Inhalant anesthetics are a cornerstone of veterinary practice, providing safe and reliable anesthesia in various species. Among these agents, isoflurane is a popular and trusted choice. Despite the introduction of newer inhalants like sevoflurane and desflurane, isoflurane remains one of the most widely used anesthetics in small animal, equine, and laboratory medicine.
Isoflurane is popular for many reasons. The agent has predictable pharmacokinetics, an excellent safety profile, and is cost-effective. Veterinarians have used isoflurane for many years, making it a familiar and practical choice. However, like all anesthetics, it can pose risks to patients and the veterinary team.
Here is a review of isoflurane's role in veterinary anesthesia, including its effects, advantages, limitations, and safe use.
Key takeaways
- Isoflurane is a widely used inhalant anesthetic with predictable effects and minimal metabolism in veterinary patients.
- Risks of isoflurane use include respiratory depression, hypotension, and occupational exposure.
- Isoflurane has a low MAC and high solubility, which means that it is potent at lower concentrations, but depth changes may occur slowly.
What is isoflurane?
Isoflurane is a volatile halogenated ether anesthetic. It is a successor to the older drug, halothane. Today, it is widely used across species in both general and specialty veterinary practice because of its reliability, safety record, and cost-effectiveness.
Isoflurane is primarily used for the maintenance phase of veterinary anesthesia, following induction with an injectable drug. Once the induction agent induces unconsciousness, the veterinary team places an endotracheal tube and delivers isoflurane, along with oxygen, via a specialized vaporizer. The vaporizer allows precise titration of anesthetic depth by adjusting the percentage of gas in the oxygen flow.
Isoflurane has been extensively studied and proven safe in small animals, horses, and laboratory species, giving veterinary professionals confidence in its use. Only a small amount of the drug is systemically absorbed, making it safe for animals across a wide range of ages and medical conditions.
Pharmacology and physiology
Isoflurane is delivered to pets via a precision vaporizer, where it is absorbed through the lungs and eliminated through exhalation. Less than 0.2% of isoflurane is metabolized by the body, making it safe for patients with hepatic or renal compromise, an advantage over older inhaled anesthetics.
The minimum alveolar concentration (MAC), or the concentration of a drug in the lungs that prevents a reaction to a painful stimulus in 50% of patients, is 1.3% for isoflurane in dogs, although this varies by species. This lower number indicates greater potency compared to other inhalants, allowing patients to be maintained at lower vaporizer settings. Premedications can lower the MAC even further.
Isoflurane is more soluble than other inhalants, which means that it stays in a pet's system longer, causing slower recovery and depth changes.
Like all inhalants, isoflurane has dose-dependent effects on the cardiovascular and respiratory systems. High concentration may cause hypotension from reduced systemic vascular resistance and myocardial contractility, and respiratory depression, leading to potential hypercapnia.
Advantages of isoflurane
Isoflurane's many advantages include:
- Reliable anesthesia – Isoflurane provides predictable veterinary anesthesia across a wide range of species. Its well-established safety profile gives clinicians confidence in its use.
- Minimal metabolism – Less than 0.2% of isoflurane is absorbed and metabolized, which makes it a safe choice for geriatric patients or those with compromised liver or kidney function.
- Cost-effectiveness – Compared with sevoflurane and other newer inhalants, isoflurane is generally less expensive, making it accessible to veterinary professionals in a wide array of clinical settings.
Isoflurane versus sevoflurane
The most common alternative to isoflurane in veterinary practice is sevoflurane, another halogenated ether anesthetic. The agents share many characteristics and a similar safety profile, but key differences influence when you might choose one over the other, including:
- Airway irritation – Sevoflurane is less pungent and less irritating to the airway, which makes it better tolerated for mask or chamber inductions when deemed appropriate.
- Speed of depth change – Recovery times after short procedures are typically faster with sevoflurane, giving it an advantage in practices that prioritize rapid surgical turnover.
- Cost and availability – Isoflurane is less expensive and more widely available than sevoflurane, making it the preferred option in busy general practices and large animal medicine.
Risks of isoflurane in veterinary anesthesia
While isoflurane is a reliable anesthetic, its safe use depends on awareness of the risks and adherence to established safety protocols and best practices. Isoflurane's limitations and risks include:
- Dose-dependent cardiovascular and respiratory depression
- Hypotension from reduced cardiac output and vasodilation
- Hypoventilation and hypercapnia
- Slower recovery and response to vaporizer setting changes than sevoflurane
Contraindications to isoflurane use align with those of other inhalant anesthetics. Veterinary teams must weigh possible contraindications against the risks of not performing surgery, particularly in emergency situations. Reasons to opt against isoflurane for elective procedures include:
- Known hypersensitivity to halogenated anesthetics
- History of malignant hyperthermia under anesthesia
- Severe cardiovascular instability or respiratory compromise
Isoflurane also poses risks to the team members providing veterinary anesthesia. According to the Occupational Safety and Health Administration (OSHA), chronic exposure to waste gases can lead to headaches, fatigue, and reproductive issues. Scavenging systems that pull waste gas from the air and remove it from the building are critical to reducing accidental exposures to staff. Maintaining anesthetic and scavenger equipment can also decrease exposure risk.
Safe use of isoflurane in veterinary anesthesia
Veterinary teams can make isoflurane safer for themselves and their patients with:
- Pre-anesthetic evaluations to identify high-risk patients and select appropriate drug combinations
- Equipment checks, including leak testing circuits, verifying oxygen levels, and confirming that vaporizers are properly calibrated for isoflurane
- Continuous monitoring of vital signs, including heart rate and rhythm, blood pressure, respiratory rate, end-tidal CO₂, oxygen saturation, and temperature, throughout the anesthetic period
- Multimodal anesthesia and pain control, including opioids, local blocks, sedatives, and NSAIDs, to lower inhalant doses
- Injectable induction instead of mask and chamber induction, which can increase stress, anesthetic gas exposure, and safety risks
Pregnant team members should not handle isoflurane and may choose to forego handling of anesthetized or recovering patients to reduce their exposure risk.
A trusted choice
Isoflurane is a widely used inhalant anesthetic in veterinary medicine because of its dependability, safety profile, and cost-effectiveness. Like all anesthetic agents, it carries risks to the patient and veterinary team that necessitate careful monitoring and thoughtful protocol designs to maintain safety.
If your team needs a refresher in veterinary anesthesia principles and safe practices, check out the Foundations of Veterinary Anesthesia and Sedation certificate course.
