酚苄明
酚苄明是一種長效、非競爭性的**α-腎上腺素受體拮抗劑**。 在獸醫學中，主要用於： - **小動物**：降低尿道內括約肌張力，以治療功能性尿道阻塞或逼尿肌無反射。它也是**嗜鉻細胞瘤**病患關鍵的術前藥物，用於預防手術操作腫瘤時引發致命的高血壓危象。 - **馬**：特殊應用包括治療早期蹄葉炎、分泌性腹瀉和膀胱輕癱。 > **臨床要點**：由於酚苄明與受體發生不可逆結合，其作用時間長，且必須等待身體合成新的受體才能恢復，此過程通常需要數天時間。
作用機制: Phenoxybenzamine covalently and irreversibly binds to **alpha-1 and alpha-2 adrenergic receptors** → non-competitive blockade of circulating epinephrine and norepinephrine → profound vasodilation and relaxation of smooth muscle. - **Urinary Tract**: Blockade of alpha receptors in the trigone and internal urethral sphincter → decreased urethral resistance → facilitation of bladder emptying. - **Cardiovascular**: Blockade of vascular alpha receptors → decreased peripheral vascular resistance → lowered blood pressure (often causing reflex tachycardia). Because the binding is irreversible, the effect is often described as a 'chemical sympathectomy'. It has no effect on beta-adrenergic or parasympathetic receptors.
各物種劑量
- Functional urethral obstruction · 2.5-7.5 mg/cat · PO · once to twice daily
- Functional urethral obstruction · 1.25-7.5 mg (total dose) · PO · q12-24h
- Functional urethral obstruction · 2.5-10 mg (total dose) · PO · q24h
- Short-term treatment of hypertension · 0.5 mg/kg · PO · q12h
- Short-term treatment of hypertension · 2.5 mg (total dose) increasing by 2.5 mg up to a maximum of 10 mg (total dose) · PO · q12h
- Short-term treatment of hypertension · 2.5-7.5 mg per cat · PO · q8-12h
- Decrease urethral sphincter tone in bladder paresis · 0.7 mg/kg · PO · 4 times a day · Used in combination with bethanechol at 0.25-0.75 mg/kg PO 2-4 times a day.
- Adjunctive treatment of laminitis (developmental phase) · 1 mg/kg · IV · q12h · for 2 doses
- Treatment of profuse, watery diarrhea · 200-600 mg · q12h
- Functional urethral obstruction · 0.25 mg/kg · PO · q12-24h · Alternative dose: 2.5-20 mg (total dose) PO q12-24h
- Functional urethral obstruction · 0.25 mg/kg · PO · q12h
劑量為持牌獸醫專業人員的臨床參考。請務必對照最新藥品說明書及個別病患確認。
給藥途徑
禁忌症
- Conditions where hypotension is undesirable (e.g., shock, unless fluid replacement is adequate)
- Horses with clinical signs of colic
- Glaucoma (relative contraindication)
- Diabetes mellitus (relative contraindication)
不良反應
- Hypotension (postural/orthostatic)
- Hypertension (rebound)
- Miosis
- Increased intraocular pressure
- Tachycardia (reflex)
- Sodium retention
- Inhibition of ejaculation
- Nasal congestion
- Weakness or dizziness
- Gastrointestinal effects (nausea, vomiting)
- Constipation (specifically noted in horses)
- Hypotension
- Tachycardia
藥物相互作用
- Epinephrine · Epinephrine reversal: If used with drugs that have both alpha- and beta-adrenergic effects, increased hypotension, vasodilation, or tachycardia may result due to unopposed beta effects.
- Phenylephrine · Phenoxybenzamine will antagonize the effects of alpha-adrenergic sympathomimetic agents.
- Reserpine · Phenoxybenzamine can antagonize the hypothermic effects of reserpine.
- Beta-blockers · Increased risk of first dose hypotensive effect; if used for phaeochromocytoma, beta-blockers must only be started after alpha blockade is in place to avoid hypertensive crisis. · major
- Diuretics · Increased risk of first dose hypotensive effect. · moderate
- Alpha-adrenergic sympathomimetics (e.g., phenylephrine) · Phenoxybenzamine will antagonize the effects of these agents. · major
監測
- Clinical efficacy (e.g., adequate urination, ease of voiding)
- Blood pressure (monitor for hypotension)
- Heart rate (monitor for reflex tachycardia)
- Blood pressure
- Heart rate and rhythm
- Urine output and ease of urination
過量
Overdosage may yield signs of **postural hypotension** (dizziness, syncope), tachycardia, vomiting, lethargy, or shock. **Treatment**: - Empty the gut if ingestion was recent and there are no contraindications. - Treat hypotension with aggressive intravenous fluid support. - **DO NOT USE EPINEPHRINE** (causes paradoxical further drop in blood pressure due to unopposed beta-adrenergic vasodilation). - Most standard vasopressors are ineffective due to the alpha-blockade. Intravenous norepinephrine (levarterenol) may be beneficial if clinical signs are severe.
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