An electrocardiogram (ECG or EKG, from the German "Elektrokardiogramm") is a non-invasive medical diagnostic recording of the electrical potentials generated by the heart muscle across each cardiac cycle. Dutch physician Willem Einthoven invented the first practical string galvanometer electrocardiograph in 1901, a breakthrough for which he received the Nobel Prize in Physiology or Medicine in 1924. Einthoven assigned the letters P, Q, R, S, and T to the distinct deflections of the cardiac tracing, establishing an international diagnostic standard that remains in clinical use today.
The heart's electrical activation begins in the sinoatrial (SA) node, situated in the upper wall of the right atrium and functioning as the natural pacemaker of the heart. The action potential spreads across both atria, causing atrial depolarization, which is recorded on the ECG as the P wave. The impulse then reaches the atrioventricular (AV) node, where a brief physiological delay (represented by the PR segment) allows the atria to complete mechanical contraction and empty blood into the ventricles. From the AV node, the electrical signal travels rapidly through the Bundle of His, the right and left bundle branches, and the Purkinje fiber network, depolarizing ventricular muscle. This rapid ventricular activation produces the sharp QRS complex. Ventricular contraction occurs during the subsequent ST segment. Finally, ventricular repolarization—the electrical recovery of ventricular muscle cells to their resting state—generates the T wave. Atrial repolarization occurs simultaneously with ventricular depolarization and is masked by the larger QRS complex.
A standard diagnostic ECG records cardiac electrical activity from twelve distinct perspectives using ten physical electrodes attached to the limbs and chest. These twelve leads include six limb leads (I, II, III, aVR, aVL, and aVF) evaluating the frontal plane and six precordial chest leads (V1 through V6) evaluating the horizontal plane. Variations in wave amplitude, duration, and intervals (such as the PR interval and QT interval) allow clinicians to detect arrhythmias, myocardial infarction (heart attack, characterized by ST-segment elevation), bundle branch blocks, and ventricular hypertrophy. For competitive examinations like UPSC and SSC, questions evaluate the pacemaker role of the SA node, wave interpretations, Einthoven's triangle, and heart rate calculations.
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Willem Einthoven invented the string galvanometer electrocardiograph in 1901 and received the Nobel Prize in Physiology or Medicine in 1924.
The sinoatrial (SA) node, located in the right atrium, acts as the primary natural pacemaker of the human heart, generating rhythmic electrical action potentials.
The P wave represents atrial depolarization, which leads to the mechanical contraction (systole) of both atria.
The PR interval measures the time taken for the electrical impulse to travel from the SA node through the atrioventricular (AV) node to the ventricles.
The AV node introduces a brief physiological delay of approximately 0.09 to 0.12 seconds to ensure complete ventricular filling prior to ventricular contraction.
The QRS complex represents rapid ventricular depolarization and normally lasts between 0.06 and 0.10 seconds in healthy adults.
Atrial repolarization occurs at the same time as ventricular depolarization and is completely masked on the ECG tracing by the larger QRS complex.
The ST segment represents the period when the ventricles are fully depolarized, corresponding to the plateau phase of the ventricular action potential.
The T wave represents ventricular repolarization, marking the electrical recovery of ventricular cardiomyocytes to their resting state.
A standard clinical ECG recording uses ten electrodes to produce twelve diagnostic leads: six limb leads and six precordial chest leads.
Einthoven's triangle is an equilateral triangle formed by imaginary electrical axes between electrodes placed on the right arm, left arm, and left leg.
Lead II, which measures electrical potential from the right arm to the left leg, closely aligns with the heart's normal electrical axis and provides the clearest rhythm strip.
ST-segment elevation is a primary clinical diagnostic marker for acute transmural myocardial infarction (STEMI).
A prolonged PR interval exceeding 0.20 seconds indicates a first-degree atrioventricular heart block.
A prolonged QT interval indicates delayed ventricular repolarization, which increases the risk of ventricular arrhythmias such as Torsades de Pointes.
Sinus bradycardia is defined as a resting heart rate below 60 beats per minute originating from the SA node.
Sinus tachycardia is defined as a resting heart rate exceeding 100 beats per minute originating from the SA node.
Standard ECG paper runs at a standard horizontal speed of 25 millimeters per second, where each 1-millimeter small box represents 0.04 seconds.
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