What is the cause of inappropriate sinus tachycardia?

• A. Abnormal automaticity
• B. Increased vagal tone
• C. Myocardial ischemia
• D. Electrolyte imbalance

Answer: (A)

Inappropriate sinus tachycardia arises when the sinoatrial (SA) node, the heart’s natural pacemaker, has increased automaticity and fires more rapidly than normal at rest. This intrinsic upregulation raises the resting heart rate and produces a sinus tachycardia that seems inappropriate for the level of activity or stress. The key idea is that the tachycardia originates from the SA node itself due to its heightened automaticity, rather than from external triggers or other conditions.

Increased vagal tone would slow the heart, not drive a fast rhythm, so that doesn't fit IST. Ischemia or electrolyte imbalance can cause tachycardia, but IST is defined by a sinus rhythm with no such contributing factors, reflecting a primary change in SA node automaticity rather than secondary triggers.