Basic Electrocardiography: Principles, Lead Systems, and Normal Waveforms
Basic Electrocardiography
This domain tests how cardiac electrical activity becomes a tracing. A positive deflection occurs when depolarisation moves toward the positive electrode; a negative deflection occurs when it moves away.
Leads and normal waveforms
- Limb leads I, II and III form Einthoven's triangle; augmented leads aVR, aVL and aVF view the heart from the same limb electrodes.
- Precordial leads V1-V6 lie across the chest and show anterior, septal and lateral activity.
- P wave: atrial depolarisation. PR interval: start of P to start of QRS; normal 0.12-0.20 s. QRS: ventricular depolarisation; normally under 0.12 s. T wave: ventricular repolarisation.
- Standard paper speed is 25 mm/s, so 1 small square = 0.04 s and 1 large square = 0.20 s. Standard gain is 10 mm/mV.
Worked example
A PR interval spans 3 small squares. At 25 mm/s, each small square is 0.04 s, so 3 x 0.04 = 0.12 s. That is at the lower limit of normal. If it spanned 6 small squares, 6 x 0.04 = 0.24 s, which is first-degree AV block.
Traps that cost marks
aVR is normally negative in sinus rhythm, so a negative P and QRS there is not automatically abnormal. Do not measure PR from the end of P to QRS; measure from the start of P. Do not mix paper speeds. A normal QRS width does not rule out ischaemia.