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ECG foundations
Waves
Find the onset and offset of the P wave, the QRS, and the T wave.
Understand the measurements
Pin the P wave
The P wave is atrial depolarisation spreading from the SA node across both atria. It is small because the atria have little muscle.
Pin the QRS
The QRS is ventricular depolarisation. It is narrow because the His-Purkinje system spreads the impulse through both ventricles in under 100 ms.
Pin the T wave
The T wave is ventricular repolarisation. Its slow upstroke and faster downstroke come from the epicardium repolarising before the endocardium.