Lesson 9 of 12 · 6 min
Electrocardiogram (ECG)
NCERT §15.3.3
The week after Sports Day, Meher gets a routine check-up and a nurse clips three wires to her wrists and ankle. A wavy line scrolls across the screen. What is it drawing?
The lesson in notes
In short
An ECG is a trace, drawn by a machine called an electrocardiograph, of the electrical events of the heart through each cardiac cycle.
For a standard ECG the patient is connected with three leads, one to each wrist and one to the left ankle, which monitor heart activity continuously. Detailed studies use multiple leads on the chest.
Each peak is named by a letter from P to T, and each corresponds to a particular electrical event.
P-wave: electrical excitation (depolarisation) of the atria, which leads to contraction of both atria.
QRS complex: depolarisation of the ventricles, which starts ventricular contraction. Contraction begins shortly after Q and marks the start of systole.
T-wave: the ventricles recover their resting electrical state (repolarisation). Where the T-wave finishes, systole finishes.
Counting QRS complexes over a known time gives the heart rate.
ECGs from different people have roughly the same shape for a given lead arrangement, so any deviation hints at an abnormality or disease; hence their clinical importance.