How do you measure the QT interval on an electrocardiogram (EKG) in a patient, particularly those with a history of heart disease or taking medications known to prolong the QT interval?

How to Measure the QT Interval on EKG

Measurement Technique

Measure the QT interval from the beginning of the QRS complex to the end of the T wave, selecting the lead with the longest, most well-defined T wave (typically V2, V3, or lead II), and use the tangent method to identify the T wave endpoint by drawing a line from the T wave peak along its steepest downslope to the baseline. 1

Starting Point

• Identify the onset of the QRS complex, which is usually readily apparent 1
• The QRS onset typically occurs up to 20 ms earlier in V2 and V3 compared to limb leads 1

Ending Point (Most Critical Step)

• Draw a tangent line from the peak of the T wave following the steepest downslope of the T wave 1
• The intersection of this tangent line with the isoelectric baseline marks the end of the T wave 1
• This tangent method is teachable and produces more accurate results than other approaches 2

Lead Selection

Choose the lead with a T-wave amplitude of at least 2 mm and the most well-defined T wave end, typically V2 or V3, and consistently use the same lead for serial measurements in a given patient. 1

• The longest QT interval is usually found in mid-precordial leads (V2 or V3) 1
• Lead II is commonly used in research and often shows a prominent positive T wave when the T wave axis is normal 1
• Lead V3 correlates most closely with the mean QT interval across all 12 leads 3
• Document which lead you are using for future comparisons 1

Special Situations

T Waves with U Waves

• If discrete U waves occur after the T wave returns to baseline, do NOT include them in the QT measurement 1
• When T and U waves are superimposed or inseparable, measure QT in leads without U waves (often aVR and aVL) 1
• Alternatively, use the tangent method to the T wave downslope, recognizing this may underestimate the true QT 1

Notched or Biphasic T Waves

• If the T wave is notched, consider the end of the entire T wave complex as the endpoint 1
• For biphasic T waves, use the tangent method from the final deflection 1

Bundle Branch Block or Wide QRS

• Subtract the difference in QRS width before and after the block from the QT measurement 1
• Alternatively, measure the JT interval (from end of QRS to end of T wave) to eliminate the confounding widened QRS 1
• Apply the chosen adjustment method consistently over time 1

Atrial Fibrillation

• Identify the shortest and longest R-R intervals, calculate QTc for each, and average the two values 1
• Alternatively, assess whether the interval from R wave to T wave peak averages more than 50% of the R-R interval, indicating QTc likely exceeds 500 ms 1
• Do not attempt rate correction when RR interval variability is large 1

Heart Rate Correction

Use linear regression formulas (such as Fridericia's formula: QTc = QT/∛RR) rather than Bazett's formula, especially at heart rates above 85 bpm, and document which correction method you use. 1, 4

Correction Formulas

• Bazett's formula (QTc = QT/√RR) overcorrects at fast heart rates and undercorrects at slow rates 1
• Fridericia's formula (QTc = QT/∛RR) is preferred and removes rate dependence more effectively 1, 4
• Despite limitations, Bazett's formula remains acceptable in clinical practice if used consistently 1

When to Correct

• Correct for heart rate when comparing QT values over time or assessing for prolongation 1
• The uncorrected QT interval lengthens with slow heart rates and shortens with fast rates 1

Measurement Tools

Manual Measurement

• Increase paper speed from 25 to 50 mm/s to enhance visualization 1
• Measure in the same lead consistently over time 1
• Average measurements from 5 beats for greater accuracy 5

Electronic Calipers

• Increase waveform size from standardization of 1 to 2,3, or 4 for better visualization 1
• Many monitor systems include built-in QTc calculators 1
• Electronic calipers provide computer-assisted measurement but still require manual endpoint identification 1

Automated Measurements

• Always visually validate computer-generated QT measurements, as algorithms may over- or underestimate the interval 1, 6
• Automated systems often measure QT from superimposed leads, yielding longer values than single-lead measurements 1
• Computer measurements are less accurate in cardiac patients than healthy controls 7

Normal Values and Thresholds

Upper Limits of Normal

• Males: QTc <450 ms (99th percentile: 470 ms) 1, 4, 8
• Females: QTc <460 ms (99th percentile: 480 ms) 1, 4, 8

High-Risk Thresholds

• QTc >500 ms is highly abnormal and significantly increases torsades de pointes risk 1, 4
• An increase of >60 ms from baseline warrants immediate attention 4, 8

Common Pitfalls

Avoid These Errors

• Do not use Bazett's formula at heart rates >85 bpm, as it produces falsely elevated values 4
• Do not measure QT in the presence of new bundle branch block without adjusting for QRS duration 4
• Do not switch leads between measurements without documentation 1
• Do not ignore the cumulative effect of multiple QT-prolonging medications 4
• Do not rely solely on automated measurements without visual confirmation 1

Measurement Variability

• Inter-reader variability ranges from 11.9 to 13.1 ms even among experienced readers 9
• Intra-reader variability ranges from 3.4 to 6.9 ms 9
• The tangent method is consistently shorter than threshold methods by up to 10 ms 9

Monitoring Recommendations

Frequency

• Measure QT at baseline before initiating QT-prolonging drugs 1, 5
• Repeat measurements 2-4 hours after each dose during drug initiation 1, 5
• Document QTc at least every 8-12 hours when monitoring for drug effects 1, 4
• Measure before and after dose increases 1

Documentation

• Record the QTc value with a rhythm strip example in the medical record 1
• Note the lead used for measurement 1
• Document the correction formula applied 1