Electrocardiogram (ECG)
மின் இதய வரைபடம் (ECG)
An ECG does not record the contraction of the heart muscle itself. It records the electrical activity that precedes the mechanical event. Confuse the two, and you lose the mark.
The idea
An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. To obtain a standard ECG, a patient is connected to the machine using three electrical leads: one to each wrist and the left ankle. The P-wave represents the electrical excitation or depolarisation of the atria. The QRS complex represents the depolarisation of the ventricles, which initiates ventricular contraction. The T-wave represents the return of the ventricles from an excited to a normal state, known as repolarisation.
Why it matters
To provide a non-invasive diagnostic tool for identifying abnormalities in the heart's electrical conduction system.
NEET lens
NEET focuses on the sequence: P-wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T-wave (ventricular repolarisation). Do not confuse 'depolarisation' with 'contraction'. Remember, the QRS complex marks the start of systole. Count the number of QRS complexes in a given time to determine the heartbeat rate.
Clinical significance
The end of the T-wave marks the end of systole. Any deviation in the shape or duration of these waves indicates clinical abnormalities like myocardial infarction or arrhythmias. Precise interpretation of these waves is the primary diagnostic tool for cardiac health.
Introduction
An electrocardiogram (ECG/EKG) is a graphical record of the electrical activity of the heart over time, recorded by electrodes on the skin surface, showing characteristic waves that correspond to atrial and ventricular depolarization and repolarization.
Core concept
ECG waves: P wave — atrial depolarization (SA node fires → atria contract). QRS complex — ventricular depolarization (large because ventricles are large; atrial repolarization buried within). T wave — ventricular repolarization (ventricles relax). Intervals: PR interval (~0.16 sec) — AV conduction time. QT interval — total ventricular electrical activity. RR interval — one complete cardiac cycle. Normal ECG shows regular waves at 70-75 cycles/min. Each cycle = P → QRS → T.
Go deeper
Why no separate atrial repolarization wave? Atrial repolarization occurs at the same time as ventricular depolarization (QRS). Since the QRS complex is much larger (ventricles are more massive → larger electrical signal), the small atrial repolarization signal is completely obscured within the QRS complex.
From Class 10
Doctors record the electrical activity of the heart as waves on a graph. This recording is an ECG (electrocardiogram) — a diagnostic tool that reveals heart health.