What is the significance of a prolonged QT (quantum time) interval on an electrocardiogram (EKG)?

Prolonged QT Interval on EKG: Clinical Significance

A prolonged QT interval on EKG represents a critical marker of increased risk for torsades de pointes, a potentially fatal polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death. 1

Definition and Measurement

The QT interval measures the duration from the onset of ventricular depolarization (beginning of QRS complex) to the end of ventricular repolarization (end of T wave). 1

Normal Values and Thresholds

• Normal corrected QT (QTc) values are ≤450 ms for men and ≤460 ms for women 2, 3
• QTc >500 ms represents markedly elevated risk for torsades de pointes regardless of sex 2, 3
• An increase of ≥60 ms from baseline is significant even if absolute QTc remains <500 ms 2, 3
• No threshold exists below which QT prolongation is completely free of proarrhythmic risk 1

Measurement Technique

• Measure in the lead showing the longest QT interval, typically V2 or V3 1
• Use the tangent method: draw a tangent to the steepest downslope of the T wave to define its intersection with baseline 2
• If measurements differ by >40 ms between leads, the measurement may be in error and adjacent leads should be considered 1
• Fridericia's formula (QT/RR^1/3) is preferred over Bazett's formula for heart rate correction, especially at higher heart rates 1, 2

Life-Threatening Arrhythmia Risk

Torsades de Pointes

Torsades de pointes is a hemodynamically unstable polymorphic ventricular tachycardia that is typically triggered by a ventricular premature beat arising from a pause-dependent increase in U wave amplitude. 1

Warning signs that place patients at immediate risk include: 1

• T wave alternans
• Polymorphic ventricular premature beats falling near the T-U portion of repolarization
• Pause-dependent enhancement of QT interval
• Nonsustained polymorphic ventricular tachycardia
• Sudden bradycardia or long pauses (e.g., compensatory pauses after ventricular ectopy)
• Enhanced U waves

Risk Factors for Torsades de Pointes

Multiple risk factors significantly increase the likelihood of developing life-threatening arrhythmias: 1

Patient Demographics

• Female sex 1, 4
• Older age 1, 4

Cardiac Conditions

• Heart disease, especially left ventricular hypertrophy 1
• Myocardial ischemia 1
• Low left ventricular ejection fraction 1
• Heart failure 1, 4

Electrolyte Abnormalities

• Hypokalemia (most critical) 1, 2, 4
• Hypomagnesemia 1, 2, 4
• Hypocalcemia 2

Cardiac Rhythm Factors

• Bradycardia 1, 4
• Slow heart rate 1

Genetic and Metabolic Factors

• Genetic predisposition to QT prolongation 1
• Baseline QT prolongation 1
• Family history of syncope, sudden death, or long QT syndrome 1
• Acquired or genetic metabolic impairment 1

Drug-Related Factors

• Concomitant use of multiple QT-prolonging drugs 1, 2, 4
• Drug-drug interactions that impair metabolism 1, 4

Drug-Induced QT Prolongation

High-Risk Antiarrhythmic Agents

The antiarrhythmic agents most likely to cause proarrhythmia include: 1

• Quinidine
• Procainamide
• Disopyramide
• Sotalol
• Dofetilide
• Ibutilide

Amiodarone often causes marked QT interval prolongation but has a low frequency of torsades de pointes. 1

Other High-Risk Medications

• Haloperidol: Cases of sudden death, QT-prolongation, and torsades de pointes have been reported, with higher than recommended doses associated with higher risk 5
• Erythromycin: Rare cases of serious cardiovascular adverse events including QT/QTc interval prolongation, cardiac arrest, torsades de pointes, and other ventricular arrhythmias have been observed 6
• Antipsychotics 5
• Certain antibiotics 6

Clinical Management Algorithm

For QTc 481-500 ms: 2

1. Implement more frequent ECG monitoring
2. Correct electrolyte abnormalities aggressively (potassium, magnesium, calcium)
3. Consider dose reduction of QT-prolonging medications
4. Avoid concomitant use of multiple QT-prolonging drugs
5. Review all medications using resources like CredibleMeds (www.crediblemeds.org)

For QTc >500 ms or increase >60 ms from baseline: 2, 4

1. Temporarily discontinue causative medications immediately
2. Correct electrolyte abnormalities urgently
3. Continue ECG monitoring until QTc normalizes
4. Implement continuous telemetry monitoring or repeat 12-lead ECG every 2-4 hours

Monitoring Requirements

When monitoring patients for drug-induced prolonged QT: 1

• Document QTc in the medical record using a rhythm strip before drug initiation
• Repeat at least every 8 hours thereafter
• Document QTc before and after increases in drug dosage
• Make QT measurements in the same lead over time

For patients on QT-prolonging medications: 2

• Obtain baseline ECG and electrolytes before starting treatment
• Repeat ECG 7 days after initiation and after any dosing changes
• Monitor QTc periodically throughout treatment

Management of Acute Torsades de Pointes

Immediate interventions: 2

1. Administer 2g IV magnesium sulfate regardless of serum magnesium level
2. Consider non-synchronized defibrillation if hemodynamically unstable
3. For bradycardia-induced torsades, consider temporary overdrive pacing or IV isoproterenol titrated to heart rate >90 bpm when temporary pacing is not immediately available

Congenital Long QT Syndrome

Patients with long-QT syndrome and associated ventricular arrhythmias require strict monitoring. 1

Torsades de pointes in these patients is typically triggered by a ventricular premature beat arising from a pause-dependent increase in U wave amplitude, with polymorphic ventricular premature beats or ventricular bigeminy often preceding longer runs by minutes or hours. 1

Beta blockers should be administered if QTc >470 ms in patients with congenital long QT syndrome. 2

Special Clinical Contexts

Patients Requiring Conscious Sedation or Anesthesia

ECG monitoring is indicated for all procedures requiring conscious sedation and should continue until patients are awake, alert, and hemodynamically stable 1

Patients with Acute Heart Failure

Continuous monitoring is recommended until signs and symptoms of acute heart failure have resolved and cardiac monitoring reveals no hemodynamically significant arrhythmias for at least 24 hours. 1

Elderly Patients

Elderly patients, particularly those with reduced renal or hepatic function, may be at increased risk for developing drug-induced hearing loss and are more susceptible to development of torsades de pointes arrhythmias than younger patients. 6

Critical Pitfalls to Avoid

• Do not rely solely on automated QT measurements—always visually validate QT-interval prolongation reported by computer algorithms 1
• Cardiac monitors do not have algorithms to measure QT intervals and sound alarms for QT prolongation, requiring manual measurement by healthcare professionals 1
• Do not attempt rate correction when RR interval variability is large (as in atrial fibrillation) or when the end of the T wave is unreliable 1
• When U waves are present and cannot be separated from T waves, measure QT in leads not showing U waves (often aVR and aVL) 1
• Patients with left bundle branch block or ventricular pacing may have artificially prolonged QT intervals that complicate assessment 2