ECG Basics

The basics section visualizes the standard paper measurements of the ECG paper, interpreting the axis of the ECG along with leads placement location on the body.

Standard Paper Measurements

1 small square (1mm) = 0.04 sec (40 ms)
1 large square = 5 small squares (5mm) = 0.2 sec (200 ms)
5 large squares = 1 second

Rate Calculation

heart rate calculation ecg
Large square method: 300 divided by no. of large squares between R-R interval
R wave method: Number of R waves (rhythm strip) X 6

Axis Interpretation

Normal Axis = QRS axis between 0 and +90 degrees | Positive QRS in lead I and aVF
Left Axis Deviation (physiologic) = QRS axis between 0 and -30 degrees | Positive QRS in lead I – Equiphasic in lead II – Negative in aVF
Left Axis Deviation (pathologic) = QRS axis between -30 and -90 degrees | Positive QRS in lead I – Negative in lead II – Negative in aVF
Right Axis Deviation = QRS axis between 90 and 180 degrees | Negative QRS in lead I – Positive in lead II – Positive in aVF
Extreme Right Axis Deviation = QRS axis between -90 and -180 degrees | Negative QRS in lead I – Negative in lead II – Negative in aVF

Leads Placement

Limb leads
Precordial leads