Chapter 13
Inhalant Anesthetics
He’s got gas!
Tamara Grubb
Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, USA
- Q. What inhalant anesthetics are commonly used in veterinary medicine?
- A. Isoflurane and sevoflurane are the most commonly used anesthetics in most countries. In the USA, isoflurane is FDA-approved for use in dogs and horses and sevoflurane is FDA-approved for use in dogs only; however, both drugs are routinely used in other species. Halothane is still used in some countries but is not available worldwide. Desflurane is used sporadically but requires a specialized heated vaporizer. Nitrous oxide is an inhaled gas that is commonly categorized with the inhalant anesthetics but which lacks the potency to produce general anesthesia in veterinary patients when used alone.
- Q. How are the inhalants supplied and how does this impact delivery?
- A. Isoflurane, sevoflurane, desflurane, and halothane are liquids at room temperature and are delivered as a vapor after being volatilized in a precision, agent-specific vaporizer. Nitrous oxide is a gas at room temperature and is delivered from a gas cylinder using a flowmeter during co-administration of oxygen.
- Q. Why is it necessary to deliver the inhalants with precision, agent-specific vaporizers?
- A. Each inhalant has a specific vapor pressure (see Table 13.1). Vaporizers are calibrated to deliver a specific vapor output (in volume %) that is based on the specific inhalant’s vapor pressure. Older inhalants had a very low vapor pressure (e.g., methoxyflurane, 22 mm Hg at 20 °C) and could be delivered in open containers that did not regulate vapor output, but the high saturated vapor pressures of the more modern inhalant anesthetics (e.g., isoflurane, 240 mmHg at 20 °C) would produce a dangerously high inhalant concentration if not regulated by a vaporizer. For instance, if isoflurane was used in an open container at sea level, a concentration of 31.6% would be achieved (240 mmHg/760 mmHg = 31.6%).
Table 13.1 Vapor pressures and blood-gas solubility of the common inhalants.
Inhalant
Vapor pressure (mmHg) at 20 °C
Solubility (blood : gas partition coefficient)
Isoflurane
240
1.41
Sevoflurane
160
0.69
Desflurane
664
0.42
Halothane
243
2.36
Nitrous oxide
39,500
0.49
Using an inhalant in a vaporizer that is not labeled and calibrated for that specific inhalant can lead to an inadequately low, or lethally high, vapor output, depending on the ratio of the vapor pressure of the inhalant that is meant for the vaporizer to the vapor pressure of the inhalant that is actually in the vaporizer. An inhalant with a lower vapor pressure than the vaporizer is designed for produces a lower concentration than predicted by the vaporizer dial setting, while an inhalant with a higher vapor pressure than the vaporizer is designed for results in a higher than expected vaporizer output.
Precision vaporizers are temperature and flow compensated so that the output remains “precise” within a fairly large range of ambient temperatures and fresh gas flows. The vaporizers should be placed outside the breathing system (vaporizer out of circle or VOC) rather than within the breathing system (vaporizer in the circle or VIC) so that the patient’s minute ventilation does not impact the vaporizer’s output.
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- Q. Why is it necessary to deliver the inhalants with precision, agent-specific vaporizers?
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