ECG Diagnosis of Acute MI in Patients with Pacemakers
ABSTRACT&COMMENTARY
Source: Sgarbossa EB, et al. Early electrocardiographic diagnosis of acute myocardial infarction in the presence of ventricular paced rhythms. Am J Cardiol 1996;77:423-424.
All patients enrolled in the gusto-i trial1composed of 41,021 patients with enzymatically confirmed acute myocardial infarction (AMI)who had ventricular-paced rhythms on the pre-randomized 12-lead electrocardiogram (ECG) were eligible for a recent study by Sgarbossa et al. Thirty-two patients (0.1%) were enrolled, with 15 patients excluded due to the presence of native rhythm and/or fusion beats (14) and left ventricular-paced rhythm (1) on the ECG. Seventeen cases (6 single- and 11 dual-chamber ventricular pacemakers) were used for analysis and compared to a similar number of randomly selected, age-matched control subjects with known, stable coronary artery disease and permanent right ventricular pacing. Classic ECG criteria for myocardial infarction in the setting of ventricular pacing2-4 were assessed between the two groups. Three ECG criteria were found to be useful in the early diagnosis of AMI: 1) discordant ST segment elevation (STE) ³ 5 mm (sensitivity, 53%; specificity, 88%; P = 0.025; 95% confidence interval, 1.26-4.39); 2) concordant STE ³ 1 mm (sensitivity, 18%; specificity, 94%; P = NS); and 3) ST segment depression (STD) ³ 1 mm in leads V1, V2, or V3 (sensitivity, 29%; specificity, 82%; P = NS). No criteria involving QRS complex or T wave morphologies were found to be useful.
COMMENT BY WILLIAM J. BRADY, MD
In the prethrombolytic era, early diagnosis of AMI was not crucial; immediate confirmation of AMI was not necessary due to the lack of time-sensitive therapies aimed at coronary revascularization and myocardial salvage. Today, rapid diagnosis of AMI, particularly transmural infarction, identifies a patient population that may benefit greatly from acute revascularization therapy. Many factors contribute to the difficult diagnosis of AMI, including presentations characterized by atypical chest pain, the various anginal equivalent complaints, and the ECG patterns that are either initially nondiagnostic or obscured by electrocardiographic patterns (e.g., left bundle branch block and ventricular-paced rhythms). Concerning the ventricular-paced ECG, several tools are available to the emergency physician to assist in the evaluation for potential AMI, including an awareness of the expected repolarization changes due to pacing and, hence, the recognition of unanticipated abnormalities, a comparison to past ECGs, the use of serial ECGs searching for evolving change, and ST segment trend monitoring. The first strategy listed is preferable in that it is not dependent on medical records or advanced technology and offers an immediate assessment of the initial ECG; further, the other tools rely in part on this awareness of the expected ST waveform morphologies in ventricular-paced rhythms.
The article by Sgarbossa et al addresses the ECG diagnosis of AMI early in the patient’s presentation in the setting of ventricular-paced rhythms. Much of the existing cardiology literature exploring this issue does not distinguish between past myocardial infarction and AMI.2-4 Further, the ECG tools recommended are cumbersome and difficult to use at the bedside. Lastly, the sensitivity and specificity of these past criteria are not impressive. The basic principle of this article can be summarized with the "rule of appropriate discordance." Uncomplicated ventricular pacing is characterized by secondary repolarization changes of opposing polarity to the predominant QRS complex deflection; in other words, the expected ST-segment position is on the opposite side of the isoelectric baseline from the predominant portion of the QRS complex. For example, during pacing from the right ventricular apex, the majority of ECG leads demonstrate an entirely negative or predominantly negative QRS complex. As such, the expected ST position is elevation. (See Figure 1A.) The various criteria suggested by Sgarbossa et al address the segment and its position relative to the predominant deflection of the QRS complex. ECG criteria #2 (see Figure 1B) and #3 (see Figure 1C) outlined above are examples of infractions of the "rule of appropriate discordance," revealing concordant STE and concordant STD (limited to leads V1, V2, or V3), respectively. The most statistically useful criterion, discordant STE of 5 mm or more (see Figure 1D), violates this rule, not with concordant ST segment changes, but with an inappropriate degree of discordant STE; repolarization changes of ventricular-paced rhythms should produce STE of less magnitude in the "normal" state.
Although the findings are limited to a certain extent by the small study population, this article is must reading for emergency physicians. It suggests the use of simple, easily applied criteria for the ECG diagnosis of AMI early in the patient’s course. It also reinforces the point that emergency physicians must be experts at ECG interpretation, including complicated cases involving ventricular-paced rhythms. (Dr. Brady is Assistant Professor of Emergency Medicine and Internal Medicine, and the Medical Director of the Chest Pain Center, at the University of Virginia in Charlottesville, VA.)
Figure 1
ECG appearance of the ST segment in right ventricular-paced rhythms in both the normal state (A) and during AMI (B, C, and D)
A: Normal discordant QRS complexST segment relationship; B: Concordant STE ³ 1 mm; C: Concordant STD ³ 1 mm limited to leads V1, V2, or V3; D: Discordant STE ³ 5 mm
References
1. The GUSTO Investigators. An international randomized trial comparing four thrombolytic strategies for acute myocardial infarction. N Engl J Med 1993;329:673-682.
2. Barold SS, et al. Primary ST and T wave abnormalities in the diagnosis of acute anterior myocardial infarction during permanent ventricular pacing. J Electrocardiol 1976;9:387-390.
3. Niremberg V, et al. Primary ST changes. Diagnostic aid in patients with acute myocardial infarction. Br Heart J 1977;39:501-507.
4. Barold SS, et al. Electrocardiographic diagnosis of myocardial infarction during ventricular pacing. Cardiol Clin 1987;5:403-417.
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