フェニレフリン
**フェニレフリン**は、強力で直接作用型の合成交感神経模倣アミンであり、獣医学においては主にその強力な血管収縮作用のために利用されます。 主な臨床応用は以下の通りです： - **低血圧およびショック**：十分な循環血液量補充後、明らかな心刺激を引き起こすことなく血圧を上昇させるために非経口的に使用されます。 - **薬物誘発性低血圧**：薬物の過剰投与や特異体質的反応（フェノチアジン系、アドレナリン遮断薬など）に続発する低血圧クリーゼに対して非常に有効です。 - **眼科用**：診断用眼科検査、後虹彩癒着形成の軽減、および複雑なぶどう膜炎に伴う疼痛の緩和のために局所投与されます。 - **鼻閉改善**：粘膜の充血を軽減するために点鼻投与されます。 *臨床のポイント*：フェニレフリンは通常用量において有意なβ-アドレナリン作用を持たないため、血圧の上昇が必要であるが心拍数や心筋仕事量の増加が禁忌である場合（例：肥大型心筋症の患者）に特に有用です。
作用機序: 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.
動物種別の用量
- 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.
投与経路
禁忌
- Severe hypertension
- Ventricular tachycardia
- Hypersensitivity to phenylephrine
- Do not apply topically once ophthalmic surgery has started (to avoid direct arterial absorption)
有害事象
- 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)
薬物相互作用
- 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
モニタリング
- 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
過量投与
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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