Special Feature
The Additional Lead Electrocardiogram in Acute Myocardial Infarction
By William J. Brady, MD
The 12-lead electrocardiogram (ecg) is a less-than-perfect indicator of acute myocardial infarction (AMI). The sensitivity of a single 12-lead ECG for the diagnosis of AMI is relatively poor. It has been suggested that the sensitivity of the 12-lead ECG may be improved if three additional body surface leads are employed in selected individuals.1,2 Acute posterior myocardial infarctions (PMI) and right ventricular myocardial infarctions (RVMI) are likely to be underdiagnosed, as the standard lead placement of the 12-lead ECG does not allow these areas to be assessed directly.3 Additional leads frequently used include leads V8 and V9, which image the posterior wall of the left ventricle, and lead V4R (alternatively labeled RV4), which reflects the status of the right ventricle. (See Figure 1 for correct placement of the additional ECG leads.) The standard ECG, coupled with these additional leads, constitutes the 15-lead electrocardiogram, the most frequently employed extra-lead ECG in clinical practice. A more detailed description of the extent of the myocardial injury may be obtained if additional leads are used to augment the standard 12-lead ECG in selected patients.
Right Ventricular Infarction
The standard 12-lead ECG findings for RVMI include ST-segment elevation in the inferior distribution as well as in the right precordial chest leads, particularly lead V1—perhaps the only lead on the standard ECG that reflects changes in the right ventricle. (See Figure 2.) At times, coexisting acute PMI may obscure the ST-segment elevation resulting from RVMI in lead V1 as seen in the patient with the acute inferoposterior myocardial infarction with right ventricular involvement. Recordings from leads placed on the right side of the chest are much more sensitive and specific in detecting the changes of RVMI. The right-sided precordial electrodes are placed across the right side of the chest in a mirror image of the standard left-sided leads. They are labeled V1R to V6R (or RV1 to RV6). The clinician may use either these six right-sided leads or the single lead V4R. Lead V4R (right fifth intercostal space mid-clavicular line) is the most useful lead for detecting ST-segment elevation associated with RVMI and may be used solely in the evaluation of possible RVMI. (See Figure 2.) The ST-segment elevation that occurs in association with RVMI is frequently quite subtle (see Figure 2), reflecting the relatively small muscle mass of the right ventricle. At other times, the ST-segment elevation is quite prominent and similar in appearance to the ST-segment changes seen in the standard 12-lead ECG. These changes are often transient, frequently resolving within 10 hours of the onset of symptoms.
Posterior Wall Infarction
On the standard 12-lead ECG, changes associated with necrosis of the posterior wall of the left ventricle are reflected in the anterior chest leads.4 These electrodes are opposite rather than adjacent to the site of damage, and the changes seen are the reverse of what one would normally expect. From the perspective of the standard 12-lead ECG, the "typical" findings indicative of transmural AMI will be reversed. This reversal results from the fact that the endocardial surface of the posterior wall faces the anterior precordial leads (V1 through V3) in the standard 12-lead ECG. ST-segment depression, prominent R waves, and upright T waves in leads V1 through V3—"when reversed"—may represent the ST-segment elevation, Q waves, and T-wave inversions, respectively, of acute PMI. If one considers the "reverse nature" of these ECG abnormalities when applied to the posterior wall, the findings assume a more recognizable, ominous meaning.
Abnormalities noted on the standard 12-lead ECG suggestive of acute PMI include the following (in leads V1, V2, and/or V3): horizontal ST-segment depression; a tall, upright T wave; a tall, wide R wave; and an R/S wave ratio > 1.0. (See Figure 3.) Further, the combination of horizontal ST-segment depression with an upright T wave increases the diagnostic accuracy of these two separate electrocardiographic findings. It must be remembered that a dominant R wave—which is equivalent to an evolving Q wave—takes a number of hours to develop and therefore is not frequently seen on the initial ECG.5,6
Summary
The use of the 15-lead ECG may help define the full extent of myocardial injury in patients with AMI. The use of additional leads may confirm the diagnosis of RVI in hypotensive patients presenting with an inferior AMI. Alternatively, in patients with inferior AMI, the detection of RVMI prior to the development of hypotension will assist the clinician in the proper, "gentle" use of vasodilating agents. In patients with ST-segment depression in leads V1-V3, the use of the additional leads V8 and V9 may help to distinguish between a PMI, anterior wall ischemia, and reciprocal changes. The discovery of an isolated PMI—i.e., the distinction from anterior wall ischemia—will enable the physician to offer appropriate therapy in the most expeditious fashion. The 15-lead ECG is recommended in any patient presenting with an inferior AMI, any infarction involving the lateral wall of the left ventricle, or any patient with ST segment depression in leads V1 - V3.
References
1. Pollack M, et al. Emergency department diagnosis of acute posterior-wall myocardial infarction using left posterior chest leads. Acad Emerg Med 1997;34:399 (abstract).
2. Melendez LJ, et al. Usefulness of three additional electrocardiographic chest leads (V7, V8, and V9) in the diagnosis of acute myocardial infarction. CMAJ 1978; 119:745-748.
3. Rich MW, et al. Electrocardiographic diagnosis of remote posterior wall myocardial infarction using unipolar posterior lead V9. Chest 1989;96:489-493.
4. Conover MB. Understanding Electrocardiography: Arrhythmias and the 12-lead ECG. 6th ed. St. Louis: Mosby-Year Book; 1992.
5. Goldberger AL. Myocardial Infarction: Electrocardiographic Differential Diagnosis. 4th ed. St. Louis; Mosby; 1991.
6. Aufderheide TP, Brady WJ. Electrocardiography in the Patient with Myocardial Ischemia or Infarction. In: Gibler WB, Aufderheide TP, eds. Emergency Cardiac Care. St. Louis: Mosby-Year Book; 1994.
22. Electrocardiographic findings associated with acute right ventricular myocardial infarction include:
a. T wave inversion in V8.
b. ST-segment depression in V4R.
c. ST-segment elevation in V4.
d. ST-segment elevation in V4R.
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