Professional and Linguistic Assessments Board (PLAB) Practice Exam

Which condition typically results in the classic "Narrow QRS Complex" on an ECG?

1. Atrial Fibrillation

2. Ventricular Fibrillation

3. Ventricular Tachycardia

4. Sinus Tachycardia

The correct answer is: Sinus Tachycardia

The classic "Narrow QRS Complex" on an ECG typically indicates that the electrical activity is originating from the atria and propagating through the normal conduction pathways of the ventricles. This results in a QRS duration of less than 120 milliseconds, which is characteristic of conditions where the impulse is generated from the sinus node or atria and is conducted normally through the His-Purkinje system. In the case of sinus tachycardia, the heart rate increases while maintaining normal ventricular conduction. This leads to a narrow QRS complex because the electrical impulses still follow the normal conduction pathway. The increase in heart rate is due to increased sympathetic stimulation or decreased parasympathetic tone, but the conduction through the ventricles remains normal. Other conditions listed result in different QRS morphologies. Atrial fibrillation typically presents with an absence of distinct P waves and can lead to irregular QRS complexes; ventricular fibrillation is characterized by chaotic electrical activity with no identifiable QRS complexes, while ventricular tachycardia involves wide QRS complexes due to abnormal ventricular conduction, often because of a focus within the ventricles. Thus, sinus tachycardia is the only condition among the choices that consistently produces narrow QRS complexes on an ECG.