Key Clinical Questions
How does the resting electrocardiogram (ECG) assist in the diagnosis and management of acute coronary syndrome (ACS)?
What is the differential for ST elevations, and what are the key differentiating electrocardiographic features of ACS versus acute pericarditis versus early repolarization?
How does the ECG assist in the diagnosis of acute pulmonary embolism?
How does the ECG assist in determining the cause of syncope?
What are the characteristic ECG findings of electrolyte disturbances?
What are the characteristic ECG findings of hypothyroidism, stroke, and drug effects?
A graphic recording of electrical potentials generated by the heart, the electrocardiogram (ECG) 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, structural changes of the myocardium, arrhythmias, 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 pre-existing heart disease have been published by the American College of Cardiology and American Heart Association (ACC/AHA).
THE NORMAL RESTING ELECTROCARDIOGRAM
The electrocardiogram is a graphical recording of the difference in potential between electrodes placed on the body surface. The standard twelve lead ECG used commonly in clinical practice includes 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. These standard twelve leads are categorized by their anatomic location into the following groups: inferior (II, III, aVF), septal (V1-V2), anterior (V3-V4), and lateral (V5-V6, I, aVL). In addition to the standard 12 leads, right-sided precordial leads (V1R-V6R) and the posterior leads (V7-V9) may be useful in the assessment of right ventricular and posterior 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 ...