An ECG shows narrow QRS complexes with no P waves in between QRS complexes, with only T waves visible. What is the most likely diagnosis?

• A. Atrial flutter
• B. Premature atrial contraction
• C. Ventricular tachycardia
• D. Sinus tachycardia

Answer: (B)

Premature atrial contraction. A PAC arises from an atrial focus outside the SA node and produces a narrow QRS because ventricular conduction is normal. The P wave for the premature beat often has an abnormal shape or is eaten into the preceding T wave, so it may not be clearly visible before the QRS, leaving the rhythm looking like a QRS complex followed by a T wave with no obvious P wave between beats. The rhythm can appear irregular because of the early beat and the subsequent compensatory pause.

This differs from other choices: atrial flutter shows a regular or irregular pattern of sawtooth atrial activity between QRS; ventricular tachycardia typically has wide QRS complexes and no preceding P waves; sinus tachycardia has normal P waves before each narrow QRS with an overall faster rate.