Isoflurane
Isoflurane is a widely used **inhalant general anesthetic** in veterinary medicine. Key clinical features include: - **Cardiovascular safety**: It produces less myocardial depression and catecholamine sensitization compared to older agents like halothane. - **Organ safety**: It undergoes minimal hepatic metabolism (only ~0.17%), making it a safe choice for patients with pre-existing hepatic or renal disease. - **Clinical Pearl**: While newer agents like sevoflurane offer faster changes in anesthetic depth and smoother mask inductions, isoflurane remains a highly reliable and cost-effective staple in veterinary anesthesia.
Mechanism: The exact mechanism of inhalant anesthetics remains partially understood, but they are believed to act on the **lipid matrix of neuronal membranes** and interact with various ligand-gated ion channels (e.g., **GABA_A receptors**) in the brain and spinal cord. Key physiological effects: - **CNS**: Reversible depression, altered temperature regulation, increased cerebral blood flow. - **Cardiovascular**: Dose-dependent hypotension (primarily via **vasodilation** rather than direct myocardial depression). - **Respiratory**: Dose-dependent respiratory depression. > **MAC (Minimum Alveolar Concentration)** values: Dog = 1.5%; Cat = 1.2%; Horse = 1.31%.
Dosing by species
- Anesthesia ยท 5% induction; 1.5-2.5% maintenance ยท Inhalation ยท Continuous ยท As needed ยท Concentrations are dependent upon fresh gas flow rate
- Anesthesia ยท 0.5-3%, inhaled ยท Inhalation ยท Continuous ยท As needed
- Maintenance of surgical anaesthesia ยท MAC is approximately 1.6% ยท Inhalational ยท Continuous ยท Duration of surgery ยท Adjust based on individual requirement and premedication.
- Anesthesia ยท Starting isoflurane at 1-2% ยท Inhalation ยท Continuous ยท As needed ยท After premed with medetomidine. Use a non-rebreathing system.
- Anesthesia ยท 4% ยท Inhalation ยท Continuous ยท As needed ยท Small birds can be anesthetized safely in 15-30 seconds
- Anesthesia ยท Induction occurs within 1-2 minutes at a concentration of 3-5%. Maintenance at 1.5-2% ยท Inhalation ยท Continuous ยท As needed ยท Anesthetic of choice for birds. Recovery is very rapid.
- Anesthesia (Mice, Rats, Gerbils, Hamsters, Guinea pigs, Chinchillas) ยท Induction: 2-3%, maintenance: 0.25-2% ยท Inhalation ยท Continuous ยท As needed ยท Using a non-rebreathing system
Routes of administration
Contraindications
- History or predilection towards malignant hyperthermia
- Known susceptibility to malignant hyperthermia
Adverse effects
- Dose-related hypotension
- Respiratory depression
- Nausea and vomiting
- Ileus
- Minimal cardiodepression
- Rare arrhythmias
- Dose-dependent hypotension (due to vasodilation, particularly in skeletal muscle)
- Dose-dependent respiratory depression
- Arrhythmias (less arrhythmogenic than halothane, but possible under certain conditions)
- Hypothermia
Drug interactions
- Aminoglycosides ยท Additive neuromuscular blockade may occur
- ACE Inhibitors or Hypotensive Agents ยท Concomitant use may increase risks for profound hypotension
- Lincosamides ยท Additive neuromuscular blockade may occur
- Non-depolarizing Neuromuscular Blocking Agents ยท Additive neuromuscular blockade may occur ยท moderate
- Succinylcholine ยท May induce increased incidences of cardiac effects (bradycardia, arrhythmias) and malignant hyperthermia in susceptible patients
- Sympathomimetics (dopamine, epinephrine, etc.) ยท May result in arrhythmias, though isoflurane sensitizes the myocardium less than halothane. Use with caution and intensive monitoring.
- Sedatives (e.g., alpha-2 agonists, phenothiazines) ยท Reduces the concentration of isoflurane required to achieve surgical anaesthesia (MAC reduction). ยท moderate
- Opioid agonists ยท Reduces the concentration of isoflurane required to achieve surgical anaesthesia (MAC reduction). ยท moderate
- Nitrous Oxide (N2O) ยท Reduces the concentration of isoflurane required to achieve surgical anaesthesia. ยท moderate
Monitoring
- Respiratory and ventilatory status
- Cardiac rate and rhythm
- Blood pressure (particularly in 'at risk' patients)
- Level of anesthesia
- Respiratory rate, rhythm, and depth (capnography/ETCO2 recommended)
- Heart rate and rhythm (ECG)
- Blood pressure (Doppler or oscillometric)
- Oxygen saturation (Pulse oximetry/SpO2)
- Body temperature
- Anaesthetic depth (eye position, palpebral reflex, jaw tone)
Overdose
Overdose risks are increased with intermittent positive pressure ventilation. Profound hypotension may occur, which usually responds to IV fluids but may require the use of vasopressors.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.