**5. Electrocardiography**

Because the pericardium is electrically inert, EKG changes found in patients with acute pericarditis are suggestive of an underlying myocardial involvement. The typical EKG evolution is seen in up to 60 % of cases of acute pericarditis [26]. EKG may reveal sinus tachycardia, diffuse ST segment elevation that is concave upwards involving any lead except aVR and V1. In pericarditis, T wave inversion occurs only after the elevated ST segment returns to baseline. ST-segment elevation associated with pericarditis should not

Perimyocarditis 109

Fig. 1. Concave upward ST segment elevation in leads II, III, Avf, V5 and V6 in a patient with perimyocarditis. Notice the focal pattern of ST segment elevation (inferolateral leads) thereby mimicking transmural myocardial infarction. Adapted from Omar et. al. [34].

reduction of the edema shown by the biopsy was also significant [39].

permission.

and collegues found that pericardial effusions are present in approximately 60% of cases of acute pericarditis, with 80% being mild, 10% being moderate, and 10% being severe [26]. Pericardial effusion was present in 38.1% of patients in the ST segment elevation group and 73.5% of the patients in the non ST segment elevation and was explained by the tendency of the larger pericardial effusions to decrease voltage including the magnitude of ST segment elevation [26]. The presence of regional wall motion abnormalities favors an ischemic process rather than acute pericarditis. Another potential echocardiographic finding in perimyocarditis is transient myocardial thickening (figure 2 and 3) which is due to interstitial edema and its presence likely predicts a poor prognosis as it has been associated with a fulminant course [38]. In a series of 25 patients with acute myocarditis who underwent echocardiogram and endomyocardial biopsy, a significant decrease in myocardial thickness was observed between the acute and the convalescent phase. The

Fig. 2. Echocardiogram on admission. Parasternal long axis view (A) and short axis (B) in diastole. There is an asymmetrical thickening of the posterior wall involving the posterior papillary muscle, and slight pericardial effusion. Adapted from reference number 38 with

result in the reciprocal depressions in aVL that accompany inferior MI, although this may not apply in some cases of localized pericarditis [33]. The most specific EKG finding for acute pericarditis is PR segment depression (PR segment elevation in aVR) which is considered an early EKG marker in the evolution of acute pericarditis. PR segment depression is due to subepicardial atrial injury and is present in more than 60 % of the patients. Acute pericarditis causes characteristic EKG changes that typically evolve through 4 stages as demonstrated in table 2.


Table 2. The 4 Electrocardiographic stages of acute pericarditis.

Nevertheless, perimyocarditis can present with focal instead of diffuse ST segment elevation mimicking transmural myocardial infarction. This, in addition to the presence of chest pain and elevated cardiac biomarkers can make the differentiation of increasing difficulty. This is important because fatal complications can occur if thrombolytic therapy is administered for a patient with acute pericarditis, or if a diagnosis of transmural myocardial infarction is missed. Omar et al. demonstrated a similar scenario where an EKG of a patient presenting with chest pain revealed focal ST segment elevation (figure 1) and the cardiac biomarkers were elevated mimicking STEMI. [34] Careful history taking, EKG interpretation and urgent echocardiogram favored the diagnosis of acute perimyocarditis.

Previous studies have reported the use of thrombolytic therapy for what was later determined to be acute pericarditis [35, 36]. The utilization of urgent coronary angiography is not uncommon in patients with acute perimyocarditis. Salisbury and colleagues described the frequency of urgent coronary angiography in 238 patients with a final diagnosis of acute pericarditis to be 16.8 % [37].
