**Core Concept**
The Valsalva maneuver involves forced expiration against a closed glottis, increasing intrathoracic pressure and altering venous return to the heart. This affects the heart rate through the baroreceptor reflex and autonomic nervous system modulation.

**Why the Correct Answer is Right**
During the Valsalva maneuver, the increased intrathoracic pressure reduces venous return to the heart, leading to a decrease in cardiac preload. This triggers a baroreceptor-mediated reflex, activating the sympathetic nervous system and inhibiting the parasympathetic nervous system. The resulting increase in sympathetic tone increases heart rate through enhanced beta-adrenergic stimulation of the sinoatrial (SA) node. This is known as the Bainbridge reflex, which normally increases heart rate with increased venous return, but in this case, the decreased venous return overrides this effect.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the Valsalva maneuver primarily affects venous return and cardiac preload, not directly influencing the rate of heart contraction.
* **Option B:** Incorrect because the decrease in intrathoracic pressure during inspiration would actually increase venous return and cardiac preload, opposite to the effect of the Valsalva maneuver.
* **Option D:** Incorrect because the Valsalva maneuver is a transient event, and the heart rate typically returns to baseline once the maneuver is stopped.

**Clinical Pearl / High-Yield Fact**
Remember that the Valsalva maneuver can be used clinically to assess the integrity of the autonomic nervous system, particularly in cases of suspected autonomic dysfunction or heart block.

**Correct Answer: C. Increased heart rate.**