A nurse is reviewing a client's ECG rhythm strip. Which of the following components should the nurse use to measure impulse conduction from the sinus node through the atrioventricular node?

Electrocardiogram (ECG) interpretation helps evaluate the electrical activity and conduction system of the heart. Different components of the ECG represent specific phases of depolarization and repolarization as impulses travel through the cardiac conduction pathway. Measuring conduction from the sinoatrial (SA) node through the atrioventricular (AV) node is important for identifying conduction delays or blocks. The PR interval specifically reflects the time taken for this impulse transmission before ventricular depolarization begins. Rationale: A. The PP interval measures the time between two consecutive P waves and is mainly used to assess atrial rhythm regularity and rate. It reflects atrial depolarization timing rather than conduction through the AV node. Although useful for rhythm interpretation, it does not specifically evaluate impulse travel from the sinus node through the AV node. B. The ST segment represents the period between ventricular depolarization and repolarization, extending from the end of the QRS complex to the beginning of the T wave. It is mainly assessed for myocardial ischemia, injury, or infarction. It does not provide information about atrial conduction or AV nodal transmission. C. The QRS complex represents ventricular depolarization and reflects the spread of electrical impulses through the ventricles. It is used to assess ventricular conduction abnormalities such as bundle branch blocks or ventricular rhythms. Since it occurs after the AV node conduction phase, it does not measure conduction from the SA node to the AV node. D. The PR interval measures the time from the beginning of the P wave to the beginning of the QRS complex, representing impulse conduction from the SA node through the atria and AV node to the ventricles. It is the key indicator for assessing AV nodal conduction time. Prolongation may indicate first-degree heart block or other conduction abnormalities.