First dose hypotension is caused by :
**First Dose Hypotension**
**Core Concept**
First dose hypotension is a common side effect of certain medications, particularly vasodilators and antihypertensive agents. It occurs due to the sudden and significant decrease in blood pressure, leading to reduced perfusion of vital organs. This phenomenon is often seen with the initial administration of medications like hydralazine, minoxidil, and nitroprusside.
**Why the Correct Answer is Right**
First dose hypotension is caused by the sudden release of nitric oxide, a potent vasodilator, which leads to the relaxation of vascular smooth muscle and subsequent vasodilation. This results in a decrease in peripheral resistance and a subsequent drop in blood pressure. The mechanism of first dose hypotension is closely related to the activation of guanylate cyclase by nitric oxide, which increases cyclic guanosine monophosphate (cGMP) levels, causing smooth muscle relaxation.
**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it does not directly relate to the pathophysiology of first dose hypotension. While histamine release can cause vasodilation, it is not the primary mechanism behind first dose hypotension.
**Option B:**
This option is incorrect because it refers to a different type of adverse effect, specifically the risk of exacerbating angina or myocardial infarction in patients with pre-existing coronary artery disease.
**Option C:**
This option is incorrect because it refers to a different type of medication-induced side effect, specifically the risk of inducing or exacerbating asthma symptoms.
**Clinical Pearl / High-Yield Fact**
To minimize the risk of first dose hypotension, it is essential to carefully monitor patients for signs of hypotension, particularly when initiating therapy with medications like hydralazine or nitroprusside. Patients with pre-existing hypovolemia or cardiovascular disease are at increased risk and may require more cautious dosing and monitoring.
**Correct Answer: A. Nitric oxide-induced vasodilation**