P waves

Abnormal

Left or right atrial hypertrophy, atrial escape (ectopic) beats

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 type I atrioventricular block, multifocal atrial tachycardia

QRS complex

Wide

Right or left bundle branch block, 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

QT interval

Short

Hypercalcemia, hypermagnesemia, Graves disease

ST segment

Depression

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 pericarditisathlete's heart), hypothermia

T wave

Tall

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

T wave

Small, flattened, or inverted

U wave

Prominent

Hypokalemia, hypomagnesemia, ischemia