Conduction

«Step 2: Rate

Step 4: Learn how to determine the heart axis»

The PQ interval

The PQ interval starts at the beginning of the atrial contraction and ends at the beginning of the ventricular contraction.

The PR duration depends on the conduction velocity in the atria, AV node, His bundle, bundle branches and Purkinje fibers.

The PQ interval (sometimes referred to as the PR interval as a Q wave is not always present) indicates how fast the action potential is transmitted through the AV node (atrioventricular) from the atria to the ventricles. Measurement should start at the beginning of the P wave to the beginning of the QRS segment.

The normal PQ interval is between 0.12 and 0.20 seconds.

A prolonged PQ interval is a sign of a degradation of the conduction system, increased vagal tone (Bezold-Jarisch reflex), or it can be pharmacologically induced.

This is called 1st, 2nd or 3rd degree AV block.

A short PQ interval can be seen in the WPW syndrome in which a faster connection exists between the atria and the ventricles.

The QRS duration

The QRS duration indicates how fast the ventricles depolarize. 

The ventricles depolarize normally within 0.10 seconds. When this is longer than 0.12 seconds, this is a conduction delay. Possible causes of a QRS duration > 0.12 seconds include:

• Left bundle branch block
• Right bundle branch block
• Electrolyte Disorders
• Idioventricular rhythm and paced rhythm

The QT interval

The QT interval indicates how fast the ventricles are repolarized and how fast they are ready for a new heart cycle
The normal value for QTc(orrected) is: 440-450ms for men and 450-470ms for women. [1]

The QT interval start at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. Click on the image for a bigger image

The QT interval comprises the QRS-complex, the ST-segment, and the T-wave.

In a (serious) prolonged QT time, is takes longer for the myocardial cells to be ready for a new cardiac cycle. There is a possibility that some cells are not yet repolarized, but that a new cardiac cycle is already initiated. These cells are at risk for uncontrolled depolarization and induce a torsade de pointes, a ventricular tachycardia.

A often used definition of the QT interval is the time between the beginning of the Q until the point where the steepest tangent line from the end of the T-wave intersects with the base line of the ECG.[2]

-

Use lead II. Use lead V5 alternatively if lead II cannot be read. Draw a line through the baseline (preferably the PR segment) Draw a tangent against the steepest part of the end of the T wave. If the T wave has two positive humps, the tallest hump should be picked. If the T wave is biphasic, the end of the tallest wave should be picked. The QT interval starts at the beginning of the QRS interval and ends where the tangent and baseline cross. If the QRS duration exceeds 120ms the amount surpassing 120ms should be deducted from the QT interval (i.e. QT=QT-(QRS width-120ms) ) Calculate QTc according to Bazett:

The difficult part is that the QT interval gets shorter if the heart rate increases. This cab be solved by correcting the QT time for heart rate using the Bazett formula::

At an RR interval 1 second, the (heart frequency 60/min) QTc=QT

The QTc calculator can facilitate QTc calculation.

<flash>file=QTc.swf|width=300|height=200|quality=best|align=right|salign=R||bgcolor=#FFF5F5</flash>

On the modern ECG machines, the QTc is given. However, the machines are not always capable of recognizing the correct QT time. Therefore, it is important to check this manually.

The following formula is indicative for normal values of QT time (uncorrected):

Difficult QT intervals

In some examples of the QT interval it can be difficult to measure a correct QT time. We have made a separate chapter: Measurement of difficult QT intervals.

Causes of a prolonged QT interval

• Medication (i.e. anti-arrhythmics, tricyclic antidepressants, phenothiazedes, for a complete list look on Torsades.org
• Inherited long QT syndrome (LQTS)
• Cerebral (subarachnoid haemorrhage, stroke, trauma)
• Post infarct

Short QT syndrome

If QTc is < 340ms short QT syndrome can be considered.

References

1. Moss AJ. Measurement of the QT interval and the risk associated with QTc interval prolongation: a review. Am J Cardiol. 1993 Aug 26;72(6):23B-25B. DOI:10.1016/0002-9149(93)90036-c |
2. LEPESCHKIN E and SURAWICZ B. The measurement of the Q-T interval of the electrocardiogram. Circulation. 1952 Sep;6(3):378-88. DOI:10.1161/01.cir.6.3.378 |

3. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920;7:353-370.

4. Gaita F, Giustetto C, Bianchi F, Wolpert C, Schimpf R, Riccardi R, Grossi S, Richiardi E, and Borggrefe M. Short QT Syndrome: a familial cause of sudden death. Circulation. 2003 Aug 26;108(8):965-70. DOI:10.1161/01.CIR.0000085071.28695.C4 |