Exercise-stress body surface ECG mapping

Thallium-201 myocardial imaging andpostexercise 12-lead ECG
Results

Group ANo patients had thallium defect and significant ST change.

Exercise variables

Group AThe end point of exercise was leg fatigue in
11 and shortness of breath in 4 patients. Workload was 77 ± 29 W. Heart rate at peak exercise
was 120 ± 12 bpm. Systolic blood pressure atpeak execise was 186 ± 24 mmHg.
Group BThe end point of exercise was anginal chest
pain in 9, ST depression in 5 and leg fatigue in 3
patients. Work load was 51 ± 19 W (P < 0.01
versus group A). Heart rate at peak exercise
was 108 ± 17 bpm (P < 0.01 versus group A).
Systolic blood pressure at peak exercise was
171 ± 19 mmHg.

Group BAll patients showed reversible thallium defect. Four patients showed reversible thallium
defect in the anterior wall, 7 patients in theposterior wall and 6 patients in both anterior
and posterior walls. All patients had significant
down-sloping ST depression. Maximal ST depression was –0.21 ± 0.08 mV.
ST-T isointegral maps

Group A
Figure 2 shows resting and postexercise STT isointegral maps of a representative patient(60-year-old man). All patients had smooth
dipolar pattern maps
both at rest and after
exercise with the positive area located overthe precordium andthe negative area over
the right chest andback. This was considered a normal response.

Fig. 4. Resting (R) and postexercise (P)QRST isointegral maps (upper and middle
panels) and the difference (D) map between resting and postexercise QRST isointegrals (lower panel) in a patient in groupA (the same patient as shown in Fig. 2).
Isointegral contours are separated by 10µV•s. Shading indicates negative areas.

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Fig. 5. Resting (R) and postexercise (P)QRST isointegral maps (upper and middle
panels) and the difference (D) map between resting and postexercise QRST
isointegrals (lower panel) in a patient ingroup B (the same patient as shown in Fig.
3d). Isointegral contours are separated by
10 µV•s. Shading indicates negative areas.
Significant area (< –10 µV•s) is present in
the postexercise map.

Group BAll patients had significant area in thepostexercise map. Seven patients (41%) had
significant area in the left-inferior and left-mid
regions in the postexercise map, 2 patients(11%) in the left-mid and left-superior regions, 4patients (24%) in all three of the left regions and 4
patients (24%) in the right-inferior, (right-mid,)left-inferior and left-mid regions. Four types ofsignificant area in the postexercise map areshown in Fig. 3. Significant area in the postexercise map was not correlated with the ischemic area determined by thallium imaging(Table 2).
QRST isointegral maps
Figure 4 shows resting and postexercise QRST
isointegral maps and the difference map between resting and postexercise QRST isointegrals in a patient in group A (the same patient as
Fig. 2). All three maps show smooth dipolar
patterns. Figure 5 shows resting and postexercise QRST isointegral maps and the QRST iso-integral difference map of a representative patient in group B (the same patient as Fig. 3d). Significant area is observed in the postexercise map.
Figures 6a and b show mean and mean –2SD difference maps between resting and postexercise
QRST isointegrals in group A. Figure 6c shows
the QRST isointegral difference map of a patient in group B (the same patient as Figs. 3dand 5), in which an extensive “–2SD area” isobserved. Figure 7 shows the relationshipbetween resting and postexercise QRST isointegral maps in a patient (group A) with a highcorrelation coefficient and in a patient (groupB) with a low correlation coefficient.

Group AAll patients showed smooth dipolar patternmaps both at rest and after exercise. The positive area covered the precordium with the maximum located in the middle of the left chest andthe minimum at the upper region of the rightchest or right back. All patients showed asmooth dipolar difference map between restingand postexercise QRST isointegrals. The nega-112

Exercise-stress body surface ECG mapping

Fig. 6. Difference maps between
resting and postexercise QRST isointegrals: Mean map in group A(a), mean –2SD map in group A (b)and a difference map of a patient ingroup B (the same patient as shownin Figs. 3d and 5) (c). Isointegral
contours are separated by 20 µV•s.
Shading indicates “–2SD area (lessthan mean –2SD values in group
A)”.

Fig. 7. Relationship between resting and postexercise QRST isointegrals: a patient (group A, the same
patient as in Figs. 2 and 4) with a high correlation coefficient (a) and a patient (group B, the same patient asshown in Figs. 3d and 5) with a low correlation coefficient (b).

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tive area covered most of the chest with theminimum located in the middle of the left chestand the maximum at the upper region of the rightchest or right back. The minimum in mean
and mean –2SD difference maps between resting and postexercise QRST isointegrals was
located in lead F4 (mean = –44.5 µV•s; mean –2SD = –80.4 µV•s). The correlation coefficient
between resting and postexercise QRST isointegrals in 87 lead points was 0.91 ± 0.06.

Group BTen patients (59%) had significant area inthe postexercise map. No patients had a nondipolar pattern map. Fifteen patients (88%) had“–2SD area” in the difference map between
resting and postexercise QRST isointegrals.
The correlation coefficient between resting andpostexercise QRST isointegrals in 87 lead
points in group B (0.28 ± 0.56) was significantly lower than that in group A (P < 0.001)(Fig. 8).

Discussion
The present study demonstrates that patientswith ischemic ST depression had a greater
decrease in the QRST isointegral values in theprecordial region than patients without ischemia and ST depression and had low similarities
between the resting and postexercise QRST isointegral maps. These findings indicate thatischemic ST depression is related to the dispersion of the exercise-induced changes in repolarization properties.
Body surface ECG mapping offers information about potential distributions around theentire thorax. In patients with coronary artery
narrowing, exercise-induced ST depression
most often occurs in the left anterior chest
leads. According to the data from exercisestress ST isopotential mapping, predicting thelocation of ischemic areas is thought to be difficult from the body surface distributions of ST
depression (Kubota et al., 1989). Montague andcolleagues (1990) studied exercise-stress ST
isointegral maps in coronary artery disease andobserved that, with exercise-induced angina,

Fig. 8. The correlation coefficient between restingand postexercise QRST isointegral maps for eachgroup, A and B. [ ], number of patients.

patients with two or three vessel disease had asignificantly greater decrease in the ST isointegral values than patients with single vessel disease. There was, however, considerable overlap among individuals. The reason for a lack ofcorrelation between body surface distributionof ST depression and anatomic site of coronary
artery disease is unclear. Exercise-induced ST
depression is commonly accompanied by adecrease in the height or inversion of the T
wave. Nakajima and coworkers (1988) studied
exercise-stress ST-T isointegral maps in patients with angina pectoris in the absence ofprevious myocardial infarction. They observedthat postexercise ST-T isointegral maps weredivided into 4 types (anterior chest, inferoposterior chest, lateral chest and global) according to the distributions of negative area, whichwere well correlated with the extent of ischemic
area determined by thallium imaging. However, our results demonstrated that significant
area in the postexercise ST-T isointegral map
was not correlated with the ischemic area. The
reason for the discrepancy is not clear. Studies
in a large number of patients are needed.
Exercise-induced ST depression is attributed to a current of injury with ischemia in subendocardial layers. T wave inversion is thought
to be related to the presence of delayed recovery. Exercise-induced ischemia causes not only114

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ST-T but also QRS changes. R wave amplitude
on the surface ECG is decreased in normal subjects while the increase is seen in patients withcoronary artery disease (Bonoris et al., 1978a,1978b). Ikeda and colleagues (1988) reported
that intraventricular conduction delay secondary to ischemia plays an important role in theincrease in R wave amplitude. ST-T isointegral
map reflects ventricular recovery sequence.
The recovery sequence is affected by activation
sequence and regional recovery properties. The
QRST isointegral map has been reported to be
useful in investigating recovery properties.
Wilson and researchers (1934) reported that the
QRST isointegral is independent of the activation sequence and dependent on repolarization
properties. They proposed the concept of aventricular gradient. Based on this concept ofthe ventricular gradient, Abildskov and workers(1980) introduced the QRST isointegral map.
They demonstrated that the QRST isointegral
map was independent of the activation sequence and useful in detecting abnormalities in
repolarization properties, even in the presenceof QRS deflection abnormalities. Althoughbody surface QRST isointegral mapping has
been used to assess repolarization abnormalities
in a variety of diseases, few reports on exercisestress QRST isointegral mapping are available.
Ohyama and others (1984) studied exerciseinduced changes in QRST isointegral map
patterns in patients with effort angina in theabsence of akinesis or dyskinesis in the left
ventricular wall motion. These investigators
observed that after exercise the maximum of themap moves far from the resting position andsplits into multiple extremes in patients withmultivessel coronary artery disease.
In this study, to assess exercise-inducedchanges in repolarization properties, we performed quantitative analysis of QRST isointegral mapping. Normal responses of QRST isointegral values to exercise have not been determined. We constructed a mean –2SD difference map between resting and postexercise
QRST isointegrals in control subjects (group
A). When the QRST isointegral value in at least
3 lead points was less than the mean –2SD, thedecrease was considered significant (“–2SD

area”). The correlation coefficient between
QRST isointegral maps is independent of thedifference in potential magnitudes and indicatesthe similarities in potential distributions(Hayashi et al., 1989). However, the relationship between resting and postexercise maps has
not been analyzed.
Control subjects (group A) showed a decrease in the maximum QRST isointegral value,
which may result from shortening of QT interval and a decrease in the R wave and T wave
amplitudes in the left precordial leads. Thesepatients showed smooth dipolar ST-T isointegral maps both at rest and after exercise. They
also showed smooth dipolar QRST isointegral
maps both at rest and after exercise and a highcorrelation coefficient between the two maps.
These findings indicate that control subjects
have higher similarities between resting andpostexercise repolarization properties.
Patients with ischemic ST depression in theabsence of previous myocardial infarction(group B) showed a high incidence of the occurrence of a significant area in the postexercise
QRST isointegral map and “–2SD area” in the
QRST isointegral difference map. In addition,these patients showed a low correlation coefficient between resting and postexercise QRST
isointegral maps. These findings indicate thatischemic ST depression is associated withremarkable changes in ST-T and QRST isointegrals and may be related to the regional abnormalities in repolarization properties.
We conclude that isointegral analysis ofbody surface ECG mapping has advantages inassessing repolarization properties in the exercise test for the detection of coronary artery disease. Further studies in patients with previous
myocardial infarction, intraventricular conduction disturbance or left ventricular hypertrophy
are needed.

Acknowledgments: We thank Drs. Takahiro Nawata
and Tetsuya Doi and all the members in the First
Dept. of Internal Medicine, Faculty of Medicine,
Tottori University for their cooperation. We also
thank Messrs. Kenichi Endo and Tsuyoshi Nishio,
Dept. of Clinical Radiology, Tottori University
Hospital for their assistance.

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(Received April 21, Accepted May 26, 1997)

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