Surgical Management

Last updated on 11/12/01

Anesthesia

Respiratory Management

Anesthesia

Anesthetic drugs are discussed on the pain management page.

One difference between the various inhalant anesthetics is the speed with which animals are anesthetized. To understand why, one must understand the vapor pressure, minimum alveolar concentration (MAC) and lipid solubilities of the drugs. 

The maximum percent of volatile anesthetic which can be vaporized at a given temperature equals the vapor pressure divided by the barometric pressure. The vapor pressure of isoflurane and halothane (at 20°C) is around 240 mm Hg, but for methoxyflurane it is 23 mm Hg. Therefore, the maximum vaporization for isoflurane and halothane is 240/760=32%, while for methoxyflurane it is 23/760=3%. 

Methoxyflurane is much more potent than the other two inhalants, however. The MAC for isoflurane is 1.4%, for halothane 0.9%, and for methoxyflurane 0.23%. But methoxyflurane is very lipid soluble. Therefore, the low vapor pressure of methoxyflurane, combined with high lipid solubility, mean that it takes much longer to get the partial pressure of methoxyflurane high enough in the brain to anesthetize the animal. This is why one can anesthetize animals relatively safely with open-drop methods using methoxyflurane than with isoflurane or halothane.

Up to 50% of methoxyflurane in the body can be metabolized, whereas only 20% of halothane and <2% of isoflurane are metabolized. This may have implications for toxicity. (Pauline Wong, DVM, DACVA, Compmed note 3/01)

Respiratory Management

Endotracheal Intubation

In addition to intubation for anesthesia, intratracheal instillation is often required for toxicology studies. Intubation of small rodents is difficult, but can be done with a 16 ga over-the-needle catheter using a blind technique, with about 90% success{4741}.

CO₂ monitoring

Anesthetized animals should be monitored, particularly if paralyzing agents are used. Typical respiratory monitoring uses end-tidal CO₂ monitors, but these can give misleading results and should be checked against blood gas monitoring if possible.{4114} 

Dalton's Law says that the sum of the partial pressures of each gas in a mixture equals the partial pressure of the mixture, in this case barometric pressure. The alveolar gas equation uses Dalton's Law to calculate what the partial pressure of arterial oxygen should be:{4114}

P_aO₂= FIO₂(Pb-PH₂O)-(P_ACO₂/RQ)

where FIO₂= fraction of inspired oxygen, i.e. 0.3 if 30% oxygen is used

        Pb=barometric pressure in mm Hg, around 680 mm Hg

        PH₂O= partial pressure of water, usually 47mm Hg

        P_ACO₂= partial pressure of alveolar CO₂, which is usually the same as end-tidal CO₂ after some time for equilibration; the goal is around 35 mm Hg maintained by adjusting the minute volume on the ventilator

        RQ= respiratory quotient, usually 1

If we substitute the constants into the equation, we get

P_aO₂= 0.3(680-47)-(P_ACO₂/1)

P_aO₂= 190-P_ACO₂     

©1999, Janet Becker Rodgers, DVM, MS

All rights reserved.

Comments? Send an email to rodgers@uky.edu