The ECG reveals evidence of ST segment elevation, most notable in limb leads I, II, and aVF, and precordial leads V3 through V6. In fact, the computer interpretation of this finding is “ACUTE MI” with an inferolateral ST segment elevation pattern. On closer inspection, however, the morphologic shape of the ST segment elevation is concave rather than the convex shape more commonly seen with infarction.
Concave ST segment elevation can be seen with benign early repolarization (BER). However, this finding is usually most evident in the right and mid-precordial leads, and is more commonly associated with prominent T waves, which is not seen in this ECG. In addition, there is evidence of PR depression in a similar lead distribution as the ST segment elevation, as well as PR elevation and ST depression in lead aVR (Figure 2). The findings on this ECG are most consistent with the diagnosis of acute myopericarditis.
Up to 90 percent of patients with acute myopericarditis have electrocardiographic abnormalities. (Cardiol Clin 1990;8:639.) These changes are a result of repolarization abnormalities from epicardial inflammation and injury in the atrium (affecting the PR segment), and ventricle (affecting the ST segment and T wave) morphologies. As depolarization is unaffected, the P wave and QRS complex are usually unchanged.
A classic four-stage progression of ECG abnormalities has been described with acute myopericarditis, though it is rare to detect all four stages. Stage I occurs during the first few days, lasts up to two weeks, and is notable for diffuse ST elevation and PR depression. Stage I abnormalities are detected more frequently and considered quasi-diagnostic, particularly PR depression. (J Emerg Med 1999;17:865.) Stage II, characterized by ST segment normalization, is variable in duration, lasting from a few days to several weeks after stage I. In addition, T wave amplitude will decrease or flatten. Stage III occurs in the second or third week of the illness and may be transient or prolonged, lasting from days to several weeks. During this time, full T wave inversion occurs. Stage IV is marked by resolution of T wave abnormalities and return to the baseline ECG.
The ST elevation of acute myopericarditis is usually less than 5 mm in height, and occurs simultaneously and diffusely in the limb and precordial leads with the exception of avR and V1 (which often have reciprocal ST depression). There is usually no clear territorial distribution, though findings can be focal, such as in the inferior distribution, when inflammation is localized (i.e., post-infarct myopericarditis). The ST elevation is concave, or obliquely flat, on its initial upslope with an indistinct J point. Differentiating ST segment elevation from other causes such as benign early repolarization (BER) and AMI or ischemia can be difficult.
Similar in morphology, ST elevation in benign early repolarization (BER) is limited primarily to the right and mid-precordial leads, with a prominent T wave. One can assess this difference using the ST/T ratio (ratio of ST segment to T wave amplitude) in V6. Using the PR segment as baseline to measure ST elevation (at the J point) and the T wave amplitude, an ST/T ratio of equal or greater than 0.25 suggests myopericarditis while less than 0.25 suggests BER (Figure 3). (Circulation 1982;65(5):1004.)
Initial ST segment morphology is often convex or obliquely flat in AMI, usually in a territorial distribution (Figure 4). T wave inversion often occurs with ST elevation in AMI, but occurs only after resolution of the ST segment abnormalities in myopericarditis. The presence of Q waves suggest AMI, while PR segment depression suggests myopericarditis. In addition, the ECG abnormalities evolve over a much shorter time with AMI (hours to a few days) than with myopericarditis.
With myopericarditis, PR depression can be seen in the same leads as ST segment elevation, and while transient, may be the earliest and most specific finding. (Am J Cardiol 1998;812:1505.) Leads aVR and V1, which may have reciprocal PR segment elevation. In assessing the PR segment, it is important to use the TP segment as baseline; otherwise the depression may be misinterpreted as ST elevation. T wave inversions are encountered diffusely in later stages only after ST elevations have resolved. This stage is often missed or not detected in the patient with myopericarditis.
The most common rhythm associated with acute myopericarditis is normal sinus or sinus tachycardia. Because of the proximity of the sinus node to the pericardium, it was previously thought the inflammation could precipitate supraventricular tachydysrhythmias, but recent studies suggest the sinus node is virtually immune to surrounding inflammation, and these dysrhythmias can be attributed to underlying cardiac disease. (J Am Coll Cardiol 1998; 32:551.)
Because of her age, the patient was admitted and ruled out for AMI. Subsequent echocardiogram revealed a small pericardial effusion, but was otherwise unremarkable. The patient recovered uneventfully with resolution of her symptoms and findings over the next two weeks, completely consistent with the diagnosis of acute myopericarditis.