Read before you practise · 3 min
Read an ECG by Electrical Event
ECG waves record electrical events in the heart; they do not directly record the mechanical sound of valves closing.
NSEJS · Body Fluids and Circulation · 3 linked questions · 100% coverage
Know this first
Electrocardiogram is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Study deeper: NCERT Science · Body Fluids and Circulation
Map waves to chambers
The P wave is atrial depolarisation; QRS is ventricular depolarisation; T is ventricular repolarisation. Counting QRS complexes over a known interval estimates heart rate.
Separate electricity from contraction
Electrical depolarisation precedes mechanical contraction. The end of QRS is not the end of ventricular systole, and “lub” is an acoustic heart sound rather than an ECG wave.
OlympIQ exam lens
- ✓P → atrial depolarisation.
- ✓QRS → ventricular depolarisation.
- ✓T → ventricular repolarisation.
- ✓QRS count → heart rate; ECG does not record heart sounds.
OlympIQ trap
Do not equate the end of an electrical wave with the end of the full mechanical phase.
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