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International Journal of Bioelectromagnetism
Vol. 5, No. 1, pp. 69-71, 2003.

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Spatial Patterns of ST-Segment Shift
During Acute Myocardial Ischemia

B Milan Horácek and Galen S Wagner

Department of Physiology & Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada
Department of Cardiology, Duke University Medical Center, Durham, NC, USA

Correspondence: B Milan Horácek, 4-P2 Sir Charles Tupper Med. Bldg., Dalhousie University, Halifax, NS, Canada B3H 4H7
E-mail: milan.horacek@dal.ca, phone 902-494-1306, fax 902-494-1685


Abstract. Standard 12-lead electrocardiogram (ECG) does not make the best use of the diagnostic information that can be acquired by noninvasive measurements of the heart 's electrical activity. The objective of this presentation is to demonstrate the role of body-surface potential mapping in identifying complementary/alternative ECG lead systems. Specifically, we will deal with the selection of the leads with which one can optimally diagnose vessel-specific acute myocardial ischemia, based on data consisting of 120 unipolar ECGs recorded during percutaneous transluminal coronary angioplasty (PTCA) in three groups of patients with single-vessel coronary artery disease. ST-segment changes were measured in all recorded leads as a difference between the ST amplitude during baseline and ischemic states. For each group of patients, mean ST changes were calculated and normalized by the corresponding standard deviation; these data were displayed as body-surface distributions and optimal vessel-specific leads were chosen.

Keywords: ECG Mapping; Acute Ischemia; Coronary Angioplasty; Diagnostic Classification; Optimal ECG Leads

1.  Introduction

Electrocardiographic ST-segment measurements have been widely used for detecting myocardial ischemia, most notably in patients with unstable coronary syndromes; for assessing thrombolytic treatment in patients with an acute myocardial infarction (MI); and for detecting sudden coronary artery closure in patients undergoing transcatheter revascularization [1]. The sensitivity and specificity for detecting myocardial ischemia can be much improved by a judicious choice of leads. In searching for the best leads, it is advantageous to use data recorded during balloon-inflation PTCA¡ªwhich provides a model for acute ischemia in humans [2]¡ªin combination with body-surface potential mapping, which provides a total picture of ECG changes [3, 4]. Our purpose here is to demonstrate how body-surface potential maps recorded during PTCA can be used to identify the optimal ECG leads for monitoring the ST-segment changes caused by acute ischemia [5].

2.  Methods

Study population consisted of 91 patients with single-vessel coronary artery disease for which they underwent PTCA [5]. For 32 of these patients, the lesion was in the left anterior descending (LAD) coronary artery; for 36 in the right coronary artery (RCA); and for 23 in the left circumflex (LCx) coronary artery. All patients had normal left-ventricular wall motion and no evidence of a recent MI. ECG data were recorded simultaneously from 120 unipolar leads; the acquisition system, subsequent signal processing, and interpolation for 352 sites on the torso have been described elsewhere [5]. The ST amplitude with respect to the TP baseline, at 60 ms after the J point, was determined at all sites during the baseline and ischemic states, and these values were subtracted from each other to produce ΔST maps. All ΔSTs pertaining to the given lead in each patient group were expressed as mean ± standard deviation (SD). Student 's t-test was used to determine the statistical significance, and mean ΔST distributions and their corresponding t distributions were plotted for each group.

3.  Optimal Leads Suggested by Body-Surface Potential Mapping Study

The relationship between the ischemia caused by an occlusion of a specific coronary artery and the body-surface distribution of ECG potentials is illustrated in reference [5], which shows the mean ΔST maps and corresponding t maps induced by inflations of the PTCA balloon. In the mean ΔST map for the LAD group, the maximum occurs just above the lead V3, and the inferior part of the map is negative, with the minimum in the mid-dorsal area. In the mean ΔST maps for the RCA and LCx groups, the positive potentials are on the inferior torso, and the negative potentials are on the superior torso, with the minimum occurring above V2 and V3, but, whereas the pattern of the LCx map resembles that of the LAD map, albeit with the opposite polarity, the pattern of the RCA map has a more vertical axis, with its maximum occurring on the lower anterior chest. The maps of t values resemble their corresponding ΔST maps, but they differ somewhat in the locations of LAD- and RCA-group extrema. Based on this evidence, optimal vessel-specific bipolar leads for detecting ischemia related to one of the coronary arteries would be: near V3 (+) and 5 cm below V8 (-) for LAD, the left iliac crest (+) and 5 cm above V3 (-) for RCA, and 5 cm below V8 (+) and 2.5 cm above V2 (-) for LCx.

4.  Limitations of Currently Used Leads

The criteria for both thrombolytic therapy and primary angioplasty require the presence of ST elevation greater than 0.1 mV in two or more contiguous leads of the 12-lead electrocardiogram (ECG) [6]. However, in many patients with a suspected MI, ST elevation is not seen on routine 12-lead ECGs; e.g., Huey and colleagues [7] observed acute ST elevation in less than one half of patients with LCx occlusion and in less than three quarters of patients with RCA or LAD occlusion. Zalenski and colleagues [8] conducted a prospective multicenter study to assess whether, in patients with a suspected MI, the diagnostic accuracy of 12-lead ECGs improves when ST elevation in posterior chest leads and right chest leads is added to the set of measurements; their conclusion was that the improvement is only modest. On the other hand, Agarwal and colleagues [9] found ST elevation in V7 - V9 useful for diagnosing LCx-related acute posterior MIs, which agrees with our finding that the optimal site for detecting LCx-related ischemia is near V8. Krucoff and colleagues [10] have used a bipolar lead with one electrode on the back for monitoring ischemia (combining a positive electrode at the V2 position with a negative one at the spinous process), and they found that LAD-related ischemia is invariably picked up by this lead. We have shown that bipolar leads with one of the electrodes on the back are most sensitive for detecting LAD- and LCx-related ischemia [5].

5.  Alternative Displays of the Standard 12-Lead Electrocardiogram

The classic display of the standard 12-lead ECG includes two segregated groups of the six limb leads (I, II, and III; and aVR, aVL, and aVF) and two integrated groups of the six precordial leads. Other methods of displaying 12-lead ECG, discussed by Pahlm-Webb and co-workers [11], can enhance the diagnostic information that is required for making the therapeutic decisions for patients with acute coronary syndromes. A "24-view ECG" features the groups of the 12 frontal and 12 transverse leads arranged like the numbers on a clock face. Thus, for instance, ECG tracings from a patient who had a transient occlusion of an LCx coronary artery during PTCA would typically show no ST elevation in any of the standard 12 leads, but there would likely be ST depression in V1-V4. The appearance of ST elevation in leads -V1 to -V4 visualizes the posterior epicardial ischemia induced by the balloon inflation; if this patient had presented with acute chest pain, use of the 24-view ECG might have facilitated true allocation to the ST-elevation category of acute coronary syndromes, rather than false one to the non-ST-elevation category. Some investigators have applied posterior thoracic electrodes in order to record ST elevation associated with posterior epicardial ischemia associated with the occlusion of the LCx coronary artery. It can be demonstrated that 24-view ECG can provide waveforms that correlate with posterior leads V8 and V9 by simply inverting precordial leads V1 and V2. In acute anterior MIs, a proximal LAD artery occlusion produces inferior ST depression [12], which is accompanied by ST elevation in leads aVL [13], and in addition in leads -II, -III, and -aVF [11]. In patients with an acute inferior MI, Birnbaum and co-workers [14] found in almost all patients ST-segment depression in lead aVL; thus truly reciprocal ST depression in the more superiorly oriented leads -II, -III, and -aVF must be present.

6.  Conclusion

The purpose of this presentation is to show the limitations of the 12-lead ECG in recognizing ischemia, and to seek alternative electrocardiographic leads, optimized for detection of ischemia originating in different regions of the ventricular myocardium. A survey of recent clinical studies shows that the electrocardiographic manifestations of acute myocardial ischemia observed on body-surface potential maps of ST-segment shifts recorded during controlled ischemia induced by balloon-inflation PTCA are in agreement with the ST-segment measurements in admission ECGs of patients with acute MI [15]. Thus, based on the electrocardiographic data recorded during PTCA, leads can be judiciously chosen, which can potentially improve the clinical electrocardiographic criteria for recognizing acute myocardial infarction and for identifying the infarct-related artery. Alternatively, new ways of displaying the standard 12-lead ECG¡ªsuch as the 24-view method¡ªcan be sought to enhance the diagnostic information required for making the therapeutic decisions for patients with acute coronary syndromes.

References

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[11] Arbane, M. & Goy, J.-J. Prediction of the site of total occlusion of the left anterior descending coronary artery using admission electrocardiogram in anterior wall acute myocardial infarction. Am J Cardiol 2000;85:487-491.

[12] Pahlm-Webb, U., Pahlm, O., Sadanandan, S. et al. A new method for using the direction of ST-segment deviation to localize the site of acute coronary occlusion: The 24-view standard electrocardiogram. Am J Med 2002;113:75-78.

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[14] Birnbaum Y, Sclarovsky S, Mager A et al. ST segment depression in aVL: a sensitive marker of acute inferior myocardial infarction. Eur Heart J 1993;14:4-7.

[15] Horácek BM and Wagner GS. Electrocardiographic ST-segment changes during acute myocardial ischemia. Card Electrophysiol Rev 2002;6:196-203.

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