18.3 Angiotensin II Receptor Blockers (ARBs)

Mechanism of Action and Efficacy of ARBs

ARBs (angiotensin II receptor blockers) lower blood pressure by blocking the action of angiotensin II, a hormone that constricts blood vessels and promotes fluid retention. They work within the same system as ACE inhibitors but target a different step in the pathway, which gives them a distinct side effect profile. For nurses, understanding how ARBs work helps you anticipate monitoring needs and educate patients effectively.

Mechanism of Action

ARBs selectively block angiotensin II type 1 (AT1) receptors. Here's what that accomplishes, step by step:

1. Blocks vasoconstriction. Angiotensin II normally binds to AT1 receptors on blood vessel walls, causing them to narrow. ARBs prevent this binding, so vessels relax and widen (vasodilation).
2. Reduces aldosterone secretion. Without angiotensin II signaling at AT1 receptors, the adrenal glands release less aldosterone. Since aldosterone tells the kidneys to hold onto sodium and water, less aldosterone means less fluid retention.
3. Lowers peripheral vascular resistance. With relaxed vessels and reduced plasma volume, the overall resistance against which the heart pumps drops, and blood pressure falls.
4. Decreases cardiac workload. The combination of vasodilation and lower fluid volume means the heart doesn't have to work as hard to push blood through the body.

The net result: lower blood pressure, improved blood flow to organs and tissues, and reduced strain on the heart.

ARBs vs. Other Antihypertensive Drugs

ARBs and ACE inhibitors (e.g., lisinopril, enalapril) both target the renin-angiotensin-aldosterone system (RAAS) and have similar blood pressure-lowering efficacy. The key clinical difference is tolerability:

• ACE inhibitors block the enzyme that produces angiotensin II, but that same enzyme also breaks down bradykinin. Bradykinin buildup is what causes the dry cough seen in up to 10-15% of patients on ACE inhibitors, and in rare cases, angioedema (swelling of the face, tongue, or throat).
• ARBs block the receptor where angiotensin II acts, so bradykinin isn't affected. This makes ARBs a go-to alternative for patients who can't tolerate ACE inhibitors due to cough or angioedema.

ARBs also have comparable efficacy to other antihypertensive classes, including calcium channel blockers (amlodipine), diuretics (hydrochlorothiazide), and beta-blockers (metoprolol). The choice among these depends on patient-specific factors like age, race, and comorbidities such as diabetes or chronic kidney disease.

Pharmacokinetics and Adverse Effects

Pharmacokinetics:

• Well-absorbed orally, with peak plasma concentrations typically reached within 1-4 hours
• Most ARBs have long half-lives, which allows for once-daily dosing (good for adherence)
• Primarily metabolized by the liver and excreted through bile and urine

Common adverse effects are generally mild:

• Dizziness, headache, fatigue
• Hypotension (especially with the first dose or in volume-depleted patients)

Serious adverse effects to watch for:

• Hyperkalemia (elevated potassium): Because ARBs reduce aldosterone, the kidneys retain more potassium. This is especially risky in patients who are elderly, diabetic, or have pre-existing renal impairment.
• Renal dysfunction: ARBs can decrease kidney perfusion in patients whose kidney function depends on angiotensin II to maintain filtration pressure (e.g., patients with renal artery stenosis).
• Contraindicated in pregnancy: ARBs can cause serious fetal harm, including renal malformations and oligohydramnios. Women of childbearing age should be counseled about this before starting therapy.

Nursing Considerations and Patient Education

Nursing Considerations for ARB Administration

Monitor blood pressure and heart rate regularly.

• Assess for orthostatic hypotension (a drop in BP when standing), particularly in elderly or volume-depleted (dehydrated) patients. Have the patient sit or lie down for a few minutes, then stand, and compare readings.
• Adjust dosage as needed to reach the target blood pressure.

Monitor renal function and electrolytes.

• Check serum creatinine and potassium levels periodically, especially after initiating therapy or changing the dose.
• Use extra caution in patients with renal impairment or those already at risk for hyperkalemia.

Watch for drug interactions.

• Avoid combining ARBs with potassium-sparing diuretics (e.g., spironolactone) or potassium supplements unless specifically ordered, due to the compounded risk of hyperkalemia.
• Use caution with NSAIDs (e.g., ibuprofen), which can blunt the blood pressure-lowering effect of ARBs and worsen kidney function.
• Do not combine ARBs with ACE inhibitors (dual RAAS blockade increases the risk of hyperkalemia, hypotension, and renal failure without added benefit for most patients).

Administer consistently.

• Give ARBs at the same time each day to maintain stable blood levels. Some can be taken with or without food; others have specific instructions (check the individual drug's labeling).

Patient Education for ARB Use

Adherence and medication management:

• Stress that ARBs control blood pressure but don't cure hypertension. Stopping the medication can cause blood pressure to rise again.
• Advise patients never to skip doses or discontinue without talking to their provider.
• Suggest practical adherence tools: pill organizers, phone alarms, or linking the dose to a daily routine (e.g., with breakfast).

Self-monitoring at home:

• Teach proper blood pressure measurement technique: seated position, arm supported at heart level, cuff on bare skin, rest for 5 minutes before measuring.
• Provide guidance on what readings to report. For example, if systolic pressure drops below 90 mmHg or rises above their provider's specified threshold, they should call their provider.

Recognizing and reporting side effects:

• Mild dizziness or lightheadedness can occur, especially early in treatment. Advise patients to rise slowly from sitting or lying positions.
• Instruct patients to seek immediate medical attention for signs of angioedema (facial or throat swelling), severe dizziness, or signs of hyperkalemia (muscle weakness, irregular heartbeat).

Lifestyle modifications:

• Low-sodium diet: Reducing salt intake (aim for less than 2,300 mg/day) supports blood pressure control.
• Regular physical activity: Brisk walking, swimming, or cycling for at least 150 minutes per week improves cardiovascular health.
• Weight management and smoking cessation: Both significantly reduce cardiovascular risk and enhance the effectiveness of drug therapy.

Pregnancy warning:

• Women of childbearing age must understand that ARBs are contraindicated in pregnancy. If a patient becomes pregnant or plans to become pregnant, they should notify their provider immediately so the medication can be changed.

Follow-up:

• Reinforce the need for regular appointments to monitor blood pressure, kidney function, and electrolyte levels. These visits are how the care team catches problems early and adjusts treatment as needed.