Learning Objectives

1. What Is the QT Interval?

The QT interval is measured from the beginning of the QRS complex to the end of the T wave. It represents the total duration of ventricular electrical depolarization and repolarization. It therefore spans most of the ventricular electrical systole.

QT duration is dependent on heart rate:

• At faster heart rates, the QT interval normally becomes shorter.
• At slower heart rates, the QT interval normally becomes longer.

2. How to Measure the QT Interval

QT measurement should be performed on a clear ECG tracing, with attention to T-wave morphology and the lead in which the T-wave termination can be identified most reliably.

1. Select the lead. Lead II or V5–V6 is commonly used. Compare suitable leads and use the lead with the longest clearly measurable QT interval.
2. Measure several consecutive beats. Record multiple cycles and use the maximum reliable QT measurement, avoiding ectopic or distorted beats when possible.
3. Identify QRS onset. Start at the earliest beginning of the QRS complex.
4. Identify T-wave termination. Use the maximum-slope tangent method when the end of the T wave is indistinct.
5. Assess U waves. A large U wave (greater than 1 mm) fused with the T wave is included in the measurement. A small or clearly separate U wave is excluded.

Maximum-slope tangent method

Draw a tangent along the steepest descending limb of the T wave. The point where this tangent intersects the isoelectric baseline is taken as the end of the T wave. If the T wave is notched, the tangent is drawn from the steepest downslope of the second notch, as described in the supplied ECG reference.

3. Corrected QT Interval (QTc)

Because the measured QT changes with heart rate, the corrected QT (QTc) estimates the QT duration at a standardized heart rate of 60 beats/min. QTc allows comparison across different heart rates and during serial ECG assessment.

Common correction formulas

Units: QT and QTc are expressed in milliseconds (ms), RR in seconds, and heart rate (HR) in beats/min. For the Framingham and Hodges equations, the QT input is in milliseconds.

Choosing a correction formula

Bazett is widely used because it is simple, but it tends to overcorrect at faster rates and undercorrect at slower rates. The supplied reference notes that Fridericia or Framingham correction may be more appropriate when the heart rate is outside approximately 60–100 beats/min. No single formula is universally optimal in every clinical setting.

When the heart rate is exactly 60 beats/min, the measured QT can be considered directly because RR is one second.

4. QTc Interpretation: Prolonged and Short QT

The supplied ECG reference gives the following practical thresholds. Values should be interpreted with the measurement method, heart rate, clinical context, and applicable diagnostic criteria in mind.

Why QTc above 500 ms is important

A QTc above 500 ms is a high-risk finding in the appropriate clinical setting, particularly with QT-prolonging drugs, bradycardia, electrolyte disturbance, or other predisposing factors. The AHA/ACCF scientific statement advises prompt evaluation when QTc exceeds 500 ms or rises by at least 60 ms from baseline during treatment with a QT-prolonging drug.

5. Causes of QTc Prolongation

Medication-related QT prolongation

Review prescribed, over-the-counter, and recently administered drugs when QTc is prolonged. Assess combinations of QT-prolonging agents, drug interactions, bradycardia, and electrolyte abnormalities. In patients receiving high-risk drugs, serial QT assessment should use a consistent measurement approach.

6. Causes of a Short QTc

• Hypercalcaemia
• Congenital short QT syndrome
• Digoxin effect

Hypercalcaemia

Hypercalcaemia shortens the ST segment and can markedly shorten the QT interval. The supplied source also notes that Osborn waves may be seen.

Congenital short QT syndrome

Short QT syndrome is a rare inherited channelopathy associated with atrial fibrillation, ventricular fibrillation, and sudden cardiac death. The supplied ECG reference describes very short QTc values (often below 300–350 ms) and tall, peaked T waves.

Features that may raise suspicion include atrial fibrillation at a young age without structural heart disease, a family member with a short QT interval, a family history of sudden cardiac death, and failure of the QT interval to lengthen appropriately as the heart rate slows.

Digoxin effect

Digoxin can shorten the QT interval and produce characteristic downsloping ST-segment depression (“reverse tick” appearance), T-wave flattening or inversion, and a range of rhythm and conduction disturbances. A short QT in a patient taking digoxin should be interpreted with the clinical context and other ECG findings.

7. Drug-Induced QT Prolongation and the QT Nomogram

In acute poisoning with QT-prolonging agents, the supplied source emphasizes assessment of the absolute QT interval together with the simultaneous heart rate, rather than relying only on a corrected QT value.

1. Measure the absolute QT interval on the ECG.
2. Determine the heart rate from the same tracing.
3. Plot the QT–heart-rate pair on the QT nomogram described by Chan and colleagues.
4. A point above the nomogram line indicates a potentially increased risk of torsades de pointes.

8. Practical ECG Interpretation Workflow

1. Confirm the ECG calibration and identify a representative sinus beat.
2. Inspect leads II and V5–V6, then select the lead with the clearest T-wave end and longest reliable QT.
3. Measure multiple consecutive beats and document the lead and method.
4. Check for T–U fusion, notched T waves, baseline instability, ectopy, and broad QRS complexes.
5. Calculate QTc and state which formula was used.
6. Review medications, potassium, magnesium, calcium, temperature, ischaemia, and relevant neurological or post-arrest context.
7. If markedly abnormal, assess symptoms, family history, serial ECGs, and the need for monitored evaluation or specialist input.

Key Takeaways

• The QT interval runs from the beginning of the QRS complex to the end of the T wave.
• QT shortens as heart rate increases and lengthens as heart rate decreases; QTc adjusts for this relationship.
• Measure the longest clearly interpretable QT, using a consistent method and careful T-wave endpoint identification.
• Bazett is common but may overcorrect at faster rates and undercorrect at slower rates; compare alternative formulas when appropriate.
• QTc >500 ms is associated with increased torsades risk, especially in the presence of additional risk factors.
• Electrolyte abnormalities, hypothermia, ischaemia, drugs, and inherited channelopathies can alter QT duration.
• In drug overdose, the absolute QT and simultaneous heart rate may be assessed using the QT nomogram.