsIF. The effectof erotic acid on the resp&se of “tierecentl~infarctedrat heart to hypothermic cardioplegia.Eur J CardiothoracSurg 1991X:82-93. 5. Yeh T Jr, RebeykaIM, Jakoi E~ et d. Orotic acid improves left ventricularrecoveryfour days after heterotopictransplantation.Ann l%orac Surg 199458:409-15. 6, RichardsSM, Fisher JL, RosenfeldtFL, ConyersRAJ,Cardiourotectionby erotic acid:mechanismofaction.J MolCell Ca&iiol(in pre~s). 7. LukomskiiPE, MeersonFZ, SolovenW, Disturbancesof contractilefun’ctionoftheheartinmyocardialinfarctionand the therapeuticuse of cofactorsof synthesisandprecursors of nucIeicacids.firdiologiia 1967;7”+11. 8. ZharovEI. Use of co-factorsof the synthesisandprecursors of nucleicacidsin myocardialinfarctpatients.Kardiologiia 1971;11:15-25. 9. StainerA, SuvaL MusilF. The determinationof erotic acid in ‘tie blood se’~m by means of the spectrophotometric method.Experiential 1968;24:116-7.

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acute myocardialinfarction:reversibledefectdue to excessive circulatingcatecholamine-induced declinein number andaffinityof beta-receptors.Am HeartJ 1981;1OM69-81. 28. Applebaum~ HouseR Rademaker&et aLCoronaryartery bypassgraftingwithinthirty days of acute myocardtalinfarction. Early and late results in 406 patients.J ,~orac CardiovascSurg1991;102:745-52. 29. VonSegesserL~ PoppJ, AmannFW, TurinaMI. Surgical revascularization in acute myocardiafinfarction.Eur J CardiothoracSurg1994;8363-9.