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Epicardial Potentials computed from the Body Surface Potential
Map using inverse electrocardiography and an individualised
torso model improve sensitivity for Acute Myocardial Infarction
diagnosis.
Michael J Daly MB MRCP a, Dewar D Finlay PhD b, Daniel Guldenring PhD b,
Raymond R Bond PhD b, Aaron J McCann PhD c, Peter J Scott MD MRCP a,
Jennifer A Adgey MD DSc FRCP a and Mark T Harbinson MD FRCP c.
a. The Heart Centre, Royal Victoria Hospital, Grosvenor Road, Belfast, Northern
Ireland UK
b. School of Computing and Mathematics and Computer Science Research Institute,
University of Ulster, Northern Ireland, UK
c. Centre for Vision and Vascular Sciences, Queen’s University, Whitla Medical
Building, 97 Lisburn Road, Belfast, Northern Ireland UK
Word Count: 3,616
Keywords: inverse electrocardiography; body surface potential mapping; acute myocardial
infarction; body mass index
Financial support: Dr Michael J Daly is supported by The Heart Trust Fund (Royal Victoria
Hospital), 9B Castle Street, Comber, Newtownards Northern Ireland BT23 5DY
Relationship with Industry/Conflict of Interest: None
Address for correspondence:
Professor Jennifer A. Adgey
The Heart Centre, Royal Victoria Hospital, Grosvenor Road, Belfast Northern Ireland
BT12 6BA
Email: [email protected]
Tel: +44 2890 633714
Fax: +44 2890 635212
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Abstract
Introduction: Epicardial potentials (EP) derived from the body surface potential map
(BSPM) improve acute myocardial infarction (AMI) diagnosis. In this study, we compared
EP derived from the 80-lead BSPM using a standard thoracic volume conductor model
(TVCM) with those derived using a patient-specific torso model (PSTM) based on body mass
index (BMI).
Methods: Consecutive patients presenting to both the ED and pre-hospital coronary care unit
between August 2009 and August 2011 with acute ischaemic-type chest pain at rest were
enrolled. At first medical contact, 12-lead ECG and BSPM were recorded. BMI for each
patient was calculated. Cardiac troponin-T (cTnT) was sampled 12h after symptom onset.
Patients were excluded from analysis if they had any electrocardiographic confounders to
interpretation of the ST-segment. A cardiologist assessed the 12-lead ECG for STEMI by
Minnesota criteria and the BSPM. BSPM ST-elevation (STE) was ≥0.2mV in anterior,
≥0.1mV in lateral, inferior, RV or high right anterior and ≥0.05mV in posterior territories. To
derive EP, the BSPM data were interpolated to yield values at 352-nodes of a Dalhousie
torso. Using an inverse solution based on the boundary element method, EP at 98 cardiac
nodes positioned within a standard TVCM were derived. The TVCM was then scaled to
produce a PSTM, using a model developed from computed tomography (CT) in 48 patients
of varying BMI, and EP re-calculated. EP >0.3mV defined STE. A cardiologist blinded to
both the 12-lead ECG and BSPM interpreted the EP map. AMI was defined as cTnT
≥0.1µg/L.
Results: Enrolled were 400 patients (age 62 ± 13 yrs; 57% male): 80 patients had exclusion
criteria. Of the remaining 320 patients, BMI was 27.8  5.6kg m-2. Of these, 180 (56%) had
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AMI. Overall, 132 had Minnesota STE on ECG (sensitivity 65%, specificity 89%) and 160
had BSPM STE (sensitivity 81%, specificity 90%). EP STE occurred in 165 patients using
TVCM (sensitivity 88%, specificity 95%, p<0.001) and in 206 patients using PSTM
(sensitivity 98%, specificity 79%, p<0.001). Of those with AMI by cTnT and EP ≤0.3mV
using TVCM (n=22), 18 (82%) patients had EP >0.3mV when an individualised PSTM was
used.
Conclusion: Among patients presenting with ischaemic-type chest pain at rest, EP derived
from BSPM using a novel PSTM significantly improves sensitivity for AMI diagnosis.
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Introduction
In ACS patients, prompt diagnosis of acute coronary occlusion with early reperfusion therapy
leads to a reduction in morbidity and mortality. However, initial 12-lead (12L) ECG has
limited sensitivity (50-60%) for STEMI diagnosis [1] with over 20% of patients categorised
as Non-STEMI on 12L ECG having an occluded artery in the early infarct setting [2].
Undiagnosed STEMI represents a high-risk patient group: at least 12.2% suffer death or nonfatal AMI at 6-months [3]. As such, rapid determination of an occluded coronary artery at
presentation is paramount.
BSPM ST-segment elevation (STE) improves sensitivity (76%) for AMI diagnosis over 12L
ECG while retaining similar specificity (92%) [4]. Furthermore, in patients with chest pain
and ST-segment depression only on initial 12L ECG, BSPM STE has sensitivity 91% and
specificity 72% for AMI and acute coronary occlusion on angiography [5]. In the OCCULTMI trial, a large prospective multicentre study, it was shown that BSPM increased STEMI
diagnosis by 27.5% over the standard 12L ECG [6]. In addition, patients in whom STE was
only detected on BSPM had increased risk (OR 3.4) of death and recurrent AMI at 30-days
post-presentation [6] due to a delayed revascularisation strategy.
Epicardial Potentials (EPs) derived from Body Surface Potentials, a process known as inverse
electrocardiography, further improves AMI diagnosis by removing the effect of the thoracic
volume on electrical conduction [7]. However, to date the method used to derive EPs assumes
that all patients have torso geometry similar to an athletic, adult male. In an increasingly
obese population, individualised inverse electrocardiography has the potential to further
improve AMI diagnosis, since an individualised torso geometry has shown potential in
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deriving more accurate EPs than those calculated using an undiscriminating male standard [810].
The aim of this research was to assess if a patient-specific inverse solution would further
improve the diagnostic sensitivity of BSPM for AMI.
Methodology
Phase 1
Torso Modelling
Review of a hospital database of patients undergoing PET-CT scanning as part of their
clinical care was undertaken (n=5450). Within each quartile (Q1 [BMI ≤ 20.0 kg/m2]; Q2
[BMI 20.1 - 30.0 kg/m2]; Q3 [BMI 30.1 - 40.0 kg/m2]; and Q4 [BMI > 40.0 kg/m2]), 12
patients were randomly selected (50% male) for analysis (n total = 48).
PET-CT scanning was undertaken using a standard hospital protocol (supine position; from
external auditory meatus to proximal femur), resulting in 260-300 regularly spaced crosssectional slices. Circumference, coronal/sagittal widths and cross-sectional area (CSA) of
each plane were calculated [IDL version 7.0.0 (ITT Visual Information Solutions)]. Five
coronal cross-sectional planes were chosen for comparison: Plane 1- sternal notch level;
Plane 3 – nipple level; Plane 5 - umbilical level; Plane 2 - the cross-sectional plane
equidistant between Planes 1 and 3; and Plane 4 - the cross-sectional plane equidistant
between Planes 3 and 5. Following three-dimensional volume reconstruction, performed by
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constructing a ‘stack’ of axial slices, values for torso volume, surface area and vertical
displacement (distance between Planes 1 and 5) were obtained.
Phase 2
Patient Population
Between August 2009 and August 2011 we studied all patients admitted to our coronary care
unit using either the emergency department or mobile coronary care unit. Those who fulfilled
the following inclusion criteria were entered into the study:
1. Typical ischaemic-type chest discomfort of ≥ 20minutes duration, occurring at rest
and presentation within 12 hours of symptom onset;
2. 12L ECG performed at first medical contact;
3. Informed consent to BSPM performed at first medical contact; and
4. Blood sampled for cardiac troponin T (cTnT) at 12hrs from symptom onset.
Patients were excluded from analysis if they had: any condition precluding assessment of the
ST-segment, i.e. left bundle branch block, right bundle branch block, left ventricular
hypertrophy, concomitant digitalis therapy or ventricular pacing; received fibrinolysis,
nitrates or glycoprotein IIb/IIIa inhibitor prior to initial 12L ECG or BSPM; or BSPM
recorded >15mins after initial 12L ECG.
In addition, demographic data and risk factors for coronary artery disease were documented.
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Twelve-lead ECG analysis
A 12L ECG was recorded at first medical contact (25mm/s and 10mm/mV). STE was
measured at the J-point by a cardiologist who was blinded to all other clinical data. STE
consistent with AMI (STEMI) was defined using the Minnesota code 9-2 [11] as ≥ 0.1mV
STE in one or more of leads I, II, III, aVL, aVF, V5, V6 or ≥ 0.2mV STE in one or more of
leads V1 – V4 [12, 13].
BSPM recording and analysis
The BSPM was recorded with a flexible plastic anterior and posterior electrode harness and a
portable recording unit (Heartscape Technologies, Inc.). The anterior harness contains 64
electrodes, including 3 proximal bipolar limb leads (Mason-Likar position) and a posterior
harness with 16 electrodes. This lead configuration enables recording of 77 unipolar ECG
signals with respect to the Wilson central terminal. During the interpretation process the
electrodes are defined to represent anterior, lateral, inferior, high right anterior, right
ventricular and posterior epicardial regions [7]. Harness application takes 3 – 4 minutes.
BSPMs were recorded over 5 – 10 seconds at a sampling rate of 1 kHz and a bandwidth of
0.05 – 100 Hz and transferred into digital format for core laboratory analysis.
The 80-lead BSPMs were uploaded and displayed on an IBM compatible computer running
PRIME™ analysis software. All 80 leads were manually checked and those of unacceptable
quality, i.e. where noise or movement artefact disallowed recognition of QRST variables,
were marked and substituted using linear grid interpolation [7]. Any BSPM with >6 leads
requiring interpolation were disregarded and these patients excluded from analysis. Printouts
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were obtained from the processed BSPM of the 80-lead ECG and a colour-contour map
displaying the amount of STE at the J point (ST0 isopotential map) and confirmed by a
cardiologist blinded to the 12L ECG result. STE was measured from the ST0 point and
defined by the following thresholds: anterior ≥ 0.2mV elevation; lateral/inferior/high right
anterior/right ventricular ≥ 0.1mV elevation; posterior ≥ 0.05mV elevation; with infarctlocation described by the ST0 isopotential colour-contour map. The result of the PRIME™
diagnostic algorithm was noted.
AMI diagnosis
Diagnosis of AMI was made when cTnT ≥ 0.1µg/L [Roche Diagnostics, Switzerland].
Phase 3
Inverse Solution
Using formulae derived from correlation analysis during Phase 1 (Figure 1), the dimensions
(total volume and surface area; and sagittal/coronal widths, circumference and cross-sectional
area at Planes 1-5) of a 352-node Dalhousie torso (Figure 2) were adjusted for each patient
based on their individual BMI. BSPM data for each patient was exported from the PRIME
software to obtain J-point values for each electrode. These were expanded using Laplacian
interpolation to yield values at all points on an individualised 352-node Dalhousie torso [1417] (MATLAB R2009a [MathWorks Inc., USA]). Inverse calculations were performed using
SCIRun (Version 4.5) software [18] and Tikhonov zero order regularisation to subsequently
derive J-point potentials at 98 epicardial nodes (EPs) [18]. STE was defined as EP >0.3mV in
any epicardial node [7, 19].
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Statistical analysis
Data are presented as number (%) and mean ± standard deviation. Group comparisons were
tested using the unpaired t test and χ2 test. Continuous clinical variables were tested by
analysis of variance. A p-value < 0.05 was considered statistically significant. Correlations
were made using Pearson’s correlation coefficient, with r2 ≥ 0.50 representing a strongly
positive correlation. Sensitivity and specificity of the various diagnostic methods were
calculated by comparing the prediction of AMI against cTnT ≥0.1µg/L. Statistical analysis
was performed using SPSS version 17.0 for Windows (SPSS Inc, Chicago, Illinois). Ethical
approval for BSPM was granted by the Local Ethics Committee.
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Results
Phase 1
Of the 48 patients studied (50% male; age 65 ± 16 yrs), results are shown in Table 1. Overall,
no statistically significant differences were observed between male and female patients in any
parameter. However, of those in Q1 (BMI ≤ 20.0 kg/m2 [n=12]), there were significant
differences in both torso surface area and volume between male and female patients (p <
0.001). In addition, there were significant differences between circumferences in Planes 1-3
inclusive (p < 0.05) in this quartile. However, there were no significant differences between
any measurements at any plane in either Q2 or Q3 (BMI 20.1 – 40.0 kg/m2). In Q4 (BMI >
40.0 kg/m2 [n=12]), all measurements showed no significant difference, except for both the
coronal diameter in Plane 1 and the sagittal diameter in Plane 4 (p < 0.05).
Analysis of correlation revealed a strongly positive relationship, i.e. r2 > 0.500, between BMI
and all parameters measured at Planes 1-5 inclusive (Figure 1). Furthermore, BMI positively
correlated with torso volume, i.e. r2 = 0.855 (Figure 1). Based on these linear relationships,
formulae (y = ax +b, where x = BMI) were derived with which to individualise the Dalhousie
torso in order to achieve a patient-specific inverse solution.
Phases 2 and 3
Of the 400 patients who entered the study, 80 were excluded from analysis due to ECG
confounders (n=72), a delay greater than15min between 12L ECG and BSPM (n=1) and
therapy prior to BSPM (n=7). No patient was excluded for having >6leads of poor quality on
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BSPM. Thus, 320 patients met the study criteria (57% male; age 62 ± 13 yrs) [Table 2], 180
(56%) of whom had AMI by cTnT definition. Patients with AMI had higher incidence of both
hyperlipidaemia and cigarette smoking than patients without AMI (Table 2). However, nonAMI patients were more likely to have a positive family history of IHD and prior history of
AMI and/or PCI than those patients with AMI. Otherwise, the baseline characteristics
between groups were similar.
Electrocardiographic diagnosis
Overall, Minnesota code 9-2 STE was detected on initial 12L ECG in 132 (41%) patients
(Table 3). Of these, 117 patients had AMI, i.e. cTnT ≥ 0.1µg/L (sensitivity 65%, specificity
89%). Overall, BSPM PRIME algorithm STE was detected in 160 (50%) patients, having
sensitivity 81% and specificity 90% for AMI diagnosis (Table 3). Upon using a standard
unadjusted Dalhousie torso for each patient, EPs derived by inverse electrocardiography
improved both sensitivity and specificity for AMI diagnosis (Table 3). However, when the
standard torso was individualised using BMI for each patient to derive EPs, sensitivity was
further improved to 98% and a reduction in specificity was observed (79%).
Of those with AMI by cTnT definition but without Minnesota STE on 12L ECG at
presentation (n=63), 29 patients had STE detected by BSPM and 41 patients had STE
detected when a standard torso was used for the inverse solution: sensitivity 65%, specificity
94% for AMI diagnosis. Of the remaining 22 patients, STE was detected in 18 patients when
an individualised torso was used to derive EPs (Figure 3).
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Discussion
Current guidelines recommend a door-to-balloon time of ≤ 90mins for STEMI patients
undergoing primary PCI [13, 20] with multiple studies having demonstrated increasing
morbidity and mortality with treatment delay in these patients [20]. Therefore, patients with
acute coronary occlusion, i.e. STEMI, not detected by 12L ECG are more likely to undergo a
delayed revascularisation strategy and are thus at increased risk of an adverse prognosis. To
date, the 12L ECG remains the initial bedside investigation in the triage of patients presenting
with acute ischaemic-type chest pain at rest despite its poor sensitivity for AMI diagnosis – a
study has confirmed that the 12L ECG STE has only 50% sensitivity for AMI diagnosis using
contrast-enhanced cardiac magnetic resonance imaging as a gold-standard [21]. In patients
with Non-STE AMI, current guidelines suggest a maximum delay to invasive strategy of 72h
from admission in those without high-risk clinical features, i.e. haemodynamic instability,
malignant arrhythmia and/or refractory ischaemia [22]. In addition, the Occluded Artery Trial
has shown no prognostic benefit of PCI for patients with an infarct artery that is occluded ≥24
hours after symptom onset [2]. It is therefore of utmost importance that stable patients with
acute chest pain, an acutely occluded coronary artery and STE ‘missed’ by the standard 12L
ECG are identified at the earliest possible time-point in order to facilitate emergent
revascularisation and improve outcomes.
BSPM has been shown to have sensitivity 76%, specificity 92% for AMI diagnosis in
consecutive patients presenting with acute ischaemic-type chest pain at rest (n=755) [4].
Improvement in sensitivity over the 12L ECG (sensitivity 68%) has been shown to be mainly
due to detection of STE in the high right anterior, posterior, and right ventricular territories
[4]. In the OCCULT-MI trial, BSPM increased STEMI detection by 27.5% over the 12L
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ECG [6]. In this OCCULT-MI trial, STE on BSPM was associated with an increased risk of
death and recurrent MI at 30-days (OR 3.4) in those without initial 12L ECG STEMI [6], due
to a non-emergent revascularisation strategy.
Previous work from our centre has shown improvement in AMI detection by an epicardial
algorithm using a standard torso in patients with a non-diagnostic 12L ECG [7]. Compared to
other electrocardiographic techniques assessed, an epicardial algorithm correctly identified
53/83 patients with a non-diagnostic 12L ECG at presentation as AMI (sensitivity 64%,
specificity 99%) - findings comparable to those presented in this study (41/63 patients;
sensitivity 65%, specificity 94%).
Furthermore, in the 427 patients with acute ischaemic-chest pain studied by Owens et al [7],
the 12L ECG in combination with an epicardial algorithm (adjusted for confounders) derived
EPs that had sensitivity 85% and specificity 98% for AMI diagnosis - using standard CK/CKMB incremental elevation as the diagnostic standard [7]. Again, this is comparable to the
results presented here, when a standard torso is used for the inverse calculation (sensitivity
88%, specificity 95%) [Table 3].
As shown in this study, there remain a significant proportion of patients with AMI, without
Minnesota STE on 12L ECG and STE on either BSPM or EPs derived from an inverse
solution using a standard torso (22/63 patients [35%]). In this study, the development of a
novel method to ‘tailor’ the standard torso for the individual patient based on BMI,
significantly improves the diagnostic sensitivity of epicardial potentials derived from the
BSPM for AMI (sensitivity 98%) [Figure 3]. However, the detection of STE in patients
without AMI when an individualised torso is used in the inverse calculation has resulted in a
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reduction in specificity by 16% (Table 3). This may be in part explained by the lack of an
individualised cardiac geometry used in our model. Further study is required in this regard, in
addition to a prospective randomised-control study of our method in a real-world population,
in order to fully assess prognostic impact.
Study Limitations
This is the first non-invasive study attempted in man to improve diagnosis of acute
myocardial infarction using inverse electrocardiography (individual torso model) related to
body habitus within the early hours of the onset of chest pain. Yes, there were limitations but
the results are encouraging.
Protocol development and patient recruitment were undertaken between 2009-11. Since this
time: (a) the 12-lead ECG criteria used for STEMI diagnosis have been further refined; and
(b) the cTnT assay used in this study to define AMI has been superseded by incremental rise
in the high-sensitivity cTnT assay. Furthermore, this study was undertaken as part of a
research protocol where coronary angiography was performed when clinically indicated.
Future work
Recently, in attempting to increase sensitivity for diagnosis of acute myocardial infarction
early after the onset of symptoms the approach taken by other research workers has been at
the expense of reduced specificity. Nevertheless, in an effort to improve the diagnostic
specificity of the novel electrocardiographic method described in this work, effort should be
made to individualise the cardiac geometry. In addition, BSPM and EP data obtained should
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be correlated with angiographic findings. Prospective studies where the novel method
described directly affects clinical decision-making should be undertaken, with long-term
outcomes evaluated.
Acknowledgements
Financial support: Dr Michael J Daly is supported by The Heart Trust Fund (Royal Victoria
Hospital), 9B Castle Street, Comber, Newtownards Northern Ireland BT23 5DY.
15
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Figures legends:
Figure 1: Axial CT slice at Plane 3 (nipple level) with torso reconstruction and Planes 1-5
shown. Graphical plots of torso volume (top right) and CSA at Plane 3 (nipple level) [bottom
right] against BMI, with both r2 values and linear equations shown.
BMI = body mass index; CSA = cross-sectional area; USN = upper sternal notch
Figure 2: 352-node Dalhousie torso
Figure 3: Case example
BMI = body mass index; BSPM = body surface potential map; cTnT = cardiac troponin-T;
ECG = 12-lead electrocardiogram; EP = epicardial potentials; STE = ST-segment elevation.
19