Electrocardiogram Fundamentals

A standard 12-lead ECG uses four limb electrodes placed on the inner wrists and ankles, and six precordial chest electrodes. Limb electrodes are positioned on both arms and legs to minimize muscle movement interference. Chest electrodes are placed at specific intercostal spaces: V1 and V2 at the fourth intercostal space along the sternal borders, V4 at the fifth intercostal space on the midclavicular line, V3 between V2 and V4, and V5 and V6 at the anterior and midaxillary lines.

The six limb leads—I, II, III, aVR, aVL, and aVF—are generated by four limb electrodes and provide different electrical views of the heart. Lead I connects the right and left arms, Lead II connects the right arm and left leg, and Lead III connects the left arm and left leg. The augmented voltage leads (aVR, aVL, aVF) provide additional perspectives from the right arm, left arm, and left leg respectively.

The P wave indicates atrial depolarization, representing the electrical activity as the atria contract. The PR interval measures the time from the start of atrial depolarization to the beginning of ventricular depolarization, reflecting the conduction delay through the atrioventricular node. Together, these components show how electrical activity travels from the atria to the ventricles during the cardiac cycle.

The QRS complex shows ventricular depolarization, the electrical activation of the ventricles. The ST segment reflects the early phase of ventricular repolarization when the ventricles begin to relax. The T wave represents the completion of ventricular repolarization. Together, these components demonstrate the complete electrical cycle of ventricular contraction and relaxation during each heartbeat.

The QT interval measures the total time for ventricular depolarization and repolarization, representing the entire electrical activity of the ventricles. Abnormal QT interval duration can indicate conduction abnormalities or other cardiac conditions. Deviations from normal QT measurements help clinicians identify potential atrial junctional and ventricular arrhythmias and assess overall ventricular electrical function.

Proper electrode placement ensures accurate electrical signal detection from different angles of the heart. Each electrode position captures a unique perspective of cardiac electrical activity. Incorrect placement can produce misleading waveforms and intervals, potentially causing misdiagnosis of cardiac conditions such as arrhythmias, conduction abnormalities, and myocardial ischemia.

The U wave represents repolarization of the Purkinje fibers, the specialized conduction tissue in the ventricles. It is often invisible on standard ECG recordings because its electrical activity is small and may be obscured by the baseline or other waveforms. When visible, the U wave appears after the T wave and indicates the final phase of ventricular electrical recovery.