serum enzyme determinations in the diagnosis and ... · serum enzymes in mi experimental myocardial...

Serum Enzyme Determinationsin the Diagnosis and Assessment

of Myocardial InfarctionBy BURTON E. SOBEL, M.D., AND WILLIAM E. SHELL, M.D.

DIAGNOSTIC ENZYMOLOGY has grownexponentially since elevated serum

amylase was first associated with pancre-atitis in 19081 and Karmen, Wroblewski,LaDue, and their associates demonstrated in1954 that SGOT* and LDH activity in serumincreased following myocardial infarction.2Following Markert's elucidation of the natureof LDH isoenzymes3 their importance indifferential diagnosis was emphasized.4 SerumCPK elevations following myocardial infarc-tion were first reported by Dreyfus and his co-workers5 in 1960, and soon confirmed by Hessand MacDonald.6 Determination of SGOT,LDH, and CPK activity rapidly becamecornerstones in the laboratory diagnosis ofacute myocardial infarction in man.

Activity of many enzymes including aldo-lase, malic dehydrogenase, isomerase, andICD may increase following myocardial in-farction.7 Serum GGT, a lysosomal enzyme,exhibits increased activity late, reaching apeak within 8 days and returning to normalapproximately 1 month following the initialinsult.8 SPK contrary to CPK is not elevatedfollowing intramuscular injections,9 while se-rum GAPDH elevation may precede increasesin conventional enzymes and aid detection ofextension of myocardial necrosis.10 However,since SGOT, LDH, and CPK determinationshave become established criteria in thelaboratory diagnosis of acute myocardial

From the Department of Medicine, School ofMedicine, University of California at San Diego, LaJolla, California.

Supported in part by U. S. Public Health ServiceMyocardial Infarction Research Unit Contract PH-43-68-1332, Project Grant HE 12373, Research andCareer Development Award 1-K4-HE-50, 179-01, andSpecial Fellowship 1 F03 HE 46505-01.

Circulation, Volume XLV, February 1972

infarction in man, material to follow willconcern those three enzymes primarily.

General ConsiderationsMeaningful interpretation of serum enzyme

determinations should be based upon severalconsiderations:

1. Elevated serum enzyme activity associ-ated with disease is generally assumed toreflect activity of enzyme released frominjured tissue. In most conditions, the sameenzymes with which an injured organ is richlyendowed are those exhibiting elevated activityin serum following organ injury.2 4 The timecourse of depletion of enzyme activity from adamaged organ parallels the time course ofincrease of activity of the same enzyme inserum following the insult."' 12 Followingexperimental coronary artery occlusion, asignificant although small myocardial arterio-venous difference of enzyme activity can bedemonstrated.'3 On the other hand, peakelevation of serum enzyme activity may notcorrelate closely with the magnitude of tissuedamage,14 and some enzymes, such as ICD,which decrease in ischemic myocardium andincrease in coronary sinus blood followingexperimental coronary artery occlusion, maynot increase concomitantly in serum.15 Thesedisparities are reconciled when the rate ofdisappearance of enzyme activity from serum,the rate of release of enzyme from ischemic

*Abbreviations: SGOT = serum glutamic oxalo-acetic transaminase; LDH = lactic dehydrogenase;CPK = creatine phosphokinase; ICD = isocitric dehy-drogenase; GGT = gamma-glutamyl transpeptidase;SPK = serum pyruvate kinase; GAPDH = glyceralde-hyde phosphate dehydrogenase; OCT = ornithine-carbamyl transferase; SGPT= serum pyruvic trans-aminase; and NADP = nicotinamide-adenine dinu-cleotide phosphate.

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myocardium, the fraction of myocardial en-zyme undergoing local denaturation, and thedistribution of enzyme released from the heartin body fluids are taken into account. Thus,ICD activity is not substantially elevated inserum following experimental myocardial in-farction because the enzyme's disappearancerate from the circulation is very rapid.1-Furthermore, when serial changes in serumCPK activity following experimental myocar-dial infarction are analyzed according to amodel based on these considerations, thecorrelation between myocardial enzyme de-pletion and increased serum enzyme activity isclose (r=0.96) over a wide range of infarctsize. 12

2. The relative value of enzyme determina-tions in the diagnosis of a specific diseaseentity should be assessed on the basis of bothspecificity and sensitivity of the test. Truepositives (instances of a positive result in apatient with the disease), false positives(instances of a positive result in a patientwithout the disease), and false negatives(instances of a negative result in a patientwith the disease) should be considered. Anenzyme test is highly specific when the ratio oftrue positives to false positives is high. It ishighly sensitive when the ratio of truepositives to false negatives is high and theincidence of false negatives is low. Obviously,sampling time and frequency will influencethe apparent specificity and sensitivity of thedetermination.

3. Enzyme determinations are assays ofactivity, not of amount or concentration of aspecific protein in serum. Accordingly, theapparent activity may be influenced by thepresence of inhibitors, cofactors, activators, orunusual amounts of substrate or product inthe serum sample.16

4. The nature of the assay procedure usedmay influence results profoundly. Since onlyinitial velocity is a reflection of enzymeactivity, kinetic assays are generally superiorto assays based on a single determination ofproduct concentration and less prone to errorintroduced by nonspecific chromogens inbiologic material.

5. Other factors related to sampling, stor-age, and laboratory technique may impair thevalidity of enzyme determinations.

6. The normal range of activity of eachserum enzyme must be defined under theassay conditions employed and is usuallydefined as the mean ± 2 SD of values from acontrol population.-' 4 When a skewed distri-bution is observed in control samples, "nor-mality" may be defined with a log normalcurve.'7 These definitions of normality meanthat the value obtained in a sample from anormal individual will fall within the normalrange 95% of the time. Stated in another way,abnormal values for serum enzyme activitywill be obtained in 5% of normal individuals.Thus, an "abnormal" value does not necessari-ly imply that the patient from whom it wasobtained harbors disease.Enzyme values vary with age, sex, and

activity.'X Accordingly, if a normal range wereestablished based on samples from 18-year-oldstudent nurses, its application to interpretationof values in serum samples from 60-year-oldmale patients in a coronary care unit would beinappropriate and misleading.

Serum Enzymes in the Diagnosisof Acute Myocardial Infarction

SGOT-Sensitivity and Specificity. In atypical patient with acute myocardial infarc-tion, SGOT activity exceeds the normal rangewithin 8 to 12 hours following the onset ofchest pain, reaches a peak elevation of two- totenfold in 18 to 36 hours, and declines to thenormal range within 3 to 4 days. As with allserum enzymes the duration of elevatedactivity depends in part on the maximumvalue attained. When other disease processesaccount for elevated SGOT, the time courseoften differs. In an extensive review ofliterature through 1960 by Agress and Kim,191692 cases of acute transmural myocardialinfarction diagnosed clinically and electrocar-diographically were tabulated. True-positiveSCOT elevations occurred in 97%. Myocardialinfarction was proven by autopsy in 119 cases.In 115 of these, SGOT had been abnormal. In


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experimental myocardial infaretion in animals,SGOT elevations are seen consistently.SGOT may increase in association with a

variety of disease processes, although oftenthe time course of elevated enzyme activitycontrasts with that seen typically in patientswith acute myocardial infaretion. Hepaticcongestion, primary liver disease, skeletalmuscle disorders, and shock may contribute tomarked and sometimes sustained SGOT eleva-tions. In 224 patients with cardiac diseaseexcluding myocardial infaretion and shock,SGOT activity was increased in 12%.2o Patientswith congestive failure exhibit an even higherproportion of false-positive SGOT elevationsapparently in proportion to the severity offailure. However, congestive heart failure inthe absence of overt hepatic decompensationis infrequently associated with striking eleva-tions of SGOT.

Myocarditis may lead to SGOT elevations,which frequently parallel the activity of thedisorder and are not precluded by steroidadministration.2' In pericarditis, SGOT iselevated in less than 15% of patients and thoseelevations may reflect subepicardial injury.7Although early reports indicated that SGOT

did not generally increase in patients withpulmonary embolism,22 it has become clearthat this conclusion is not justified. Coodleynoted that four of 17 patients with definitepulmonary embolism and 17 of 25 patientswith probable pulmonary embolism manifest-ed increased SGOT activity.23 In other re-ports, between 25 and 50% of patients withemboli proven angiographically exhibited in-creased SCOT.24

Increased SGOT activity follows tachyar-rhythmia in more than 50% of cases when theheart rate exceeds 140 beats/min for at least30 min in the absence of acute myocardialinfarction. Since OCT is often elevated in suchcases,25 the SGOT appears to reflect hepaticdecompensation secondary to hemodynamicconsequences of the arrhythmia. Accordingly,CPK does not rise nearly as frequently.7 Onthe other hand, the incidence of arrhythmiawithin the first 24 hours following myocardialinfarction correlates with the magnitude ofCirculation, Volume XLV, February 1972

SGOT elevation26 perhaps because bothSGOT and the incidence of arrhythmia reflectthe extent of myocardial injury.Approximately 25% of patients treated with

direct-current countershock (electrical cardio-version) may exhibit increased SGOT activityusually less than threefold above normal.Since SGPT and isoenzymes of LDH identi-fied with liver are not elevated in suchpatients,27 and since OCT, an enzyme identi-fied almost exclusively with liver, does notincrease28 it appears that the SGOT rise is dueto enzyme release from skeletal muscle. CPKelevations, commonly seen, are both morefrequent and more marked than SGOTelevations in this setting.28

Several other conditions, frequently ofinterest to the cardiologist, may be associatedwith elevated SGOT activity. Patients withdisease of the biliary tract who receiveintravenous or intramuscular narcotic injec-tions may exhibit transient increases in SGOTactivity.29 Usually, the time course of elevationis much shorter and the magnitude much lessthan that seen typically following myocardialinfarction. Concomitantly increased alkalinephosphatase activity is a valuable confirma-tory sign indicating that the enzyme elevationis related to cholestasis potentiated by narcoticadministration rather than to myocardialdamage.

Approximately 11% of females taking oralcontraceptive medication exhibit increasedSGOT, usually late in the menstrual cycle,3j'and SGOT may be increased in patientstreated with clofibrate. SGOT and CPKincreased in 8% of 60 patients treated withlarge doses of clofibrate, and elevated enzymeactivity and an associated myalgia waxed andwaned with drug administration or its discon-tinuance, respectively.31 False-positive SGOTelevations following erythromycin administra-tion are a problem only when the colorimetricenzyme assay is employed.32SGOT may increase following cardiac cath-

eterization. In both adults and children, themagnitude of serum enzyme elevations ap-pears to be related to the duration and extentof dissection required during the procedure.33

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Typically, SGOT rises following operation inthe experimental animal and in as many as 95%of patients following general surgical proce-dures.34 Although increased SGOT may resultfrom hepatic damage by anesthetic agents insome cases, 34 its magnitude appears to dependprimarily on the extent of dissection.Serum enzyme elevations may become

clinically important indices of rejection follow-ing cardiac transplantation. In a recentexperimental study, discrete SGOT and CPKelevations appeared to provide sensitive cri-teria of incipient rejection in the dog.35

LDH-Senrsitivity and Specificity. In atypical patient with acute myocardial infarc-tion serum LDH activity exceeds the normalrange within 24 to 48 hours, reaches a peakelevation of two- to tenfold in 3 to 6 days, anddeclines to the normal range within 8 to 14days.36 True-positive LDH elevations oc-curred in 86% of 282 patients with myocardialinfarction diagnosed clinically and in all 39patients with myocardial infarction proven atautopsy.19 Results in other series are similar.37The specificity of LDH as an index of acute

myocardial infarction depends on the popula-tion at risk. Increased LDH occurs in 30% ofpatients with congestive heart failure,20 and itstime course may resemble that following acutemyocardial infarction.Serum LDH activity may increase markedly

under the following conditions: hemolysis, inthe sample, or in vivo; megaloblastic anemiaor anemia associated with ineffective erythro-poiesis; leukemia; acute and chronic liverdisease; and renal disease, especially renalinfarction. Striking serum LDH elevations areseen in many patients with neoplastic disease,and elevations in cerebrospinal, pleural, orascitic fluid may be indicative of localneoplasm. LDH increases even more common-ly than SGOT following pulmonary embo-lism.22-24 In the dog, increased serum LDHactivity is a concomitant of acute pulmonaryembolism produced by administration ofautologous thrombus despite the absence ofpulmonary infarction and underlying heartdisease.38 Many conditions associated with

SCOT elevation including cardiac catheteriza-tion, myocarditis, and shock may give rise toincreased serum LDH activity as well. Inaddition, LDH may increase following severeexercise apparently because of release ofenzyme not only from skeletal muscle but alsofrom liver and myocardium..39 LDH does notincrease as commonly as SGOT followingelectrical cardioversion, but it may be releasedfrom skeletal muscle and/or liver judging bythe serum isonenzyme profile seen.27' 28

CPK-Sensitivity and Specificity. In atypical patient with acute myocardial infarc-tion, serum CPK activity exceeds the normalrange within 6 to 8 hours, reaches a peak oftwo- to tenfold in 24 hours, and declines to thenormal range within 3 to 4 days following theonset of chest pain. The upper limit of normalCPK in females is only about two thirds ofthat in males.40 CPK is a sensitive index ofacute myocardial infarction, but results varymarkedly with the assay technique employed.Both sensitivity and the incidence of falsepositives increase when thiol activation isused. Nevertheless, such activation is essentialto assure reproducibility and prevent rapiddecay of CPK activity in serum.17 Unfortu-nately, interpretation of CPK results fromsome studies are clouded by other difficultiesincluding use of the forward instead of theback reaction. When CPK is assayed by theback reaction with thiol activation, the sensi-tivity of CPK is impressive, of the order of 90%or more. On the other hand, when CPK isassayed by other techniques sensitivity suffers.As with other enzymes, specificity of CPK

elevation as an index of acute myocardialinfarction is far from unequivocal. Myocardi-um, skeletal muscle, brain, and thyroidcontain complements of CPK. Because of thepaucity of CPK in other parenchymal organs,CPK offered promise as a relatively specificindex if cardiac injury. However, serum CPKis markedly increased in most patients withmuscular dystrophy, inflammatory disease ofmuscle, alcohol intoxication with or withoutdelerium tremens, diabetes mellitus with orwithout ketoacidosis, convulsions, psychosis,


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and intramuscular injections, apparently be-cause of muscle release.4' 4 Serum CPK israrely affected by renal disease.

Contrary to SGOT and LDH, CPK activityusually remains normal in patients withneoplastic disease. However, X-ray therapyinvolving the heart consistently producedCPK elevations in serum. CPK did notincrease in patients exposed to comparableamounts of radiation(2000-5000 rads within 5weeks) confined to regions excluding themyocardium but including skeletal muscle.45Serum CPK does not generally increase in

patients with uncomplicated congestive heartfailure.17 It may increase with pericarditis;butelevations are rare and those that do occur areusually slight.17 46 In myocarditis increasedCPK activity often parallels the underlyingseverity of the inflammatory process.'6' 17

CPK has been considered an effectivediscriminant in the differential diagnosis be-tween acute myocardial infarction and acutepulmonary embolism. However, even in earlyclinical reports, occasional patients with pul-monary embolism exhibited increased serumCPK activity.'7' 46 When CPK activity isassayed with thiol activation, increased activi-ty can be detected frequently.47 Perkofffocused attention on this in a report of threeof seven patients with acute pulmonaryembolism who exhibited increased serum CPKactivity. CPK activity is demonstrable in lungextracts from man and experimental animals.47In experimental pulmonary thromboembolismin dogs, serum CPK activity increases consis-tently two- to threefold despite only transientand minimal hemodynamic changes and theabsence of pulmonary infarction.48 Thus,modest, transient serum CPK elevations areprobably relatively frequent following acutepulmonary embolism.Although SCOT and LDH elevations fol-

lowing tachyarrhythmia appear to be due toenzyme from liver, recent observations in 12patients demonstrated increased serum CPKactivity49 suggesting possible myocardial en-zyme release. Since tachycardia increasesmyocardial oxygen consumption and infarctsize experimentally,50 CPK elevations mayCirculation, Volume XLV, February 1972

reflect occurrence of some myocardial necrosisin patients with tachyarrhythmia. Increasingheart rate by means of ventricular pacing inotherwise normal dogs to two and one halftimes the control rate failed consistently toincrease serum CPK activity. However, in-creasing heart rate from 100 to 180 beats/minaugmented infarct size and led to associatedelevations in serum CPK in five of sixconscious dogs with coronary artery occlu-sion.51 Electrical cardioversion produces two-to threefold increases of serum CPK activity inup to 75% of cases.28CPK elevations occur in several other

conditions of particular interest to the cardiol-ogist. Selective coronary angiography33 mayincrease serum CPK. Following cardiac cathe-terization the incidence and magnitude ofserum CPK elevations appear to depend onthe extent of muscle dissection involved in theprocedure.33' 5'2Two- to threefold CPK elevations are

frequent following intramuscular injections ofnarcotics, phenothiazines, barbiturates, antibi-otics, diuretics, and analgesics even withoutovert signs of inflammation.44 4 CPK, likeother serum enzymes, may increase withsevere exercise apparently because of alteredproperties of cell membranes.53LDH Isoenzymes-Sensitivity and Specifici-

ty. LDH isoenzyme determinations haveenhanced diagnostic accuracy remarkably.The five common LDH isoenzymes are namedin order of rapidity of migration toward theanode in an electrophoretic field. Accordingly,LDH1 is the fastest and LDH5 the slowest inconventional systems. Each isoenzyme is atetrameric unit composed of four subunits oftwo possible types. The physical properties ofindividual isoenzymes are determined by therelative percentages of each type of subunitcontained.3 Extracts of heart contain primarilyLDH1, while those from liver or skeletalmuscle contain mostly LDH4 and LDH5.Characteristic LDH isoenzyme profiles inspecific tissues may be related to the degree ofaerobic metabolism typical of the organ. SinceLDH1 and LDH2 are susceptible to inhibition

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by pyruvate, this substrate may be preferen-tially shunted into mitochondria and metabo-lized aerobically in organs rich in theseisoenzymes, such as heart. However, it is notclear that this mechanism operates in vivo,54and myocardial LDH profiles may changeunder altered physiologic states.+5

Following acute myocardial infarction in-creased serum LDH1 activity frequently pre-cedes increased total LDH and, in fact, istypically present in the first available bloodsample obtained from patients hospitalized foracute myocardial infarction.'0 Since many

patients with increased LDH, exhibit totalLDH activity remaining within the normalrange, increased LDH, activity is a more-

sensitive, early index of acute myocardialinfarction than is total LDH. Its sensitivityexceeds 95%.

In most other conditions in which totalserum LDH is increased the isoenzyme profilediffers considerably from that seen followingacute myocardial infarction. While LDH1 isthe most prominent serum isoenzyme follow-ing infarction, LDH-, is typically increasedwhen congestive failure occurs without infarc-tion.4 The isoenzyme profile typical of acutemyocardial infarction is rarely seen followingacute pulmonary embolism usually associatedwith elevated LDH9 and LDH3.16 Whenexperimental pulmonary embolism is pro-

duced by administration of autologous throm-bus, increased activity of LDH3 accounts formost of the elevated total serum LDHactivitv.38

In cardiac patients, hemolysis associatedwith jet lesions or prosthetic valves frequentlyleads to increased serum LDH1. LDH, may

increase in patients with muscular dystrophy,since myopathic tissue contains an unusualproportion of LDH1. Furthermore, LDH1increases in patients with renal disease,especially renal infarction.56The ratio of LDH, to LDH2 is increased in

young females, particularly during pregnancy,

compared to postmenopausal females or to

males.'8 Furthermore, physical activity of thesubject may influence isoenzyme profiles. Inwell-conditioned males, LDH5 increases fol-

lowing exercise,57 presumably because ofrelease of enzyme from skeletal muscle.LDH1, LDH,, and LDH.5 increase in exercisedrats, probably because of release from heart,skeletal muscle, and liver since tissue LDHdecreases in all three organs.P."Although patients with arrhythmia may

exhibit increased total serum LDH activity,the isoenzyme profile is quite different fromthat seen following acute myocardial damage.Thus, LDH, activity did not increase in any of29 patients with tachyarrhythmia.-- Direct-current countershock may increase total LDHactivity because of release of skeletal musclecomiponents, but LDH, remains normal.28LDH, is usually not elevated in patients withpericarditis, but in approximately 25% ofpatients with myocarditis LDH, activity inserum may increase.4 In patients with hearttransplants, rejection episodes are frequent-ly characterized by increased LDH1/LDH2ratios in serum.589 Cardiac catheterization, withor without coronary angiography, usually doesnot lead to significantly increased total or"heart" LDH isoenzyme activity.33 Thus, LDHisoenzyme analysis may be particularly valu-able in assessing the presence or absence ofconcomitant myocardial infarction in patientswith arrhythmia or pericarditis, or those un-dergoing electrical cardioversion or cardiaccatheterization.Most conditions in which total LDH

activity is significantly increased includingliver disease, skeletal muscle injury, and shockwith skeletal muscle and hepatic ischemia arereadily distinguishable from acute myocardialinfarction since the isoenzyme profile typicallyshows predominance of slow components.When coexistent myocardial infarction occursin patients with these disorders, increasedLDH, may be diagnostically useful in theabsence of marked changes in total LDH.

Activities of many serum enzymes areelevated in patients with myxedema.9 Twomechanisms appear to account for this phe-nomenon: (1) enzyme release from myopath-ic muscle, and (2) diminished catabolism ofcirculating enzyme. The LDH isoenzymepattern seen in serum from patients with


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myxedema typically exhibits elevated LDH4and LDH5. Thus, the pattern is readilydistinguishable from that seen with ischemicmyocardial injury.

Efforts directed toward detecting "heart"LDH isoenzymes by rapid chemical proce-dures rather than electrophoretic techniquesexploit properties of the dominant LDHsubunit seen in heart LDH isoenzymesincluding heat stability, susceptibility to inhi-bition by pyruvate, resistance to inhibition byurea, and affinity for alpha-hydroxybutyricacid as a substrate. Alpha-hydroxybutyricdehydrogenase (a-HBD) should not beviewed as a specific enzyme but rather asenzymatic activity residing in that fraction ofLDH with high affinity for alpha-hydroxy-butyric acid.

Electrophoretic analysis of LDH isoen-zymes continues to provide a more-sensitiveand specific diagnostic index compared to thechemical techniques. However, precautionsrequired include assay of enzyme activitywithin the range of linearity of the stainingprocedure used and control of temperaturethroughout the supporting medium. Themajor difficulty with chemical methods is thatspecific isoenzymes are not identified. Appar-ent enzyme activity with the chemical meth-ods depends on total LDH activity and totalsubunits of each type rather than the isoen-zyme profile per se. Thus, chemical inhibitionmethods may fail when total LDH activity ishigh. Lack of agreement between results of"heart" LDH isoenzyme activity estimated byurea inhibition and that determined electro-phoretically occurs in as many as 24% ofsamples, with frank contradiction in as manyas 10%."10 Despite these qualifications, LDHfractionation by chemical techniques led totrue-positive results in a combined total of 457of 464 patients from several centers.'06 61, 62

Thus, results with chemical fractionationtechniques are more specific and sensitivethan total LDH determinations, while thoseobtained with isoenzyme separation by elec-trophoresis surpass both.Circulation, Volume XLV, February 1972

Activity of Serum Enzymes in Patientswith Coronary Insufficiency

Insufficient coronary blood flow may beassociated with several clinical syndromes andpathologic consequences. Yet, elucidation ofthe relationships between each of thesesyndromes and changes in serum enzymes hasbeen difficult. In order to examine theserelationships, it is useful to consider foursyndromes of apparently decreasing severity:(1) definite transmural myocardial infarctionwith evolution of Q waves; (2) prolongedchest pain due to myocardial ischemia andassociated with transient S-T-segment and T-wave changes; (3) prolonged chest pain dueto ischemia but associated with no electrocar-diographic changes; and (4) typical anginapectoris. Compilation of data from severalstudies indicates that 92, 58, 27, and 5% ofpatients in these four categories, respectively,exhibit elevated SGOT, CPK, LDH, and/orLDH1.4 7, 8, 10, 16, 17, 19, 46, 60, 61, 63-65

Several conclusions appear warranted. Theincidence and extent of enzyme elevationsvary widely in patients with insufficientcoronary blood flow without transmural myo-cardial infarction, probably because criteriafor patient classification vary. The incidenceof increased serum enzyme activity is greatestin patients with transmural myocardial infarc-tion, decreases in those with chest pain andassociated electrocardiographic changes, andis least in those with prolonged chest pain orangina pectoris without any electrocardio-graphic changes. This trend is consistent withthe possibility that prevalence of myocardialnecrosis in these patient groups follows thesame trend and that enzyme elevations reflectnecrosis.Enzyme elevations that do occur are modest

in patients with insufficient coronary bloodflow without transmural myocardial infarc-tion.16' 19 Mortality following definite myocar-dial infarction parallels, in general, themagnitude of peak enzyme elevation acute-ly.66 Thus, available clinical data are consis-tent with the interpretation that serumenzyme elevations following myocardial isehe-mia are indicative of myocardial necrosis.

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Clearly, the wide spectrum of necrosis seenclinically may include massive infarction on

the one hand, and miniscule islands ofmyocardial-cell death on the other, withprofoundly different prognostic and therapeu-tic implications.

Quantitative Relationships betweenthe Extent of Myocardial Necrosisand Changes in Serum Enzymes

In unselected patients with acute infarctionwhen SGOT, LDH, and/or CPK activityincreases more than tenfold, mortality risesmarkedly.;6' 67 This correlation may resultfrom a general dependence of both mortalityand peak enzyme activity on the extent ofmyocardial necrosis. In one large autopsyseries, infarcts were graded as small, medium,and large by morphologic criteria. The medianpeak SGOT and LDH values in patientsfalling into these respective groups increasedcorrespondingly.68 Infarct size is large inpatients dying with cardiogenic shock associ-ated with myocardial infarction.6" Undercertain circumstances inflammatory or regen-erative responses within a damaged organ

may increase serum enzyme activity beyondthat anticipated from the amount of enzyme

initially present in the entire damaged organ.

However, this phenomenon does not appear

to occur with myocardial damage.12Although peak serum enzyme activity corre-

lates with infarct size in groups of animalsdespite inprecision in anatomic criteria lead-ing to scatter, the correlation may not be closein the same animal.15 70 In order to elucidatethe basis of this apparent disparity, we havestudied serum and myocardial CPK activityfollowing experimental myocardial infarction.CPK was chosen because this enzyme is notpresent to a significant extent in cells partici-pating in the inflammatory response in myo-

cardium. In initial studies in rabbits, depletionof myocardial CPK correlated closely withinfarct size assessed morphologically 24 hoursafter coronary artery occlusion.7' CPK deple-tion in selected regions from dog hearts 24hours after coronary artery occlusion corre-

lated closely with the extent of acute cellularinjury in the same site measured independent-ly with the use of epicardial electrocardio-graphic recordings.-" A method based solelyon serial changes of serum CPK activity wasthen developed for quantitative assessment ofinfarct size in the conscious animal.12 CPKreleased from the infarct was determinedsolely on the basis of serial serum CPKmeasurements analyzed with a model account-ing for the rate of CPK disappearance fromserum, the rate of release of CPK frommyocardium undergoing infarction, the frac-tion of myocardial CPK degraded locally inthe heart, and the distribution space intowhich CPK is diluted following release intothe circulation, all of which were determinedindependently. Infarct size calculated on thebasis of observed changes in serum CPKactivity was compared with infarct sizedetermined directly by analysis of myocardialCPK depletion in 22 animals. The correlationwvas close in each case. Results from allexperiments fit a regression line with narrowconfidence limits and a correlation coefficientof 0.96.12 These findings are consistent withthe bulk of earlier experimental work andclinical observations that support the hypothe-sis that elevation of serum enzyme activityfollowing myocardial infarction is a reflectionof enzyme release from myocardial cellsundergoing necrosis. Thus, although the rela-tionship between changes in serum enzymeactivity and infarct size is complex it appearsto be quantitative. Accordingly, analysis ofchanges in serum enzyme activity followingmyocardial infarction should permit the accu-rate assessment of the extent of myocardialnecrosis and its progression, so that the potentsurgical tools becoming available for thepatient with coronary artery disease can befocused effectively. 2

Conclusions

SCOT, LDH, CPK, and LDH isoenzymedeterminations are of immense value in theassessment of patients with coronary artery

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disease. Meaningful interpretation of suchdeterminations requires knowledge of sam-pling technique, assay conditions, and physio-logic and pathologic states capable of influenc-ing serum enzyme activity. The time course,magnitude, and pattern of serial changes arehelpful in identifying responsible etiologicfactors. Following acute myocardial infarc-tion, serum LDH1, CPK, and SGOT risepromptly. Total LDH activity increases laterand appears to be a somewhat less-sensitiveindex. All enzymes suffer from some lack ofspecificity. Total LDH and LDH1 are particu-larly prone to spurious elevations whenhemolysis is present in vivo or in vitro. LDH1and CPK are less likely than SCOT and totalLDH to increase in patients with congestiveheart failure. LDH1 is particularly useful indifferentiating acute myocardial infarctionfrom acute pulmonary embolism and inexcluding myocardial injury in patients treat-ed with direct-current countershock or studiedinvasively in whom serum activity of SGOT,CPK, and LDH derived from skeletal musclemay increase. Extensive evidence supports theconclusion that serum enzyme elevationsfollowing myocardial ischemia are relatedquantitatively to enzyme release from irrever-sibly injured cells. Continued elucidation ofchanges in serum enzyme activity should be ofconsiderable practical value in detecting andassessing the extent of infarction and itsprogression in individual patients with coro-nary artery disease.

References1. WOHLGEMUTH J: Uber eine neue Methode zur

quantiativen Bestimmung des diastatischenFerments. Biochem Ztschr 9: 1, 1908

2. KARMEN A, WROBLEWSKI F, LADUE JS: Transam-inase activity in human blood. J Clin Invest34: 126, 1954

3. MARKERT CL: Lactate dehydrogenase isozymes:Dissociation and recombination of subunits.Science 140: 1329, 1963

4. COHEN L, DJORDJEvrICH J, ORNIISTE V: Serumlactic dehydrogenase isozyme patterns incardiovascular and other diseases, with particu-


lar reference to acute myocardial infarction. JLab Clin Med 64: 355, 1964

5. DREYFUS JC, SCHAPIRA G, RESNAIS J, SCEBAT L:Le creatine-kinase serique dans le diagnosticde l'infarctus myocardique. Rev Franc EtudesClin Biol 5: 386, 1960

6. HESS JW, MACDONALD RP: Serum creatinephosphokinase activity. Mich Med 62: 1095,1963

7. XVEST M, ESHCHAR J, ZIMMERMAN HJ: Sertumenzymology in the diagnosis of myocardialinfarction and related cardiovascular condi-tions. Med Clin N Amer 50: 171, 1966

8. HEDWORTH-WHITTY RB, WHITFIELD JB,RICHARDSON RW: Serum -y-glutamyl transpep-tidase activity in myocardial ischaemia. BritHeart J 29: 432, 1967

9. SHAFT FR, BAN RW, IMFELD H: Serum pyruvatekinase in acute myocardial infarction. Amer JCardiol 26: 143, 1970

10. KARLINER JS, GANDER MP, SOBEL BE: Elevatedserum glyceraldehyde phosphate dehydrogen-ase activity following acute myocardial infarc-tion. Chest 60: 318, 1971

1 1. JENNINGS RB, KALTENBACH JP, SMETTERS GW:

Enzymatic changes in acute myocardial is-chemic injury. Arch Path (Chicago) 64: 10,1957

12. SHELL WE, KJEKSHUS JK, SOBEL BE: Quantita-tive assessment of the extent of myocardialinfarction in the conscious dog by means ofanalysis of serial changes in serum creatinephosphokinase (CPK) activity. J Clin Invest.In press

13. PASYK S, BLOOR CM, KHOuRI EM, GREGG DE:Systemic and coronary effects of coronaryartery occlusion in the unanesthetized dog.Amer J Physiol 220: 646, 1971

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BURTON E. SOBEL and WILLIAM E. SHELLInfarction

Serum Enzyme Determinations in the Diagnosis and Assessment of Myocardial

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