Corrected QT Interval (QTc)

• Toggle unit to use msec, small boxes or large boxes
  • 1 small box (1 mm) = 0.04 seconds (40 msec) at 25 mm/sec
  • 1 large box (5 mm) = 0.2 seconds (200 msec) at 25 mm/sec

• Toggle unit to use msec, small boxes or large boxes
  • 1 small box (1 mm) = 0.02 seconds (20 msec) at 50 mm/sec
  • 1 large box (5 mm) = 0.1 seconds (100 msec) at 50 mm/sec

#EM3 East Midlands Emergency Medicine Educational Media summarizes actionable resources.

The Life in the Fastlane website also provides a concise QTc overview with examples See article called QT Interval (Burns & Buttner, 2022) for more details.

 Here are a few key points:

• QTc is prolonged if ≥ 440ms in men or ≥ 460ms in women
• QTc > 500 is associated with an increased risk of torsades de pointes (TdP)
• Viskin (2009) proposes the use of a QT interval scale to aid diagnosis of patients with short and long QT syndromes (once reversible causes have been excluded).
• Drug-induced QT-prolongation 
  • In the context of acute poisoning with QT-prolonging agents, the risk of TdP is better described by the absolute rather than corrected QT.
  • More precisely, the risk of TdP is determined by considering both the absolute QT interval and the simultaneous heart rate (i.e. on the same ECG tracing).
  • These values are then plotted on the QT nomogram (developed by Chan, 2007) to determine whether the patient is at risk of TdP.
  • The QT nomogram is a clinically relevant risk assessment tool that predicts arrhythmogenic risk for drug-induced QT prolongation can be used for risk stratification
  • A QT interval-heart rate pair that plots above the line indicates the patient is at risk of TdP.

Consider potential causes of prolonged QT:

• Electrolyte abnormalities:
  • Hypokalemia
  • Hypocalcemia
  • Hypomagnesemia
• Medications:
  • Psychotropic drugs
  • Anti-arrhythmics
  • Other drugs
• Central causes:
  • Hypothyroid
  • Hypothermia
  • Raised intracranial pressure
  • Autonomic dysfunction
• Intrinsic cardiac causes:
  • Myocardial ischemia
  • Severe bradycardia, high-grade AV block
  • Congenital long QT syndrome
  • ROSC post-cardiac arrest
  • Coronary artery disease (CAD)
  • Cardiomyopathy

Viskin (2009) proposes the use of a ‘QT interval scale’ to aid diagnosis of patients with short and long QT syndromes (once reversible causes have been excluded).

The Life in the Fastlane website provides a concise overview with examples in an article called QT Interval (Burns & Buttner, 2022). Here are a few key points:

Potential causes of a short QTc

• Hypercalcaemia
• Congenital short QT syndrome
• Digoxin effect

Short QT syndrome may be suggested by the presence of:

• Lone atrial fibrillation in young adults
• Family member with a short QT interval
• Family history of sudden cardiac death
• ECG showing QTc < 350 ms with tall, peaked T waves
• Failure of the QT interval to increase as the heart rate slows