Recognizing Those Electrocardiograms That Distinguish You as a Smart Clinician

Abstract and Introduction

Abstract

After 100 years, the 12-lead electrocardiogram (ECG) remains the most accessible and inexpensive noninvasive diagnostic and prognostic tool in cardiology. Almost every hospitalized patient will undergo electrocardiography and patients with known cardiovascular disease will do so many times. Today, ECGs also are routinely used for insurance purposes, for occupational fitness determination, and in a variety of other settings. Most ECG machines today are able to read the tracings to some degree (with varying accuracy), but it is still incumbent on cardiologists to understand and interpret the ECG tracings.

Introduction

The initial recognition of cardiac electrical signals was reported by Waller in 1887 followed by Einthoven's development and description of the ECG in 1902.^[1] Professor Hein Wellens has been the driving force for many breakthroughs in clinical cardiac electrophysiology. With the knowledge derived from a 12-lead ECG, combined with the recordings made with invasive electrophysiology, he has composed many new ideas leading to major contributions in clinical cardiac electrophysiology and, more generally, in arrhythmology.

In the setting of acute myocardial infarction (AMI), recognition of the area at risk, and severity of the ischemia is critically important to establish the aggressiveness of therapy. Because the earliest ECG changes of acute cardiac ischemia are in the ST-T segment, emphasis for early diagnosis and risk classification should be on ST-T segment changes.

Two approaches have been introduced to use ST-T segment changes for early diagnosis and decision-making in AMI:

ST-segment deviation score - is calculated by adding the number of millimeters that the ST segment deviates (elevation or depression) from the isoelectric line in all 12 ECG leads. In both anterior and inferior AMI, the total amount of ST-segment elevation and depression, as well as the number of leads in which these changes are present, are proportional to infarct size. ^[2,3]

ST-segment deviation vector - also called the vector of ischemia, was introduced more recently to predict the site of occlusion in the coronary artery and thus the approximate size of the area at risk; the more proximal the location of the occlusion in the coronary artery, the greater the area at risk. ^[3,4]

The ST-segment deviation vector is determined in the same way as the QRS vector in that clinicians should look for the isoelectric ST segment in the limb leads to determine the lead axis perpendicular to the ST-segment vector, and then find the most positive or most negative ST-segment deviation to determine the direction of the ST-segment vector (Figure 1). The direction of the ST-segment deviation vector varies in different occlusion sites in the left anterior descending coronary artery (LAD) (Figure 2). Note that in an anterior wall MI, the ST-deviation vector in the frontal plane indicates the occlusion site in the LAD. Of the precordial leads only V1 is of value, ST elevation in this lead of more than 2 mm indicates occlusion proximal of the first septal branch and therefore ischemia of the upper part of the interventricular septum and a high chance of ischemia of the sub-AV nodal conduction system.

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Figure 1.

Acute Anterior Myocardial Infarction

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Figure 2.

Direction of the ST-Segment Deviation Vector in Different Occlusion Sites in the LAD

In infero-posterior MI, the behavior of the ST deviation vector usually allows for the differentiation between a right coronary artery (RCA) or a circumflex (CX) coronary artery occlusion (Figure 3). That differentiation can be facilitated by recording lead V4R, which also gives information about the site of the coronary occlusion in the RCA, above or below the right ventricular branch. In lead V4R, the ST segment is elevated with a positive T wave in a proximal RCA occlusion, while the ST segment is isoelectric in case of a distal RCA occlusion. In the CX occlusion, V4R shows ST depression and a negative T wave. Recognition of a proximal RCA occlusion is important because it indicates right ventricular infarction and identifies patients having about a 50% risk of developing atrioventricular (AV) nodal block.

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Figure 3.

Behavior of the ST-Segment Deviation Vector in Infero-Posterior MI

(Dr. Wellens emphasizes that the ST deviation in lead V4R usually disappears within 10 hours after onset of pain, which means that it is important to record this lead on admission.)

The presence or absence of Q waves is important, too, with one proviso. Although Q waves have been thought to indicate myocardial necrosis, extensive ischemia can, in fact, result in transient Q waves due to conduction delay in the zone under the electrode. Significant myocardial salvage can be accomplished in patients with new pathological Q waves and, even after 2 hours, infarction size can be limited by therapy. This indicates that especially large anterior wall infarctions are still evolving at that time. Thus, Dr. Wellens said patients should not be excluded from reperfusion therapy simply because Q waves are present.

How well are emergency departments doing in getting an ECG reading in AMI patients? In February 2006, a team of U.S. researchers analyzed data from 63,478 patients (42% women) with high-risk non-ST-segment elevation acute coronary syndromes (ACS) enrolled in the CRUSADE Quality Improvement Initiative.^[5] 10 minutes from hospital arrival) or nondelayed (<10 minutes).

Overall, median time to electrocardiography was 15 minutes with ECG acquisition delayed (median 25 minutes) in 65.2% of patients. Women were more likely than men to have delayed ECG acquisition (69% vs 62%), and female gender was the most significant predictor of delayed ECG acquisition. With only 33% of high-risk patients with non-ST-segment elevation ACS having an initial ECG obtained within 10 minutes of arrival as recommended, the authors advised that emergency departments focus on decreasing the time to initial ECG acquisition, especially in female patients, to improve treatment of ACS.

ECGs in Other Settings

The ECG in the setting of heart failure may give important information in relation to prognosis and therapy. This holds for markers such as sinus tachycardia, atrial fibrillation, ventricular arrhythmias, number of leads showing QR or QS complexes, the width of the QRS complex, and a prolonged QTc interval.

QRS width is a useful predictor of outcome in patients with heart failure, especially in patients with left bundle branch block. This may be used to select patients most likely to benefit from resynchronization of ventricular activation by left ventricular pacing with or without synchronized right ventricular pacing. Often both risk estimation and response to therapy can be derived from ECGs recorded during treatment of heart failure.

In supraventricular tachycardias, the 12-lead ECG usually allows identification of where the arrhythmia originates and frequently the mechanism. The 12-lead ECG often is now sufficient to differentiate among atrial tachycardia, atrial flutter, AV nodal tachycardia, and tachycardias incorporating an accessory AV pathway in the tachycardia circuit, and to identify whether catheter ablation is possible.

In this interview, Prof. Hein Wellens discusses determining the size of area at risk following AMI and the site of occlusion in the culprit coronary artery, identifying the site of conduction abnormalities, and how an ECG can predict good responders to resynchronization therapy.

Cardiosource © 2006  American College of Cardiology

Cite this: Recognizing Those Electrocardiograms That Distinguish You as a Smart Clinician - Medscape - Oct 02, 2006.