What is the formula to calculate JTc (corrected JT interval)?

How to Calculate JTc (Corrected JT Interval)

The Formula

The JTc is calculated by first measuring the JT interval (from the J-point at the end of the QRS complex to the end of the T wave), then correcting it for heart rate using the same correction formulas applied to QT intervals, most commonly Bazett's formula: JTc = JT/√RR (where RR is measured in seconds). 1, 2

Understanding the JT Interval

• The JT interval represents ventricular repolarization time independent of depolarization, measured from the J-point (junction between QRS complex and ST segment) to the end of the T wave 3
• This measurement is particularly useful when QRS prolongation is present (such as in bundle branch blocks), as it excludes the depolarization component that artificially lengthens the QT interval 2, 4

Step-by-Step Measurement Approach

1. Identify the J-Point and T-Wave End

• Locate the J-point where the QRS complex ends and the ST segment begins 3
• Identify the end of the T wave using the "Teach-the-Tangent" or "Avoid-the-Tail" method: draw a tangent to the steepest downslope of the T wave and mark where it intersects the baseline 3, 1, 2
• Measure in leads II, V5, or V6, which typically provide the best T-wave delineation 3, 2

2. Measure the Raw JT Interval

• Count the number of small squares (1 mm = 0.04 seconds or 40 milliseconds) from the J-point to the end of the T wave 4
• Convert to milliseconds by multiplying the number of small squares by 40 4

3. Measure the RR Interval

• Measure the RR interval (distance between consecutive R waves) in the same units 3
• If sinus arrhythmia is present with beat-to-beat variation, use an average of multiple RR intervals 3

4. Apply the Correction Formula

• Use Bazett's formula: JTc = JT/√RR (with RR in seconds) 3, 2
• For example: if JT = 320 ms (0.32 seconds) and RR = 0.81 seconds, then JTc = 0.32/√0.81 = 0.32/0.90 = 356 ms 3

Important Limitations and Caveats

Heart Rate Considerations

• Bazett's formula overcorrects at heart rates >90 beats/min and undercorrects at heart rates <50 beats/min 3, 1
• For heart rates <50 beats/min, have the patient perform mild aerobic activity to achieve a heart rate closer to 60 beats/min before measuring 3, 1
• For heart rates >90 beats/min, allow additional resting time to achieve a lower heart rate 3, 1

Alternative Correction Formulas

• While Bazett's remains the clinical standard, linear regression functions or Fridericia's cube-root formula (JTc = JT/∛RR) may provide more accurate correction, particularly at extreme heart rates 1, 5, 6
• The American College of Cardiology suggests that linear regression functions are preferable to Bazett's formula for more accurate rate correction 1

Common Pitfalls to Avoid

• Do not include discrete U waves that arise after the T wave returns to baseline 2, 4
• Always use the same lead for serial measurements in the same patient, as JT length varies across the 12 leads 2, 4
• Do not rely solely on automated computer measurements without visual validation 1, 2
• The JT interval shows significant relationship to heart rate, making correction essential for accurate interpretation 7

Clinical Context

• The JT interval is particularly valuable when QRS prolongation exists (bundle branch blocks, ventricular pacing, pre-excitation), as it isolates the repolarization component 2, 7
• In left bundle branch block specifically, specialized formulas have been developed that may provide more accurate assessment than standard correction methods 7