Electrocardiographic diagnosis of posterior myocardial infarction revisited: A new approach using a multivariate discriminant analysis and thallium-201 myocardial scintigraphy

J. ELECTROCARDIOLOGY 19 (1), 1986, 33-40

Electrocardiographic Diagnosis of Posterior Myocardial Infarction Revisited: A New Approach Using a Multivariate Discriminant Analysis and Thallium-201 Myocardial Scintigraphy By PASQUALEF. NESTICO, M.D., A-HAMID HAKKI, M.D., EA.C.C., ABDULMASSIII S. ISKANDRIAN, M.D., EA.C.C., AND GARY J. ANDERSON, M.D., EA.C.C.

SUMMARY T h i s s t u d y e x a m i n e d t h e feasibility of using a m u l t i v a r i a t e d i s c r i m i n a n t a n a l y s i s to design a useful e l e c t r o c a r d i o g r a p h i c (ECG) model to diagnose p o s t e r i o r m y o c a r d i a l infarction (MI). Thallium-20) s c i n t i g r a p h y was used as a reference s t a n d a r d to i d e n t i f y p o s t e r i o r scar (fixed p e r f u s i o n defects}. T h e m o d e l was d e r i v e d f r o m 111 p a t i e n t s of w h o m 37 h a d fixed p o s t e r i o r d e f e c t s and 74 h a d n o r m a l images, a n d its v a l i d i t y was s u b s e q u e n t l y t e s t e d

in a separate group of 180 patients. In the initial group of patients, the fixed perfusion defects involved the posterior left ventricular wall alone in 15 patients, and the p o s t e r i o r and inferior walls in 22 p a t i e n t s . S t e p w i s e m u l t i v a r i a t e d i s c r i m i n a n t a n a l y s i s of 26 E C G v a r i a b l e s p r o d u c e d a model of two v a r i a b l e s (Q-wave d u r a t i o n in a V F a n d T - w a v e a m p l i t u d e in V l} which provided a sensitivity of 78%, a specificity of 89%, and a predictive a c c u r a c y of 86% for the diagnosis of p o s t e r i o r MI. T h i s model, w h e n t e s t e d in the s e c o n d g r o u p of 180 p a t i e n t s , yielded an overall prediction a c c u r a c y of 82% (sensitivity 65%, specificity 85%). T h u s , t h e c o m b i n a t i o n of Q - w a v e in a V F a n d u p r i g h t T w a v e in V 1 is the b e s t E C G p r e d i c t o r of p o s t e r i o r MI. T h e s e two variables reflect the f r e q u e n t association of p o s t e r i o r M I with inferior MI, and the reciprocal repolarization changes in the r i g h t precordial leads.

T h e e l e c t r o c a r d i o g r a p h i c (ECG) d i a g n o s i s of p o s t e r i o r myocardial i n f a r c t i o n (MI) m a y be difficult. 1-3 P r e v i o u s s t u d i e s show t h a t some E C G p a t t e r n s have a high specificity b u t a low sensitivity.L 3-5 A n E C G a l g o r h y t h m t h a t c a n r e a d i l y d i a g n o s e p o s t e r i o r M I will be useful to clinicians. We have previously derived an E C G a l g o r h y t h m to predict the presence or absence of a n t e r o s e p t a l M I in p a t i e n t s w i t h p o o r R wave progression. 6 This s t u d y uses a stepwise discriminant analysis of E C G m e a s u r e m e n t s to select the variable(s) t h a t b e s t predict the p r e s e n c e or absence of p o s t e r i o r MI.

MATERIALS AND METHODS The study group consisted of 37 patients who had fixed perfusion defects involving the posterior left ventricular (LV) wall and 74 patients who had normal images. The 37 patients (9 women and 28 men, aged 40 to 76 years, mean 61) were selected from a larger group of patients with unstable angina pectoris or acute MI who had rest thallium-201 imaging. Of the 37 patients, nine had acute MI using standard criteria {prolonged ischemic chest pain, ECG changes and enzyme changes}, and 28 patients had unstable angina pectoris. Patients were excluded if they had perfusion defects that did not involve the posterior LV wall or if the defects were in multiple locations (i.e. posterior as well as anterior) or if the defects were reversible (ischemic), or if the patients had left bundle branch block, right bundle branch block, nonspecific intraventricular conduction delay with a QRS duration of>~110 milliseconds, or left anterior or posterior fasicular block. These exclusions were felt to be necessary, since these changes may modify the ECG vectors in patients with posterior MI. Each of the 37 patients had rest thallium-201 scintigrams seven to fourteen days after admission to the coronary care unit. The remaining 74 patients were included to assess the specificity of the ECG criteria. There

From the LikoffCardiovascular Institute of Hahnemann University and Hospital, Philadelphia, Pennsylvania. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "'advertisement" in accordance with 18 U.S.C. w solely to indicate this fact. Reprint requests to: Abdulmassih S. Iskandrian, M.D., ttahnemann University and ttospital, Likoff Cardiovascular Institute, Broad and Vine Streets, Philadelphia, PA 19102.

33

34

NESTICO ET AL

were 38 women and 36 men, aged 49--+12 y e a r s (mean_standard deviation). These patients had a low probability of coronary artery disease (CAD) based on their age and atypical symptoms, and all had normal resting 12-lead electrocardiograms, normal exercise elect r o c a r d i o g r a m and n o r m a l exercise thallium-201 scintigrams. In this study, posterior MI was defined as fixed t h a l l i u m 201 p e r f u s i o n defects i n v o l v i n g the posterelateral segment in the 300 left anterior oblique projection or the inferoposterior segment in the 650 left anterior oblique projection, or both. Previous studies suggest that fixed defects represent myocardial scar, and t h a t these two segments correspond to the posterior LV wall. 9~ A separate group of 180 patients were selected to test the validity of the ECG model which was derived from the initial group of 111 patients. There were 64 women and 116 men, aged 50_+11 years (range 24 to 74). Thirty-six patients (20%) gave a history of previous MI. The 180 patients were consecutively studied with exercise thallium-201 imaging in our laboratory for the evaluation of chest pain syndrome which we suspected was caused by CAD. The technique of rest and exercise thallium-201 imaging and the interobserver and intraobserver variability "have been previously reported. 1~ Briefly, images were obtained in three projections: anterior, 30 ~ left anterior oblique, and 65 ~ left anterior oblique projections. The rest images were obtained after an overnight fast, 20 minutes following intravenous injections of 2 mCi of thallium-201. None of the patients had angina within 24 hours preceding the thallium imaging. In patients who had exercise images, the thallium-201 was injected at peak exercise, and images were obtained 5 to 10 minutes after completion of the exercise. Redistribution images were obtained four hours after the injection in the projection showing initial abnormalities. Images were assessed qualitatively and quantitatively for the presence of perfusion defects in each of five segments in each projection (Fig. 1). The presence of fixed perfusion defects indicates the presence of myocardial scar. Fixed defects were defined as perfusion defects which were present on the initial studies and were unchallenged in the four hour delayed images, i.e., no redistribution. Fixed defects in the posterolateral segments (segments 9 or 10) or inferoposterior segments (segments 14 or 15) indicated the presence of posterior myocardial scar. Fixed defects in the inferior wall (segments 4 or 5) indicated the presence of inferior scar. In each patient, 26 ECG measurement (Table I) were made from 12-lead electrocardiograms obtained immediately before thallium-201 imaging. Electrocardiography. A standard 12-lead ECG was obtained from patients using a Hewlett-Packard threechannel recorder. The duration and amplitude of various ECG waveforms were measured by visual inspection or with hand-held calipers by one of the authors (PEN) (Table I). To assess intra- and interobserver agreement, 248 measurements in 15 ECGs were obtained four weeks later by the same observer and independently by another

Anterior

30 ~

LAO

6 5 ~ I..AO

Fig. 1. Qualitative assessment of thallium-201 images in 3 projections.

cardiologist (Fig. 2). The intraobserver and intembserver a g r e e m e n t s were excellent (r=0.99 and r = 0 . 9 8 , respectively). Statistical analysis. Univariate and multivariate statistical analyses were performed with programs from BMDP la (Health Services Computing Facilities, University of California, Los Angeles). The presence or absence of posterior fixed perfusion defects as determined by thallium-201 imaging was considered to be the dependent variable, and the ECG variables were considered to be independent variables. Stepwise discriminant analysis was performed to derive a multivariate function to classify patients into outcome categories (posterior scar or no posterior scar). Independent variables were considered candidates for forward entry into the function and backward elimination from the function, if their association with the dependent variable and their contribution to the discriminant function were significant (f=2.4). A discriminant function was calculated from the derivation sample of the initial group, and was tested in a second group of patients. Statistical significance was determined using the chi-square analysis with Yates' correction and Fisher's exact test. Results are expressed as mean _+standard deviation (SD) when appropriate. Definition of Terms. From the electrocardiogram and thallium imaging results the following terms are defined: True positive (TP): the presence of posterior MI by ECG and posterior scar by thallium imaging. True negative (TN): absence of posterior MI by ECG and absence of posterior scar by thallium imaging. False positive (FP): presence of posterior MI by the ECG but no posterior scar by thallium imaging. False negative (FN): absence of posterior MI by ECG but presence of posterior scar by thallium imaging. Sensitivity (%) =

Sensitivity (%) =

TP

TP +

FN

TN

TN +

FP

Predictive accuracy (%) =

x 100

x 100

TP + TN .x 100 TP -F TN -I- FP -I- FN

J. ELECTROCARDIOLOGY 19(1), 1986

POSTERIOR M Y O C A R D I A L I N F A R C T I O N

35

Table I

RESULTS

Electrocardiographic Variables Used in the Step. wise Discrminant Analysis Q amplitude in lead II amplitude in lead aVF Q duration in lead II duration in lead III s duration in lead aVL Q duration in lead aVF Q duration in lead V 5 duration in lead V 6 R amplitude in lead I R amplitude in lead II R amplitude in lead III R amplitude in lead aVF R amplitude in lead V~ R amplitude in lead V2 R duration in lead V 1 R duration in lead V 2 S amplitude in lead I S amplitude in lead aVF S amplitude in lead V~ S amplitude in lead V2 T amplitude in lead V~ T amplitude in lead V2 S-T segment deviation in lead V~ S-T segment deviation in lead V2 R/S ratio in lead V1 R/S ratio in lead V2

01 r-

where Q aVF is Q-wave duration (in seconds} in lead aVF, and T V~ is T-wave amplitude (in millimeters) in lead V r Using a binary cutoff at a discriminant score of -0.5, values below this level correctly classified 30 of the 37 patients with posterior scar {sensitivity 81%}, while values above this level correctly classified 66 of 74 patients with no posterior scar {specificity 89%}.

15elm

W el

(IB

12

am

eeee

9

e9

9

e ~ e

O

U.

9-

P 12Q) w J~

we 9

.m

"5

Equation 1 y=0.65--65.1 (Q aVF) - 0 . 6 (T V])

15

"0 G)

Initial Group. The rest thallium-201 images showed fixed perfusion defects involving the posterolateral or inferoposterior segments or both in 15 patients. These 15 patients may thus be considered to have isolated posterior MI. The remaining 22 patients had fixed defects in the inferior wall, in addition to defects in one or both of the above locations. These patients may be considered to have combined posterior and inferior MI. The sensitivity, specificity, and predictive accuracy of the ECG criteria for posterior MI in the initial group of patients are shown in Table II. Stepwise discriminant analysis of ECG criteria resulted in a two variable model consisting of the Q-wave duration in aVF, and T-wave amplitude in V]. The coefficients and associated f values are listed in Table III. The performance of the model on the original data set from which it was derived is shown in Fig. 3. The results based on two variables are expressed in the following equations:

:>, .Q

m e

9

9"

~ m e

e m emmme

a)

E

s

6. 9 eoee

W G)

:i

_

9

9

o LU

Q

E

m o o

m

9 oo

era, t o

O.

W

(a

e

o m

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R = 0.995 P
eeBm

' 1'2 ' 16 ECG M e a s u r e m e n t of Second Reading

39 "*

(3 I,U

" ' "

.m,**~

r = 0.ge

P
Ob ' ' ' 1'2 ' 16 ECG M e a s u r e m e n t by Observer # 2

Fig. 2. Intraobserver variability (panel A) and interobserver variability (panel B). *, indicates that many patients had similar measurements.

J. ELECTROCARDIOLOGY 19(1), 1986

36

NESTICO ET A L

Table II Sensitivity, Specificity, and Predictive Accuracy of Electrocardiographic. Criteria for Inferoposterior Myocardial Scar in the Initial Group Inferoposterior scar (n:37)

Normals (n:74)

QIR ratio>~1.0 lead

II AVF

R-duration>~0.04 + upright T wave

V1

R-duration>_-0.04 + R/S ratio>_-1.0

V1 or V2

2

(5)

1

(99)

68

R-duration>_-0.04 +R/S ratio>~l.0 +upright Our model

V1 or V2 V1 or V2 T wave V1

1

(3)

3

(100)

65

30

(81)

8

(89)

86

lead II AVF R-durations>0.04 lead V1

V2 VI o r V 2 VI

Upright T wave S-T depression

V~

R/S ratio>_-1.0

V~

V2 v, orv

(n) 1 1 1 21 21 12 66 1 2 3 15 15 0 9 0

(Specificity %) (99) (99) (99) (72) (72) (84) (11) (99) (97) (96) (80) (80) (100) (88) (100)

(n) 7 17 2 6 7 24 28 2 3 5 13 14 9 15 1

Q-duration>~0.03

(Sensitivity %) (19) (46) (5) (15) (19) (65) (76) (5) (8) (14) (35) (38) (24) (41) (3)

Total (n:111) Predictive Accuracy % 72 81 68 59 54 77 32 68 68 68 65 66 75 72 68

Abbreviations: n, number of patients.

of the 74 normal subjects were correctly classified, resulting in a sensitivity of 73%, a specificity of 89%, and a predictive accuracy of 87% {Table IV). Other ECG criteria were either not sensitive or not specific for posterior scar. Validation Results. Of the second group of 180 patients, 20 patients {11%} had posterior scar by thallium imaging. The perfusion defects involved the inferior wall in addition to the posterior s e g m e n t s in 12 of the 20 patients. The E C G changes in patients with and without posterior scar are shown in Table V. The results show poor sen-

Compared to other ECG criteria, the model had a high sensitivity, specificity and predictive accuracy (see Table II). The discriminant model was then reconstructed to separate the 15 patients with isolated posterior scar from the 74 subjects with normal images as follows: Equation 2 y=0.14 - 0 . 7 5 (T V~} - 4 3 (Q aVF) where T V 1 is T-wave amplitude (in millimeters} in V~, and Q aVF is Q-wave duration (in seconds} in aVF. Using this model, 11 of the 15 patients with isolated posterior scar by thallium imaging and 66

9 9 9 9

I

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!

-3.5

I

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I

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I

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O.7

1.4

2.1

Fig. 3. The result of discriminant analysis of 2-variable model to predict posterior myocardial infarction. The closed circles represent patients with infarction and open circles represent patients without infarction. The binary cutoff point of -0.5 provided a sensitivity of 81% and a specificity of 89%.

J. ELECTROCARDIOLOGY 19(1), 1986

POSTERIOR MYOCARDIAL INFARCTION

37

Table III Predictive Function of Variables Used in the Model to Predict Posterior Myocardial Infarction in the Initial Group

Inferoposterior scar Coefficient F-value

Variables Q-duration in lead AVF T-wave amplitude in lead V1

- 65.1 - 0.6

Isolated posterior scar Coefficient F-value

42.5 24.1

- 43 - 0.75

4.6 36.0

*, constant=0.65; + , constant =0.14

sitivity but good specificity. The discriminant model (Equation 1) when applied to the 180 patients resulted in a sensitivity of 65%, a specificity of 84, and a predictive accuracy of 82%.

DISCUSSION Using a multivariate discriminant analysis of the ECG measurements listed in Table I, a 2-variable model was derived to diagnose posterior MI with .a reasonable predictive accuracy: Q-wave duration in aVF and T wave amplitude in V,. The model

had higher sensitivity and comparable specificity to the results obtained using conventional ECG criteria. The two variables reflect the frequent association of inferior MI with posterior MI (Q aVF), and reciprocal repolarization changes in the right precordial leads (T wave amplitude in V~}. The 2-variable model (Equation 1} when applied subsequently to 180 consecutive patients with suspected coronary artery disease performed better than conventional ECG criteria {Table V}. Perloff ~ examined the ECG changes in 20 patients with posterior MI and 100 normal subjects and concluded that the sensitivity and specificity

Table IV Sensitivity, Specificity, and Predictive Accuracy of Electrocardiographic Criteria for Isolated Posterior Scar in the Initial Group Isolated posterior scar (n:15)

Q-duration>~0.03 lead

II AVF R-duration>~0.04 lead V1 V2 V 1 or V 2 Upright T wave V1 V2 S-T depression V1 V2 R/S ratio>_-1.0 V1 V2 V 1 or V 2 Q/R ratio>~l.0 lead II AVF R-duration>~0.04 + upright T wave

(n) 3 3 1 3 3 10 11 1 2 3 7 7 3/14 4/14

(Sensitivity %) (20) (20) (7) (20) (20) (67) (73) (7) (13) (20) (47) (47) (21) (29)

Normals (n: 74) (n) 1 1 1 21 21 12 66 1 2 3 15 15 0 9/71

Total (n:89)

(Specificity %) (99) (99) (99) (72) (72) (84) (11) (99) (97) (96) (80) (80) (100) (87)

Predictive Accuracy % 85 85 83 63 63 81 21 83 83 83 74 74 88 78

V1

1

(7)

0

(100)

84

R-duration>_.0.04 +R/S ratioel.0

V 1 or V 2

1

(7)

1

(99)

83

R-duration>~0.04 +R/S ratio>~l.0 +upright T wave

V 1 or V 2 V 1 or V 2 V1

1

(7)

0

(100)

84

11

(73)

8

(89)

87

Our model Abbreviation: n, number of patients.

J. ELECTROCARDIOLOGY 19(1), 1986

38

N E S T I C O ET A L

of the ECG changes depend on the individual lead examined. Perloff I found that increased R duration in leads V~ and V2, increased R/S ratio in leads V~ or V2, and upright or isoelectric T-waves in lead V~ to be the most common changes in posterior MI. The diagnosis of posterior.MI was based on autopsy in four patients, and on vectorcardiography in 16 patients. Arkin et al 3 correlated the ECG findings with wall motion abnormalities assessed by contrast left ventriculography in 200 patients with CAD. Posterior asynergy was considered to represent posterior MI. They found that the combination of R-wave duration _> 40 milliseconds in V~ and an upright T-wave in V~ were the best predictors of posterior wall asynergy (a sensitivity of 12%, a specificity of 98%, and a predictive accuracy of 80%}. In our study, this combination yielded a sensitivity of 7%, a specificity of 100%, and a predictive accuracy of 84%. Savage et al. TM correlated the ECG changes with histopathologic findings and found that 11 of 12 patients with posterior MI had Q-waves in the in-

ferior leads. They were not able to distinguish inferior from 'true' posterior MI by ECG changes. In our study, 22 of the 37 patients (59%} with posterior myocardial scar (fixed thallium-201 perfusion defects} had involvement of the inferior wall. This may explain the importance of the Q-wave duration in aVF in the identification of patients with posterior MI. In fact, when patients With inferior MI were excluded, the Q-wave duration in aVF was a less powerful predictor of the presence of isolated posterior MI than the T wave amplitude. Ward et al2 evaluated a QRS scoring system to estimate posterolateral myocardial infarct size in 20 patients studied at necropsy. They found that increased R-wave in leads V~ or V2 was present in nine patients (45%} and that Q-waves in leads II or aVF indicative of inferior MI were present in five patients (20%}. Despite differences in patient population between our study (antemortem) and that of Ward et al. (postmortem}, these ECG criteria were not different from our findings in 15 patients with isolated posterior MI, where increased R-waves in V~ or V2 were present in 47% of patients and QTable V

Sensitivity, Specificity, and Predictive Accuracy of Electrocardiographic Criteria for Inferoposterior Myocardial Scar in the Test Group Inferoposterior scar (n:20)

No Inferoposterior scar (n: 160)

2 3 26 26 3 9

(Specificity %) (98) (94) (98) (76) (74) (76) (6) (100) (99) (98) (84) (84) (98) (94)

Predictive Accuracy % 88 88 88 73 72 75 5 89 88 89 78 78 91 89

(15)

0

(100)

91

3

(15)

4

(98)

88

3

(15)

0

(100)

91

13

(65)

26

(84)

82

(n) 2 8 3 10 10 13 0

(Sensitivity %) (10) (40) (15) (50) (50) (65) (100)

0

(0)

0

0 3 7 7 6 10

(0) (15) (35) (35) (30) (50)

V1

3

R-duration>_-0.04 + R/S ratio>/1:0

V1 or V2

R-duration>_-0.04 +R/S ratio>_-1.0 +upright T wave Our model

V~ or V2 V1 or V2 V1

Q-duration>/0.03

II AVF R-duration>_-0.04 lead V1 V2 V1 or V2 Upright T wave V1 V2 S-T depression V1 V2 R/S ratio>~l.0 V1 V2 V~ or V2 Q/R ratio>_.1.0 lead II AVF R-duration>~0.04 + upright T wave

lead

Total (n:180)

(n) 3 10 4 39 41 38 151

Abbreviations: n, number of patients.

J. ELECTROCARDIOLOGY 19(1), 1986

POSTERIOR MYOCARDIAL INFARCTION

waves in leads II or aVF were present in 20% of patients. The value of the T-wave amplitude in lead V~ as it relates to posterior MI was not addressed by Ward et al. 6, although we '5 and others '.3. ,6 have shown a relation between T-wave amplitude in V 1 and posterior MI, asynergy or disease of the left circumflex or right coronary arteries. Mechanisms of the ECG manifestation of posterior myocardial infarction. Tall and broad R-waves in right precordial leads in patients with posterior MI are probably due to the dominance of the unopposed anterior LV wall forces due to the loss of late posterior wall vectors. However, tall R-waves in V1.3 may be associated with other conditions such as right bundle branch block, right ventricular dominance, type A Wolff-Parkinson-White syndrome, muscular dystrophy, and dextracardiaP In this study, we excluded patients with the conditions listed above. Posterior MI may result in tall symmetrical Twaves in right precordial leads, probably due to the T-wave vector being directed anteriorly away from the infarcted posterior LV wall. In a previous s t u d y ~5we have found that upright T-waves in the right precordial leads correlated with the presence of left circumflex disease, and less frequently with right coronary artery disease. These vessels supply the posterior LV wall. In contrast to the T-wave vector, the S-T segment vector is directed towards the area of myocardial injury. This results in S-T segment depression in V,. 3. Gibson et al! 7 and Goldberg et alp studied patients after acute inferior MI and found that reciprocal S-T segment depression correlated with extensive myocardial damage, primarily involving the posterior or posterolateral LV wall, and was not related to concomitant ischemia in the anterior wall or left anterior descending artery disease. The reason that S-T depression in right precordial leads was not incorporated in the model is probably because very few of our patients had acute MI. Several features of our s t u d y deserve comment. The diagnosis of posterior MI was based on the presence of fixed thallium-201 perfusion defects. The images were obtained at rest in the 37 patients in the initial study, b u t during exercise in the 180 patients in the validation study. Fixed defects four hours post exercise represent scar, and therefore the use of rest images in one s t u d y and redistribution images in the second s t u d y should not change the result. The pharmacokinetics of thallium-201 indicates that this is an acceptable method because reperfusion is nearly complete four hours after injection of thallium-201! 9

J. ELECTROCARDIOLOGY 19(1), 1986

39

The patient population obviously differs, since 37 patients in the initial group with posterior defects were sicker and had either acute MI or rest angina pectoris, while the second group of patients were healthier and were being evaluated for chest pain suspected of being due to chronic CAD. Thallinm-201 imaging was used to assess the location and extent of MI. Although fixed thallium-201 perfusion defects have been shown to be specific and sensitive markers of myocardial fibrosis due to CAD, occasionally diseases other than CAD m a y produce perfusion defects.2~Also, small infarcts may not be detected by thallium imaging, and this may explain why the sensitivity of the model used was lower in the second group of patients.2~ It should however be pointed out that alternative methods to define MI, such as the use of autopsy, contrast angiography and vector cardiography have their own limitations. Thus, although pathologic studies may accurately localize the site and extent of MI, it provides information based on a select group of patients who did not survive and assumes no changes in ECG or MI size between the ECG s t u d y and autopsy. Similarly, LV asynergy by contrast angiography m a y be a s s o c i a t e d with ischemic b u t viable myocardium.22-2~ In summary, the ECG model described hero is useful in the diagnosis of isolated posterior M I or combined postorior MI and inferior MI. The model is sensitive and specific in patients admitted to the coronary care unit in whom this information is most needed. In addition, the model is still highly specific b u t not sensitive for posterior MI in patients with s y m p t o m a t i c b u t stable CAD. Acknowledgment: The authors thank Eric M. Umile for his assistance in preparing the manuscript. REFERENCES 1.

PERLOFF,J R: The recognition of strictly posterior myocardial infarction by conventional scalar electrocardiography. Circulation 30:706, 1964 2. P~ulvr, R D, DENNIS, E W AND KINARD, S A: The difficult electrocardiographic diagnosis of myocardial infarction. Prog Cardiovasc Dis 6:85, 1963 3. ARKIN, B M, HUETER, D C AND RYAN, T J: Predictive value of electrocardiographic patterns in localizing left ventricular asynergy in coronary artery disease. A m Heart J 97:453, 1979 4. WARD, R M, WIIITE, R D, IDEKER, R E, HINDMAN, N B, ALOnSO, D R, BisiioP S P, BLOOR, C M, FALLON, J T, GOTTLmU, G J, HACKEL, D B, HUTCmNS, G M, PIIILLIPS,H R, REIMER, K A, ROARK, S F, ROCIILANI, S P~ ROGERS, W J, RUTII,W K, SAVAGE, R M, WEISS,

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J. ELECTROCARDIOLOGY 19(1), 1986