Patients presenting to the emergency department with chest pain of potentially ischemic origin are evaluated with three principle tools: the history of the event, the 12-lead electrocardiogram (ECG), and, in suspected cases of acute myocardial infarction, serum markers of cardiac injury. An accurate clinical history is pivotal in evaluating these patients, and is a prerequisite for correctly interpreting the ECG within the context of the clinical situation.
The ECG is important in evaluating patients with chest pain, yet this same important technology has significant limitations in this application. In patients who present with a clinical history compatible with AMI, fewer than 50 percent of patients will demonstrate a diagnostic ECG consistent with ST segment elevation AMI (STEMI). One of the explanations for this significant shortcoming of the 12-lead ECG is its somewhat limited anatomic perspective. Considering the left ventricular anatomic segments, the 12-lead ECG evaluates the anterior wall quite well; the inferior and lateral walls are reasonably imaged, though these regions can “hide” STEMI with minimal ST segment elevations of questionable morphology. The posterior wall and the right ventricle are clearly “under-imaged.” These “electrocardiographically silent” areas, including the posterior, inferior, and lateral walls of the left and the right ventricles, represent a potential area of diagnostic improvement in early STEMI detection.
Body surface mapping (BSM) is an electrocardiographic technique that can provide a solution to this diagnostic dilemma. BSM provides a more complete depiction of the heart by applying numerous electrocardiographic leads; it is thought that this increased electrocardiographic surveillance provides a greater ability for detecting STEMI. (Am J Cardiol 2000;85:934; J Am Col Cardiol 2002;39:332A; Am J Cardiol 2003;92:252.)
Rather than replacing the 12-lead ECG, the BSM ECG is an extension of the traditional 12-lead electrocardiogram, a strategy which can further define acute coronary syndrome-related injury. In the body map electrocardiographic system, clinical information is obtained and displayed in three separate formats: the 12-lead ECG (Fig. 2), an 80-lead ECG (Fig. 3), and color contour, or body, maps (Fig. 4). The 12-lead ECG is the standard 12-lead electrocardiogram while the 80-lead ECG uses 80 different leads with 64 anterior and 16 posterior leads (Fig. 3). These two data displays are interpreted in a similar fashion to standard electrocardiography with an analysis of the ST segment and T wave.
The color contour maps, a markedly different means of imaging the heart in the potential ACS patient, depict the anterior and posterior thorax; these maps can be presented as “flat” maps or torso maps. The two-dimensional flat map is a basic projection of the anterior and posterior thorax; the torso map is an anatomically oriented, 3-D depiction of the thorax with anterior and posterior segments. Color coding is used to depict the magnitude and deflection of the ECG signal, namely the QRS complex or ST segment: green for isoelectric position, red for positive deflection, and blue for negative deflection. The most clinically useful torso map is the ST isopotential measured at the J point or “0” seconds into the ST segment. Other torso maps include the QRS isointegral map, the STT isointegral map, and the ST60 isopotential map.
Body surface mapping can be used in ACS patients to detect undiagnosed STEMI (via the 12-lead ECG). Body surface mapping should not be the initial ECG performed in the chest pain patient. Rather, the typical evaluation should occur, including the 12-lead ECG. In this evaluation, if the 12-lead ECG demonstrates a nondiagnostic pattern (i.e., “nondiagnostic” in the sense that a STEMI is not noted), the clinician then must consider whether this patient is experiencing a non-STEMI ACS presentation or was a STEMI occurring in an electrocardiographically silent area. If the answer is a possible STEMI in any of these electrocardiographically silent areas, then BSM ECG is a reasonable next step. Menown and colleagues applied the body map ECG to evaluate the patient with suspected ACS and isolated electrocardiographic ST segment depression, and noted that numerous patients with serum marker-confirmed AMI demonstrated ST segment elevation on body map regions not viewed by the standard 12-lead ECG, including the inferior, lateral, posterior, and right ventricular regions. (Am J Cardiol 2003;92:252.)
In a second application, the body mapping electrocardiogram can further define the extent of ACS injury in patients with known STEMI. In this situation, the 12-lead ECG reveals, for example, an inferior wall STEMI. At this point, some clinicians might consider the evaluation largely complete. The clinician, however, can opt to perform a BSM ECG to evaluate the STEMI presentation more completely. Additional anatomic segments of the heart can be involved, including the posterior wall of the left and the right ventricles. With increasing anatomic segment involvement, an increased risk of acute cardiovascular complication, poor post-AMI cardiac function, and death is encountered. Demonstration of this additional involvement is difficult if only the 12-lead ECG is used.
Ornato (J Am Col Cardiol 2002;39:332A) and Menown (Am J Cardiol 2000;85:934) have done just that. Ornato et al compared the 12-lead and BSM ECGs in patients suspected of AMI. In this multicenter trial, the authors found that the body surface mapping electrocardiogram was more sensitive than 12-lead ECG in detecting STEMI; specificities of the two ECGs were similar in this patient group. (J Am Col Cardiol 2002;39:332A.) In another STEMI population, Menown and colleagues investigated the BSM's ability to investigate both posterior and right ventricular wall myocardial infarction in patients with diagnosed inferior wall STEMI. The authors compared the BSM with the additional lead ECG, using V2R, V4R, V7, and V9 as well as standard 12-leads. The authors noted that ST segment elevation over the right ventricle and posterior wall was detected in significantly more patients with BSM than additional lead ECG, and that BSM more directly visualized the right ventricle and posterior wall of the left ventricle, increasing the diagnostic rate of STEMI in these poorly imaged regions of the heart. (Am J Cardiol 2000;85:934.)
Body map electrocardiography undoubtedly provides a more complete anatomic description of the heart in patients with ACS. While more complete descriptive imaging of the heart is valuable, the actual clinical impact of this additional information remains unknown. Most acute reperfusion trials, including fibrinolysis and percutaneous coronary intervention, have focused on STEMI of the anterior, inferior, and lateral walls of the left ventricle.
In the 52-year-old woman presented at the beginning of this column, the initial 12-lead ECG did not demonstrate significant findings, worrisome for ACS. Yet the ECG did reveal subtle findings suggestive of a posterior wall event. This ECG, when considered in an ill patient, concerned the involved clinicians and suggested ACS as a likely diagnosis. The body map with 80-lead electrocardiogram demonstrated ST segment elevation in the posterior leads (Fig. 3); the torso body map (Fig. 4) revealed a red color in the posterior region, consistent with ST segment elevation of the posterior wall. Cardiology was consulted, which admitted the patient to the coronary care unit. Cardiac catheterization revealed distal right coronary artery obstruction, which was successfully opened with an intra-coronary stent. The patient had an uneventful hospital course, and was discharged on hospital day four with the diagnosis of posterior wall myocardial infarction.