What is the underlying cause of multifocal atrial tachycardia?

What is the underlying cause of multifocal atrial tachycardia?

• A. The development of additional pacemaker sites in the ventricles
• B. Atrial automaticity foci developing resistance to overdrive suppression by the SA node
• C. Increased heart rate due to sympathetic stimulation
• D. Valvular heart disease

Answer: (B)

Multifocal atrial tachycardia (MAT) occurs when multiple ectopic atrial foci become active, leading to rapid atrial contractions that are not initiated by the sinoatrial (SA) node, which is the natural pacemaker of the heart. This arrhythmia is characterized by varying P-wave morphologies on an electrocardiogram, reflecting the different pacemakers firing in the atria.

The correct answer highlights how these atrial automaticity foci acquire resistance to the overdrive suppression typically exerted by the SA node. In normal physiology, the SA node drives the heart rate, and when it fires rapidly, it usually suppresses other pacemaker sites. However, in the case of MAT, the ectopic foci become independent and can outpace the SA node, creating a situation where they can fire and lead to the tachycardia. This resistance enables these secondary pacemaker sites to dominate the heart's rhythm, not allowing the SA node to maintain control in a typical manner.

Understanding this mechanism is crucial, as it differentiates MAT from other forms of tachycardia, where typically one single pacemaker might predominate. The identification of multiple P-wave morphologies is a key diagnostic feature of