What lies beneath: Posterior ST elevation myocardial infarction with underlying right ventricular-paced rhythm

Abstract

A 66-year-old man with a history of coronary artery disease, stage V chronic kidney disease, peripheral arterial disease and a dual-chamber pacemaker experienced persistent chest and shoulder discomfort following his daily hemodialysis treatment. Treatment with clopidogrel had been discontinued three days previously due to impending vascular surgery. Electrocardiography revealed a right ventricular-paced rhythm with ST abnormalities indicative of posterior ST elevation myocardial infarction. The patient underwent urgent cardiac catheterization and required percutaneous coronary intervention for an acutely occluded coronary artery. The present case report emphasizes the importance of careful and timely review of the electrocardiogram of any patient with a ventricular-paced rhythm who experiences signs and symptoms consistent with acute coronary syndrome. Certain characteristic electrocardiographic abnormalities have been demonstrated to predict acute myocardial infarction in such patients.

Patient outcomes following acute ST elevation myocardial infarction (STEMI) have improved in recent years due, in part, to the availability of rapid percutaneous coronary intervention techniques that can often quickly restore flow to the infarct-related artery. An increasing number of patients with known or occult coronary artery disease are also living with permanent pacemakers or implantable cardioverter defibrillators. Correct and accurate interpretation of the presenting electrocardiograms (ECGs), including those of patients with ventricular-paced rhythms, is central to the implementation of optimal and rapid treatment for patients with STEMI. In the present article, we report a case involving a patient with acute posterior STEMI and an underlying right ventricular (RV)-paced rhythm that complicated the initial clinical assessment.

CASE PRESENTATION

A 66-year-old man with a complex history including diabetes mellitus, coronary artery disease with multiple previous cardiac catheterizations and percutaneous coronary interventions, peripheral arterial disease, stage V chronic kidney disease and a dual-chamber pacemaker was hospitalized for left lower extremity peripheral arterial disease with critical limb ischemia. Clopidogrel was discontinued on hospital admission due to anticipated vascular surgery. On the third day of hospitalization, the patient developed diaphoresis and left shoulder and chest pressure while undergoing his routine hemodialysis.

An initial ECG showed RV-paced rhythm with ST depression in leads V2 and V3 (Figure 1). The RV-paced rhythm was unchanged compared with previous ECGs; however, the anterior ST depressions were new. The patient’s hemodialysis session was terminated but he continued to experience chest and shoulder discomfort and diaphoresis and was treated with sublingual nitroglycerin. A cardiology consultation was obtained and emergent cardiac catheterization was performed, which revealed acute occlusion of a stent to the right posterior descending artery that had been placed 11 months previously. During the procedure, results of tests of the patient’s cardiac markers during the episode of chest pain became available: troponin I 0.11 ng/mL (reference range 0 ng/mL to 0.04 ng/mL) and myoglobin 209 ng/mL (reference range 17 ng/mL to 106 ng/mL). The level of troponin I later rose to a maximum of 42.09 ng/mL before beginning to decline. The patient underwent percutaneous coronary intervention and a single drug-eluting stent was placed in the right posterior descending artery to treat 100% in-stent thrombosis within a previously stented segment of the artery. Clopidogrel treatment was reinitiated (600 mg loading dose followed by 75 mg per day). Following percutaneous coronary intervention, an ECG showed RV-paced rhythm with resolution of the anterior ST depression (Figure 2). The patient initially fared well, but his subsequent hospital stay was complicated by critical limb ischemia and he died from cardiac arrest following vascular surgery 12 days after his MI.

Figure 1).

Electrocardiogram obtained 30 min after the onset of persistent chest and shoulder pressure. Note the ST segment depression observed most prominently in leads V2 and V3

Figure 2).

Electrocardiogram obtained following resolution of symptoms after percutaneous coronary intervention involving placement of a drug-eluting coronary stent in a 100% occluded right posterior descending artery

DISCUSSION

Evaluation of the ECG for ischemic changes in the presence of a ventricular-paced rhythm is challenging. For most dual-chamber pacemakers, placement of the RV lead is usually near the RV apex, which produces a pattern similar to a left bundle branch block (LBBB) in the septal and anterior precordial leads, with a predominantly negative QRS complex followed by ST elevation and positive T-waves (1).

Unlike right bundle branch block or nonspecific intraventricular conduction delays, LBBB can significantly challenge attempts to diagnose acute myocardial infarction (AMI), and STEMI in particular. Electrocardiographical criteria to diagnose an AMI in the setting of an LBBB have been developed, and are known as the Sgarbossa Criteria (2). These include ST segment elevation ≥1 mm in an ECG lead with a positive QRS complex, ST segment depression ≥1 mm in lead V1, V2 or V3, or ST-segment elevation ≥5 mm in a lead with a negative QRS complex (2).

RV-paced rhythm presents similar difficulties in ECG interpretation in the setting of a possible AMI. When the ST segments must be evaluated for possible ischemia in the setting of ongoing ventricular pacing, the criteria used are the same as those used in the presence of a LBBB (1,3,4). In the Global Utilization of Streptokinase and Tissue plasminogen activator for Occluded coronary arteries (GUSTO) trial, which involved 26,003 patients with AMI confirmed by abnormal cardiac markers, ECGs of patients with RV pacing were analyzed using the Sgarbossa criteria. ST segment depression in leads V1, V2 or V3 had a specificity of 82% for acute infarction, although the sensitivity was only 29% (1,4), perhaps due to the small number of paced ECGs available for analysis. Follow-up angiographic data were not available for individual patients; therefore, whether these were posterior STEMIs or anteroseptal ST depression (subendocardial) infarcts, both of which could produce ST depression in these ECG leads, could not be determined (4). We are not aware of specific data regarding ECG criteria to differentiate posterior STEMI from an anteroseptal subendocardial infarction in the setting of an RV-paced rhythm.

AMI in patients with LBBB or ventricular-paced rhythms has been categorized as ‘undetermined type’ and, in a large prospective study of AMI patients, was observed in 6.5% of patients. Patients with ‘undetermined type’ AMI were found to have a high intrahospital mortality rate of 11.8% (5). Consequently, prompt diagnosis and treatment should be considered at least as important in this subgroup of patients as in individuals without LBBB or paced rhythms.

Options for evaluating a patient with RV-paced rhythm for acute ST abnormalities include temporary termination of pacing (by reprogramming the indwelling pacemaker) or analysis of the ECG with ongoing ventricular pacing. Temporary termination of pacing is sometimes possible, but requires the presence of specialized equipment (a pacemaker interrogation device) and personnel trained to alter pacemaker settings. Reprogramming of the pacemaker to avoid ventricular pacing could also be dangerous if the patient has complete atrioventricular block – the ‘pacemaker-dependent’ patient (1,3). Also, the phenomenon of ‘T-wave memory’, nonischemic T-wave repolarization changes that can occur temporarily following termination of tachycardia, ventricular pacing or a LBBB rhythm, can sometimes obscure ST changes caused by myocardial injury (6). Based on the present case report, the diagnosis of a posterior STEMI can potentially be made emergently using an ECG even in the presence of an RV-paced rhythm or LBBB. However, it may not be possible to distinguish between a true posterior myocardial infarction and anteroseptal subendocardial ischemia by examination of the ECG.

CONCLUSION

The presence of an RV-paced rhythm, similar to a native LBBB pattern, can significantly complicate the diagnosis of AMI. Pre-existing abnormal repolarization, particularly in the septal and anterior ECG leads, makes detection of septal, anterior or posterior ischemia more difficult. It is important for clinicians to be aware of criteria for the diagnosis of AMI in the setting of an LBBB and that these criteria can be applied to patients with a RV-paced rhythm. Also, it is important to note that the sensitivity and specificity for diagnosing AMI using these criteria are low. Further evaluation with emergent echocardiography to assess ventricular wall motion abnormalities may be necessary or, if clinical suspicion is high, urgent diagnostic cardiac catheterization may be required to reach a more definitive diagnosis.