How do you diagnose and manage arrhythmias using an electrocardiogram (ECG)?

Diagnosing and Managing Arrhythmias Using ECG

Systematic ECG Analysis for Arrhythmia Detection

A systematic approach to ECG interpretation is essential for accurate arrhythmia diagnosis, requiring sequential analysis of rate, rhythm, P-wave morphology, PR interval, QRS duration, and QT interval. 1

Step 1: Calculate Heart Rate and Assess Rhythm Regularity

• Calculate heart rate by counting large squares between consecutive R waves (300÷number of large squares) or count QRS complexes in a 6-second strip and multiply by 10. 1
• Evaluate rhythm regularity by examining R-R interval consistency—irregular R-R intervals suggest atrial fibrillation, multifocal atrial tachycardia, or frequent ectopy. 1
• Normal heart rate is 60-100 bpm; bradycardia <60 bpm and tachycardia >100 bpm require further characterization. 1

Step 2: Identify P Waves and Their Relationship to QRS

• Examine P wave morphology: normal P waves are upright in leads I, II, aVF and biphasic in V1, with duration <120 ms and amplitude <2.5 mm. 1
• Determine if every P wave is followed by a QRS complex and if every QRS is preceded by a P wave—this distinguishes sinus rhythm from other arrhythmias. 2
• Absent P waves suggest atrial fibrillation or junctional rhythm; variable P wave morphology indicates multifocal atrial tachycardia or wandering atrial pacemaker. 2

Step 3: Measure Critical Intervals

• Measure PR interval (normal 120-200 ms or 3-5 small squares) to assess AV conduction—prolonged PR indicates first-degree AV block. 1
• Measure QRS duration (normal <120 ms or <3 small squares)—wide QRS (≥120 ms) suggests ventricular origin, bundle branch block, or aberrant conduction. 1
• Calculate corrected QT interval (QTc): normal <450 ms for men, <460 ms for women—prolonged QTc increases risk for torsades de pointes and sudden death. 1, 3

Step 4: Classify the Arrhythmia Mechanism

If QRS is narrow (<120 ms), the arrhythmia is almost always supraventricular; wide QRS requires differentiation between ventricular tachycardia and supraventricular tachycardia with aberrancy. 2

For Regular Narrow-Complex Tachycardia:

• No visible P waves with regular RR intervals: AVNRT is most likely—look for pseudo-R waves in V1 or pseudo-S waves in inferior leads. 2
• P wave in ST segment separated from QRS by >70 ms: AVRT is most likely. 2
• RP interval longer than PR interval: consider atypical AVNRT, permanent junctional reciprocating tachycardia, or atrial tachycardia. 2

For Irregular Rhythms:

• Irregularly irregular rhythm with absent P waves: atrial fibrillation. 2
• Irregular rhythm with variable P wave morphology (≥3 different forms): multifocal atrial tachycardia. 2

ECG Monitoring Strategies for Arrhythmia Diagnosis

When Initial ECG is Non-Diagnostic

Serial ECGs should be performed at 15-30 minute intervals when clinical suspicion for arrhythmia remains high but the initial ECG is normal or non-diagnostic. 2, 4

• Compare with previous ECGs when available—new changes are more significant than isolated findings. 2
• A normal ECG does not exclude arrhythmia: up to 6% of patients with evolving acute coronary syndrome and arrhythmias are discharged with normal ECGs. 2

Extended Monitoring Indications

Holter monitoring (24-48 hours) is indicated for patients with structural heart disease and frequent symptoms or high pre-test probability of arrhythmia. 2

• External loop recorders are appropriate when symptoms occur weekly; implantable loop recorders are indicated for recurrent syncope with injury or when inter-symptom interval exceeds 4 weeks. 2
• Ambulatory ECG monitoring is recommended at initial assessment in hypertrophic cardiomyopathy to detect non-sustained ventricular tachycardia (present in 25% of adults) and paroxysmal supraventricular arrhythmias (up to 38%). 2

Management Based on ECG Findings

Immediate Actions for Life-Threatening Arrhythmias

Patients with new ST-elevation, ST depression, or new left bundle branch block should be treated according to STEMI and NSTE-ACS guidelines immediately. 2

• Obtain supplemental leads V7-V9 when posterior MI is suspected (often electrically silent on standard 12-lead ECG). 2
• Ventricular pauses >3 seconds while awake, Mobitz II or third-degree AV block, or rapid paroxysmal ventricular tachycardia are diagnostic and require immediate intervention. 2

High-Risk ECG Features Requiring Specialist Referral

All patients with wide-complex tachycardia of unknown origin require immediate referral to an arrhythmia specialist. 2

• Patients with Wolff-Parkinson-White syndrome (pre-excitation with arrhythmias) should be referred for electrophysiology study due to risk of sudden death. 2
• Conduction abnormalities (AV block, bundle branch blocks) that may progress to complete heart block require urgent evaluation. 3
• QT prolongation suggesting torsades de pointes risk mandates immediate specialist consultation. 3

Diagnostic Testing Beyond Standard ECG

Invasive electrophysiological study is indicated when initial evaluation suggests arrhythmic cause (abnormal ECG, structural heart disease, syncope with palpitations, or family history of sudden death). 2

• Exercise testing is reasonable for exercise-related arrhythmias or suspected chronotropic incompetence. 3
• Echocardiography should be performed when structural heart disease is suspected based on ECG findings (left ventricular hypertrophy, pathologic Q waves). 2, 3

Critical Pitfalls to Avoid

Computer-generated ECG interpretations must always be verified by a qualified physician—automated systems have different specifications that result in significant measurement differences. 2, 1

• Do not base management solely on a single normal ECG—left ventricular hypertrophy, bundle branch blocks, and ventricular pacing may mask ischemia or arrhythmias. 2
• Asymptomatic arrhythmias detected on Holter monitoring without symptom correlation have unclear clinical significance and should not guide therapy alone. 2
• Age and gender affect normal parameters: QT intervals are longer in women, and arrhythmia frequency increases with age. 1
• Certain medications (antiarrhythmics, psychotropics) significantly affect ECG findings and must be considered in interpretation. 1