The genesis of indeterminate axis: A quantitative vectorcardiographic analysis

J. ELECTROCARDIOLOGY 15 (3), 1982, 221-226

The Genesis of Indeterminate Axis: A Quantitative Vectorcardiographic Analysis BY ARV L.

GOLDBERGER,M.D.*

SUMMARY The vectorcardiographic basis of indeterminate QRS axis has not been elucidated. Accordingly, Frank lead VCGs were obtained on ten consecutive subjects with biphasic QRS complexes in the six extremity leads. The frontal plane vector in each case showed a very narrow loop inscribed in a symmetrical fashion around the E point. The maximal width of the frontal plane loop (0.20+0.10 mV, mean _ S D ) was significantly smaller (P<0.02) than previously published normal values (0.29__+0.15 mV, n----100). Terminal forces were oriented to the right and superiorly. The transverse plane vector also showed a characteristic pattern with posterior and rightward orientation of terminal forces; the 50 ms QRS vector was significantly (P<0.0005) more posterior and rightward (258_+29 ~ in subjects with indeterminate axis compared with established normal values (307+28 o, n=510). The precordial leads reflecting these terminal, rightward forces showed prominent S waves in lead V5 (0.86__+0.48 mV) and V6 (0.38__+0.27 mV) which were significantly greater (p<0.0005) than the amplitude of S waves in lead V5 (0.24--+0.20 mV) and V6 (0.06-+0.09 mV) established for normals (n=424). These findings suggest that: 1) the indeterminate nature of the frontal plane QRS axis is an epiphenomenon resulting from orientation of the plane of the QRS loop relatively perpendicular to the frontal plane; and 2) the underlying factor is the posterior, rightward and superior orientation of terminal QRS forces, which may result from a number of causes. more, i n d e t e r m i n a t e axis is associated with characteristic posterior, rightward and superior displacement of the cardiac vector which m a y result from a variety of etiologies. This distincrive vector loop produces prominent terminal S waves in the lateral chest leads, a previously unrecognized correlate of indeterminate frontal plane QRS axis.

The label "indeterminate QRS axis" is applied to ECGs in which all six frontal plane leads show biphasic (RS or QR) complexes with equal, or nearly equal positive and negative components. A l t h o u g h the p a t t e r n is widely recognized in clinical practice, the vectorcardiographic features of indeterminate axis have received scant attention in standard textbooks ~3. The characteristics of the frontal plane QRS loop responsible for the unusual axis have not been elucidated. On the basis of biphasic complexes in each of the ext r e m i t y leads, one might predict a relatively "circular" frontal plane loop. The following vectorcardiographic s t u d y demonstrates, contrary to expectations, t h a t the frontal plane QRS vector responsible for the indeterminate axis has a very narrow, almost linear configuration. Further-

MATERIALS A N D METHODS Ten consecutive subjects (all males} with an indeterminate QRS axis were prospectively identified from the electrocardiographic laboratory at the San Diego Veterans Administration Medical Center. Salient clinical data on these subjects are presented in Table 1. Frank lead VCGs* were obtained and the following variables were analyzed: 1. QRS duration (measured as maximal duration in lead X, Y or Z recorded at 50 mm]sec paper speed}. 2. Amplitude of the S waves in leads V5 and V6 on the scalar ECG. 3. QRS loop inscription in the frontal, transverse and right sagittal planes, identified as clockwise, counterclockwise, or figure of 8.

*Assistant Professor of Medicine, UCSD, Director of ECG Graphics, San Diego Veterans Administration Center. ~rhe 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. w1734 solely to indicate this fact. Reprint requests to: Ary L. Goldberger, M.D., Cardiology Division {IliA}, Veterans Administration Medical Center, 3350 La Jolla Village Drive, San Diego, CA 92161.

*Instruments for Cardiac Research, Inc., Syracuse, New York.

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Fig. 1. Case 2. ECG shows S1 $2 $3 pattern. NoLe characteristically narrow symmetrical frontal plane loop' with posterior and rightward orientation of terminal QRS forces. Note terminal S waves V5 and V6. Vector loop time marks inscribed at 2.5 ms intervals. Long arrows indicate general direction of QRS vector. 4. Maximal width Of the QRS loop in the frontal plane, measured as the greatest perpendicular distance between the efferent and afferent loops 1. 5. Location of the 20ms and 50ms QRS vectors in the transverse plane. By convention 1, the positive pole of lead X was taken as 0 ~ and the positive (posterior) pole of lead Z was taken as 270 ~ (Fig. 1). 6. Direction of terminal forces in the frontal and right sagittal planes. Statistical Methods Maximal width of the frontal QRS loop, the orientation of the 20ms and 50ms transverse plane QRS vectors and the amplitude of the S waves in leads V5 and V6 in the group with indeterminate axis were compared to previously published normal values 4~ using a two-tailed t test. Results are expressed as mean values _ one standard deviation (SD). Statistical significance is defined as P<0.05.

RESULTS Clinical d a t a on the ten s u b j e c t s w i t h indeterm i n a t e QRS axis are briefly s u m m a r i z e d in T a b l e

I. Of n o t e is t h e v a r i a b i l i t y in c a r d i o p u l m o n a r y s t a t u s , including s u b j e c t s w i t h severe chronic l u n g disease, c o r o n a r y or n o n - c o r o n a r y h e a r t disease, as well as t h o s e w i t h minimal or no apparent pathology.

Scalar E C G Findings. M e a n Q R S d u r a t i o n was 104___16 ms (range 83-125). Q R S d u r a t i o n was ~110 ms in all b u t two cases (Table II). F r o n t a l p l a n e leads in all cases s h o w e d the c h a r a c t e r i s t i c findings of i n d e t e r m i n a t e axis. T h e classic S 1 S 2 S 3 p a t t e r n 7 was p r e s e n t in eight cases. I n one case, a n r S r ' was p r e s e n t in lead I w i t h S w a v e s in leads 2 and 3. One case (Fig. 2) s h o w e d an S1Q3 m o r p h o l o g y . In all ten cases, t h e r e were p r o m i n e n t S w a v e s in the lefL c h e s t leads (V5 or V6). T h e a m p l i t u d e of these S w a v e s in b o t h lead V5 ( 0 . 8 6 _ 0.48mV) a n d lead V6 (0.38+0.27mV) were significantly g r e a t e r J. ELECTROCARDIOLOGY

1 5 ('3), 1 9 8 2

GENESIS OF INDETERMINATE AXIS

TABLE I. CLINICAL D A T A Patient

Age

Diagnosis

1. RL

70

Severe chronic obstructive pulmonary disease

2. RS

51

Idiopathic cardiomegaly

3. AA

50

Chronic obstructive pulmonary disease, pulmonary emboli

4. VS

53

Hypertension, angina pectoris

5. KF

58

Coronary artery disease, anterior infarct

6. MH

35

Hypertension

7. JE

55

Normal

8. HL

31

Straightback syndrome, pulmonic flow murmur

9. EB

i56

Carcinoma of lung

10. JL

40

Severe aortic insufficiency

(p<0.0005) than previously published 4 normal values for the S waves in these leads (Table 2).

Vectorcardiographic Data Vectorcardiographic findings are summarized in Table III. The frontal plane QRS loops were ins c r i b e d in a v a r i a b l e d i r e c t i o n (clockwise, counterclockwise, or figure of 8). The configuration of the frontal plane QRS vector in each case, however, was strikingly similar, showing a very narrow loop inscribed in a relatively symmetrical fashion around the E point (Figs. 1 and 2). The maximal width of the frontal plane QRS loop (0.20-+0.10mV) was significantly less (P
223

(0.29_+0.15mV). Terminal forces in the frontal plane were invariably oriented to the right. The horizontal plane vector also showed a characteristic appearance. The early forces were directed to the left in seven of ten cases. Although there was considerable variability in the orientation of the 20 ms vector (range 110 ~ to 290 ~ the mean value (15_+59 o) was significantly different (P<0.001) from previously published 6 normal values (81_+26 ~ n=510). Terminal forces in the horizontal plane were consistently oriented rightward and posteriorly. The 50 ms QRS vector was significantly more (p<0.0005) posterior and rightward (258_+29 ~ in subjects with indeterminate axis compared to normals (307-+280) 6. In all but one case, the 50 ms vector was oriented ~<270 o in the rightward, posterior quadrant of the transverse plane. In the one exception (case 8), the 50 ms vector was oriented at 330 ~ but the 60 ms vector was markedly rightward (225~ In all cases, the vector in the horizontal plane was inscribed in a counterclockwise direction. The right sagittal plane QRS vector was inscribed in a clockwise direction. In all cases, the terminal forces were oriented posteriorly and in all but one case the terminal forces were oriented superiorly. In that partial exception (case 9), late forces were directed superiorly, although the very terminal forces were oriented inferiorly. DISCUSSION This quantitative vector analysis provides insight into the genesis of indeterminate axis. First, the primary factor underlying the indeterminate frontal plane axis is the orientation of terminal QRS forces rightward, posteriorly, and superiorly, reflected on the scalar ECG by promi-

TABLE I1: ECG D A T A SUBJECT

1 2 3 4 5 6 7 8 9 10

ORS DURATION (ms)

UMB LEADS

$1 S2 $3 Sl $2 $3 Sl $2 $3 $1 82 $3 $1 S2 $3 $1 S2 Q3 Sl $2 $3 $1 $2 $3 $1 $2 $3 rSr'l $2 $3 mean +SD

83 100 89 100 135 95 95 110 105 125 104 16

SV5 (mY) O.4O O.9O 0.50 0.35 1.30 1.60 0.55 0.90 0.50 1.55 0.86 0.48

Normats (n=424) mean 0.24 (Ref 4) +SD 0.20 P< O.O005

J. ELECTROCARDIOLOGY 15 (3), 1 9 8 2

SV6 (mV) O.20 O.3O 0.30 0.30 0.55 1.OO 0.30 0.55 0.25 O.00 0.38 0.27 0.06 0.09 0.0OO5

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F i g . 2. C a s e 6. E C G s h o w s $1 Q 3 p a t t e r n , s i m u l a t i n g i n f e r i o r w a l l i n f a r c t i o n . F r o n t a l p l a n e l o o p is n a r r o w a n d s y m m e t r i c a l as in Fig. 1, with terminal posterior and rightward forces. Note very prominent S waves in V5 and V6.

TABLE FRONTAL SUBJECT

WIDTH (mV)

1 2 3 4 5 6 7 8 9 10

0.09 O. 1 2 O. 1 3 0.20 0.09 0.31 0.25 0.35 0.30 O. 1 5 mean 0.20 _+SD 0.10 N o r m a l s (N = l O0)(Ref 6) mean 0.29 • SD 0.15 P< 0.02

PLANE

ROTATION

TRANSVERSE

TERMINAL FORCES

Fig 8 CCW CW CW Fig 8 Fig 8 CW Fig 8 CW Fig 8

III. VECTORCARDIOGRAPHIC

R/S R/S R/S R/S R/S R/S R/S R/S R/S-I R/S

20 ms Vector (degrees)*

110 60 335 40 280 20 0 13 60 20 290 mean 15 +_SD. 59 N o r m a l s (n = 5 1 0 ) [ R e f 5) mean 81 _+ SD 26 P< 0 . 0 0 1

50 ms Vector (degrees)

263 259 270 228 238 240 244 330 240 263 258 29

DATA PLANE ROTATION

RIGHT

SAGITTAL

ROTATION

PLANE TERMINAL FORCES

CCW CCW CCW CCW CCW CCW CCW CCW CCW CCW

CW CW CW CW CW CW CW CW CW CW

P/S P/S P/S P/S P/S P/S P/S P/S P/S-I P/S

307 28 0.0005

I=inferior; P=posterior; Abbreviations: CCW=counterclockwise rotation; CW=clockwise r o t a t i o n ; fig 8 = f i g u r e - o f - e i g h t ; R =rightward; S =superior. * T o c a l c u l a t e t h e m e a n + SD f o r 2 0 m s v e c t o r s , p o i n t s at n o lying in t h e q u a d r a n t b e t w e e n 0 and 2 7 0 o w e r e c o n s i d e r e d equivalent t o ( n - 3 6 0 o)

J. ELECTROCARDIOLOGY

1 5 (3),

1 982

GENESIS OF INDETERMINATE AXIS

nent S waves in V5 or V6. Despite the rightward displacement of the QRS vector, no subject showed an actual right bundle branch block configuration. Furthermore, QRS duration was either normal or only slightly prolonged in most cases. Second, indeterminate axis does not, as might be presupposed, reflect a circular frontal plane loop. The "indeterminate" QRS axis is due instead to the relatively orthogonal orientation of the plane of the QRS loop vis-a-vis the frontal plane. As a result, the frontal plane loop has a very narrow, symmetrical configuration resulting in biphasic (RS or QR) complexes in each of the extremity leads. QRS voltage will be greatest in those leads most directly aligned with the long axis of this loop and least in leads lying at right angles tolit. Therefore, the indeterminate nature of the fr0ntal plane axis is actually an epiphenomenon, a result of the orientation of the plane of the QRS loop nearly perpendicular to the frontal plane. The primary factor is the posterior, rightward, and superior orientation of terminal QRS forces, which was seen in all cases. This characteristic rightward and posterior displacement of the QRS vector may result from a variety of causes including normal (positional) variants, right ventricular hypertrophy, right ventricular conduction delay, ischemia and noncoronary heart disease. The association of indeterminate axis with right ventricular conduction delay has been noted previously. Grant 7 in his discussion of the $1S2S3 pattern commented that it reflected a "conduction variation of the right ventricle" most commonly observed in patients with cot pulmonale or congenital heart disease. The occurrence of this pattern in normal subjects and in those with coronary disease was also noted. Grant further suggested that this pattern was typically associated with an R' in V1 as part of the right ventricular conduction delay. However, none of the eight cases reported here with indeterminate axis and S1S2S3 pattern showed evidence of an rSr' in V1, despite the presence of prominent left precordial S waves. Therefore, anterior displacement of terminal QRS forces does not appear to be a general feature of the indeterminate axis variant. In addition, indeterminate axis is not always associated with an $1S2S3 pattern. One of the cases in the current series showed an $1Q3 pattern (Fig. 2). Posterior and rightward orientation of terminal forces was also present in this case. The $1 Q3 pattern has also been described as a J. ELECTROCARDIOLOGY 1 5 (3), 1982

225

transient finding with acute cor pulmonale s, another clinical setting where displacement of the QRS vector posteriorly and rightward due to a right ventricular conduction delay might be expected. It should be emphasized, however, that while all cases of indeterminate axis in the current series showed either an $1S2S3 or $1Q3 pattern, not all ECGs showing either of these patterns will necessarily have an indeterminate axis 9,'~ Finally, from a clinical viewpoint, understanding the vectorcardiographic basis of indeterminate axis is important. This entity is occasionally confused with left anterior hemiblock, since both patterns may be associated with RS complexes in the inferior limb leads. However, with left anterior hemiblock, terminal forces in the frontal plane are oriented superiorly and leftward, in contrast to the late rightward forces seen with indeterminate axis. Indeterminate axis may also simulate inferior infarction, as shown in Case 2, pr6ducing prominent QR waves in inferior leads III and aVF. Initial forces in such cases are oriented to the left, in contrast to the superior orientation of early forces with inferior infarction.

Acknowledgements: The authors thank Ms. Sue Connolly for her expert secretarial help and Dr. Robert Slutsky for his help in identifying patients for this study.

REFERENCES 1. BENCHIMOL,A: Vectorcardiography. Williams and Wilkins, Baltimore, 1973 2. CHOU,T C, HELM, R A AND KAPLAN, S: Clinical Vectorcardiography, 2nd edition. McGraw-Hill, New York, 1974 3. COOKSEY,J D, DUNN, M AND MASSIE, E: Clinical Electrocardiography and Vectorcardiography, 2nd Edition. Year Book Medical Publishers, Chicago, 1977 4. SIMONSON,E: Differentiation between Normal and Abnormal in Electrocardiography. C V Mosby, St. Louis, 1961, p 295 5. MCCALL, B W, WALLACE, A G AND ESTES, E H: Characteristics of the normal vectorcardiogram recorded with the Frank lead system. Am J Cardiol 10:514, 1962 6. DRAPER,H W, PEFFER, C J, STALLMAN,F W, LIvrMANN,D AND PIPBERGER, H V: The corrected orthongonal electrocardiogram and vectorcardiogram in 510 normal men (Frank lead system). Circulation 30:853, 1964 7. BECKWITtt, J R: Grant's Clinical Electrocardiography, 2nd Edition. McGraw-Hill, New York, 1970, pp131-133

226

GOLDBERGER

8. McGINN,S AND WtIITE, P D: Acute cor pulmonale resulting from pulmonary embolism: Its clinical recognition. JAMA 104:1473, 1935 9. GOLDBERGER, E AND SCIIWARTZ, S: Electrocardiograms in which the main ventricular deflections

are directed downward in the standard leads. Am Heart J 29:62, 1945 10. FRIEDMAN, H H: Diagnostic Electrocardiography and Vectorcardiography, 2nd Edition. McGrawHill, New York, 1977, pp 352-356

J. ELECTROCARDIOLOGY 1 5 (3), 1 982