去氧腎上腺素
**去氧腎上腺素 (Phenylephrine)** 是一種強效、直接作用的合成擬交感神經胺，在獸醫學中主要利用其強大的血管收縮特性。 主要臨床應用包括： - **低血壓與休克**：在充分補充血容量後，以腸胃外途徑給藥來提升血壓，且不會引起明顯的心臟刺激。 - **藥物引起的低血壓**：對於因藥物過量或特異性反應（如吩噻嗪類、腎上腺素阻斷劑）引起的低血壓危機非常有效。 - **眼科應用**：局部應用於診斷性眼科檢查、減少虹膜後沾黏的形成，以及緩解複雜性葡萄膜炎相關的疼痛。 - **鼻腔去充血**：鼻內給藥以減少黏膜充血。 *臨床要點*：由於去氧腎上腺素在正常劑量下缺乏顯著的 β-腎上腺素活性，當需要升高血壓但禁忌增加心率或心肌耗氧量時（例如肥厚型心肌病患者），此藥物特別有用。
作用機制: Phenylephrine is a highly selective **alpha-1 adrenergic receptor agonist**. - **Vascular Smooth Muscle**: Binds to post-synaptic **alpha-1 receptors** → activates **Gq-proteins** → stimulates **Phospholipase C (PLC)** → cleaves PIP2 into **Inositol Triphosphate (IP3)** and **Diacylglycerol (DAG)** → IP3 triggers the release of intracellular calcium from the sarcoplasmic reticulum → profound **vasoconstriction**. - **Cardiovascular Effects**: Intravenous administration causes peripheral vasoconstriction, leading to significant increases in both diastolic and systolic blood pressures. This sudden increase in blood pressure often triggers a **baroreceptor-mediated reflex bradycardia** (which can be blocked by anticholinergics like atropine). - **Organ Perfusion**: Constricts most vascular beds (renal, splanchnic, pulmonary, cutaneous), though coronary blood flow is typically increased due to elevated aortic diastolic pressure.
各物種劑量
- To increase blood pressure by vasoconstriction in pronounced systemic vasodilation or when increasing myocardial contractility is disadvantageous (e.g., hypertrophic cardiomyopathy) · 1-2 micrograms/kg/minute · IV · CRI · Infusions of 1 microgram/kg/min significantly increased mean arterial pressure without a change in cardiac output. At 2 micrograms/kg/min, cardiac index also was increased with an increase in stroke volume index.
- As a constant rate infusion for profound vasodilation due to septic shock · 2-10 micrograms/kg/minute · IV · CRI
- As a vasopressor in catastrophic stages of hypovolemic shock · 1-3 micrograms/kg/min · IV · CRI
- As a CRI to increase peripheral vascular resistance and mean arterial blood pressure · Low dose is 1 microgram/kg/min; high dose is 3 micrograms/kg/min · IV · CRI · May see reflex bradycardia, and vasoconstriction can lead to excessive decreases in blood flow to liver, GI tract, and kidneys, although coronary blood flow is increased.
- Diagnosis of Horner's syndrome (denervation hypersensitivity) · 1% solution topically to both eyes · topical · once · single dose · Use lower concentrate solutions in cats to avoid systemic hypertension.
- General use · 5 mg · IV · Single dose · ARCI UCGFS Class 3 Drug
- As a CRI to increase peripheral vascular resistance and mean arterial blood pressure · Low dose is 1 microgram/kg/min; high dose is 3 micrograms/kg/min · IV · CRI · May see reflex bradycardia, and vasoconstriction can lead to excessive decreases in blood flow to liver, GI tract, and kidneys, although coronary blood flow is increased.
給藥途徑
禁忌症
- Severe hypertension
- Ventricular tachycardia
- Hypersensitivity to phenylephrine
- Do not apply topically once ophthalmic surgery has started (to avoid direct arterial absorption)
不良反應
- Reflex bradycardia
- CNS effects (excitement, restlessness, headache)
- Arrhythmias (rare)
- Severe extravasation injuries (tissue necrosis and sloughing)
- Hypertension
- Tachycardia
- Tissue necrosis and sloughing (if IV extravasation occurs)
藥物相互作用
- Alpha-Adrenergic Blockers (phentolamine, phenothiazines, phenoxybenzamine) · Higher dosages of phenylephrine may be required to attain a pressor effect if these agents have been used prior to therapy.
- Anesthetics, General (halogenated) · Potentially may induce cardiac arrhythmias when used with halothane anesthesia.
- Atropine (and other anticholinergics) · Block the reflex bradycardia caused by phenylephrine.
- Beta-Adrenergic Blockers · The cardiostimulatory effects of phenylephrine (seen at high doses) can be blocked.
- Digoxin · Use with phenylephrine may cause increased myocardium sensitization. · major
- Monoamine Oxidase (MAO) Inhibitors (e.g., amitraz, selegiline) · Should not be used with phenylephrine because of a pronounced pressor effect.
- Oxytocin · When used concurrently with oxytocic agents, pressor effects may be enhanced.
- Sympathomimetic Agents (epinephrine) · Tachycardia and serious arrhythmias are possible.
- Volatile anaesthetic agents · Increased risk of cardiac arrhythmias · major
- Oxytocic agents · Enhanced pressor effects leading to severe hypertension · major
監測
- Cardiac rate and rhythm
- Blood pressure
- Blood gases (if possible)
- IV catheter site (for signs of extravasation)
- Blood pressure (systemic use or high-concentration topical use)
- Heart rate and rhythm (ECG)
- Pupillary response and time to dilation (for Horner's syndrome testing)
- IV catheter site for signs of extravasation
過量
The margin of safety with phenylephrine overdose is fairly wide, especially after oral administration. **Clinical Signs**: - Common findings in dogs include vomiting, lethargy, depression, hyperactivity, and tachycardia. - Severe overdosage can cause hypertension, seizures, paresthesias, ventricular extrasystoles, and cerebral hemorrhage. **Treatment**: - Vomiting is commonly seen and may self-decontaminate oral exposures. - Cardiovascular changes often respond well to IV fluids. - Beta-blockers or nitroprusside may be indicated when signs (tachycardia, severe hypertension) are refractory to fluids.
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