Medetomidine
**Medetomidine** is a potent, synthetic **alpha-2 adrenergic agonist** widely used in veterinary medicine for its profound sedative, analgesic, and muscle-relaxant properties. Key clinical features include: - **Biphasic cardiovascular response**: Initial peripheral vasoconstriction leads to transient hypertension, followed by a reflex bradycardia and subsequent normotension or mild hypotension. - **Reversibility**: Its effects can be rapidly and completely reversed using specific alpha-2 antagonists like **atipamezole**. - **Stereochemistry**: Medetomidine is a racemic mixture of two stereoisomers. The active enantiomer is **dexmedetomidine**, which has largely replaced racemic medetomidine in many modern veterinary markets. - **Synergism**: Highly synergistic with opioids, ketamine, and other anesthetic agents, allowing for significant dose reductions of concurrent anesthetics (MAC-sparing effect). > **Clinical Pearl**: While it provides excellent somatic and visceral analgesia, the sedative effects typically outlast the analgesic effects. Patients may suddenly arouse if stimulated by loud noises or rough handling, even if they appear deeply sedated.
Mecanismo: Medetomidine produces its effects by binding to and activating **pre- and postsynaptic alpha-2 adrenergic receptors** in the central and peripheral nervous systems. - **Central Nervous System**: Activation of alpha-2 receptors in the **locus coeruleus** → inhibition of adenylyl cyclase → decreased cAMP → efflux of potassium (hyperpolarization) → decreased release of **norepinephrine**. This suppression of sympathetic outflow results in profound sedation, anxiolysis, and analgesia. - **Cardiovascular System**: Activation of peripheral **alpha-1 and alpha-2b receptors** in vascular smooth muscle → profound vasoconstriction → increased systemic vascular resistance (hypertension) → baroreceptor-mediated increased vagal tone → **reflex bradycardia**. - **Endocrine/Metabolic**: Inhibits insulin release from pancreatic beta cells → transient **hyperglycemia**. Decreases antidiuretic hormone (ADH) release → increased diuresis. Medetomidine is highly selective, with an alpha-2:alpha-1 selectivity ratio of 1620:1 (approximately 10 times more specific than xylazine).
Dosificación por especie
- Sedation/analgesia · 40-80 micrograms/kg IM · IM · Single dose · Higher doses increase duration, not depth of sedation
- For use with an IM opioid · 5-10 micrograms/kg · IM · Single dose
- Sedation/analgesia · 0.001-0.01 mg/kg (1-10 micrograms/kg) IV, IM or SC · IV/IM/SC · Single dose
- Large, exotic cat (tigers, etc.) immobilization · Midazolam (0.1 mg/kg) plus medetomidine (0.05-0.07 mg/kg) IM followed by ketamine (4-10 mg/kg) IM, if needed. May antagonize with atipamezole (0.25-0.35 mg/kg) IV, SC. · IM · Single dose
- Premedication (in combination with an opioid) · 5-20 μg (micrograms)/kg · IV/IM/SC · single dose · Analgesia lasts approx 1 hour at 10 μg/kg · Use lower end of dose range for IV administration.
- Management of excitation in the recovery period · 1-2 μg/kg · IV · single dose · Animals must be monitored carefully following administration.
- Perioperative analgesia and rousable sedation · 2-4 μg/kg/h · IV · CRI · perioperative period · Particularly useful when administered as an adjunct to opioid-mediated analgesia.
- Sedative/analgesic · 15 minutes prior to medetomidine, give atropine (0.05 mg/kg) or glycopyrrolate (0.01 mg/kg) then give medetomidine at 60-80 micrograms/kg IM or SC. · IM/SC · Single dose · Sedation lasts for up to 3 hours · May be reversed with atipamezole (400 micro-grams/kg IM)
Vías de administración
Contraindicaciones
- Cardiac disease
- Respiratory disorders
- Liver or kidney diseases
- Shock or severe debilitation
- Animals stressed due to heat, cold, or fatigue
- Pregnancy (insufficient safety data; use only if benefits clearly outweigh risks)
- Cardiovascular disease
- Systemic disease
- Geriatric patients
- Pregnant animals
- Conditions where vomiting is contraindicated (e.g., gastrointestinal foreign body, raised intraocular pressure)
- Diabetes mellitus
Efectos adversos
- Bradycardia (often profound)
- Atrioventricular (AV) blocks
- Decreased respiratory rate and potential apnea
- Hypothermia
- Increased urination (diuresis)
- Vomiting (especially in cats)
- Hyperglycemia
- Pain on intramuscular injection
- Blanched or cyanotic mucous membranes (due to peripheral vasoconstriction)
- Rarely: prolonged sedation, paradoxical excitation, hypersensitivity, death from circulatory failure
- Bradycardia
- Decreased cardiac output
- Initial hypertension followed by normotension/hypotension
- Vomiting (especially common after IM administration)
- Diuresis (due to ADH suppression)
Interacciones farmacológicas
- Atropine / Glycopyrrolate · Controversial. Using anticholinergics to treat medetomidine-induced bradycardia can lead to severe hypertension, increased myocardial work, and arrhythmias. Concomitant use is generally discouraged, especially at higher medetomidine doses (>20 mcg/kg).
- Opiates (Fentanyl, Butorphanol, Meperidine) · Synergistic enhancement of sedation and analgesia. Adverse cardiovascular and respiratory effects may also be pronounced. Reduced dosages and careful monitoring are advised.
- Propofol · When used after medetomidine, severe hypoxemia may occur. Significant dosage reductions of propofol are required along with adequate respiratory monitoring.
- Yohimbine · May reverse the effects of medetomidine, but atipamezole is the preferred and more specific reversal agent.
- Other anesthetic agents (e.g., propofol, alfaxalone) · Significantly reduces the dose required for induction and maintenance of anesthesia. · major
- Volatile anesthetics (e.g., isoflurane, sevoflurane) · Reduces the dose required to maintain anesthesia by up to 70%. · major
- Sympathomimetic amines · Contraindicated; may cause severe cardiovascular complications. · major
- Opioids · Synergistic sedation and analgesia (beneficial interaction). · moderate
Monitorización
- Level of sedation and analgesia
- Heart rate and rhythm (ECG recommended in high-risk patients)
- Body temperature (monitor for hypothermia)
- Blood pressure
- Respiratory rate and depth
- Pulse oximetry (SpO2)
- Heart rate and rhythm (bradycardia is expected, but monitor for severe arrhythmias)
- Blood pressure (initial hypertension followed by normotension/hypotension)
- Oxygen saturation (SpO2)
- Body temperature (risk of hypothermia due to reduced metabolic rate and peripheral vasoconstriction)
- Depth of sedation
Sobredosis
Single doses of up to **5X (IV)** and **10X (IM)** have been tolerated in dogs, though severe adverse effects (profound bradycardia, AV blocks, respiratory depression) can occur. Death has occurred rarely in dogs (1 in 40,000) receiving 2X doses. > **Important**: Treatment of medetomidine-induced bradycardia with anticholinergic agents (atropine or glycopyrrolate) is **not recommended** due to the risk of severe hypertension, increased myocardial oxygen demand, and arrhythmias. **Atipamezole** (an alpha-2 antagonist) is the safest and most effective choice to reverse medetomidine-induced cardiovascular and sedative effects.
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