Prazosin
Prazosin is a highly selective **alpha-1 adrenergic receptor antagonist** (sympatholytic). While historically utilized as a balanced vasodilator (reducing both preload and afterload) for the adjunctive treatment of congestive heart failure (CHF) and systemic or pulmonary hypertension, its primary application in modern veterinary medicine is the management of **lower urinary tract disease**. **Clinical Pearls:** * **Urethral Relaxation:** Prazosin is highly effective at relaxing the smooth muscle of the internal urethral sphincter. It is a staple in treating functional urethral obstruction, particularly in "blocked cats" (feline idiopathic cystitis/FLUTD) and dogs with reflex dyssynergia or prostatic disease. * **Cardiovascular Profile:** Unlike hydralazine, prazosin dilates both the arterial and venous sides and does not typically trigger reflex tachycardia or increase renin release.
กลไกการออกฤทธิ์: **Prazosin** competitively binds to and blocks **post-synaptic alpha-1 adrenergic receptors**. * **Genitourinary Tract:** Blockade of alpha-1 receptors in the trigone of the bladder and the proximal urethra → **smooth muscle relaxation** → decreased urethral resistance → facilitates micturition and relieves functional obstruction. * **Cardiovascular System:** Blockade of alpha-1 receptors on vascular smooth muscle → **vasodilation** of both arterioles and veins → decreased peripheral vascular resistance (afterload) and decreased venous return (preload) → lowered systemic blood pressure and reduced right atrial pressure.
ขนาดยาตามชนิดสัตว์
- Decrease urethral resistance · 0.5 mg (total dose) PO q8h or 0.03 mg/kg IV · PO/IV · q8h (PO)
- Decrease urethral resistance · 0.03 mg/kg IV · IV
- Functional urethral obstruction · 0.25-0.5 mg/cat (total dose) PO q12-24h · PO · q12-24h
- Urethral spasm · 0.25-1 mg/cat (total dose) PO q8-12h · PO · q8-12h · Initially give for 5-7 days then wean off if possible
- Reduce urethral tone · 0.5 mg/cat (total dose) PO once to two times a day · PO · q12-24h
- Adjunctive treatment of heart failure · 1 mg PO three times daily for dogs weighing less than 15 kg; 2 mg three times daily PO for dogs weighing more than 15 kg · PO · q8h
- Hypertension · 1-4 mg (total dose) PO q12-24 hours · PO · q12-24h
- Hypertension in a large dog · 1 mg (total dose) PO q8-12h · PO · q8-12h
- Decrease urethral resistance · 1 mg per 15 kg of body weight PO q8h · PO · q8h
- Functional urethral obstruction · 1 mg/15 kg of body weight PO q8-24h · PO · q8-24h
- Decrease urethral resistance · 1 mg per 15 kg of body weight PO q12-24h · PO · q12-24h
วิธีการให้ยา
ข้อห้ามใช้
- Preexisting hypotensive conditions
- Pre-existing hypotension
- Hypovolemia
- Known hypersensitivity to alpha-blockers
อาการไม่พึงประสงค์
- Hypotension (including orthostatic hypotension/syncope)
- Lethargy
- Dizziness
- Nausea
- Vomiting
- Diarrhea
- Constipation
- Tachyphylaxis (drug tolerance)
- Hypotension (especially first-dose effect)
- Lethargy / Weakness
- Dizziness / Syncope
- Gastrointestinal upset (vomiting, diarrhea)
- Reflex tachycardia
อันตรกิริยาระหว่างยา
- Beta-blocking agents (e.g., propranolol) · May enhance the postural hypotensive effects seen after the first dose of prazosin
- Clonidine · May decrease prazosin antihypertensive effects
- Sildenafil (and other PDE inhibitors) · May increase risk for hypotension
- Verapamil or Nifedipine · May cause synergistic hypotensive effects when used concomitantly with prazosin
- Amlodipine · Additive hypotensive effects · moderate
- ACE Inhibitors (e.g., Benazepril, Enalapril) · Increased risk of severe hypotension · moderate
- Beta-blockers (e.g., Propranolol) · Exacerbation of postural hypotension · moderate
- Sildenafil · Profound hypotension · major
การติดตาม
- Baseline thoracic radiographs
- Mucous membrane color
- Capillary refill time (CRT)
- Arterial blood pressure
- Venous PO2 (if possible)
- Systemic blood pressure
- Heart rate
- Urine output and ease of urination (if used for urethral spasm)
การได้รับยาเกินขนาด
Based on ASPCA APCC data (2008-2009): * **Dogs:** Common findings include hyperactivity, tachycardia, hyperthermia, panting, and agitation. * **Cats:** Common findings include tachycardia. **Treatment:** * Evacuate gastric contents and administer activated charcoal using standard precautionary measures if ingestion was recent and cardiovascular status is stable. * Monitor heart rate and blood pressure. * Treat shock using volume expanders and pressor agents if necessary. * Monitor and support renal function.
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