All are true statement about digitalis action EXCEPT:
**Core Concept**
Digitalis is a cardiac glycoside used to treat heart failure and atrial fibrillation. It increases the force of cardiac contractions (inotropic effect) and slows the heart rate (negative chronotropic effect) by inhibiting the Na+/K+ ATPase pump, leading to increased intracellular calcium levels and enhanced contractility.
**Why the Correct Answer is Right**
Digitalis acts by competitively inhibiting the Na+/K+ ATPase pump, which increases the intracellular concentration of sodium. This increase in sodium triggers a series of events that ultimately lead to an increase in intracellular calcium levels. The increased calcium binds to troponin and tropomyosin, causing a greater number of myosin heads to bind to actin, resulting in enhanced contractility. The negative chronotropic effect is mediated by increased vagal tone, which is a result of increased parasympathetic activity in the heart.
**Why Each Wrong Option is Incorrect**
**Option A:** While it is true that digitalis increases the force of cardiac contractions, this is not the only effect of digitalis. Digitalis also has a negative chronotropic effect, which is a significant aspect of its action.
**Option B:** Digitalis does indeed have a positive inotropic effect, but this is not the only statement about digitalis action that is true.
**Option C:** This is a true statement about digitalis action. Digitalis increases the refractory period of the cardiac muscle, which is a result of its negative chronotropic effect.
**Clinical Pearl / High-Yield Fact**
A key side effect of digitalis toxicity is the development of atrioventricular (AV) block, which can progress to complete heart block. This is often seen in patients with renal impairment, as digitalis is primarily excreted by the kidneys.
**Correct Answer:** A.