Hypothermia Minimally Decreases Nitrous Oxide Anesthetic Requirements

Abstract

Over the 38-28 degrees C range, changes in minimum alveolar anesthetic concentration (MAC) parallel changes in lipid solubility of the anesthetics studied. We hypothesized that there would be minimal change in N2OMAC, since N2O lipid solubility is relatively unaltered by temperature changes. We determined N2OMAC in rats using a hyperbaric chamber. In Group N (normothermia, n = 10) rectal temperature was maintained at 37.5 +/- 1 degrees C (mean +/- SD). In Group H (hypothermia, n = 9) temperature was maintained at 29.7 +/- 1.8 degrees C. The hyperbaric chamber was pressurized with N2O and oxygen partial pressure was 0.4 +/- 0.1 atm. Chamber pressure was adjusted approximately 15% up or down, stabilized for approximately 15 min, and the noxious stimulus (electrical current) was applied. This process was continued until two N2O partial pressures were determined which just prevented and just permitted gross, purposeful movement. Nitrous oxide MAC for Group N and Group H were 1.9 +/- 0.2 atm and 1.6 +/- 0.2 atm, respectively, P < 0.01. Temperature and MAC correlated: r = 0.59, P < 0.01. We conclude that hypothermia minimally decreases N2OMAC, which is consistent with the effects of hypothermia on N2O solubility in lipid membranes.

Isoflurane MAC determination in dogs using three intensities of constant-current electrical stimulation

“…Electrical stimulation may be an option when the tail clamping stimulus cannot be employed, such as when the animal is in a hyperbaric chamber (Antognini et al. ) or during investigations of segmental MAC‐sparing effects (Valverde et al. ).…”

“…The minimum alveolar concentration (MAC) has been used as an index of inhalant anesthetic agent potency since 1963 (Merkel & Eger 1963), and is the most widely employed index of the depth of anesthesia when such agents are administered. MAC has also facilitated investigations of the physiological effects of inhalation agents at equipotent concentrations and has helped to elucidate the mechanism of action of inhalant anesthetics (Saidman & Eger 1964;Quasha et al 1980;Hornbein et al 1982;Antognini & Schwartz 1993;Rampil et al 1993;Antognini et al 1994;G omez de Segura et al 1998;Jinks et al 2002;Gerstin et al 2003).…”

“…In addition to tail clamping, electrical stimulation represents a noxious stimulus that is frequently used for MAC determination (Valverde et al 1989(Valverde et al , 2003Antognini et al 1994;Soares et al 2004;Seddighi et al 2011). Electrical stimulation has some advantages over tail clamping in that it facilitates easier quantification, standardization and reproducibility (Le Bars et al 2001).…”

“…Electrical stimulation has some advantages over tail clamping in that it facilitates easier quantification, standardization and reproducibility (Le Bars et al 2001). Electrical stimulation may be an option when the tail clamping stimulus cannot be employed, such as when the animal is in a hyperbaric chamber (Antognini et al 1994) or during investigations of segmental MAC-sparing effects (Valverde et al 1989).…”

“…Different anatomic sites have been used for electrical stimulation in determinations of MAC in animals. These include the pelvic limb (Valverde et al 1989(Valverde et al , 2003Soares et al 2004), thoracic limb (Valverde et al 1989;Seddighi et al 2011), tail (Laster et al 1993;Antognini et al 1994;Valverde et al 2003) and oral mucosa (Valverde et al 2003;Levionnois et al 2009). Isoflurane and halothane MAC in the dog did not differ when 50 V was applied to the pelvic or thoracic limbs, or the oral mucosa (Valverde et al 1989(Valverde et al , 2003.…”