Anesthesia & Analgesia:
Cardiovascular Anesthesia: Society of Cardiovascular Anesthesiologists
Minimum Alveolar Anesthetic Concentration of Volatile Anesthetics in Normal and Cardiomyopathic Hamsters
Vivien, Benoit MD; Langeron, Olivier MD; Coriat, Pierre MD; Riou, Bruno MD, PhD
Laboratoire d'Anesthesiologie, Departement d'Anesthesie-Reanimation, Centre Hospitalier Universitaire Pitie-Salpetriere, Universite Paris VI, Paris, France.
Supported in part by Association Francaise contre les Myopathies and Societe Francaise d'Anesthesie et de Reanimation. BV was the recipient of a fellowship grant from the Fondation pour la Recherche Medicale.
Accepted for publication December 7, 1998.
Address correspondence and reprint requests to Pr. B. Riou, Departement d'Anesthesie-Reanimation, Groupe Hospitalier Pitie-Salpetriere, 47 Blvd de l'Hopital, 75651 Paris Cedex 13, France.
Minimum alveolar anesthetic concentrations (MAC) values of volatile anesthetics in cardiovascular diseases remain unknown.We determined MAC values of volatile anesthetics in spontaneously breathing normal and cardiomyopathic hamsters exposed to increasing (0.1%-0.3% steps) concentrations of halothane, isoflurane, sevoflurane, or desflurane (n = 30 in each group) using the tail-clamp technique. MAC values and their 95% confidence interval were calculated using logistic regression. In normal hamsters, inspired MAC values were: halothane 1.15% (1.10%-1.20%), isoflurane 1.62% (1.54%-1.69%), sevoflurane 2.31% (2.22%-2.40%), and desflurane 7.48% (7.30%-7.67%). In cardiomyopathic hamsters, they were: halothane 0.89% (0.83%-0.95%), isoflurane 1.39% (1.30%-1.47%), sevoflurane 2.00% (1.85%-2.15%), and desflurane 6.97% (6.77%-7.17%). Thus, MAC values of halothane, isoflurane, sevoflurane, and desflurane were reduced by 23% (P < 0.05), 14% (P < 0.05), 13% (P < 0.05), and 7% (P < 0.05), respectively in cardiomyopathic hamsters. Implications: Minimum alveolar anesthetic concentrations of volatile anesthetics were significantly lower in cardiomyopathic hamsters than in normal hamsters.
(Anesth Analg 1999;88:489-93)
Many pathophysiological conditions can modify the minimum alveolar anesthetic concentration (MAC)  of volatile anesthetics, including hypothermia , in the elderly [3,4], during pregnancy , and in diseases such as hypo- or hyperthyroidism , diabetes , and sepsis . Although anesthetics can induce marked cardiovascular effects that could differ in patients with cardiovascular diseases, no data are presently available concerning MAC values of volatile anesthetics in cardiovascular diseases.
Hamsters with genetically induced cardiomyopathy have been used as an experimental model of cardiomyopathy to compare the effects of anesthetics in normal and diseased myocardium [9-11]. However, MAC values have been determined in many rodents species [12-15], but not in the hamster, and no data are available concerning MAC values in cardiomyopathic animals. Therefore, we designed the present study to determine MAC values of halothane, isoflurane, sevoflurane, and desflurane in normal and cardiomyopathic hamsters.
Forty normal Syrian hamsters (strain F1B) and 40 cardiomyopathic Syrian hamsters (strain BIO 14.6) were used in the current study (Bio Breeders, Fitchburg, MA). In this strain, all animals of both sexes develop hypertrophic cardiomyopathy from the age of 6 wk. Care of the animals conformed to the recommendations of the Helsinki Declaration, and the study was performed in accordance with the regulations of the official edict of the French Ministry of Agriculture. All animals were 6-7 mo old.
MAC was determined using the tail-clamp technique, as previously described [12,16]. Animals were tested at the same time of day-2:00-8:00 PM-to minimize variations in anesthetic requirements induced by circadian rhythm . Eight to twelve hamsters were used in each experiment (i.e., at the same time); only one drug was used during the same experiment; and at least three experiments were necessary to complete each anesthetic study. A minimal interval of 4 days was required between two experiments involving the same animals. Animals were restudied for different anesthetics; in a preliminary study, we verified that the MAC value of halothane in normal hamsters was not significantly modified after a 4-day interval (n = 10; 1.12% +/- 0.11% vs 1.14% +/- 0.11%). We therefore concluded that tissue damage from multiple tail-clamp applications was not important.
Spontaneously breathing hamsters were exposed to halothane, isoflurane, sevoflurane, or desflurane in a 80 x 40 x 10-cm chamber, closed by a thin plastic sheet. The volatile anesthetic was administered with a calibrated vaporizer (Fluotec 4, Isotec 3, Sevotec 5, or Tec 6; Ohmeda, Steeton, UK) in 100% oxygen as the carrier gas, with a fresh gas flow of 12 L/min. Concentrations of the volatile anesthetic in the chamber were continuously measured by using an infrared calibrated analyzer (Artema MM 206SD; Taema, Antony, France). The infrared analyzer was calibrated daily according to manufacturer guidelines using anesthetic gas mixtures of known concentration. The anesthetic mixture (volatile anesthetic in oxygen) was rewarmed to 30.0[degree sign]C before entering the chamber, and the temperature of the chamber was continuously monitored. The body temperature of one animal in each experiment was continuously monitored with a rectal probe (Harvard Apparatus, Inc, South Natick, MA). When the rectal temperature of this monitored hamster dropped by 1.0[degree sign]C, the chamber was rewarmed using a heating lamp until its temperature was restored to 37.0[degree sign]C.
Before applying test stimuli, hamsters were exposed for 2 h to a constant anesthetic concentration of almost 80% halothane, isoflurane, sevoflurane, or desflurane MAC values previously determined at 37.0[degree sign]C in rats, which were 1.11%, 1.38%, 2.50%, and 7.71%, respectively [12,18,19]. The 2-h exposure time was chosen to achieve inspiratory/alveolar ratios (FI/FA) close to 1.0  and to maintain the total anesthetic exposure time at <or=to8 h. However, it became obvious after the first experiment with halothane that cardiomyopathic hamsters did not require this initial anesthetic concentration; therefore, the initial concentration of the four anesthetics tested applied to the cardiomyopathic hamsters was reduced to almost 50% of MAC values previously determined in rats; i.e., 0.6%, 0.7%, 1.3%, and 3.8%, respectively.
A 6-in. hemostatic clamp was applied for 45 s to the first rachet position on the mid-portion of the tail . The hemostatic clamp was applied through a small hole so as not to modify the anesthetic concentration in the experiment chamber. An animal was considered to have moved if it made a "gross purposeful muscular movement" , usually of the hind limb and/or the head, as opposed to increasing its rate or depth of respiration. The anesthetic concentration was increased in 0.1% (halothane and isoflurane) to 0.3% steps (sevoflurane and desflurane), and the testing sequence was repeated after 30 min of each concentration exposure, meaning that steady-state FI/FA ratios close to 1.0 could be reasonably achieved. No experiment required exposure to more than seven consecutive increased anesthetic concentrations; therefore, the total anesthetic exposure time was kept to <or=to8 h. However, MAC determination is not affected by the duration of anesthetic exposure . At the end of the procedure, anesthetic administration was stopped, and hamsters awoke while breathing 100% oxygen. During MAC determination, no hamster exhibited respiratory distress, and all recovered without obvious untoward effects. Moreover, no death occurred during each experiment and the next 2 days.
The original MAC concept of Eger et al.  used a "bracketing approach" in humans and animals. In animals studies, it is possible to apply the tail-clamp stimuli on multiple occasions. Thus, an appropriate mathematical technique to quantify the relationship between MAC and response/no response data is the logistic regression analysis. This produces values for MAC comparable to those produced with the bracketing technique and enables an extrapolation of the probability of response to any given anesthetic concentration within the curve . For each strain and for each volatile anesthetic, median MAC values were calculated (n = 30) using logistic regression (NCSS 6.0; Statistical Solutions, Cork, Ireland), and the 95% confidence interval limits were calculated . All P values were two-tailed, and a P value <or=to0.05 was considered significant.
(Table 1) shows the values of logistic regression parameters (beta (0) and beta1). In normal hamsters, inspired MAC values (95% confidence intervals) of volatile anesthetics were: halothane 1.15% (1.10%-1.20%), isoflurane 1.62% (1.54%-1.69%), sevoflurane 2.31% (2.22%-2.40%), and desflurane 7.48% (7.30%-7.67%). In cardiomyopathic hamsters, inspired MAC values of volatile anesthetics were: halothane 0.89% (0.83%-0.95%), isoflurane 1.39% (1.30%-1.47%), sevoflurane 2.00% (1.85%-2.15%), and desflurane 6.97% (6.77%-7.17%). These results indicate that MAC values of halothane, isoflurane, sevoflurane, and desflurane were reduced by 23% (P < 0.05), 14% (P < 0.05), 13% (P < 0.05), and 7% (P < 0.05), respectively, in cardiomyopathic hamsters (Figure 1).
The magnitude of MAC reduction in cardiomyopathic hamsters seems to be a function of the solubility of the anesthetics, particularly its affinity to a lipid-like phase (Figure 2).
We observed that MAC values of volatile anesthetics were significantly reduced in hamsters with genetically induced hypertrophic cardiomyopathy (Figure 1). Further, the decrease differed among anesthetics, being greater for anesthetics with a greater affinity to lipids (Figure 2).
Our MAC values in normal hamsters were consistent with MAC values previously determined in other rodents [12,18,19]. Our measurements of inspired anesthetic concentrations should reasonably reflect the partial pressures of anesthetic at their site of action. During MAC determination, hamsters breathed 100% oxygen to prevent hypoxemia and did not exhibit respiratory distress; afterward, all recovered without noticeable untoward effects. PCO2 values observed in previous studies [7,12,23] in spontaneously breathing rats remained well within the range (15-95 mm Hg) in which anesthetic potency is not altered . As in most previous determinations of MAC in rodents, inspired rather than alveolar anesthetic concentrations were measured. We did not use correction factors to calculate the exact MAC values . Indeed, these correction factors are unknown for sevoflurane and desflurane, which might be different in the hamster, which is only one-third to one-fourth the size of the rat. Because the equilibration time was long, we assumed that FI/FA ratios were very close to 1.0.
Several hypotheses can be proposed to explain the decreased MAC in cardiomyopathic hamsters. First, cardiomyopathic hamsters are subject to heart failure with hypertrophy and/or dilation and, therefore, hypotension [25,26], which has been reported to reduce anesthetic requirements . Therefore, the decreased MAC values of cardiomyopathic hamsters could be explained by a hemodynamic participation. However, the decreased MAC values observed in diabetic  or septic  rodents occur without altered hemodynamics. Second, MAC is decreased in the elderly [1,23], and Ottenweller et al.  found similarities in the cardiovascular status between cardiomyopathic and normal aging hamsters. Third, excessive intracellular calcium occurs in the heart and brain of cardiomyopathic hamsters , and the anesthetic potency of halothane in cats directly varies with the cerebrospinal fluid calcium ion concentration . Therefore, brain Ca2+ accumulation might contribute to the decreased MAC in cardiomyopathic hamsters. Furthermore, anesthetic requirements are reduced in several rats models manifesting reduced brain synaptic plasma membrane Ca2+-ATPase activity , and the activity of the plasma membrane Ca2+-ATPase is decreased in cardiac muscle cells of cardiomyopathic hamsters . Such an alteration in the brain of cardiomyopathic hamsters could be involved in the reduced MAC value in this strain. However, none of these hypotheses can clearly explain why the magnitude of MAC reduction in cardiomyopathic hamsters seems to be a function of the lipid solubility of the anesthetic (Figure 2).
The negative inotropic effects of volatile anesthetics are more pronounced in diseased myocardium [31,33]. However, the fact that the MAC of volatile anesthetics could be decreased in animals with diseased myocardium was not considered in these previous studies. Because our study demonstrated that MAC values of volatile anesthetics are decreased in cardiomyopathic hamsters, their cardiovascular effects should also be compared for equipotent concentrations, adjusted for healthy and diseased myocardium. According to such a correction, the negative inotropic effects of halothane and isoflurane in cardiomyopathic hamsters are not greater than those in normal hamsters . However, a decreased MAC, or a more pronounced effect of volatile anesthetics expressed as vol%, are two ways of considering the lower safety margin of volatile anesthetics in diseased myocardium.
In conclusion, we demonstrated that MAC values of halothane, isoflurane, sevoflurane, and desflurane are decreased in cardiomyopathic hamsters. The main questions raised by the present study are as follows. Is this effect caused by genetic differences between the two strains and, consequently, what is its underlying mechanism? Is this effect rather due to cardiomyopathy per se and, consequently, does it occur in other cardiovascular diseases? Why is this effect not equal across anesthetics, causing more of a change with anesthetics having a greater lipid solubility?
The authors thank Edmund I Eger II, MD, for helpful criticism.
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