Vectorcardiographic assessment of anterior and apical myocardial infarction

J. ELECTROCARDIOLOGY 13 (3), 1980, 245-252

Vectorcardiographic Assessment of Anterior and Apical Myocardial Infarction BY STEVEN PORT, M.D., FORTUNE A. DUGAN, M.D., JOHN W. STARR, M.D., JOSEPH C. GREENFIELD, JR., M.D. AND GALEN S. WAGNER, M.D.

SUMMARY When anterior myocardial infarction is extensive it m a y involve the left ventricular apex circumferentially. Previous studies have suggested t h a t apical involvement is indicated by clockwise deformities of the vector loop in the transverse plane. Vectorcardiograms (VCG) of 799 p a t i e n t s with c o r o n a r y a r t e r y disease, catheterized during a five-year period, were reviewed for the presence of areas of clockwise inscription in the transverse plane. Anteroapical myocardial infarction was defined by the presence of anteroapical akinesia or dyskinesia on left ventriculogram. One hundred seventy-one VCGs were identified from patients with ventriculograms adequate for analysis. Of these, 96 could be classified according to criteria published by Young et al. 4 for anteroapical asynergy but only 59 of the 96 had anteroapical akinesia or dyskinesia, a predictive accuracy of 61%. Application of Young's criteria to 64 patients with left ventricular hypert r o p h y but w i t h o u t c o r o n a r y disease or wall m o t i o n abnormalities falsely predicted the presence of anteroapical akinesia or dyskinesia in 28%, i.e., a specificity of 72%. Simplified criteria for anteroapical infarction were defined and shown to have a predictive accuracy of 91% in patients with coronary artery disease. The specificity of the criteria in the group with left ventricular hypertrophy and no wall motion abnormalities was 95%. In 100 patients found to be normal at cardiac catheterization, and in 200 healthy volunteers the specificity was 100%. ventricular infarction. 3 Subsequently, Young et al. studied a group of patients with anterior infarcts and described vectorcardiographic criteria for severe asynergy which also included the apex. 4 However, as s t a t e d by Young et al., a prospective study of clockwise abnormalities of the transverse plane QRS loop was necessary to substantiate their predictive value. The purpose of the present investigation was: 1) to prospectively evaluate the performance of clockwise abnormalities of the transverse plane QRS vector loop as p r e d i c t o r s of a n t e r i o r a n d a p i c a l 'myocardial infarction; and 2) to define vectorcardiographic criteria for the presence of anterior and apical infarction and '~o examine their predictive accuracy, sensitivity and specificity.

Recently, vectorcardiographic criteria for the diagnosis of anterior myocardial infarction have been developed in our laboratory by correlation with anterior wall motion abnormalities as defined by left ventriculography. 1 These criteria, however, do not permit quantitative assessment of the extent of infarcted m y o c a r d i u m . P o s t m o r t e m e x a m i n a t i o n of h e a r t s w i t h a n t e r i o r i n f a r c t s h a s demonstrated t h a t the apex m a y be involved circumferentially and t h a t this is indicated on the 12-lead electrocardiogram by Q waves or m a r k e d l y diminished R waves in the lateral precordial leads. ~ One would anticipate t h a t these changes would produce clockwise deformity of the vector loop in the horizontal plane. Hugenholz in 1965, recognized clockwise abnormalities of the transverse plane QRS vector loop in six patients with extensive left

From the Department of Medicine, Division of Cardiology, Duke University Medical Center, Durham, North Carolina. 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 solelyto indicate this fact. This work was supported in part by SCOR grant HL17670 from the National Heart and Lung Institute. Reprint requests to: Steven Port, M.D., Box 3022, Duke University Medical Center, Durham, North Carolina 27710.

MATERIALS AND METHODS The data bank of the Duke Clinical Epidemiology section was used to identify patients who had a left ventriculogram obtained at catheterization for suspected cm'onary artery disease (CAD) and whose evaluation included a VCG obtained within one week of catheterization. Patients were excluded if the VCG showed a left bundle branch block. Seven hundred and ninety-nine patients catheterized during a five-year period fulfilled these criteria. 245

246

PORT ET AL

A second group included 64 patients with aortic valve disease and left ventricular hypertrophy (LVH) as defined by the criteria of Phillips et al. 5 These patients had neither significant CAD nor focal contraction abnormalities. A control group included 200 healthy volunteers aged 20-29 without a history of heart disease or hypertension and 100 patients who underwent cardiac catheterization for evaluation of chest pain in whom both nbrmal ventricular contraction pattern and n o r m a l c o r o n a r y a r t e r y a n a t o m y were demonstrated. Vectorcardiograms were recorded using the Frank lead system and either a Hewlett-Packard 1507A vectorcardiograph or an Instruments for Cardiac Research VC61B vectorcardiograph modified with a Hewlett-Packard oscilloscope and camera. The chest electrodes were placed in the fourth intercostal space as recommended for the supine position. A calibration of 1 mV per 2 to 3 cm deflection was used, depending on the size of the QRS loop. Initial QRS forces were recorded with a calibration of 1 mV per 10 cm deflection with P and T loops excluded. The VCG trace was interrupted each 2.5 msec (Hewlett-Packard) or 2.0 msec (Instruments for Cardiac Research). Transverse plane QRS loops were used for analysis. A transverse plane amplitude ~>1.8 mV was used as the VCG criterion 5 for LVH. Routine 12-lead ECGs were obtained at the same time as the VCG. ECGs were analyzed for the presence of Q waves ~>.03 seconds in duration in leads V4-6 and for the presence of diminished R waves defined as: average R wave in V4-6 <5mm or any R wave in V4-~ ~<3mm. An initial screening identified transverse loops with any clockwise inscription. These loops were then reviewed and, when possible, assigned to one of the categories A through F as defined by Young et al. 4 Loops not assigned to one of the categories A through F were termed unclassifiable. Representative loops are presented in Fig. 1. Cardiac catheterization was performed using the standard technique. Selective coronary angiography was performed in multiple views using Judkin's technique. Left ventriculography was performed in the right anterior oblique position. Ven-

Type A

Type B

Type C

Type 0

Type E

Type F

Fig. 1. Diagrammatic representation of Young's groups A through F.

triculograms of those patients who demonstrated any clockwise inscription in the transverse plane vector loop were reviewed and evaluated. Anterior and apical wall motion was categorized as normal, hypokinetic, akinetic, or dyskinetic2 A wall received the classification of its most abnormal segment. Only those ventricles with contraction abnormalities of both anterior and apical walls which were most certainly due to myocardial infarction were considered diagnostic. The predictive accuracy of VCG criterion was defines as true positives true positives + false positives where true positives are those patients with the VCG criteria in question and anteroapical akinesia or dyskinesia and false positives are those with the VCG criteria but without anteroapical akinesia or dyskinesia. 7 The term "maximum sensitivity" is used in this study and is defined as true positives true positives + false negatives It was necessary to refer to maximum sensitivity because the true size of the false negative group was unknown. It is possible that there are additional patients with anteroapical akinesia or dyskinesia whose VCGs did not have any clockwise inscription and consequently would not have been included in the false negative group of this study. Thus, this evaluation identifies the maximum sensitivity attainable. Specificity is defined as true negatives true negatives + false positives where true negatives are those with neither the VCG criteria in question nor anteroapical akinesia or dyskinesia.

RESULTS O f t h e 799 VCGs available for analysis, 186 (23%) h a d clockwise inscription of at least one s e g m e n t of the t r a n s v e r s e loop. One h u n d r e d s e v e n t y - o n e p a t i e n t s h a d b o t h V C G s a n d vent r i c u l o g r a m s w h i c h were a d e q u a t e for evaluation. N i n e t y - s i x (56%) of t h e s e could be classified into one of Young's g r o u p s A-F. A c o m p a r i s o n of t h e d i s t r i b u t i o n of V C G s into t h e s e groups in b o t h Y o u n g ' s a n d t h e p r e s e n t s t u d y is s h o w n in Table 1. I n t h e p r e s e n t s t u d y t h e r e were no p a t i e n t s in g r o u p F a n d r e l a t i v e l y i n s i g n i f i c a n t n u m b e r s (2 a n d 3) in groups B a n d E. The d i s t r i b u t i o n a m o n g g r o u p s A, C a n d D were s i m i l a r in t h e two p a t i e n t populations. T h e s e "classifiable" loops were c o n t r a s t e d w i t h t h e r e m a i n i n g 75 "unclassifiable" loops in Table 2. The p e r c e n t a g e of classifiable loops w h i c h m e t L V H c r i t e r i a (>/1.8 mV) w a s 24%, a n d a l m o s t all (92%) w h i c h were ~<1.8 m V m e t c r i t e r i a for a n t e r i o r m y o c a r d i a l infarction. C r i t e r i a for L V H were m e t in a m a j o r i t y

J. ELECTROCARDIOLOGY, VOL. 13, NO. 3, 1980

VCG ASSESSMENT OF ANTEROAPICAL MI

TABLE I Distributions of VCGs with Clockwise Inscriptions in the Transverse Plane into Young's GroupsA-F.

247

TABLE III Patients with LVH But Neither CAD Nor Wall Motion Abnormalities: Prevalence of False Positive VCG Patterns.

Group

Young et al.

Present Study

VCG Group

Number

A

54 (42%)

36 (37%)

A

5/84 (8%)

B

9 (7%)

2 (2%)

B

0

C

10 (8%)

14 (15%)

C

10/64 (16%)

O

42 (34%)

41 (43%)

D

2/64 (3%)

E

9 (7%)

3 (3%)

E

1/64 (2%)

F

2 (2%)

0

F

0

126

Total Classifiable

96

TABLE IV Predictive Accuracyof VCG Groups.

TABLE II Comparative Occurrence of Loops Meeting LVH and AMI Criteria in "Classifiable" and "Unclassifiable" Groups.

23/96 (24%)

67/73 (92%)

Unclassifiable

44/75 (58%)

19/31 (61%)

(58%) of the "unclassifiable" groups of loops and, of the rest, 61% met anterior myocardial infarction criteria. Of the 96 patients with "classifiable" VCGs, 59 had anteroapical akinesia or dyskinesia, giving an overall predictive accuracy of 61% (range, 43% in group C to 86% in group A). The predictive accuracy in groups A, B, C and E was 73%. The lack of predictive accuracy of the unclassified loops suggested by the low i n c i d e n c e of VCG c r i t e r i a for a n t e r i o r myocardial infarction was confirmed by an extremely low prevalence (19%) of anteroapical akinesia or dyskinesia. Within the group of 171 patients with a clockwise inscription in the transverse plane, there were 73 with both anterior and apical akinesia or dyskinesia. Fifty-nine of the 73 had VCGs which could be classified into one of the types A-E, indicating a maximum sensitivity of 81%. Since VCG criteria for LVH were met by 24% of the loops classified into types A-F and by 58% of the u n c l a s s i f i a b l e loops, the specificities of loop p a t t e r n s A-E were evaluated in a group of 64 patients with LVH but with neither CAD nor wall motion abnormalities. Eighteen of these patients had loops which could be classified into one of the types A-E (Table 3). Thus, in the presence of LVH, the Young criteria falsely predicted anteroapical infarction in 28% (a specificity of 72%). This observation suggested re-evaluation of both predictive accuracy and maximum senJ. ELECTROCARDIOLOGY, VOL. 13, NO. 3, 1980

Anterior and Apical Akinesia or Dyskinesia (t>1.8 mV Excluded)

Group

Anterior and Apical Akinesia or Diskinesia

A

31/36 (86%)

B

1/2 (50%)

1/1 (100%)

C

6/14 (43%)

4/8 (50%)

D

19/41 (49%)

17/37 (46%)

E

2/3 (67%)

2/3 (67%)

Total Classified (A-E)

59/96(61%)

47/74 (64%)

Unclassified

14/75 (19%)

LVH (~> 1.8 mV) AMI (LVH Excluded) Young GroupsA-E

18/64 (28%)

23/25 (92%)

TABLE V Maximum Sensitivity of VCG Groups. VCG Group

Sensitivity (LVH Included)

Sensitivity (LVH Excluded)

A

31/73 (42%)

23/73 (32%)

B

1/73 (2%)

1/73 (2%)

C

6/73 (8%)

4/73 (5%)

D

19/73 (26%)

17/73 (23%)

E

2/73 (3%)

2/73 (3%)

Total (A-E)

59/73 (81%)

47/73 (64%)

Unclassified

14/73 (19%)

sitivity of p a t t e r n s A-E after loops wit}, transverse plane amplitude I>1.8 mV were excluded (Tables 4 and 5). The predictive accuracy is minimally enhanced from 61% to 64%, while maximum sensitivity decreases significantly from 81% to 64%.

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PORT ET AL

Each of the VCG loop types A-E was then considered separately to determine its suitability for inclusion in VCG crieria for anteroapical infarction. Group D was the most difficult pattern to evaluate. The high sens i t i v i t y of 25% (second only to group A) suggested that the D loop should be included in any criteria for anteroapical infarction. However, the unacceptable predictive accuracy of 49% made this inclusion untenable. Furthermore, assignment of vectors to group D is complicated by the requirement of Young et al. that the maximal leftward deviation of the loop be more negative than - 4 0 ~ in the transverse plane. Elimination of that constraint lowered the predictive accuracy even further (49% to 40%) and only minimally enhanced the sensitivity (26% to 34%). The exclusion of vectors with LVH did not appreciably alter the results in these groups. LVH was noted in 43% of patients with type C loops (Table 3). Predictive accuracy for identification of anteroapical a k i n e s i a or dyskinesia was low (43%) and was not significantly improved (5(F/o) when only the patients without LVH were considered. There were insignificant n u m b e r s of patients within groups B and E to permit evaluation of the importance of these patterns. Thus, only the type A loop had sufficient sensitivity and predictive accuracy for consideration as a criterion for anterior and apical infarction. The high incidence of LVH, however, (31%, Table 3) necessitated examination of the influence of this parameter upon diagnostic performance of this group (Table 6). The predictive accuracy could be slightly improved (from 86% to 92%) by elimination of those loops meeting criteria for LVH. This resulted, however, in eliminating eight of the 31

TABLE VI Influence of LVH Upon Predictive Accuracyand Maximum Sensitivity of the Type A Loop. Number

Predictive Accuracy

Maximum Sensitivity

Total Group A

36

86%

42%

With LVH

11

73%

11%

Without LVH

25

92%

31%

truly positive loops, further decreasing (42% to 31%) the low sensitivity of group A. It was observed that in patients with type A loops and transverse plane magnitudes t>1.8 mV, the earliest posterior forces were almost always to the right of the origin. This was true even in the presence of initial anterior forces (<0.1 mV). Using this additional criterion, 7 of 11 patients with LVH and a type A loop were identified (Table 7). Six of the seven had anteroapical akinesia or dyskinesia. Of the four patients who had LVH and a type A loop that were not identified by the new criterion, TABLE VII Performance of Modified VCG Criteria. Number With Anteroapical Akinesia or Dyskinesia Group

Number

Number Meeting VCG Criteria

Predictive Accuracy

No LVH

25

23/25

92%

LVH

11

6/7

86%

Total

36

29/32

91%

+

Fig. 2. Representative transverse plane loop showing completely clockwise efferent and afferent limbs. Maximum transverse plane magnitude is 1.05 mV. Ventriculogram showed severe anterior and apical dyskinesia.

Fig. 3. Transverse plane loop showing completely clockwise efferent and afferent limbs. Maximum transverse plane magnitude is 3.4 mV. Note that earliest posterior forces are to the right of the null point. Ventriculogram showed severe anterior and apical dyskinesia. J. ELECTROCARDIOLOGY, VOL. 13, NO. 3, 1980

VCG A S S E S S M E N T O F A N T E R O A P I C A L MI

249

f

Fig. 4a. Transverse plane loop showing mostly clockwise efferent limb and completely clockwise afferent limb. Maximum transverse plane magnitude is 1.80.

Fig. 4b. Enlargement of the initial forces of the loop shown in Fig. 4a. Note that earliest posterior forces are to the left of the null point. Ventriculogram showed completely normal wall motion and coronary arteries were normal.

two had anteroapical akinesia or dyskinesia. Hence, this modification improved the predictive accuracy of type A loops from 86% to 91% and lowered the sensitivity minimally (42% to 40%). Our proposed vectorcardiographic criteria for identification of anteroapical infarction are as follows (in the absence of complete left bundle branch block) (Figs. 2-4). 1) In the transverse plane, both efferent and afferent limbs of the QRS loop are mostly or completely clockwise. 2) I f t h e m a x i m u m t r a n s v e r s e p l a n e amplitude is/>1.8 mV, then the earliest posterior forces must be to the right of the point of origin. These criteria were then applied to groups of patients without angiographic evidence of CAD (64 with and 100 without LVH) and to 200 h e a l t h y volunteers. The specificity in those with LVH was 95% and in the other groups 100%. ECGs were available in 95 of the 96 patients with types A-E VCG loops. The Q and R wave analysis of left precordial leads is shown

according to vector loop t y p e in Table 8. Sixty-six percent of patients with type A loops had a Q wave of/>.03 seconds in one of the V4~ leads compared to 15% and 30% for types C and D loops respectively. Q w a v e s ~>.03 seconds in duration were present in 40 of the 95 patients. Anteroapical akinesia or dyskinesia was present in 35 of the 40, giving a predictive accuracy of 87.5% (Table 9) for the Q criterion. Diminished R waves in V4-6 were present in 21 patients of whom 12 (58%) had anteroapical akinesia or dyskinesia. R waves of normal amplitude were present in 34 patients of whom 8 (23%) had anteroapical akinesia or dyskinesia. Of the 73 patients with anteroapical akinesia or dyskinesia, 39 had a Q wave/>.03 seconds in one of the V4-61eads for a maximum sensitivity of 53%. Using both the Q criterion and the diminished R wave criterion increased maximum sensitivity to 67% b u t lowered predictive accuracy to 80%. TABLE IX Predictive Accuracy of ECG Criteria. Anteroapical Akinesia or Dyskinesia

Predictive Accuracy

Average R wave in V~.6 > 5 mm (n = 34)

8

23%

Average R wave in V4-6 0-5 mm or R~<3 (n = 21)

12

58%

O.~ > .03 sec V4.s (n = 40)

35

87.5%

TABLE VIII Analysis of Lateral Precordial ECG Leads in Vector Groups A-E. Type Loop

Normal Average RV4.6<5mm Q~>.03 R Waves or Any R~< 3mm in V4, 5, or e

A (n = 36)

4 (12%)

B (n = 2)

2 (100%)

C (n = 13)

8 (62%)

3 (23%)

2 (15%)

D (n = 41)

20 (49%)

9 (21%)

12 (30%)

1 (33%)

2 (67%)

E (n = 3)

8 (22%)

24 (66%)

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DISCUSSION A recent study from this institution has demonstrated the reliability of akinesia or dyskinesia of the ventricular wall as predictors of pathologically demonstrable myocardial infarcts, s Hypokinesia did not prove as specific an i n d e x of m y o c a r d i a l fibrosis. C o n s e q u e n t l y , a n t e r o a p i c a l a k i n e s i a or d y s k i n e s i a w e r e c h o s e n a s m a r k e r s of anteroapical infarction in this study. Vectorcardiographic criteria for anterior infarction have been defined previously by correlation with left ventricular wall motion abnormalities in patients with CAD, subtotal or total occlusion of the LAD coronary artery and a history of a previous myocardial infarct i o n 2 More recently, S a v a g e et al. 2 have documented the correlation between a u t o p s y - p r o v e n a n t e r o a p i c a l infarcts and qualitative changes on 12-lead ECG. In that study, p a t i e n t s with anterior and circumferential apical involvement could be distinguished from those with infarcts limited to the anterior myocardium by the presence of Q waves or markedly reduced R waves in leads V4-6. This confirmed the findings previously reported by Hoffman, Pruitt and Edwards 9 that QRS changes in leads V4-6 indicated infarction of the apex rather than the lateral free wall of the left ventricle. However, those studies did not report the VCG findings. H u g e n h o l z et al. did r e p o r t an a u t o p s y series in which VCGs had shown clockwise abnormalities in the horizontal plane in six cases. 3 Two were classified as anterolateral infarcts and five with figure-eight patterns as lateral infarcts. In each of these cases, the autopsy revealed extensive left ventricular infarction. The more recent report of Young et al. 4 showed that clockwise QRS loop rotation in the t r a n s v e r s e plane could serve as a marker for severe asyngergy which included the apex as well as the anterior wall. The present study confirms the correlation between clockwise rotation in the transverse plane of the vector and anteroapical wall motion abnormalities. However, when prospectively examined, the vectorcardiographic patterns identified by Young et al. have an accuracy for identification of anteroapical a k i n e s i a or d y s k i n e s i a of only 61% w h e n applied to our group of patients with CAD. This does not prove a low specificity of those criteria for identification of "severe anteroapical asynergy" since a quantitative evaluation of wall motion was employed in their study while absence of motion or paradoxical motion has been required in the present study. The m a x i m u m sensitivity of those vectorcardiographic patterns for identification of anteroapical akinesia or dyskinesia was only

80%. Thus, a p p r o x i m a t e l y one-fifth of patients with infarction of the anterior and apical walls sufficient to be associated with no apparent systolic contraction do not have any of the m a r k e r s of clockwise transverse plane rotation described by Young et al. The type A loop had the highest predictive a c c u r a c y (86%) of a n y of t h e loops w i t h clockwise rotation. By dividing the group with type A loops into those with and those without LVtt, it was noted that the group with LVH had a 28% rate of false positives. By modifying the VCG criteria in this subgroup, the rate of false positives was reduced to 14% and the predictive accuracy of the whole group was increased to 91% while the m a x i m u m sensitivity remained essentially the same. The specificity of the simplified criteria was examined in the 64 patients with L V H and no CAD or wall motion abnormalities and was found to be 95%. In the control groups of 100 patients, proved normal at catheterization, and 200 healthy volunteers, the modified criteria are 100% specific. Types C and D loops had predictive accuracies which were much lower than group A (43% and 49% vs 86%). These were not improved sufficiently after exclusion of vectors meeting criteria for left ventricular hypertrophy. Therefore, these patterns cannot be considered comparable to the above criteria for indication of anterior a n d apical infarction. It is possible that these patterns of QRS changes are analogous to diminished R waves in leads V4-~ on ECG, a finding which was associated with r circumferential" apical involvement of anterior infarcts at postmortem evaluation. 2 However, that hypothesis was not confirmed in this study since only 23% and 21% of types C and D loops respectively had diminished R wave in leads V4-6. Sixty-two percent of type C loops and 49% of type D loops had normal R waves in leads V4-6. Recent angiographic studies have demonstrated a relationship between the number of leads with Q waves or diminished R waves and the extent of myocardial infarction as suggested by the left ventricular ejection fraction. l~ Those findings are supported by this study in which the Q wave's extension to the lateral precordial leads was associated with anteroapical akinesia or dyskinesia in 87.5% of the cases. Diminished R waves in V4-6 were not as reliable in predicting anteroapical akinesia or dyskinesia (58%). Comparison of the proposed VCG criteria and the E C G show that the predictive accuracy of the VCG (91%) is better than either the ECG Q wave criterion (87.5%) or the Q wave plus diminished R wave criteria (77%). However, the m a x i m u m sensitivity of the J. ELECTROCARDIOLOGY, VOL. 13, NO. 3, 1980

VCG ASSESSMENT OF ANTEROAPICAL MI

E C G Q w a v e c r i t e r i o n is 53%, s o m e w h a t higher t h a n the VCG criteria (42%). It is a p p a r e n t t h a t extensive ant er i or infarction which includes the left v e n t r i c u l a r apex can be identified by the most complete a l t e r n a t i o n of t h e QRS in t h e t r a n s v e r s e plane; i.e., totally clockwise rotation on VCG or totally absent R waves on ECG. Less extensive apical involvement m a y be indicated by clockwise deformity which is less t h a n total or by r e t e n t i o n of diminished R waves. F u r t h e r studies are necessary to confirm this correlat i o n f o r t h e e n t i r e s p e c t r u m of s i z e s o f myocardial infarcts. Acknowledgments: The authors would like to acknowledge the important technical assistance of Mary Riggsbee and Betty Cawthorne and offer their appreciation to Virginia Utley for the preparation of the manuscript. REFERENCES 1. STARR, J W, WAGNER, G S, DRAFFIN, R, GREENFIELD,J C, JR, BEHAR,V S, REED, J AND WALSTON,A: Vectorcardiographic criteria for the diagnosis of anterior myocardial infarction. Circulation 53"229, 1976 2. SAVAGE, R M, WAGNER, G S, IDEKER, R E , PODOLSKY, S A AND HACKEL,D B: Correlation of postmortem anatomic findings with electrocardiographic changes in patients with typical anterior and posterior infarcts. Circulation 55:279, 1977 3. HUGENHOLZ, P G, FORKNER, C E, JR AND LEVINE, J: A clinical appraisal of the vectorcardiogram in myocardial infarction. Circulation 24-825, 1961

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