What does QTc prolongation look like on an EKG?

QTc Prolongation on EKG: Characteristics and Recognition

QTc prolongation on an EKG is characterized by a prolonged QT interval exceeding 450 ms in men or 460 ms in women, with values above 500 ms considered particularly concerning for risk of torsades de pointes. 1

Identifying QTc Prolongation on EKG

Normal vs. Prolonged QTc Values

• Normal QTc values:
  • Men: ≤450 ms
  • Women: ≤460 ms 1
• Concerning QTc values:
  • Borderline/Gray zone: 440-500 ms (men) / 460-500 ms (women) 1
  • High risk: >500 ms (both sexes) 1

Visual Characteristics of QTc Prolongation

QTc prolongation appears on an EKG as:

• Extended distance from the beginning of the QRS complex to the end of the T wave
• The T wave may appear distorted or have an abnormal morphology
• U waves may become more prominent and sometimes merge with T waves 1

Warning Signs of Impending Torsades de Pointes

Several ECG features indicate high risk for progression to torsades de pointes:

• T wave alternans (beat-to-beat changes in T wave amplitude or morphology)
• Enhanced U waves
• Ventricular ectopy falling near the peak of the T-U complex
• Pause-dependent enhancement of the QT interval (QT prolongation after pauses)
• Short-long-short R-R sequences preceding polymorphic ventricular tachycardia 1

![Characteristic ECG pattern showing QT prolongation with distorted T-U complex, followed by a short-long-short cycle sequence triggering torsades de pointes] 1

Measurement Technique

To accurately measure the QTc interval:

1. Select a lead with a well-defined T wave end (typically lead II or V3-V5)
2. Measure from the beginning of the QRS complex to the end of the T wave
3. Use the same lead for serial measurements
4. Apply heart rate correction using Bazett's formula: QTc = QT/√RR (where RR is in seconds) 1

Risk Factors for QTc Prolongation

Understanding risk factors helps identify patients at higher risk:

Patient-Related Factors

• Female sex
• Advanced age
• Genetic predisposition (congenital LQTS)
• Heart disease (LV hypertrophy, heart failure, myocardial ischemia)
• Bradycardia or heart block 1

Modifiable Factors

• Electrolyte abnormalities:
  • Hypokalemia
  • Hypomagnesemia
  • Hypocalcemia 1, 2
• Medications:
  • Antiarrhythmics (quinidine, sotalol, amiodarone)
  • Certain antibiotics (macrolides, fluoroquinolones)
  • Antipsychotics
  • Multiple QT-prolonging drugs concurrently 1, 3, 4

Clinical Significance and Management

When QTc prolongation is identified:

• If QTc >500 ms or increase >60 ms from baseline: consider discontinuing offending medications 1
• Correct electrolyte abnormalities
• Monitor ECG regularly in high-risk patients
• For patients with signs of impending torsades de pointes, administer IV magnesium sulfate and consider temporary pacing if bradycardia is present 1

Common Pitfalls in QTc Assessment

• Measurement errors: Incorrect identification of T wave end, especially when U waves are present
• Lead selection: Using inconsistent leads for serial measurements
• Heart rate effects: Not accounting for heart rate (using uncorrected QT)
• U wave inclusion: Mistakenly including U waves in QT measurement
• Drug interactions: Failing to recognize combined effects of multiple QT-prolonging medications 1, 5

By understanding these characteristics and monitoring parameters, clinicians can better identify QTc prolongation and take appropriate steps to prevent the potentially fatal arrhythmia of torsades de pointes.