Select the option that correctly describes the ECG shown in the following image.

DSSSB October 2024

Medical & Surgical Nursing

Hard

Appeared in: DSSSB October 2024

Explanation

The key finding in the ECG is complete atrioventricular (AV) dissociation, where atrial and ventricular activities are independent.
The P waves (atrial depolarization) occur at a regular and faster rate compared to the QRS complexes.
The QRS complexes (ventricular depolarization) also occur regularly but at a much slower 'escape' rhythm rate.
There is no consistent PR interval; the P waves and QRS complexes have no relationship, which is the defining characteristic of a third-degree or complete heart block.

Option B: Second-degree type 1 block (Wenckebach) involves a progressive lengthening of the PR interval until a QRS complex is dropped. The provided ECG does not show this progressive prolongation.
Option C: Second-degree type 2 block (Mobitz II) is characterized by a constant PR interval for all conducted beats, with intermittent non-conducted P waves (dropped QRS complexes). The ECG shows a completely variable PR interval, not a fixed one.
Option D: Supraventricular tachycardia (SVT) is a rapid heart rhythm (typically greater than 150 bpm). The ECG in the image clearly shows a slow heart rate (bradycardia), which is the opposite of tachycardia.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain ECG interpretation of atrioventricular (AV) blocks as background academic context rather than a clinical decision trigger.
Third-degree heart block is a medical emergency because the slow ventricular escape rhythm may not be sufficient to maintain adequate cardiac output, leading to symptoms like dizziness, syncope (fainting), and hypotension.
Patients with symptomatic third-degree heart block require immediate intervention, which often includes transcutaneous pacing followed by the insertion of a permanent pacemaker.
What if? If the patient with this ECG was asymptomatic and the block was discovered incidentally, they would still require urgent cardiology consultation and likely a pacemaker, as the risk of sudden cardiac arrest (asystole) is high, even without symptoms.

How to Approach the Question

First, analyze the overall heart rate on the ECG. In this case, the ventricular (QRS) rate is slow (bradycardia), which immediately makes tachycardia less likely.
Next, identify the P waves and QRS complexes and assess their regularity. Here, both P-P intervals and R-R intervals are regular.
The most crucial step is to determine the relationship between the P waves and the QRS complexes. Look at the PR interval.
Observe that there is no consistent PR interval. The P waves 'march through' the QRS complexes without any association.
This finding of AV dissociation, where atria and ventricles beat independently, is the classic sign of a third-degree complete heart block.

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 1 p. 1966-1968

E6 Physiology Ganong 27e Vol 2 Part 1 p. 77-79