A QTc of 460 milliseconds is a number, not a verdict. Whether it is normal, borderline, or prolonged depends on who it belongs to, what rhythm the heart was in, and how wide the QRS complex is — and the standard limits assume a normal rhythm, a narrow QRS, and an adult. This guide sets out the limits from the American Heart Association statement, explains what each band means, and then works through the three situations that change the reading: atrial fibrillation, bundle branch block, and paced rhythms. Nothing here is a diagnosis: a QTc value must be interpreted by a qualified clinician using the full ECG and the clinical context.

The limits, from the AHA/ACCF/HRS statement

The most widely used limits come from the 2009 AHA/ACCF/HRS recommendations for the standardisation and interpretation of the ECG, with the 99th-percentile values from the AHA/ACCF statement on preventing torsade de pointes. They are sex-specific after puberty, because women have a slightly longer QT than men, and they are stated for adults.

QTc limits in adults (AHA/ACCF/HRS 2009; AHA/ACCF 2010 for the 99th percentile and the 500 ms band).

Many calculators, including the one on this site, quote a simpler pair of limits — 450 ms for men and 470 ms for women — as the practical line, with 440 to 460 ms called borderline. Those numbers sit between the AHA’s prolonged limit (450 ms in men, 460 ms in women) and its 99th-percentile values (470 ms and 480 ms): they are a shorthand for the same range, not a second standard. The 500 ms line is the one that carries the most weight: a QTc over 500 ms is associated with a two- to three-fold higher risk of torsade de pointes in congenital long-QT data, and it is the threshold that most often prompts action. The companion guide explains the Bazett and Fridericia formulas and when to use each, which is how the QTc itself is calculated.

What ‘prolonged’ means

Prolonged means the ventricles are taking longer than usual to repolarise — to reset after each beat — and the concern is that a long repolarisation leaves a window in which an early extra beat can trigger torsade de pointes, a twisting ventricular tachycardia that can degenerate into ventricular fibrillation. The risk rises with the QTc: a QTc just over the prolonged limit carries a small excess risk, and a QTc over 500 ms carries a substantially larger one.

But risk depends on far more than the number. It depends on whether the prolongation is congenital or acquired, on potassium and magnesium, on the drugs in use, and on whether the reading is persistent or a single borderline value on a noisy tracing. A single borderline reading is very different from a persistent QTc above 500 ms, which is why the full clinical picture matters and why no single number is a diagnosis.

Atrial fibrillation: average, do not cherry-pick

In atrial fibrillation the RR interval varies from beat to beat, so there is no single RR to correct by — and the QTc from any one beat is a lottery. The standard approach is to average: measure the QT and the preceding RR over several consecutive beats — commonly five to ten — and use the averages in the formula. Some clinicians use the longest RR, some the shortest, and some the average, and the three give different answers, which is itself the point: the QTc in AFib is an estimate with a range, not a precise figure.

Two cautions follow. First, do not cherry-pick the beat that gives the answer you want: the shortest RR gives the highest QTc by Bazett, and the longest RR gives the lowest, so a single-beat measurement can be made to say almost anything. Second, remember that the correction formulas were derived in sinus rhythm, and their accuracy in AFib is lower — a QTc in AFib should be read as approximate, and a borderline value in AFib is a reason to repeat the measurement, not to act on it.

Bundle branch block: the wide QRS problem

A bundle branch block widens the QRS complex, and the QT interval includes the QRS — so a wide QRS inflates the QT without any change in repolarisation. Measuring the QTc in the usual way overstates the repolarisation time, and the overstatement is roughly the width of the block: a QRS of 160 ms carries about 40 ms of extra width over the normal 120 ms, and that extra width sits inside the QT.

The correction is to remove the depolarisation time and look at repolarisation alone. The JT interval does that: JT = QT − QRS duration, which is the time from the end of the QRS to the end of the T wave. The stepwise approach used by QT specialists is the same idea in a different form: if the QRS duration exceeds 120 ms, subtract the excess over 120 ms from the QT before correcting — a QT of 500 ms with a QRS of 160 ms becomes an adjusted QT of 460 ms. Either way, the principle is the same: the QTc is a repolarisation measure, and the depolarisation time does not belong in it.

Paced rhythms: the standard QTc does not apply cleanly

A paced rhythm produces a wide QRS, for the same reason a bundle branch block does: the ventricles are activated from one point rather than through the conduction system, so depolarisation takes longer and the QRS widens. The QT is inflated by the same mechanism, and the standard QTc formulas — derived in narrow-QRS sinus rhythm — do not apply cleanly to a paced beat. The JT interval is again the useful measure, and modified QT approaches exist for paced rhythms, but they are specialist tools, not bedside ones.

The practical rule is that a QTc from a paced rhythm should be read with caution and compared with the patient’s own previous values rather than with the population limits. A pacemaker does not itself prolong repolarisation, but it changes the measurement, and a QTc that looks prolonged on a paced tracing may be an artefact of the pacing. Whether it matters is a question for the clinician who knows the patient, the device, and the tracing.

Children: different ranges

Adult QTc bands do not apply to children. Before puberty there is no sex difference in the QT, and the upper limit of normal is lower than in adults — around 440 to 460 ms depending on age — with age-specific ranges that a paediatric cardiologist applies. Paediatric QTc interpretation uses those age-specific ranges and often requires specialist input, particularly where congenital long QT syndrome is suspected.

The calculator shows the arithmetic for any input, but its adult bands do not apply to a child, and a QTc that would be borderline in an adult can be abnormal in a child, or the reverse. Any concern about a child’s QT should go to a paediatric cardiologist, not to a calculator.

What to do with a long QTc

A long QTc is a reason for clinical review, not for self-treatment. The review starts with the tracing itself — repeat the ECG, measure by hand, and check whether the value is persistent — and then with the two commonest reversible causes: the medication list, and potassium and magnesium. Many antiarrhythmics, some antibiotics, antipsychotics and antidepressants, methadone, and several other drugs can prolong the QT, and low potassium or magnesium makes the prolongation more dangerous. None of that is a reason to stop a drug on your own: stopping a needed drug is itself a risk, and the decision belongs to the clinician who prescribed it.

Above 500 ms, the review is prompt rather than routine, because that is the band where the risk of torsade de pointes rises substantially. Below it, the question is usually whether the prolongation is real, whether it is persistent, and what is causing it — and the answer comes from the full ECG, the electrolytes, and the medication list, not from the number alone.

Frequently asked questions

What is a normal QTc on an ECG?

For most adults, up to about 450 ms in men and 470 ms in women is considered normal, with 440 to 460 ms often called borderline. The AHA/ACCF/HRS statement sets prolonged at 450 ms or more in men and 460 ms or more in women, with about 470 ms in men and 480 ms in women at the 99th percentile. Above 500 ms is widely regarded as markedly prolonged and warrants prompt clinical review.

What is QTc prolongation?

QTc prolongation is a QTc above the prolonged limit — 450 ms or more in men, 460 ms or more in women by the AHA/ACCF/HRS statement. It means the ventricles are taking longer than usual to repolarise, which leaves a window for torsade de pointes. It can be congenital or acquired, and the risk depends on how long the QTc is, on electrolytes, and on drugs — a single borderline reading is very different from a persistent QTc above 500 ms.

Can you measure QTc in atrial fibrillation?

Yes, by averaging. In AFib the RR interval varies beat to beat, so measure the QT and the preceding RR over several consecutive beats — commonly five to ten — and use the averages in the formula. Do not use a single beat: the shortest RR gives the highest QTc and the longest RR the lowest, so a single-beat measurement can be made to say almost anything. The result is an estimate, and a borderline value should be repeated.

Can you measure QTc in a paced rhythm?

It can be measured, but the standard QTc does not apply cleanly. A paced beat has a wide QRS, which inflates the QT without any change in repolarisation, so the usual formulas overstate it. The JT interval — QT minus QRS duration — is the more useful measure, and modified approaches exist, but they are specialist tools. A paced QTc should be compared with the patient’s own previous values, not with the population limits.

How do you measure QTc with a left bundle branch block?

Remove the depolarisation time first. Either use the JT interval — QT minus QRS duration — or subtract the excess of the QRS over 120 ms from the QT before correcting: a QT of 500 ms with a QRS of 160 ms becomes an adjusted QT of 460 ms. The principle is the same either way: the QTc measures repolarisation, and the extra QRS width from the block does not belong in it.

What is a high QTc level?

Over 500 ms is considered highly abnormal in both sexes, and it is associated with a two- to three-fold higher risk of torsade de pointes in congenital long-QT data. A QTc above 500 ms warrants prompt clinical review — a repeat ECG, a check of the medication list, and potassium and magnesium. Between 450 and 500 ms the reading is prolonged but the risk is lower, and the question is usually whether it is persistent and what is causing it.

Limits: AHA/ACCF/HRS recommendations for the standardisation and interpretation of the ECG (Circulation 2009); AHA/ACCF prevention of torsade de pointes (2010). Educational information only, not medical advice.