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A typical ECG displays three major waveforms: the P wave, which represents atrial depolarization; the QRS complex, a triangular wave indicating rapid ventricular depolarization; and the T wave, which shows ventricular repolarization. The T wave is flatter and wider than the QRS complex because repolarization occurs more slowly than depolarization.

An ECG records electrical signals by placing electrodes on the arms, legs, and chest. An electrocardiograph amplifies these signals and produces 12 distinct tracings that capture the heart's electrical activity during each heartbeat. This non-invasive method allows clinicians to visualize the spread of action potentials through the heart's chambers.

Enlarged P waves may suggest atrial enlargement, while high-voltage QRS complexes indicate ventricular enlargement. A flatter-than-normal T wave may suggest oxygen deprivation in heart muscle, whereas an elevated T wave could indicate high blood potassium levels. An enlarged Q wave may signal myocardial infarction.

The P-R interval, also called the P-Q interval, represents the conduction time from the start of atrial excitation to the start of ventricular excitation. This interval reflects the time between atrial and ventricular depolarization, providing clinicians with information about the heart's electrical conduction pathway and timing.

The S-T segment represents the plateau phase when ventricular contractile fibers are depolarized. An elevated S-T segment may indicate acute myocardial infarction, while a depressed S-T segment might suggest insufficient oxygen supply to the heart muscle. These changes are critical diagnostic indicators of cardiac stress or damage.

The Q-T interval extends from the start of the QRS complex to the end of the T wave, representing the complete cycle of ventricular depolarization and repolarization. This measurement helps clinicians assess the total time the ventricles spend in electrical activity and can reveal repolarization abnormalities affecting cardiac function.

Continuous ambulatory electrocardiography, using a Holter monitor worn for 24 hours, records continuous ECG data to detect brief or unpredictable heart issues. Stress testing evaluates the heart's response to physical exercise, identifying whether narrowed coronary arteries can meet increased oxygen demands during strenuous activity.