Adrenergic Blocking Agents Lecture Practice Flashcards

Classification and General Action of Adrenergic Blocking Agents

• General Nomenclature and Characterization:

  • These agents are primarily referred to as sympatholytic drugs because they perform lysis—effectively breaking, blocking, or lyzing the natural effects of the sympathetic nervous system.

  • This contrasts with sympathomimetic drugs, which mimic the effects of the sympathetic nervous system.

• Primary Mechanism of Action:

  • The drugs function by occupying receptor sites to prevent norepinephrine from activating them.

  • The therapeutic and adverse effects are determined by several factors regarding receptor interaction:

    • The degree of blockade (full vs. partial).

    • The nature of the interaction (e.g., competitive antagonism).

    • Affinity for specific receptor sites.

• General Physiological Effects:

  • A decrease in systemic blood pressure.

  • A potential decrease in respiratory rate.

  • A potential increase in heart rate as an associated response.

  • Vasodilation: Since the sympathetic nervous system's constrictive effects are blocked, the vessels dilate, leading to improved perfusion in areas such as the kidneys.

Nonselective Adrenergic Blocking Agents (Alpha and Beta)

• Drugs in this Subgroup:

  • Amiodarone, carbinolol, and labetalol.

• Specific Drug Indications:

  • Carbidolol (Carbidolol/Carbentol): Primarily used for adults with heart failure because it is effective at controlling symptoms. It is available only in an oral format.

  • Labetalol: Utilized for patients with extreme hypertension, hypertensive emergencies/crises, or those with pheochromocytoma. It is also indicated for managing hypertension in patients undergoing clonidine withdrawal. It is available in both IV and oral forms.

  • Amiodarone: Classified as an antiarrhythmic (or antidiarrhythmic). It is used in emergency situations for rate control. It is available as an IV medication.

• Pharmacokinetics and Duration:

  • Amiodarone Half-life: Extremely long, estimated at approximately $60$ days. Due to this duration, effects persist long after the drug is discontinued.

  • Labetalol (IV): Onset is nearly immediate, occurring within $5$ minutes. Effects last for approximately $5.5$ to $6$ hours.

  • Labetalol (Oral): Peaks in $1-2$ hours and lasts for $8-12$ hours. The half-life for the oral form is $6-8$ hours. Dosing is typically scheduled twice a day.

• Contraindications and Cautions:

  • CHF (Congestive Heart Failure): Generally contraindicated because vasodilation and heart rate changes can exacerbate the condition.

  • Bradycardia/Heart Block: Contraindicated because these drugs further decrease the heart rate.

  • Pregnancy and Lactation: These are cautions rather than absolute contraindications. Risks must be weighed against benefits (e.g., in cases of pre-eclampsia).

  • Fetal Risks of Amiodarone: This specific drug crosses the placenta and can cause fetal bradycardia, neonatal bradycardia, fetal hypothyroidism, growth retardation, and future birth adverse effects.

  • Diabetes: Caution is required as these agents can mask the symptoms of hypoglycemia (e.g., hiding diaphoresis, dizziness, and tachycardia).

• Drug-Drug Interactions:

  • Gas Anesthetics (e.g., Halothane): May lead to extreme hypotension.

  • Diabetic Agents: Increased risk of masked hypoglycemia.

  • Calcium Channel Blockers: Can cause a significant drop in heart rate.

Nonselective Alpha Adrenergic Blocking Agents

• Primary Agent: Cantolamine (Phentolamine).

• Indications:

  • Extravasation: Used to counteract the tissue damage caused by IV extravasation of dopamine, norepinephrine, or epinephrine. It is injected into the area to plug the tissue and block alpha receptors.

  • Pheochromocytoma: Management of severe hypertension associated with this tumor, including during surgery.

• Contraindications:

  • Coronary Artery Disease (CAD) or MI: Because of underlying atherosclerosis, it can trigger angina, myocardial infarction, or stroke (CVA).

• Adverse Effects:

  • Angina, MI, stroke, orthostatic hypotension, arrhythmias, and weakness.

• Interactions:

  • Counteracts ephedrine and epinephrine.

  • Reacts with alcohol to increase effects.

Alpha-1 Selective Adrenergic Blocking Agents

• Drugs in this Subgroup: Doxazosin and Plazosin.

• Primary Indications:

  • BPH (Benign Prostatic Hypertrophy): These are the most common treatments for BPH as they decrease the vascular tone and help manage prostate hypertrophy.

  • Hypertension: Plazosin is specifically noted for this use.

• Nursing Considerations:

  • Lactation: This is a contraindication as the drug enters breast milk and affects the infant.

  • Fluid Volume: Use caution in patients with CHF or renal failure, as vasodilation may lead to increased volume and fluid overload.

  • Interactions: Avoid concurrent use with other antihypertensives or vasodilators to prevent profound hypotension.

Nonselective Beta-Adrenergic Agents

• Drugs in this Subgroup: Nabilol, propranolol, natalol, temolol, sotalol, and tylenol (noted in context of glaucoma/beta-blocker list).

• Mechanism: Competitive antagonists that block both Beta-1 and Beta-2 receptors in the heart, lungs, and nephrons.

• Uses beyond Hypertension:

  • Cardiovascular issues and angina.

  • Migraine Prophylaxis: Propranolol is specifically indicated for preventing migraine headaches.

  • Anxiety: Propranolol is used as an adjunct for situational anxiety (e.g., public speaking).

  • Glaucoma: Timolol (temolol) is administered via eye drops.

• Contraindications:

  • Asthma and COPD: Since these block Beta-2 receptors, they can cause significant bronchospasm.

  • Pregnancy/Lactation: Can prevent the fetus/infant from utilizing necessary receptor sites.

• Adverse Effects and CNS Concerns:

  • Erectile Dysfunction: A major reason for non-compliance in younger men.

  • CNS Issues: Depression, fatigue, sleep disturbances, and a general feeling of being "low."

  • NSAID Interaction: Concurrent use with NSAIDs can lead to fluid and sodium retention, exacerbating or causing CHF.

Beta-1 Selective Adrenergic Agents

• Drugs in this Subgroup: Atenolol, metoprolol, and esthol (Esmolol).

• Clinical Advantage: These agents selectively target Beta-1 receptors in the heart, having significantly less effect on Beta-2 receptors in the lungs. They are the preferred beta-blockers for patients with asthma, COPD, or a history of smoking.

• Specific Deliverables:

  • Asmolol: Only available as an IV.

  • Metoprolol/Atenolol: Available in both PO and IV formats.

  • IV Onset: Almost immediate; peaks in $5$ minutes and lasts $12$ to $24$ hours.

• Drug Interactions:

  • Agonists: Giving a beta-agonist with these will decrease the effectiveness of both.

  • NSAIDs: Increases risk of CHF.

  • Rifampin: Decreases the effectiveness of beta blockers.

  • Barabinil: Decreases heart rate further.

Questions & Discussion

• Q: Will these medications cause a decrease in respiratory rate and blood pressure?

  • A: Yes, decreasing blood pressure and respiratory rate are expected effects, though heart rate may also increase depending on the drug/response. Vasodilation occurs because the sympathetic nervous system is blocked.

• Q: Can these cause angioedema since they affect renin?

  • A: While theoretically possible (since renin is inhibited), it is not a major concern. Renin is still produced but not activated at the level seen in the RAS (Renin-Angiotensin System) pathway. The speaker noted never seeing angioedema with these specific agents in clinical practice.

• Q: What is the threshold for acting on vital sign changes?

  • A: A common guideline is if the blood pressure or heart rate goes up or down by $25\%$ from the patient's documented baseline.

• Q: What is the difference between an allergy and a sensitivity?

  • A: An allergy involves a specific immune reaction; a sensitivity may allow for the drug to still be given with careful monitoring. Always ask the patient what their specific reaction is (e.g., rash vs. upset stomach).

• Anecdote regarding Beta-blockers and Asthma:

  • The speaker shared a story about their mother, who has a history of asthma and PVCs (premature ventricular contractions). A cardiologist prescribed metoprolol. After a few days, she experienced severe chest tightness due to bronchospasm. The speaker emphasized that even with "selective" agents, clinicians must be extremely vigilant with respiratory histories because some pulmonary effects can still occur, especially with environmental triggers.

General Nursing Considerations and Patient Education

• Assessment:

  • Take a thorough history: focus on asthma, COPD, bronchospasm, and diabetes.

  • Obtain a baseline EKG to monitor changes in the heart's conduction system, particularly with amiodarone.

  • Obtain baseline vital signs to determine the patient's normal range at home.

  • Monitor labs: CBC, chemistry, liver enzymes (since many are metabolized in the liver), and renal function tests.

• Administration Guidelines:

  • Hold Parameters: Generally, if the heart rate is less than $50$ bpm, the medication should be held.

  • Neuro Checks: Crucial for agents like phentolamine due to the risk of CVA/stroke; monitor orientation and affect.

• Patient Education:

  • Fall Risk: Patients (especially those on new prescriptions) are at high risk for orthostatic hypotension. Instruct them to rise slowly and call for assistance before getting out of bed.

  • Monitoring: Teach patients how to monitor their own blood pressure and heart rate at home.

  • Compliance: Discuss potential side effects like depression and sexual dysfunction to encourage transparency and adherence.