How to read an Electrocardiogram (ECG) - Part 5, The QRS Complex

The QRS complex represents activation of the ventricle. Special conducting bundles spread the wave of depolarization rapidly over the ventricle.

The QRS complex is normally less than 0.10 seconds in length, two and a half boxes. Lengthening of the QRS indicates some blockage of the electrical action in the conducting system. This may be due to ischemia, necrosis of the conducting tissue, electrolyte abnormality, or hypothermia.

If the first deflection of the QRS is downward, it’s called a Q wave. The Q wave represents activation of the ventricular septum. The electricity spreads from right to left through the septum.

Q waves may be normal. For example in lead I, a Q less than 1/4 of the R height, and less than one box wide, is considered normal. This is the early activation of the septum. This activation goes left — away from lead I — and is therefore negative on the ECG. “Septal Qs” are normal in I, F, V5 and V6. Qs are also generally innocent in lead III and lead V1 if no other abnormality is seen.

Q waves are “significant” if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave. Significant Q waves indicate either myocardial infarction or obstructive septal hypertrophy (IHSS).

The first upward deflection of the QRS is called the R wave. Most of the ventricle is activated during the R wave. The R wave may be prolonged if the ventricle is enlarged, and may be abnormally high (indicating strong voltage) if the ventricular muscle tissue is hypertrophied.

The S wave is any downward deflection following the R wave. Like the R wave, an abnormally large S wave may indicate hypertrophy of the ventricle.

If a second upward deflection is seen, it’s called an R-prime wave. R-prime waves are never normal, but indicate a problem in the ventricular conduction system.

QRS complexes may be described by naming the waves that form them. For example, a complex with an R, an S, and an R’ is called an RSR’ complex.


Part 1 PQRST Waves
Part 2 ECG Graph Interpretation
Part 3 The P Wave
Part 4 The PR Interval
Part 5, The QRS Complex
Part 6, The ST Segment

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