Positional changes of spatial QRS- and ST-segment variables in normal subjects: Implications for continuous vectorcardiography monitoring during myocardial ischemia

Journal of Electrocardiology Vol. 33 No. 1 2000

Positional Changes of Spatial QRS- and ST-Segment Variables in Normal Subjects: Implications for Continuous Vectorcardiography Monitoring During Myocardial Ischemia Bjarne L. Norgaard, MD, Birthe M. Rasmussen, MD, Mikael Dellborg, MD, and Kristian Thygesen, MD, FACC, FESC

Abstract: Electrocardiographic QRS- and ST-segment changes are to be expected during changes in body posture. We prospectively analyzed the influence of changes in body position on continuous vectorcardiography monitoring of QRS-vector difference (QRS-VD) and ST change-vector magnitude (STC-VM) according to the currently used criteria of myocardial ischemia in 21 normal subjects. Fifteen (71%) and 6 (29%) subjects had significant positional QRS-VD and STC-VM changes, respectively. Vectorcardiography changes were most frequent and pronounced in the left lateral position. An alternative to the existing criterion of ischemia is proposed to improve the specificity of STC-VM. Subjects with positional QRS-VD changes had higher mean STC-VM values as compared with those without such changes. Otherwise no characteristics among those with positional vectorcardiography changes could be identified. There was no statistically significant association between positional QRS-VD and STC-VM changes (R = .13, P = .57). We conclude that the clinical use of QRS-VD in its present form for continuous vectorcardiography monitoring of myocardial ischemia seems to be of limited practical value, because of the presence of frequent "pseudo-ischemic" changes. STC-VM seems to have a significant potential of continuous vectorcardiography monitoring. However, an indicator of body position change or even an algorithm enabling on-line correction for positional vectorcardiography changes seems to be essential to improve the accuracy of this technique in identifying myocardial ischemia. Key w o r d s : vectorcardiography, QRS-segments, ST-segments, myocardial ischemia, monitoring, body position.

From the Department of Medicine and Cardiology, Aarhus University Hospital, Denmark; and Department of Medicine and Cardiology, Sahlgrenska University Hospital, Ostra, Gothenburg, Sweden. This study was supported by a grant from the Danish Heart Foundation, Copenhagen, Denmark. Reprint requests: Bjarne L. Norgaard, MD, Department of Medicine and Cardiology, Aarhus University Hospital, Tage Hansens Gade 2, DK-8000 Aarhus C, Denmark. Copyright © 2000 by Churchill Livingstone ® 0022-0736/00/3301-0003510.00/0

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Journal of Electrocardiology Vol. 33 No. 1 January 2000

Continuous vectorcardiography monitoring is a well-established method for the evaluation of patients with known or suspected myocardial ischemia (1-6). However, changes in body position may lead to significant alterations of the QRS- and ST-segment, potentially reducing the specificity of the method. In a previous investigation the present study group reported that changes in body position may cause "pseudo-ischemic" alterations of STvector magnitude (ST-VM) in certain subjects (7). Whether changes in body position may lead to significant alterations of vectorcardiographic variables, such as QRS-vector difference (QRS-VD) and ST change-vector magnitude (STC-VM), both of which take spatial vectorial changes into account, has not been hitherto systematically investigated to the best of our knowledge. Previous data have indicated that QRS-VD is extremely sensitive with regard to detecting transient episodes of myocardial ischemia, however, the specificity seems to be low (3,8,9). In contrast, beside its high sensitivity, STC-VM seems to add high specificity (3,8-11). This study describes the range of normal QRS-VD and STC-VM alterations after changes in body position in normal subjects and evaluates currently used vectorcardiography monitoring criteria for myocardial ischemia accordingly.

trocardiographic signals are sampled for 1-minute periods via a bedside data acquisition unit and averaged to form mean vectorcardiography complexes in 3 orthogonal leads, X, Y, and Z. The mean vectorcardiography complexes are continuously conveyed to a central compatible personal computer for storage and analysis. The following vectorcardiographic variables were studied (Fig. 1): QRS-VD, which reflects the changes in the shape within the QRS complex, and STC-VM, which is the length of the difference vector between the initial ST vector and the current ST vector. The sensitivity of the system is 1 /zV. The orthogonal leads are recorded and analyzed, and QRS-VD and STC-VM are calculated at the end of each averaging period. Data are updated continuously and displayed as trend curves during the whole recording period.

m

Referencecomplex

Positional complex

Ax, Ay. Az= [ ~

ORS-VD= V

Ax=+Ay2+Az2

ST - Vector Maqnitude

Materials and Methods

X

Y

Z

Study Subjects Twenty-one (10 women and 11 men) supposedly healthy h u m a n volunteers, all employees in our institution, participated in the present study. Study subjects were not allowed to use any medication and were known not to be suffering from any medical disease. All were nonsmokers. Sinus rhythm and absence of conduction disturbances were required in an electrocardiogram (ECG) taken before the study period.

Continuous Vectorcardiography Monitoring Continuous vectorcardiography monitoring with the MIDA1000 or Coronet systems (Ortivus Medical AB, T~iby, Sweden) has been described previously (1). In brief, the system consists of a dataacquisition module and a personal computer. The signal is continuously recorded from 8 body surface electrodes positioned according to Frank (12). Elec-

ST-VM = V Xi ~Yi =+ Zi=

STC - Vector MaqnRude

Fig. 1. Top panel, The average reference (°supine") QRS complex (thick line) and the average QRS complex in the left lateral position (thin line). Shaded area of difference between the reference and positional complexes, summated for leads X, Y, and Z, is the QRS-VD. Middle panel, ST-VM is the summated deviation from the isoelectric level, measured 60 msec after the J-point. Bottom panel, STC-VM is the summated (spatial) difference vector between the T vector of the reference ("supine") complex and the T vector of the positional complex, measured 60 msec after the J-point.

Positional Changes of Spatial QRS- and ST-Segment Variables

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or during the recording sessions. Thorough preparation of the skin was performed before each recording to minimize the skin-electrode impedance. Monitoring sessions were led, in each subject, by 1 of 2 investigators w h o were aware of the purpose of the study. Written informed consent was obtained from all subjects. The protocol was approved by the regional Scientific Ethical Committee.

Values from the recorded trend curves were read by 2 investigators.

Definitions As for other electrocardiographical items, criteria for ischemia during continuous vectorcardiography monitoring are empirical. However, based on dinical experience and previous data (5), significant positional changes were defined as follows: (i) QRS-VD = reversible increase to >15 /J,V when changing from the supine to the lateral body position and (2) STG-VM = reversible increase >50/J,V when changing from the supine to the lateral body position. In addition, reversible changes of QRSVD > 15/zV and of STC-VM > 50/zV, respectively, from the individual baseline (period 2, supine position) were evaluated as alternatively "pseudoischemic" criteria.

Data Analysis Mean QRS-VD and STC-VM (and heart rate) values in the supine positions during period 1 and period 2, respectively, were compared. All QRS-VD and STC-VM measurements were based on mean trend values measured from minute 7 to 11 after the beginning of period 1 and after each change in body position during period 2, respectively (Fig. 2). Positional QRS-VD and STC-VM changes were evaluated from values obtained during period 2 (reference: supine position). Body surface area in square meters was based on height and weight according to a standard formula (13). Data are presented as means (standard deviation [SD] and range). Categorical variables were compared by using the Ghi-square or Fischer's exact test as appropriate. Group comparisons of continuous response variables were made by unpaired and paired t tests as appropriate. The association b e t w e e n positional QRS-VD and STG-VM changes were evaluated by using simple linear regression. A P value < .05 was considered statistically significant.

Study Design The protocol has previously been described in detail (7). In brief, a specific body positional schedule was followed: period 1: Supine, right lateral, left lateral; and period 2: Supine (reference), right lateral, and left lateral position, respectively. The monitoring time was approximately 20 minutes in each position, thus, each monitoring period lasted approximately 135 minutes (Fig. 2). All recording sessions were preceded by 10 to 15 minutes of rest. Intake of food or beverages was not permitted for 2 hours before

k QRS-VD--

--

pVsec

STC-VM I.tV

600

600 I

Period 1

I

Period 2

50,0

40.0

•

•

Supine 30.0

1

Right lateral

.'

/

25

L..

lateral

so. I

(reference)

,gh, lLe lateral

lateral •

20.0

•

500

400

300

200

-100

10.0

0 09:40

10:00

10:20

i0:40

11:00

11:20

Fig. 2. Illustrative QRS-VD (thick line) and STC-VM (dotted line) trend curves during continuous vectorcardiography monitoring in a subject with significant positional vectorcardiography changes. The monitoring time was approximately 20 minutes in each position. STC-VM, ST change-vector magnitude; QRS-VD, QRS-vector magnitude.

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Journal of Electrocardiology Vol. 33 No. 1 January 2000

Results

T a b l e 2. The Influence of Different Body Positions on QRS-VD

Subject Characteristics

MeanValue

M e a n age (range) was 47 (35 to 64) years. T w o subjects i n d u d e d in the study, i w o m a n (48 years of age) and one m a n (58 years of age), h a d ECG manifestations of left ventricular h y p e r t r o p h y according to the criteria by S o l o k o w and Lyon (14) in a n ECG obtained before the study period. A physical e x a m i n a t i o n of these subjects s h o w e d n o a b n o r malities. In particular, b o t h h a d arterial blood pressure m e a s u r e m e n t s w i t h i n n o r m a l limits. ECGs f r o m the rest of the study p o p u l a t i o n w e r e w i t h i n n o r m a l limits (15). There was a statistically significant fall in the m e a n h e a r t rate (SD, range) f r o m the supine position in period 1 to tile supine position in period 2, 69 (13, 4 0 - 9 0 ) versus 64 (11, 43-94) b e a t s / m i n (P < .0001). There was n o t e n d e n c y for the h e a r t rate to be faster or slower at a n y o n e position during period 2.

Positional QRS-VD Changes There was a statistically significant increase in the m e a n QRS-VD (SD, range) w h e n c o m p a r i n g the values in the supine positions, 2 (2, 1-9) /xVsec in period 1 versus 4 (2, 2 - 1 3 ) in period 2 (P < .0001). Fifteen subjects (71%) had a significant positional QRS-VD change. Of 42 positional shifts, 23 (55%) resulted in a significant QRS-VD change as s h o w n in Table 1. Positional QRS-VD changes w e r e m o s t f r e q u e n t in the left lateral position. The q u a n t i t a t i v e influence of different b o d y positions on QRS-VD is s h o w n in Table 2. A significant change (reference: supine position) could be s h o w n in b o t h side-lying positions. The m e a n STC-VM level was higher in subjects w i t h positional QRS-VD changes t h a n in subjects w i t h o u t such changes as s h o w n in Table 3. Otherwise, subjects w i t h positional QRS-VD changes did not differ significantly f r o m those without QRS-VD changes with respect to sex, age, b o d y surface area, a n d change in h e a r t rate, respectively.

T a b l e 1. Frequency of Positional QRS-VD and STC-VM Changes Positional Shift

Supine versus side Supine versus right Supine versus left

QRS-VD (%)

S T C - V M(%)

23 (55) 8 (19) 15 (36)

8 (19) 2 (5) 6 (14)

Twenty-one subjects performed 42 positional shifts. QRS-VD change >- 15 #,Vsec. STC-VM change --> 50 p,V.

SD

/xVsec /xVsec Supine Right lateral position Left lateral position

4 11 20

2 5 6

Range /xVsec

P Value

2-13 3-21 10-34

P < .0001" P < .0001"

Values shown are mean-values. SD, and ranges for QRS-VD in different body positions. * QRS-VD in the lateral positions are compared with QRS-VD in the supine position.

Alternatively, w e tested the i m p a c t of changing the " p s e u d o - i s c h e m i c " QRS-VD criterion on the o c c u r r e n c e of positional QRS-VD changes. W h e n e m p l o y i n g a reversible c h a n g e of QRS-VD > 15/~V f r o m the individual baseline, 13 subjects (62%) still had significant positional QRS-VD changes. All positional QRS-VD changes ceased i m m e d i a t e l y on r e t u r n to the supine position. No positional QRS-VD changes w e r e p r e s e n t a m o n g the two subjects w i t h ECG m a n i f e s t a t i o n s of left v e n t r i c u l a r hypertrophy.

Positional STC-VM Changes M e a n STC-VM (SD, range) increased f r o m 11 (8, 1-32) p,V in the supine position during period 1 to a m e a n of 17 (10, 3 - 4 7 ) /xV in the supine position during period 2 (P < .0001). Six subjects (29%) had significant positional STC-VM changes. Of 42 positional shifts, 8 (19%) resulted in significant STC-VM changes as s h o w n in Table 1. Positional STC-VM changes w e r e m o s t f r e q u e n t in the left lateral position. The q u a n t i t a t i v e influence of different b o d y positions on STC-VM is

T a b l e 3. Characteristics A m o n g Subjects With or Without Positional QRS-VD Changes

No Positional

Positional

Change

Change

P Value

6 5 (83) 48 (38-58) 1.89 (0.24) 0

.61 .78 .79 .70

No. 15 Men, No. (%) 9 (60) Mean age (range), years 43 (35-64) Mean BSA (SD), m 2 1.87 (0.17) Change in heart rate +2 from supine to the LL position, % Mean STC-VM (SD) in 48 (30) the LL position, p.V

34 (15)

.03

BSA, body surface area; LL, left lateral; SD, standard deviation; STC-VM, ST change-vector magnitude.

Positional Changes of Spatial QRS- and ST-Segment Variables

Table 4. The Influence of Different Body Positions on STC-VM MeanValue, p.V

SD,

Range,

b~V

/zV

P Value

17 24 42

10 14 27

3-47 6-58 11-I 17

P < .0001" P < .0001"

Supine Right lateral position Left lateral position

Values shown are mean-values, SD, and ranges for STC-VMin different body positions. * STC-VM in the lateral positions is compared with STC-VMin the supine position.

s h o w n in Table 4. A significant change (reference: supine position) could be s h o w n in both side-lying positions. Subjects with positional STC-VM changes did not differ significantly from those w i t h o u t with regard to sex, age, b o d y surface area, change in heart rate, and QRS-VD, respectively, as indicated in Table 5. If a reversible change of STC-VM > 50 /zV from the individual baseline was used as a "pseudoischemic" criterion, only 3 subjects (14%) had significant positional STC-VM changes. All positional STC-VM changes ceased immediately u p o n r e t u r n to the supine position. No positional STC-VM changes w e r e present a m o n g the 2 subjects with ECG manifestations of left ventricular hypertrophy.

Relationship Between Positional QRS-VD and STC-VM Change Five subjects had simultaneous positional QRS-VD and STC-VM changes in the left lateral position. One subject had a positional STC-VM change w i t h o u t a simultaneous positional change

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of QRS-VD. As s h o w n in Table 3, m e a n STC-VM in the left lateral position was h i g h e r in subjects with as c o m p a r e d with subjects w i t h o u t positional QRS-VD changes. However, n o difference in QRS-VD could be s h o w e d a m o n g subjects with as c o m p a r e d to those w i t h o u t positional STC-VM changes as seen in Table 5. There was n o statistically significant correlation b e t w e e n the change in QRS-VD and STC-VM w h e n changing from the supine to the left lateral position as s h o w n in Figure 3, R = . 1 3 ( P = . 5 7 ) .

Discussion The usefulness of a n y m o n i t o r i n g device depends on b o t h the sensitivity and specificity of the test, but data regarding the specificity of c o n t i n u o u s vectorcardiography m o n i t o r i n g is limited (3,7,8). Alterations of the QRS- and ST-segment because of changes in body posture h a v e b e e n described in patients with ischemic heart disease as well as a m o n g n o r m a l subjects, respectively (16-21 ). Various mechanisms explaining positional electrocardiographical changes have b e e n proposed, Me, differences in ventricular v o l u m e (22), anatomical change of the heart in relation to the recording surface electrodes (23), latent heart disease (18), and changes of the vagal and sympathetic t o n e (18). Such "pseudo-ischemic" changes m a y cause problems during ECG-ischemia m o n i t o r i n g and definitively lead to the (mis-)diagnosis of silent m y o cardial ischemia. In a previous investigation w e reported that changes in b o d y position m a y cause

35 0

0

3o

Table 5. Characteristics Among Subjects With or Without Positional STC-VM Changes Positional

No Positional

Change

Change

No. M e n , No. (%) 1 M e a n age (range), years 45 M e a n BSA (SD), m 2 1.91

6 (17) (36-56) (0.19)

Change in heart rate from supine to the LL position, % Mean QRS-VD in the LL position, v,Vsec

+5

25

0

~-~ a

20

¢/} n-

15

Oo

I0

0

>.

P Value

15 10 (67) 48 (35-64) 1.78 (0.15)

P = .11 P = .46 P = .15

0

P = .30

0° oo

0

O

0

0

20

o 0

r--0.13 p:0.57

i

i

J

i

i

40

60

80

100

120

STC-VM (IJV)

19 (3)

20 (7)

P = .39

BSA, b o d y surface area; LL, left lateral; SD, s t a n d a r d deviation; QRS-VD, QRS-vector m a g n i t u d e .

Fig. 3. Relationship between the mean QRS-VD and the mean STC-VM values in the left lateral body position. STC-VM, ST change-vector magnitude; QRS-VD, QRSvector magnitude.

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Journal of Electrocardiology Vol. 33 No. 1 January 2000

"pseudo-ischemic" alterations of ST-VM in certain subjects (7). In the present study we extended these investigations to involve QRS-VD and STC-VM, both of which add the dimension of spatial vectorial changes and both suitable indicators of myocardial ischemia (3,5,8-11,24).

QRS-Vector Difference QRS-vector difference is a highly sensitive indicator of myocardial ischemia during coronary angioplasty (9,24); however, the specificity of this variable seems to be low (3,8,9). In the present study, we found that positional QRS-VD changes occurred frequently. More than half of the study subjects had "pseudo-ischemic" QRS-VD changes in the left lateral position according to the presently accepted QRS-VD ischemia criterion (5). The positional QRS-VD changes were most pronounced in the left lateral position. These findings are consistent with the results of a study by Jernberg et al. (20) in which the positional influence on continuous 12-lead ECG monitoring among 36 patients suspected of having acute myocardial ischemia was investigated. In the latter, QRS-VD was calculated from the leads Vl, II, and V5, thus, rudely corresponding to the orthogonal leads Z, Y, and X. In 2 previous studies, 1 including 144 and the other including 10 patients with angina pectoris in w h o m coronary angioplasty was performed (9,24), QRS-VD increased from a baseline m e a n value (-+SD) of 4 _+ 3 to a m a x i m u m value during balloon occlusion of 26 _+ 12 and from 1 _+ 0.5 to 15 _+ 11 /zVsec, respectively. In comparison, we found that QRS-VD increased from a baseline mean value (-SD) (supine position) of 4 +_ 2 to a m e a n value of 20 _ 6 /zVsec in the left lateral position. Thus, the transient QRS-VD increase during induced transmural myocardial ischemia in patients with ischemic heart disease does not seem to differ from what may be expected during positional body changes in healthy subjects. The specificity of this variable was only marginally improved by testing an alternative QRS-VD cut off, with the subject's own baseline as reference. In the present investigation mean STC-VM in the left lateral body position was higher in subjects with positional QRS-VD changes as compared with subjects without. Otherwise, we were unable to identify any characteristics among subjects with positional QRS-VD changes as compared to subjects without.

ST Change-Vector Magnitude Previous data involving patients with ischemic heart disease undergoing coronary angioplasty and animal studies indicate that STC-VM is sensitive in identifying myocardial ischemia (9-11). Indeed, these data indicate that the relative STC-VM is more sensitive in identifying myocardial ischemia than the absolute ST-VM. In a previous investigation, including 10 patients scheduled for elective coronary angioplasty, STC-VM increased from a baseline mean value (_+SD) of 7 _ 3 to 132 _+ 83 /zV during balloon inflation (9). In comparison, STC-VM increased from a baseline mean value (-SD) (supine position) of 17 _+ 10 to a mean value of 42 _ 27 /zV in the left lateral position in the present study. Thus, ischemic STC-VM changes seem to be much more pronounced, as compared with positional changes, indicating a high specificity of STC-VM. However, according to the presently accepted STC-VM ischemia criterion (a reversible change >50/,V) (5) we found that almost one third of presumably healthy subjects had significant positional STC-VM changes. In a previous study we reported that almost 10% (2 of 21) of the present study population had "pseudo-ischemic" ST-VM changes in the left lateral position (7). Thus, when using the presently accepted criteria for myocardial ischemia our data indicate a poorer specificity of STC-VM as compared with ST-VM. These findings, however, are in contrast to the results from 2 previous studies, 1 involving patients with noncardiac chest pain (3) and another including normal subjects (8), in which no subject had STC-VM episodes. Explanations for the discrepancies between the results of the latter studies and the findings of the present may be 2-fold (1); in the previous studies the STC-VM (as well as ST-VM) ischemia criteria were defined as a reversible change > 100 p,V, which differs from the cut-off value evaluated in the present study; and (2) The influence of body positional change on STC-VM was not systematically assessed in any of these studies. The significant change in STC-VM from period 1 to a steady state level well above zero in period 2 is concordant with clinical experience and previous data (9); however, the origin of this p h e n o m e n o n is yet unclear. In the present study all external and physiological conditions potentially influencing the ST-segment were maintained as constant as possible, however, a fall in heart rate corresponding to a more physically relaxed state was registered from period 1 to period 2. An association between changes in heart rate and ST-segment changes has

Positional Changes of Spatial QRS- and ST-Segment Variables

been shown previously (%25). However, whether such a trivial change in heart rate as seen in the present study may lead to the rather substantial STC-VM change is highly speculative. Because of the initial STC-VM change, we tested a secondary and nonestablished set of limits for "pseudo-ischemic" changes during which the subject's own baseline was used as the reference for further STC-VM changes. Only 14% of the study population had significant positional changes by using a reversible increase in STC-VM -- 50 /~V from the individual baseline. The value of using this alternative STC-VM criterion for myocardial ischemia and especially the impact on the sensitivity, should be tested in future studies. However, the most appropriate way in which the specificity of STC-VM can be improved is to allow the computer or the reader to correct for such positional STsegment episodes. Previous studies have indicated that positional ST-segment deviations during continuous ECG monitoring may arise in patients with certain ECG characteristics (7,20). We found in a recent study that positional ST-VM episodes among normal subjects primarily arose in those with a positive T-wave in lead Z or a negative T-wave in lead V1 (7). Although, the 2 subjects with "pseudoischemic" ST-VM changes in the latter study also had significant positional STC-VM changes, no ECG characteristics could be identified among those with as compared to those without positional STC-VM changes in the present study. Moreover, we were unable to identify any demographical or physical characteristics among subjects with positional STC-VM changes as compared to subjects without.

Relationship Between Positional QRS-VD and STC-VM Change If characteristic coherences between positional and ischemic QRS-VD and STC-VM (or ST-VM) changes could be established, on-line identification of (or even construction of mathematical algorithms correcting for) positional ST-segment changes during vectorcardiography monitoring may be possible. As shown in the present study, most positional STC-VM changes are accompanied by simultaneous changes of QRS-VD; however, we found no statistically significant relationship between the m a x i m u m QRS-VD and the m a x i m u m STC-VM in the left lateral body position. A fair correlation between the m a x i m u m QRS-VD and ST-VM value during epicardial balloon occlusion was showed in a previous study (R = .69, P < .001) (24). However, whether this correlation exists be-

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29

tween ischemic QRS-VD and STC-VM changes is not known at present. Thus, employing quantitative trend analyses we are at present unable to identify any characteristic positional versus ischemic QRS-VD and STC-VM relationship. Thus, this subject needs to be further investigated. In addition, it might be speculated that a qualitative analysis correlating QRS-vector loop and ST-vector changes during nonpathological versus cardiac ischemic circumstances may show characteristic coherences.

Conclusions The rather low specificity of QRS-VD in clinical studies is probably caused by "pseudo-ischemic" alterations because of changes in body position. The clinical use of QRS-VD in its present form for long-term vectorcardiography monitoring of myocardial ischemia seems to be of limited practical value. On the other hand, STC-VM seems to have a significant potential, as a supplement or even alternative to ST-VM, during vectorcardiography monitoring. To improve the specificity of STC-VM, an alternative ischemia criterion in which the STC-VM change is compared with the base-line level is proposed. However, an indicator of body position or even on-line identification of true positional "pseudo-ischemic" ST-variable changes would undoubtedly improve the accuracy of this technique in detecting myocardial ischemia.

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