SEVOFLURANE:  THE FACT OF THE MATTER
Dr. Julie Smith, Diplomate ACVA, Chief of Anesthesia, Iams Pet Imaging Center

Sevoflurane is the newest inhalant anesthetic available for use in veterinary medicine.  Its effects on blood pressure, heart rate, and respiratory rate are very similar to the effects of isoflurane.  Both agents are relatively safe, but cause a similar decrease in mean arterial blood pressure at high clinical concentrations, due to myocardial depression and peripheral vasodilatation.  Dose dependent respiratory depression is also similar for both agents, with a marked hypoventilation and hypercapnia seen at higher percentages.

The blood/gas partition coefficient of sevoflurane is half that of isoflurane (0.65 vs 1.4).  This difference in solubility relates to the speed of induction, the time to extubation and recovery, and the rapidity with which depth changes can be made during anesthetic maintenance.  Mask inductions with sevoflurane are smoother and faster than those with isoflurane.  It is easier to get into trouble when increasing the depth of anesthesia with sevoflurane, but then it is easier to get out of trouble when it is detected.  Recoveries tend to be similar for both agents.

Sevoflurane is less potent than isoflurane.  Because its MAC is twice that of isoflurane (2.4 vs 1.2) more sevoflurane is necessary to achieve the same anesthetic depth.

Approximately 0.2% of the absorbed isoflurane is actually metabolized by the patient.  This compares to 1.0 to 5.0% for sevoflurane.   Neither agent undergoes significant metabolism.  (Halothane is reported to undergo from 20 to 50% metabolism of amount absorbed.)

The cost of sevoflurane is approximately 7 times that of isoflurane on a per ml basis.  This cost difference is less initially if a free vaporizer is provided.  New vaporizers cost about $2000.

So what does all this mean??

It means that sevoflurane is similar in many ways to isoflurane.  The main advantages are relative to the speed of induction and recovery.  However, depending on your anesthetic practices, these advantages may not be fully appreciated.  If you commonly use a balanced anesthesia protocol, incorporating premedication with tranquilizers/analgesics and IV induction agents, the rapid induction is not clinically important.  If you choose to mask induce the majority of your patients, then sevoflurane will provide a much smoother and faster induction than isoflurane.  Though a common practice, the higher oxygen flow and agent concentration necessary for mask induction is wasteful and may have deleterious effects in some cases.  Not to mention the waste gas issues.

If a lot of your cases are outpatient type procedures, then the speed of recovery is important.   A fast recovery is not desirable in all circumstances.  Tranquilizers and analgesics affect the