Median effective dose of isoflurane, sevoflurane, and desflurane in green iguanas

Abstract

Over time, MACs decreased for all 3 agents. Final MAC measurements were similar to values reported for other species. The decrease in MACs over time may be at least partly explained by limitations of anesthetic uptake and distribution imposed by the reptilian cardiorespiratory system. Hence, for a constant end-tidal anesthetic concentration in an iguana, the plane of anesthesia may deepen over time, which could contribute to increased morbidity during prolonged procedures.

Minimum Alveolar Concentration of Isoflurane and Arterial Blood Gas Values in Anesthetized Green Iguanas, Iguana iguana

“…The first M A C is o f lu ra n e , determined at >4hr post-induction, was 2.4 ± 0.7% , and is higher than the values obtained in both the M o sley , e t al, stu d y an d o u rs. T he se c o n d an d th ird M A C is o f lu ra n e determinations (Barter, et al, 2006) were lower and suggested a decrease o f minimum alveolar concentration values over time, but comparison o f these later values with ours is problematic. By definition, minimum alveolar concen tration should stay the sam e over tim e i f the stim ulus is supramaximal and other factors are kept constant, which the authors reported (Merkel and Eger, 1963).…”

“…Two minimum alveolar concentration determinations studies conducted six or more days apart on the same experimental group o f igua nas yielded minimum alveolar concentration values o f 2.0 ± 0.6% and 2.1 ± 0.6% (Mosley, et al, 2003). In a different study, M A C is o f lu r a n e determinations made on the same ani m als d u rin g a sin g le a n e s th e s tic e p is o d e th a t la s te d approximately 10 horns, yielded the following results: 2.4 ± 0.7%, at about four to fives hours post induction (PI), 2.1 ± 0.5 % at eight to nine hours PI, and 1.8 ± 0.3% at 10 -11 hr PI; (Barter, et al, 2006). In the latter study minimum alveolar concentration values were initially high and only decreased over tim e to levels com parable to those o f M osley, et al, (2003) experiment.…”

“…By definition, minimum alveolar concen tration should stay the sam e over tim e i f the stim ulus is supramaximal and other factors are kept constant, which the authors reported (Merkel and Eger, 1963). Barter, et al (2006) hypothesized that the decrease o f M A C is o f lu r a n e values over time reflected a delay for isoflurane brain level to equilibrate with lung level, but this may be less clinically relevant since anesthesia o f such duration would only occur under exception al circumstances. Our M A C is o f lu ra n e o f 1.6 ± 0.4% is similar to that o f Dumeril's monitors (1.54 ± 0.17%) (Bertelsen, et al, 2005), and is lower than that o f desert iguanas for which the M A C is o f lu ra n e was 2.82 at 20°C (68°F), 3.10 at 30°C (86°F), and 3.14 at 35°C (95°F) (Dohm and Brunson, 1993).…”

“…The mean minimum alveolar concentration value in this study is lower but overlaps, within one standard deviation, with that o f Mosley, et al, 2003, andBarter, et al, 2006, values. A lower m inim um alveolar concentration value m ay reflect either m inor, undetected differences in m ethodology, or reflect intraspecific variation combined with a relatively small sam ple size. This study used seven animals, Mosley, et al, 2003 used ten animals, andBarter, et al, 2006 used six animals.…”

“…Furthermore, reptiles possess a single ventricle and dual aor tic arches, so that arterial blood gas values will vary with the collection site Sonnenschein, 1964, Oldham andSmith, 1975). In this study, we determined blood gas values in green iguanas for samples obtained from the right common carotid artery, a direct branch o f the right aortic arch, and compared values to those o f arterial blood collected from the left carotid artery (Barter, et al, 2006), the femoral artery (M osley, et al, 2004), and the ventral coccygeal vessels (Mosley, et al, 2003, Mosley, et al, 2004 under similar con ditions. The purpose o f this study was therefore two-fold: to provide additional data and insight pertaining to the minimum alveolar concentration o f isoflurane and arterial blood gas values in green iguanas and to compare results with those o f previous publications.…”