How is a short QT interval (QTc) defined on a standard 12‑lead ECG?

Definition of Short QT Interval

A short QT interval is defined as a corrected QT (QTc) of 390 ms or shorter for both men and women 1, 2.

Guideline-Based Definition

The AHA/ACCF/HRS consensus guidelines provide the authoritative threshold for clinical practice. This definition applies to adults and uses rate-corrected QT intervals, with the recommendation that adjustments for both heart rate and gender be incorporated into the measurement 1, 2.

Key Measurement Considerations:

• Rate correction is essential: The QT interval must be corrected for heart rate using an appropriate formula
• Gender-specific thresholds apply only to prolonged QT: For short QT, the same cutoff (≤390 ms) applies to both men and women 1, 2
• Age considerations: These adult thresholds may not apply to children, where different percentile distributions exist 1

Important Clinical Caveats:

Formula Selection Matters

Bazett's formula commonly overestimates QTc and can produce false results, particularly at extreme heart rates 3, 4. The guidelines acknowledge that linear regression functions or power functions of RR interval provide more accurate rate adjustment 1, 2. When using automated ECG machine measurements with Bazett's correction, manual verification is strongly recommended.

The "Short QT Syndrome" Distinction

While the guideline defines short QT as ≤390 ms, research on "Short QT Syndrome" (the inherited channelopathy associated with sudden death) has used more extreme cutoffs:

• Some studies define the syndrome using QTc <300 ms 5, 6
• Others use QTc <340-350 ms 7, 8
• The syndrome itself is extremely rare: One large study of >100,000 patients found zero cases with QTc <300 ms 5

Clinical Implications of the 390 ms Threshold:

Research suggests that individuals with short QT intervals (using the guideline definition) have significantly increased risk of atrial arrhythmias, including atrial flutter (21%) and atrial fibrillation (18%) 7. This makes the 390 ms threshold clinically relevant beyond just identifying the rare inherited syndrome.

Practical Measurement Algorithm:

1. Measure the QT interval manually from lead II or V5 (whichever shows the clearest T-wave end)
2. Correct for heart rate using Fridericia's formula or a linear regression method (avoid Bazett's if possible)
3. Apply the threshold: QTc ≤390 ms = short QT for both genders
4. Adjust for QRS duration if needed: In bundle branch blocks, use JT interval instead 1
5. Consider clinical context: Exclude technical errors, verify with multiple leads, and assess for underlying conditions

Common Pitfalls to Avoid:

• Don't rely solely on automated machine measurements: Up to 100% of machine-identified short QT intervals <300 ms were found to be errors due to pacemaker artifacts, incorrect T-wave detection, or cycle length miscalculation 5
• Don't use Bazett's formula at extreme heart rates: It systematically overcorrects and produces unreliable values 3
• Don't ignore the clinical picture: A short QT measurement should prompt evaluation for arrhythmia history, family history of sudden death, and consideration of atrial arrhythmia risk 7