## PR Interval: Definition, Components, and Clinical Significance ### Correct Statements (True — Not the Answer) **Option B (True):** The PR interval is measured from the onset of the P wave to the onset of the QRS complex. It encompasses atrial depolarization (P wave) plus the conduction delay through the AV node and bundle of His. This is the standard textbook definition. **Option C (True):** The normal PR interval is 120–200 ms (0.12–0.20 sec). In infants and children, the PR interval is shorter (80–140 ms) due to smaller cardiac dimensions and faster conduction velocity. This is well-established in standard physiology texts. **Option A (True):** A PR interval exceeding 200 ms in an adult is the standard ECG criterion for first-degree AV block. While the word "diagnostic" can be debated philosophically, by standard ECG criteria (Harrison's, Braunwald's), PR > 200 ms defines and diagnoses first-degree AV block. This statement is considered true for examination purposes. ### The Incorrect Statement (Correct Answer — Option D) **High-Yield:** The **PR segment** (the isoelectric line from the end of the P wave to the beginning of the QRS complex) represents conduction through the AV node and bundle of His — this is **partially correct but importantly incomplete and misleading**. **Why Option D is the EXCEPT answer:** - The PR **segment** is an isoelectric line on the surface ECG because there is **no net electrical force large enough to be recorded** on the surface ECG during this period — not because it represents a specific conduction pathway. - The PR segment reflects conduction through the AV node, bundle of His, **and bundle branches** — the statement omits the bundle branches. - More critically, the PR **segment** is distinct from the PR **interval**: the PR interval includes the P wave (atrial depolarization) + PR segment (AV nodal + His-Purkinje conduction). Attributing the PR segment solely to "AV node and bundle of His" is an oversimplification that is factually incomplete. - According to **Harrison's Principles of Internal Medicine (21e, Ch. 226)**, the PR segment represents the period of electrical silence on the surface ECG during which conduction traverses the AV node, His bundle, and bundle branches — the omission of bundle branches makes Option D the false statement. ### PR Interval vs. PR Segment | Component | Boundaries | Represents | |-----------|-----------|------------| | P wave | Onset to end of P | Atrial depolarization | | PR segment | End of P to onset of QRS | AV node + His bundle + bundle branches (isoelectric) | | PR interval | Onset of P to onset of QRS | Atrial depolarization + entire AV conduction system | **Clinical Pearl:** The PR segment is isoelectric because the electrical forces during AV nodal and His-Purkinje conduction are too small to register on the surface ECG — not because no conduction is occurring. Elevation or depression of the PR segment can indicate atrial injury (e.g., pericarditis, atrial infarction). [cite: Harrison 21e Ch 226; Braunwald's Heart Disease 12e Ch 12]
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