Interpretation of Abnormal ECGs

P waves

Abnormal

Left or right atrial enlargement, premature/ectopic atrial complexes, ectopic atrial rhythm, multifocal atrial tachycardia, hyperkalemia

P waves

Absent

Atrial fibrillation, sinus node arrest or exit block, hyperkalemia (severe)

P-P interval

Varying

Sinus arrhythmia

PR interval

Long

First-degree atrioventricular block

PR interval

Varying

Mobitz (second-degree) type I (Wenckebach) atrioventricular block, multifocal atrial tachycardia

P-QRS relationship

Absent

Third-degree atrioventricular block (atrioventricular dissociation)

QRS complex

Wide

Premature ventricular complexes, right or left bundle branch block or other aberrant conduction ventricular tachycardia, ventricular flutter, ventricular fibrillation, hyperkalemia

QT interval

Long

Myocardial infarction, myocarditis, stress cardiomyopathy, hypocalcemia, hypokalemia, hypomagnesemia, hypothyroidism, subarachnoid hemorrhage, intracerebral hemorrhage, stroke, congenital long QT syndrome, antiarrhythmics (eg, sotalol, amiodarone, quinidine), tricyclic antidepressants, phenothiazines, other medications, antiarrhythmics (eg, sotalol, amiodarone, quinidine), tricyclic antidepressants, phenothiazines, other medications

QT interval

Short

Hypercalcemia, hypermagnesemia, Graves disease, digoxin, digoxin

ST segment

Depression

Myocardial ischemia; acute posterior myocardial infarction; digoxin; ventricular hypertrophy; Myocardial ischemia; acute posterior myocardial infarction; digoxin; ventricular hypertrophy;pulmonary embolism; left bundle branch block in leads V5–V6 and possibly in I and aVL; right bundle branch block in leads V1–V3 and possibly in II, III, and aVF; hyperventilation; hypokalemia

ST segment

Elevation

Myocardial ischemia, acute myocardial infarction, left bundle branch block in leads V1–V3 and possibly in II, III, and aVF, acute pericarditis, left ventricular hypertrophy, hyperkalemia, pulmonary embolism, digoxin, normal variation (eg, , left ventricular hypertrophy, hyperkalemia, pulmonary embolism, digoxin, normal variation (eg,athlete's heart), hypothermia

T wave

Tall

Hyperkalemia, acute myocardial infarction, left bundle branch block, stroke, ventricular hypertrophy

T wave

Small, flattened, or inverted

Myocardial ischemia, myocarditis, age, race, hyperventilation, anxiety, drinking hot or cold beverages, left ventricular hypertrophy, certain medications (eg, digoxin), pericarditis, pulmonary embolism, conduction disturbances (eg, right bundle branch block), electrolyte disturbances (eg, hypokalemia), stress cardiomyopathyMyocardial ischemia, myocarditis, age, race, hyperventilation, anxiety, drinking hot or cold beverages, left ventricular hypertrophy, certain medications (eg, digoxin), pericarditis, pulmonary embolism, conduction disturbances (eg, right bundle branch block), electrolyte disturbances (eg, hypokalemia), stress cardiomyopathy

U wave

Prominent

Hypokalemia, hypomagnesemia, ischemia