What does a wave on an electrocardiogram (EKG) indicate?

What Does a Wave on an EKG Indicate?

A wave on an EKG represents specific phases of cardiac electrical activity: the P wave indicates atrial depolarization, the QRS complex shows ventricular depolarization, the T wave reflects ventricular repolarization, and the U wave (when present) represents delayed repolarization phenomena. 1

Core ECG Wave Components and Their Meaning

P Wave

• Represents atrial depolarization - the electrical activation that causes the atria to contract 2
• Normal P waves indicate proper sinus node function and atrial conduction 2

QRS Complex

• Represents ventricular depolarization - the electrical activation that causes the ventricles to contract 1, 2
• Normal duration is <120 ms or <3 small squares 2
• Widened QRS complexes indicate abnormal ventricular conduction such as bundle branch blocks 1, 3

T Wave

• Represents ventricular repolarization - the recovery phase after ventricular contraction 1
• In adults ≥20 years old, normal T waves are upright in leads I, II, and V3-V6; inverted in aVR; and variable in aVL, III, and V1 1
• T wave abnormalities can indicate primary repolarization changes (actual changes in myocyte repolarization from ischemia, electrolyte abnormalities, drugs) or secondary changes (due to altered depolarization sequence like bundle branch blocks) 1

U Wave

• Represents a mechanoelectric phenomenon occurring after the T wave - thought to be delayed Purkinje system repolarization 1, 4
• Most evident in leads V2-V3 with amplitude approximately 0.33 mV or 11% of T wave height 1
• Heart rate dependent: rarely present at rates >95 bpm, present in 90% of cases at rates <65 bpm 1

Critical Clinical Patterns to Recognize

Tall/Peaked T Waves

• Peaked T waves are the earliest ECG finding in hyperkalemia, typically appearing at potassium levels >5.5 mmol/L and indicating severe cardiotoxicity requiring immediate treatment 5
• Most prominent in precordial leads V2-V3 5
• Giant T waves can also represent hyperacute myocardial infarction before ST elevation develops, requiring emergent cardiac catheterization 5, 6

Inverted T Waves

• Can indicate myocardial ischemia, infarction, or other primary repolarization abnormalities 1
• Must distinguish from secondary T wave inversions due to bundle branch blocks or ventricular hypertrophy 1, 3

Abnormal U Waves

• Inverted U waves in leads V2-V5 are always abnormal and may indicate acute ischemia or hypertension 1
• Prominent U waves can result from hypokalemia, quinidine-like drugs, or fusion with T waves in long QT syndromes 1
• Report U waves when inverted, merged with T wave, or amplitude exceeds T wave 1

Primary vs. Secondary Repolarization Abnormalities

Primary Abnormalities

• Result from actual changes in myocyte action potential without changes in depolarization sequence 1
• Caused by ischemia, myocarditis, drugs, electrolyte abnormalities (especially calcium and potassium), hyperventilation, position changes, or catecholamines 1
• Clinically more significant as they indicate true changes in ventricular myocyte repolarization characteristics 1

Secondary Abnormalities

• Result from altered depolarization sequence (changed QRS) affecting repolarization pattern 1
• Seen with bundle branch blocks, ventricular pacing, preexcitation, and ectopic beats 1, 3
• ST-T changes are directed opposite to the abnormal QRS vector 1
• Do not require changes in individual cell action potentials 1

Critical Pitfalls to Avoid

• Never rely solely on computer interpretation - ECGs must be reviewed by a qualified physician as automated readings frequently contain errors 2
• Always interpret ECG in clinical context - the same findings have different significance depending on symptoms and patient presentation 2, 6
• Check for electrode misplacement, especially precordial leads, which can dramatically alter interpretation and cause false diagnoses 2
• Absence of ECG changes does not rule out dangerous conditions - some patients with chronic hyperkalemia develop tolerance and may not show typical ECG findings 5
• Compare to previous ECGs when available - serial changes are often more informative than isolated findings 6, 3
• Recognize that left bundle branch block, ventricular pacing, and left ventricular hypertrophy reduce the ECG's ability to detect acute ischemic changes 3

Immediate Action for Concerning Findings

When peaked T waves are present:

• Check potassium level immediately while preparing for emergency treatment - do not wait for results if ECG shows severe changes like absent P waves, prolonged PR, widened QRS, or sine wave pattern 5
• Assess for additional hyperkalemia ECG changes indicating progressive cardiotoxicity 5
• Consider hyperacute MI if clinical presentation suggests acute coronary syndrome 5, 6