Which features on the electrocardiogram characterize atrial tachycardia?

• A. P waves are always visible before QRS with sinus-like morphology.
• B. P waves morphology differs and may be before QRS or hidden in T wave; there are isoelectric lines between P waves; AV block does not affect tachycardia.
• C. QRS complexes are wide with absent P waves.
• D. P waves are absent.

Answer: (B)

Atrial tachycardia shows atrial activity that comes from an ectopic focus, so the P waves reflect atrial depolarization that is not the normal sinus pattern. Because the impulse originates outside the SA node, the P-wave morphology is different from normal and can occur before each QRS with a non-sinus shape, or sometimes be hidden within the preceding T-wave when the rate is very fast. Between successive P waves you see an isoelectric baseline, showing organized atrial activity. The ventricular rate then depends on how the AV node conducts; even if AV block occurs, the atrial tachycardia can continue, producing a variable or slowed ventricular response. This combination—atypical P-wave morphology, possible P waves buried in the T wave, and an isoelectric interval between P waves with ongoing atrial activity—best characterizes atrial tachycardia. In contrast, patterns like wide QRS with no P waves or absent P waves point to other rhythms such as ventricular or junctional tachycardias, not atrial tachycardia.