The Normal ECG Trace
Overview
Square Sizes
The standard ECG is recorded at a speed of 25mm/sec and gain of 10mm/mV.- 40ms = 1mm = 1 small square
- 200ms = 5mm = 1 large square
- 0.1mV = 1mm = 1 small square
The ECG Trace
- A normal ECG trace includes a P wave, a QRS complex and a T wave.
Look For
PQRST Complex
- P wave - indicative of atrial depolarisation
- QRS complex - indicative of ventricular depolarisation
- T wave - indicative of ventricular repolarisation
Intervals
- PR interval - the time between atrial depolarisation and ventricular depolarisation, suggestive of AV conduction time
- QT interval - the time between ventricular depolarisation and repolarisation
ECG Leads
- A standard 12-lead ECG includes bipolar limb leads, unipolar limb leads and chest leads.
Bipolar Limb Leads
- Electrical potential difference is measured between two limb electrodes.
Electrodes Direction Complex I RA → LA 0 Lateral II RA → LL +60 Inferior III LA → LL +120 Inferior Unipolar (Augmented) Limb Leads
- Electrical potential difference is measured between one limb electrode and a combination of the other limb electrodes.
Electrodes Direction Complex aVL LA -30 Lateral aVR RA -150 Right atrial aVF LL +90 Inferior Unipolar Chest Leads
- Electrical potential difference is measured between the chest electrode and the average of the limb electrodes.
Complex V1 Right atrial V2 Septal V3 Anterior V4 Anterior V5 Lateral V6 Lateral
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