**Question:** Cardiac arrhythmias are most commonly seen during administration of:

**Core Concept:** Cardiac arrhythmias are abnormalities in the heart's electrical activity that lead to changes in heart rate and rhythm. They are often caused by disturbances in the normal electrical conduction system of the heart.

**Why the Correct Answer is Right:**

Correct Answer: **D. Digitalis**

Digitalis is a group of cardiac glycosides derived from the foxglove plant (Digitalis purpurea). These drugs are commonly used as diuretics and to treat congestive heart failure. However, digitalis is a potent inhibitor of the sodium pump (Na+/K+-ATPase) in the cardiac myocytes, which can lead to increased intracellular sodium and potassium levels. This disrupts the normal cardiac action potential and impairs the heart's electrical conduction system, resulting in various types of arrhythmias, such as bradycardia, sinus tachycardia, atrial fibrillation, and ventricular tachycardia.

**Why Each Wrong Option is Incorrect:**

A. **Inotropes (e.g., dopamine, dobutamine):** These drugs primarily affect myocardial contractility, increasing cardiac output, and are usually used to treat hypotension and shock states rather than causing arrhythmias.

B. **Calcium channel blockers (e.g., verapamil, diltiazem):** These drugs are used to treat hypertension, angina, and arrhythmias by blocking calcium entry into cardiac myocytes, which reduces myocardial contractility and heart rate.

C. **ß-blockers (e.g., propranolol, atenolol):** B-blockers are used to treat hypertension, angina, and arrhythmias by blocking the effects of catecholamines on cardiac myocytes, reducing heart rate and contractility.

D. **Digitalis (e.g., ouabain):** As mentioned earlier, digitalis interferes with the cardiac conduction system, leading to arrhythmias when given in high doses or improperly.

**Clinical Pearl:**

Digitalis-induced arrhythmias are a common cause of ECG changes, such as prolonged QT interval, U waves, and ST-T wave abnormalities. These arrhythmias are dose-dependent, meaning they become more common with higher doses and are reversible when the drug is discontinued or dose is decreased. Close monitoring of ECG and electrolyte levels (potassium, magnesium) is crucial when administering digitalis to minimize the risk of arrhythmias.

Understanding the mechanism of action and potential side effects of each medication can help prevent and manage complications during treatment.