Isoflurane

Introduction

Isoflurane is currently the most commonly used inhalation agent in veterinary practice. Similarly to halothane it's main use is as a maintenance agent after induction with an injectable agent but, again, can be used to induce patients. However, it does not have a pleasant odour and so many patients will breath hold. Isoflurane is licenced in most companion animals.

Pharmacokinetics

Isoflurane is a nonflammable and stable anaesthetic that, at room temperature, is a liquid and so requires passage through a vaporiser. Unlike halothane, it does not require a preservative, nor does it undergo ultraviolet degradation. The blood:gas partition coefficient is lower than that of halothane, meaning that is poorly blood soluble. This means that it causes rapid induction , recovery and depth of anaesthesia. The MAC for isoflurane is approximately 1.3% in dogs and 1.6% in cats, making it less potent than halothane, but it is less tissue soluble. There is minimal metabolism to isoflurane, but any that occurs is in the liver.

Adverse Effects

Central Nervous System

• Isoflurane does not mar the cerebral circulation's response to carbon dioxide. This means that hyperventilation can be used to decrease ICP in these patients.

Cardiovascular System

• Myocardial contractility depression.
• Heart rate may increase, which helps control cardiac output in the face of depression of myocardial contractility.
• Decresase in arterial blood pressure due to decreased vascular resistance.

Respiratory System

• Ventilation depression.

Other Systems

• Decreases flow through the hepatic portal vein but increased flow through the hepatic artery, so hepatic damage is less likely.
• Like halothane, isoflurane can cause malignant hyperthermia in susceptible patients.

Contraindications

• Isoflurane should not be used in patients with a susceptilbility to malignant hyperthermia.
• It potentiates non-depolarising neuromuscular blocking agents.