A patient on lithium therapy was found to be hypertensive also. Which of the following antihypertensive drugs is contraindicated in a patient on lithium therapy in order to prevent toxicity ?
**Core Concept**
Lithium therapy is associated with a narrow therapeutic index, and its toxicity can be exacerbated by various factors, including antihypertensive medications. The kidneys play a crucial role in lithium excretion, and any medication that impairs renal function can increase lithium levels, leading to toxicity.
**Why the Correct Answer is Right**
Nonsteroidal anti-inflammatory drugs (NSAIDs), particularly those that are potent inhibitors of prostaglandin synthesis, can reduce renal blood flow and glomerular filtration rate (GFR). This decrease in renal function can lead to increased lithium levels, thereby increasing the risk of lithium toxicity. The mechanism involves the inhibition of prostaglandin I2 (PGI2), a potent vasodilator that helps maintain renal blood flow. By inhibiting PGI2, NSAIDs can cause vasoconstriction, reducing renal perfusion and GFR.
**Why Each Wrong Option is Incorrect**
**Option A:** Diuretics, such as furosemide, can actually increase lithium clearance and are often used to manage lithium-induced nephrogenic diabetes insipidus. Therefore, they are not contraindicated in patients on lithium therapy.
**Option B:** Calcium channel blockers, such as nifedipine, do not have a significant effect on renal function and are not contraindicated in patients on lithium therapy.
**Option C:** Angiotensin-converting enzyme (ACE) inhibitors, such as enalapril, can actually increase lithium levels by reducing renal blood flow and GFR. However, they are not typically considered a significant risk factor for lithium toxicity.
**Clinical Pearl / High-Yield Fact**
When managing patients on lithium therapy, it is essential to avoid NSAIDs, particularly those with potent anti-inflammatory effects, as they can increase the risk of lithium toxicity. Instead, diuretics or other antihypertensive agents that do not significantly impair renal function should be used.
**Correct Answer:** A. NSAIDs, particularly those that inhibit prostaglandin synthesis, are contraindicated in patients on lithium therapy to prevent toxicity.