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The ST segment represents the plateau phase of ventricular repolarization when ventricles are uniformly depolarized. Deviations (elevation or depression) indicate ischemia, injury, or infarction. Atrial activity is represented by P waves, QRS by ventricular depolarization, and T wave by repolarization. ST analysis is crucial for diagnosing acute coronary syndromes.

The PR interval begins at the onset of the P wave (atrial depolarization) and ends at the start of the QRS complex (ventricular depolarization). It reflects conduction through the atria, AV node, bundle of His, and Purkinje system. A normal PR interval is 0.12–0.20 seconds. Prolonged PR indicates first-degree AV block; shortened PR may suggest pre-excitation (e.g., Wolff-Parkinson-White). The QRS duration reflects ventricular depolarization. The QT represents total ventricular depolarization and repolarization. The ST segment represents early ventricular repolarization. Accurate measurement of intervals is a core skill for CCTs in rhythm analysis.

For quick screening of right ventricular involvement, obtaining V4R alone (fifth intercostal space, right midclavicular line) is commonly sufficient and practical. Many protocols add a complete right-sided set (V1R–V6R) when more information is needed, but the single most sensitive lead for acute right ventricular infarction is V4R. Standard left-sided limb leads remain unchanged; only the specified chest lead(s) move. Mirroring all precordials is valid for a full right-sided ECG but is not required when a targeted V4R is requested. Elevating leads by two interspaces distorts anatomic representation and may create false ST changes. Proper labeling on the tracing (e.g., “V4R” or “Right-sided ECG”) is essential to avoid misinterpretation. Right-sided leads are particularly important in inferior STEMI scenarios where right ventricular infarction affects preload sensitivity and nitroglycerin use. Prompt, accurate placement supports appropriate hemodynamic management.

A junctional rhythm originates from the AV junction with a rate of 40–60 bpm. P waves are absent, inverted, or retrograde. QRS complexes are narrow since ventricular conduction is intact. Sinus bradycardia has visible sinus P waves. Idioventricular rhythm is wide-complex and slower (20–40 bpm). AF is irregular, not regular. Recognizing junctional rhythms is essential for monitoring patients with sinus node dysfunction or digoxin effect.

Holter monitors are excellent for detecting frequent arrhythmias but may miss rare events outside the 24–48-hour window. Event monitors or implantable recorders are better for infrequent symptoms. Holters are noninvasive, can record multiple leads, and correlate symptoms with rhythm via diaries. Understanding limitations helps providers select the right test.