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The q t interval
1. The Q-T Interval
Jast Arnaldo T. Tejada III MD
Cardiology Fellow
Mediatrix Heart Institute
2. Objectives
• To discuss important concepts & how to interpret Q-T Interval
on ECG
• To discuss important concepts of the causes & etiology of Q-
T prolongation
• To provide ECG samples of Abnormal Q-T intervals for
different pathology.
3. QT interval
• QT interval includes the QRS complex ,ST segment ,T wave
• Corresponding to phases 0 to 3 of the action potential
• Beginning of the QRS complex and end of T wave
4. QT interval
• Presence of a U wave is not included in the measurement
• Multiple leads should be selected
• QT interval is the longest QT in ECG
5. QTc: QT interval
• Affected by heart rate.
• Longer when the heart rate is slower and shorter
• Should always be corrected for heart rate
8. QTc: Bazett Formula
• (Normal) <0.42 seconds in men
• (Normal) <0.44 seconds in women
• (Prolonged) >0.44 seconds in men
• (Prolonged) >0.46 seconds in women and
• children
10. QTc: Bazett Formula
Step1: Measure the QT
interval: The QT interval
measures 10 small
blocks. This is
equivalent to
0.40 seconds
Step2:Measure
the R-R interval:
The R-R interval
measures 14
small blocks,
which is
equivalent to
0.56 sec. The
square root of
0.56 seconds is
0.75 sec
Step3:Calculate the
QTc: Using the
Bazett formula as
shown below: QTc
= 0.40 ÷ 4 0.75 =
0.53 seconds. The
QTc is prolonged.
11. Importance of the QT Interval
• Prolonged QT - increased risk for Ventricular Arrythmias
especially Torsades de Pointes
• Congenital short QT – associated with paroxysmal atrial &
ventricular fibrillation & Sudden cardiac death
The QT interval includes the QRS complex, ST segment, and T wave corresponding to phases 0 to 3 of the action potential. It is measured from the beginning of the QRS complex to the end of the T wave.
Note that the presence of a U wave is not included in the measurement. In
assessing the duration of the QT interval, multiple leads should be selected and the QT interval is
the longest QT that can be measured in the whole 12-lead ECG recording.
QTc: The QT interval is affected by heart rate. It becomes longer when the heart rate is slower and
Shorter when the heart rate is faster. The QT interval therefore should always be corrected for heart rate.
The corrected QT interval is the QTc.
Figure 2.8: The QT Interval. The QT interval is measured from the beginning of the QRS complex to the end of the
T wave. When the heart rate is >70 bpm, one can “eyeball” that the QTc is normal if the QT interval is equal to or
less than half the R-R interval. When this occurs, no calculation is necessary. If the QT interval is more than half
the R-R interval, the QTc may not be normal and should be calculated.
The simplest and most commonly used formula for correcting the QT interval for heart rate is
the Bazett formula shown here.
The normal QTc should not exceed 0.42 seconds in men and 0.44 seconds in women. The
QTc is prolonged when it measures >0.44 seconds in men and >0.46 seconds in women and
children.
An easy rule to remember in calculating the QTc when the heart rate is >70 bpm is that the
QTc is normal (<0.46 seconds) if the QT interval is equal to or less than half the R-R interval
Figure 2.9: Calculating the QTc. If a calculator is not available, the QTc can be calculated by using Table 2.1.
The preceding R-R interval is measured because the QT interval is dependent on the previous R-R interval.
In this figure, the QT interval (10 small blocks) is more than half the preceding R-R interval (14 small blocks),
thus the QTc may not be normal and should be calculated as shown in the text. The right panel is a reminder
that the QT interval is equivalent to the total duration of the action potential (phases 0 to 3).
First: Measure the QT interval: The QT interval measures 10 small blocks. This is equivalent to
0.40 seconds (Table 2.1, column 1, QT interval in small block).
Second: Measure the R-R interval: The R-R interval measures 14 small blocks, which is
equivalent to 0.56 seconds. The square root of 0.56 seconds is 0.75 seconds (see Table 2.1).
Finally: Calculate the QTc: Using the Bazett formula as shown below: QTc = 0.40 ÷ 4 0.75 =
0.53 seconds. The QTc is prolonged.
An abnormally prolonged QT is associated with an increased risk of ventricular arrhythmias, especially Torsades de Pointes.
Congenital short QT syndrome has been found to be associated with an increased risk of paroxysmal atrial and ventricular fibrillation and sudden cardiac death.
Apparent QTc 500ms – prominent U waves in precordial leads (hypokalaemia (K+ 1.9))
Hypokalaemia causes apparent QTc prolongation in the limb leads (due to T-U fusion) with prominent U waves in the precordial leads.
QTc 510 ms secondary to hypomagnesaemia
QTc 510ms due to hypocalcaemia
Hypocalcaemia typically prolongs the ST segment, leaving the T wave unchanged.
QTc 620 ms due to severe hypothermia
Severe hypothermia can cause marked QTc prolongation, often in association with bradyarrhythmias (especially slow AF), Osborn waves and shivering artefact.
QTc 495 ms due to hyperacute MI
Myocardial ischemia tends to produce a modest increase in the QTc, in the 450-500 ms range.
This may be useful in distinguishing hyperacute MI from benign early repolarization (both may produce similar hyperacute T waves, but benign early repolarisation (BER) will usually have a normal QTc).
QTc 630ms with widespread T wave inversion due to subarachnoid haemorrhage
A sudden rise in intracranial pressure (e.g. due to subarachnoid haemorrhage) may produce characteristic T wave changes (‘cerebral T waves’): widespread, deep T wave inversions with a prolonged QTc.
QTc 550ms due to congenital long QT syndrome
There are several congenital disorders of ion channels that produce a long QT syndrome and are associated with increased risk of torsades de pointes and sudden cardiac death.
Marked shortening of the QTc (260ms) due to hypercalcaemia
Hypercalcaemia leads to shortening of the ST segment and may be associated with the appearance of Osborne waves.
Very short QTc (280ms) with tall, peaked T waves due to congenital short QT syndrome
Congenital short QT syndrome (SQTS) is an autosomal dominant inherited disorder of potassium channels associated with an increased risk of paroxysmal atrial and ventricular fibrillation and sudden cardiac death.
The main ECG changes are very short QTc (<300-350ms) with tall, peaked T waves.
Digoxin produces a relative shortening of the QT interval, along with downward sloping ST segment depression in the lateral leads (‘reverse tick’ appearance), widespread T-wave flattening and inversion, and a multitude of arrhythmias (ventricular ectopy, atrial tachycardia with block, sinus bradycardia, regularized AF, any type of AV block).
