HEART SPECIALISTS IN H S R LAYOUT BANGALORE
A systematic description of ECGs
The following eight short steps will enable most ECGs to be described correctly:
1 Check the paper speed and calibration markers.
2 Measure or estimate the heart rate.
3 Estimate the rhythm.
4 Look for P waves.
5 Measure the PR interval.
6 Examine the QRS complex.
7 Check the ST segment.
8 Measure the T wave.
ECG interpretation should always be as restrained as practicable, taking into account the
clinical context where known and comparison with previous tracings where possible. The possibility
of Prinzmetal’s electrocardiographic heart disease must always be borne in mind—that
is, do not assume that an abnormal ECG always means heart disease.2
Paper speed and calibration markers
The standard paper speed is 25 mm/second. This means that 1 mm (small square) = 0.04 seconds
and 5 mm (large square) = 0.20 seconds. Provided that the grid is shown, this gives the time
scale regardless of the actual image magnification used.
Voltage is measured on the vertical axis: 10 mm = 1 mV, as shown in the calibration artefact
Leads are often described in groups that correspond approximately to the area of the heart
n Leads 1 and aVL are (high) lateral leads.
n Leads 2, 3 and aVF are inferior leads.
n Leads 1, 2, 3, aVL, aVF and aVR are collectively called limb or frontal plane leads. Leads 1,
2 and 3 are standard limb leads, while leads aVL, aVF and aVR are augmented limb leads.
n Leads V1 and V2 are anteroseptal leads.
n Leads V3 and V4 are anterior leads.
n Leads V5 and V6 are anterolateral leads.
n Leads V1–V6 are collectively called chest, precordial or horizontal plane leads.
3• AN OVERVIEW OF CLINICAL ELECTROCARDIOGRAPHY 49
By definition, sinus tachycardia is a heart rate ≥ 100/minute and sinus bradycardia is a heart
rate ≤ 50/minute.3
To calculate the heart rate from the ECG, the R-R interval in mm can be divided into 1500.
For example, an R-R interval of 20 mm gives a rate of 75/minute and an R-R interval of 15 mm
gives a rate of 100. Similarly, large 5 mm squares can be divided into 300; thus three squares give
a rate of 100/minute. In regular rhythms, any two congruous points of the P-QRS-T sequence
can be used to estimate the rate. An ECG ruler has a scale that enables rapid rate measurement
and calculation of other intervals. With practice, the rate can be estimated at a glance.
Begin by looking for P waves. They are best seen in lead 2 (L2) (which is calculated electrocardiographically
as the arithmetic sum of leads 1 and 3), aVR (where everything including the