A commonly cited disadvantage of isoproterenol therapy is tachycardia that limits diastolic filling time.

• A. Tachycardia that limits diastolic filling
• B. Decreased risk of arrhythmias
• C. Improved diastolic filling
• D. Increased SVR

Answer: (A)

The key idea is that isoproterenol stimulates beta receptors to boost heart rate (chronotropy) and contractility, and it also causes vasodilation. This rapid heart-rate increase often shortens the diastolic period, which is when most ventricular filling occurs. When diastole is shortened, the ventricle has less time to fill, so preload drops and stroke volume may not rise as much as the heart rate does. The net effect can limit diastolic filling and even blunt the intended rise in cardiac output, making tachycardia a notable disadvantage of this drug.

In this context, the other options don’t fit: reducing the risk of arrhythmias isn’t correct because isoproterenol can provoke tachyarrhythmias; improved diastolic filling isn’t accurate since filling time is actually reduced with faster heart rates; and increasing systemic vascular resistance isn’t typical—isoproterenol usually lowers SVR through beta-2–mediated vasodilation.