The QRS complex is the main group of waves on the ECG. The QRS complex represents ventricular depolarisation. Within this group, a Q wave is a negative wave before an R wave, an R wave is positive and an S wave is a negative wave after an R wave. Not all of these waves are always present.
Key features of a QRS complex include:
- Width: A normal QRS is narrow (70-100 msec wide). If it is narrow, the ventricles must be depolarised via the normal conduction highways. If it is wide, there is either a ventricular arrhythmia or there is aberrant conduction of a supraventricular rhythm through the ventricles (e.g. bundle branch block, hyperkalemia…).
- Height: A normal QRS is neither too short nor too tall. Tall QRS complexes can be caused by left or right ventricular hypertrophy (more muscle = more electrical signal). Short QRS complexes may be caused by a loss of muscle (infarction) or a barrier between the heart and the recording electrodes (e.g. pericardial effusion).
- R wave progression: The QRS complexes normally progress from being negative in V1 to positive in V6, with a transition point around V3-4.
- Component waves: There may be normal or abnormal Q waves, extra waves (e.g. delta, epsilon, Osborn, fragmented), R waves where there shouldn’t be (e.g. V1, aVR) or delayed time to the peak of the R wave.
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