What indicates the presence of a third-degree AV block on an ECG?

• A. Atrial flutter with variable ventricular response
• B. Complete dissociation between atrial and ventricular activity
• C. Prolonged PR interval with dropped beats
• D. A wide QRS complex

Answer: (B)

The presence of a third-degree AV block, also known as complete heart block, is characterized by a complete dissociation between atrial and ventricular activity. In this condition, the electrical signals generated in the atria do not propagate to the ventricles. Instead, the atria and ventricles beat independently; the atria generate their own rhythm typically at a normal rate (often sinus), while the ventricles may beat at a much slower intrinsic rate originating from the bundle of His or Purkinje fibers. This dissociation is visually apparent on the ECG, where the P waves (representing atrial contraction) and the QRS complexes (representing ventricular contraction) occur at different rates and do not align, indicating a lack of communication between the atria and ventricles.

This understanding is essential for differentiating third-degree AV block from other types of heart blocks, and recognizing the characteristics of the ECG waveform can aid in diagnosing the severity and management of this condition. Since the other choices describe different arrhythmias or characteristics not specific to third-degree AV block, they do not fulfill the criteria necessary to confirm this particular diagnosis.