Variation in the precordial QRS transition zone in normal subjects

JOURNAL OF ELECTROCARDIOLOGY, 21 (1), 1988, 25-30

Variation in the Precordial QRS Transition Zone in Normal Subjects BY LEO G. HORAN, M.D.,* MARANDAPALLI R. SRIDHARAN,M.D.,$ R. CHRIS HAND,** AND NANCY C. FLOWERS, M.D.*

SUMMARY From body surface potential map data for 51 normal young men (with QRS axis between 0 and 90 degrees) both the spatial QRS area vector and the isoarea map of the QRS were obtained. Acting on Grant's assumption that the transition zone defined a plane perpendicular to the spatial QRS vector, we determined the angular shift in altitude and azimuth required to move the spatial vector of each individual to the position of the group mean. We then shifted the precordial map of the transition zone of each individual with the same angular correction. These resulting transition zone boundaries clustered much closer to each other, but did not move into absolute coincidence. We interpreted the nearness-to-fit to be an estimate of the degree to which the precordial QRS configurations conformed to a common simple vector or dipolar pattern. T h e r e is a wide v a r i a t i o n in n o r m a l s u b j e c t s in r i g h t or l e f t w a r d s e t t i n g of t h e t r a n s i t i o n zone, or line of d e m a r c a t i o n , b e t w e e n n e t n e g a t i v e a n d n e t p o s i t i v e d e f l e c t i o n s for t h e p r e c o r d i a l Q R S complex. T h i s h a s b e e n a t t r i b u t e d to a n a t o m i c variation in o r i e n t a t i o n of t h e h e a r t w i t h r e s p e c t to t h e b o n y l a n d m a r k s which specify electrocardiographic l e a d locations. 1,2 T h e p r e s e n t s t u d y e x a m i n e s t h e c o n t o u r line of t h e t r a n s i t i o n z o n e over t h e anterior t h o r a x in n o r m a l s u b j e c t s a n d relates it to t h e QRS area vector determined from body surface p o t e n t i a l m a p s . T h e close r e l a t i o n s h i p b e t w e e n t h e b o u n d a r y line of t h e t r a n s i t i o n p l a n e a n d t h e Q R S - a x i s - v e c t o r m a y s u g g e s t a m e a n s of reduci n g clinically i n s i g n i f i c a n t d i s t i n c t i o n s within a n y g r o u p of n o r m a l s u b j e c t s .

100 msec or less and frontal QRS axes were between 0 and +90 degrees. The maps were constructed from 142 unipolar leads, digitized, serially averaged over 50 consecutive heart beats, and collated with a P D P 11/34 laboratory digital processor as previously described. 8 For each subject, isoarea maps 4 were constructed by summing the voltages recorded at each electrode site throughout QRS and throughout ST-T, i.e., from the end of the QRS to the end of the T wave {Fig. 1). The 142-point maps were expanded to 202 points by interpolating a row between the top map row and the neck (north pole} electrode and two rows between the bottom row and the left foot (south pole) electrode This positioned the potential distribution on the computational sphere in a manner topographically analogous to that on the thorax. The interpolation was accomplished from a transform which serially calculated the missing six voltages from the 16 known voltages of each of the possible 22 on each meridional circle. The transform was based on the first four sines and cosines in the Fourier series for the electrode positions--equally spaced about the longitudinal circle. 5 The expanded map array was applied to spherical projection so that the voltage at each site weighted a local surface normal vector: the resulting x, y, and z moments for all 202 "electrode" sites were summed to yield the x, y, and z coordinates of the mean heart vector for QRS {Fig. 2). The heart vectors of each set were rotated until they matched the mean for the whole set; the amount of azimuth and altitude required to produce the rotation was calculated. 6 This shift was applied to the projection of the corresponding null or transition zone contour, shifting its position on the map surface. As illustrated diagrammatically in Fig. 3a (left}, the angle/3 of azimuth required to move the sample heart vector to the mean vector position was applied to move the

MATERIALS AND METHODS Body surface potential maps were obtained for 51 normal male subjects, ranging in age between 19 and 55. Electrocardiograms were normal with QRS durations of

*Professor of Medicine ~Associate Professor of Medicine **Electronics Technician From the Veterans Administration Medical Center and the Section of Cardiology, Department of Medicine, Medical College of Georgia, Augusta, Georgia. Supported by the Veterans Administration and the National Heart, Lung, and Blood Institute Grants 33715 and 33692. Reprint requests to: Leo G. Horan, M.D., Section of Cardiology, Medical College of Georgia, Augusta, Georgia 30912.

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HORAN ET AL

RB

Group

FRONT

LB

Mean

F

RB

Fig. 1. Body surface potential isoarea map during the inscription of the QRS complex of a normal man. The isopotential contour lines are spaced at intervals of 5 mVms; the bold contour is the null boundary between positivity and negativity and traces the transition zone of equiphasic QRS complexes on the chest surface.

the frontal projection of the respective span for each contour is proportional to the cosine of a. The increment or decrement in cosine value was applied to enlarge or shrink the vertical dimension of the transition zone and similarly to locate its upward or downward displacement. The combined effect of these two anamorphisms is shown in Fig. 3b, where a rightwardly placed contour line (a) associated with a relatively vertical heart vector is shifted left and expanded to match the group mean vector (m); similarly, a leftwardly placed contour associated with a more horizontal vector (b) is shifted to the right and appropriately shrunk--again to more closely approximate the mean.

RESULTS

H

Fig. 2. Frontal (F) and horizontal (H) projections of the QRS area vectors computed for 51 normal young men. On the right is the mean for the group.

map of the transition zone to the right or left with respect to the axillary reference boundaries of the anterior thorax. On the right, a diagrammatic view of the angles % and a m of altitude respectively of the sample heart vector and the mean for the normal group is shown. Note that

Fig. 4 shows t h e lines of t r a n s i t i o n b e t w e e n negative a n d positive p o t e n t i a l fields s u p e r i m p o s e d on a t o r s o outline (frontal view} for each of the 51 s u b j e c t s . N o t e t h a t for QRS, the principal patt e r n is t h a t of a diagonal c o n t o u r sweeping up in an S - c u r v e from the low r i g h t a n t e r i o r thorax, and p a s s i n g left of t h e s t e r n u m over to the left shoulder. T h e h e a r t v e c t o r p r o j e c t i o n s to t h e r i g h t of the collection of c o n t o u r s show t h a t t h e equivalent cardiac dipoles for t h e QRS isoarea m a p s p o i n t largely to the lower left t h o r a x and slightly p o s t e r i o r l y - - o r away from the anterior aspect of the r i g h t shoulder. Similarly, the S T - T isoarea t r a n s i t i o n c o n t o u r s s u r r o u n d a large r o u n d m o u n d of p o s i t i v i t y centered b e t w e e n the left p a r a s t e r n a l and midclavicular lines at midprecordial level--as also indicated b y the associated S T - T isoarea vectors. T h e results

JOURNAL OF ELECTROCARDIOLOGY 21 (1), 1988

VARIATION IN QRS T R A N S I T I O N Z O N E

Fig. 3a. Diagrammatic representation of the way in which an individual QRS area vector was moved toward the group mean and, in turn, correspondingly moved the map of the accompanying mull-line or transition zone. On the left is a horizontal view showing the azimuthal angle, fl, of deviation of the individual or sample vector from the group mean. When the sample is moved through the angle fl to the standard or mean position, the frontal plane projection is simultaneously rotated through the same angle to move the transition zone. On the right, the angle a measures the difference between the angles of altitude for the individual sample and group mean. When the vector of the sample is moved through a, the tilt and span of the corresponding transition zone is appropriately transformed.

__z samplevector ~~anvector I~ :..............~_.~~O~, ! / I/.,-"//\ ~---/~,

ii

~"71

"~E'~ le \ .o,"~/ I " %"***o "~'vector

':........... " ' ~ /

I ~

Azimuth

~ ~'/mj/ean

Altitude

vector

F~

.9

27

b

Fig. 3b. Two individual contours transition zones, a to the right and to the left of the mean transition zo~ for the group. The corresponding sp tial vector frontal and horizontal pr jections are shown beside the tors On the right, the effect of moving tt two spatial vectors, a and b, toward tt group mean (m) and corresponding] transforming the transition zone bou: dary so that it too moves toward coiJ cidence with that of the group mea is shown.

Fig. 4. Superimposed frontal plane maps of the QRS transition zone boundaries for 51 normal young men before {left) and after (right) moving the QRS spatial vectors into coincidence. The point at which each contour line cresses an arbitrary horizontal line (set at the level of the lead V3 electrode) is marked with a small circle. The arrows indicate that the crossing points group more closely after the respective contours are moved to match the mean spatial vector. See Table I.

JOURNAL OF ELECTROCARDIOLOGY 21 (1), 1988

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H O R A N ET A L

are shown in Fig. 5. Note t h a t b o t h the right-toleft and head-to-foot variations have been greatly reduced by the same vector-collecting procedure and t h a t consequently the band or range of transition contours are narrowed. As not e d in Table I, we looked for the position of the point of transition zone crossing at the lead V 3 level; we chose this level as a compromise because precordial leads V 1 and V 2 are a half-interspace higher and leads V4_6 are a half-interspace lower. The ratios of variances (i.e., co mp ar in g raw crossing to corrected crossing} were 4.03 for QRS and 6.46 for ST-T. These ratios b o t h exceeded the requirement of a ratio of 2.13 for significance at the 0.5% levelY

DISCUSSION In an earlier study, ~ we examined the relationship in the dog between the anatomic vector normal to the right ventricular aspect of the interventricular sept um and t he spatial mean QRS vector: we found t h a t the rotational t r a n s f o r m a t i o n t h a t brought the anatomic vectors to a common site did not similarly focus the associated electrical vectors, indicating the electrical orientation was dependent on ot her factors beside the simple anatomic orientation. However, in this study, we compare two electrical phenomena, t he hum an mean spatial vector and t he transition zone, which reasonably may be expected to be closely related. A crude analogy

Fig. 5. Superimposed frontal plane maps of ST-T transition zone boundaries for the same young men shown in Figs. 1 and 4. On the left, the original contour lines and on the right, the contour lines after moving the spatial ST-T vectors into coincidence and the respective transition zone boundaries with them. Again, the arrows indicate the closing in of the points where the line of transition crosses the horizontal level of the lead V3 electrode.

TABLE I Spatial Isoarea Vectors and Transition Zone Crossings (in degrees*) VECTOR

QRS Area ST-T Area

TRANSITION ZONE

Altitude

Azimuth

Raw Crossing

Corrected Crossing

34.5 + 18.3 27.2 _+ 15.0

- 2 9 . 9 +_ 24.6 37.4 ___ 24.2

52.0 _+ 22.7 136.4 _+ 30.5

53.2 _+ 11.3 135.9 _+ 12.0

*The altitude is reported in degrees of elevation or depression from the horizontal plane: positive toward the foot and negative toward the head. The azimuth is read as positive rotation toward the front from the left axilla, negative toward the back. Thus, the left axilla is at 0 ~ midsternum at +90 ~ right axilla at +180 ~ and midvertebral line - 9 0 ~ Transition zone crossings at the level of lead V 3 are also in azimuth, and since positive all on the anterior chest, the QRS is mainly on the left chest (less than +90 ~ and the ST-T mainly on the right chest (greater than +90~

JOURNAL OF ELECTROCARDIOLOGY 21 (1), 1988

VARIATION IN QRS TRANSITION ZONE

is that the transition zone is like the rim of an open umbrella and the mean spatial vector is like the umbrella handle; moving the handle correspondingly moves the rim. The relationship between orientation of the electrical axes of QRS and ST-T and the corresponding transition zones was observed by Ashman 6 and popularized by Grant and Estes2 We have utilized this relationship to reduce the apparent wide variation in morphology of QRS and ST-T deflections in a group of normal subjects. We interpret the constriction of contour patterns upon aligning the related heart vectors as compatible with the postulate that the patterns of ventricular activation and recovery in the electrocardiograms of normal subjects (without extremes of axis deviation) are relatively uniform. We conclude that the normal spectrum in electrocardiographic appearance and transition zone location is more a sequel of subtle variations in heart position and orientation with respect to the bony contours of the thorax than of subtle variations in electrical behavior. Thus, merely redirecting the dipole or vector, which accounts for most of the electrical source, made the major correction, b u t the remaining elements of the complex electrical p a t t e r n 1~ (both the higher order components in the heart source and the individual variations in the shape and constitution of the b o d y volume conductor) cannot be resolved b y mere geometric orientation, TMand so contributed to the remaining variability in pattern. Major electrophysiologic departures from the common normal pattern of activation, as fascicular block, are quite distinct and may belong to a different constellation. 13 Wyndham showed that the epicardial breakthrough patterns in normal subjects followed the Durrer prototype, ~4 b u t quite distinctly different patterns were found in bundle branch and fascicular block. 15 Were each of these subgroups treated the same way as this normal group without extreme axis deviation, appropriate reduction in the variability of expression in each subgroup may be expected.

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JOURNAL OF ELECTROCARDIOLOGY 21 (1), 1988

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