Chapter 102. The Resting Electrocardiogram

1. How does the resting electrocardiogram (ECG) assist in the diagnosis and management of acute coronary syndrome (ACS)?

2. What are the key differentiating electrocardiographic features of ACS versus acute pericarditis and versus early repolarization?

3. How does the ECG assist in the diagnosis of acute pulmonary embolism?

4. How does the ECG assist in determining the cause of syncope?

5. What are the characteristic ECG findings of electrolyte disturbances?

6. What are the characteristic ECG findings of hypothyroidism, stroke, and drug effects?

A graphic recording of electrical potentials generated by the heart, the electrocardiogram is the most commonly performed cardiovascular laboratory procedure in the United States. As a noninvasive, versatile, reproducible, and inexpensive test, the ECG has utility in the evaluation of a range of signs and symptoms encountered by the hospitalist, including acute chest discomfort, breathlessness, syncope, and palpitations. While the ECG is useful in the detection of myocardial ischemia, arrhythmias, structural changes of the myocardium, and conduction system disease, clinicians should also be able to recognize normal variants that may mimic cardiac disease and electrocardiographic manifestations of noncardiac illness. Guidelines for the use of electrocardiograms in patients with and without preexisting heart disease have been published by the American College of Cardiology and American Heart Association (ACC/AHA) and have changed little in recent years. An understanding of the clinical context and availability of prior electrocardiograms for comparison improves diagnostic accuracy when interpreting electrocardiograms.

The electrocardiogram is a graphical recording of the difference in potential between electrodes placed on the body surface. Twelve conventional leads that generate such a recording include the six extremity (limb) and six chest (precordial) leads. The chest leads (V1–V6) record electrical activity in a horizontal plane, and the limb leads (bipolar leads I, II, and III; and unipolar leads aVR, aVL, and aVF) record potentials transmitted on the frontal plane. Right-sided precordial leads (V1R–V6R) and electrode locations posterior to V6 (V7–V9) may be useful in the assessment of right ventricular and posterior-lateral infarctions, respectively. If a wave of depolarization spreads toward the positive pole of a lead, a positive deflection is recorded in that lead. Conversely, if a wave of depolarization spreads toward the negative pole of a lead, a negative deflection is recorded in that lead. The components of the normal resting electrocardiogram are the P wave, generated by atrial contraction; the PR interval, representing conduction through the AV node; the QRS complex, generated by biventricular contraction; and the ST-T wave, reflecting biventricular recovery.

The initial diagnostic evaluation of the patient with acute chest discomfort centers on the recognition of life-threatening conditions, including acute coronary syndrome and pulmonary embolism.

Acute Coronary Syndrome

Electrocardiography is an indispensable tool in the diagnosis and management of acute coronary syndromes (ACS). Deviation of the ST-segment is generally the earliest electrocardiographic manifestation of myocardial injury. Elevation of the ST-segment ...