Digitalis does not affect which parameter of heart –
**Core Concept**
Digitalis is a cardiac glycoside that increases the force of cardiac contractions and slows the heart rate by inhibiting the Na+/K+-ATPase pump, leading to increased intracellular calcium and enhanced contractility. This mechanism affects various parameters of the heart, but there's one parameter that remains unchanged.
**Why the Correct Answer is Right**
Digitalis increases the force of cardiac contractions by increasing the sensitivity of the contractile apparatus to calcium. However, it does not affect the **stroke volume**, as the volume of blood ejected per beat is determined by the preload (end-diastolic volume) and afterload (peripheral resistance), which are not directly influenced by digitalis.
**Why Each Wrong Option is Incorrect**
**Option A:** The **heart rate** is actually decreased by digitalis due to its vagolytic effects, which inhibit the release of acetylcholine from the vagus nerve.
**Option B:** The **contractility** of the heart is increased by digitalis due to its ability to increase intracellular calcium levels.
**Option C:** The **electrocardiogram** may show changes such as a prolonged PR interval and widened QRS complex due to digitalis toxicity, but this is not a parameter that digitalis does not affect.
**Clinical Pearl / High-Yield Fact**
Digitalis toxicity is often associated with hyperkalemia, which can be life-threatening. Remember the "digitalis toxicity" mnemonic: "Dysrhythmias, Iritis, Inhibited gut, Hypokalemia, and Hyperkalemia".
**Correct Answer: C.