Abstract
OBJECTIVE To evaluate whether the sedative effects of a combination of dexmedetomidine and ketamine differed when it was administered IM in a hind limb versus a forelimb of leopard geckos (Eublepharis macularius).
DESIGN Randomized crossover study.
ANIMALS 9 healthy adult leopard geckos.
PROCEDURES Each gecko received a combination of dexmedetomidine (0.1 mg/kg [0.045 mg/lb]) and ketamine (10 mg/kg [4.5 mg/lb]; DK), IM, in a forelimb and hind limb in a randomized order and with a 7-day interval between treatments. All geckos received atipamezole (1 mg/kg [0.45 mg/lb], SC) 45 minutes after DK administration. Palpebral and righting reflexes, jaw tone, and superficial pain and escape responses were each assessed on a 3-point scale, and the scores for those variables were summed to calculate a sedation score. Those variables and heart and respiratory rates were evaluated at predetermined times before and for 1 hour after DK administration.
RESULTS For the forelimb treatment, mean sedation score was higher and mean heart rate was lower than the corresponding values for the hind limb treatment at most time points after DK administration. The righting reflex remained intact for all 9 geckos following the hind limb treatment but became absent in 7 geckos following the forelimb treatment.
CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that the extent of DK-induced sedation was greater when the combination was injected IM in a forelimb versus a hind limb of leopard geckos, likely owing to a hepatic first-pass effect following hind limb injection. In reptiles, IM hind limb administration of drugs that undergo hepatic metabolism and excretion is not recommended.
Leopard geckos (Eublepharis macularius) are commonly kept as companion animals, used as research models, and maintained in zoological institutions. Therefore, studies regarding sedation, anesthesia, and analgesia of that species are warranted.
In reptiles, parenteral drug administration in the caudal half of the body is controversial.1–6 Venous blood from the tail, and to varying and minor degree from the hind limbs, flows directly to the kidneys via the renal-portal system, whereas blood from the hind limbs drains predominately into the ventral abdominal vein, which enters the liver before reaching the systemic circulation.4 Concerns following injection of drugs into the caudal half of the body of reptiles include hepatic and renal first-pass effects, which can result in nontherapeutic systemic plasma drug concentrations. Multiple studies2,3 have investigated the effect of renally excreted antimicrobials in turtles, and results of those studies indicate that blood drug concentrations did not differ significantly between hind limb and forelimb administration. Consequently, it was widely recommended that, in reptiles, injection site was irrelevant in regard to drug kinetics and that most drugs could be administered at any site, including the caudal aspect of the body.1–3 The renal-portal system of reptiles has been extensively researched; however, hepatic first-pass effects on drug kinetics following hind limb administration in reptiles have garnered much less attention, despite the fact that, from a clinical aspect, their pharmacodynamic relevance may be equivalent to or more important than that of renal first-pass effects.5,6
In chelonians, lizards, and crocodilians, venous blood from the hind limbs flows into the ventral abdominal veins, then directly into the liver or hepatic portal vein, which subsequently enters the liver.4,7,8 Hence, any drug administered in a hind limb of most reptiles enters the hepatic circulation before reaching the systemic circulation. In turtles and crocodiles, administration of anesthetics and analgesics that undergo hepatic metabolism into a hind limb results in significantly lower plasma drug concentrations and decreased clinical efficacy, compared with forelimb administration, likely owing to hepatic first-pass effects.6,9,10,a Despite published evidence that hepatic first-pass effects can significantly affect the pharmacokinetics and pharmacodynamics of hepatically metabolized drugs, such as alfaxalone or voriconazole, following administration in the caudal half of the body of reptiles, some pharmacokinetic and pharmacodynamic studies9,11 failed to account for the effect of injection site in the study design.
The objective of the study reported here was to evaluate whether the sedative effects of DK in leopard geckos differed significantly when the drug combination was administered in a forelimb versus hind limb. Because both dexmedetomidine and ketamine undergo hepatic metabolism, we hypothesized that geckos would be less sedate following hind limb administration of DK than after forelimb administration owing to hepatic first-pass effects.
Materials and Methods Animals
The study was approved by the University of Wisconsin Institutional Animal Care and Use Committee. Nine university-owned adult leopard geckos (5 male and 4 female) were used in the study. The mean ± SD body weight was 58.6 ± 10.8 g for males and 45.5 ± 8.4 g for females. The geckos were individually housed in terrariums, and the ambient temperature was maintained between 26.7° and 27.8°C (80° and 82°F). Geckos had ad libitum access to water and were fed a commercial insect-based dietb and appropriately sized mealworms. The mealworms were fed a high-calcium, gut-loading formula and were dusted with calcium and multivitamin supplements prior to being fed to the geckos.
Study design and procedure
The study had a randomized crossover design. All geckos received a combination of dexmedetomidinec (0.1 mg/kg [0.045 mg/lb]) and ketamined (10 mg/kg [4.5 mg/lb]) in the muscles of the proximal portion of the left forelimb (forelimb treatment) and left hind limb (hind limb treatment) with a minimum of 7 days between treatments. The order in which the treatments were administered to each gecko was randomized by means of commercial randomization software.e The 2 drugs were combined in 1 syringe for administration. The doses used were determined on the basis of results of pilot studies conducted to identify a DK combination that would provide reliable deep sedation in geckos following IM administration in a forelimb. All geckos received atipamezolef (1 mg/kg [0.45 mg/lb], SC) in the axillary region 45 minutes after DK administration to antagonize the effects of dexedetomidine and facilitate recovery.
Data collection
For each treatment (forelimb and hind limb), the respiratory rate, palpebral reflex, righting reflex, jaw tone, superficial pain response, and escape response were measured at 5 minutes before (baseline) and every 5 minutes after DK administration for 1 hour. Heart rate was measured at baseline and at 15, 30, 45, and 60 minutes after DK administration. Respiratory rate was measured by counting visible coelomic excursions. Heart rate was measured with a Doppler flow probe that was placed over the pectoral girdle. The palpebral reflex was evaluated by touching either the medial or lateral canthus of each eye with a small cotton-tipped applicator. To assess the righting reflex, each gecko was positioned in dorsal recumbency for 15 seconds; if the animal could not right itself within that period, the reflex was considered absent. Jaw tone was assessed by the placement of gentle downward traction on the mandible. The superficial pain response was evaluated by gently pinching the skin of the hind limbs with padded forceps. The escape response was assessed by the animal's unwillingness to be restrained or attempts to flee from the investigator's hand. The palpebral and righting reflexes, jaw tone, and superficial pain and escape responses were each scored on a 3-point scale, where 0 = normal, 1 = reduced, and 2 = absent. Those scores were summed to calculate a total sedation score at each data acquisition time. A particular reflex or response was recorded as reduced or absent only if that change was observed at 2 consecutive data acquisition points (ie, 5-minute intervals). Throughout the observation period, the person (DMF) who measured and recorded all variables was unaware of (blinded to) the site of DK administration (ie, treatment) for each gecko.
Statistical analysis
Descriptive statistics were generated. Outcomes of interest were sedation score, respiratory rate, and heart rate. The data distribution for each outcome of interest was assessed for normality by the Shapiro-Wilk test and for equal variance by the Brown-Forsythe test. The effects of treatment (forelimb or hind limb) and time on each outcome were evaluated with a 2-way ANOVA for repeated measures. The Holm-Sidak method was used for post hoc comparisons. Values of P < 0.05 were considered significant. All analyses were performed with commercial statistical software.g
Results
Descriptive data for the palpebral reflex, righting reflex, superficial pain response, jaw tone, and escape response following forelimb and hind limb DK administration were summarized (Table 1). Initial evidence of sedation was apparent for 5 of the 9 geckos at 5 minutes after the forelimb treatment. None of the geckos appeared sedate at 5 minutes after the hind limb treatment, and only 2 of the 9 geckos appeared sedate at 10 minutes after the hind limb treatment. The mean sedation score for the forelimb treatment was significantly (P < 0.01) greater than that for the hind limb treatment at all observation points between 5 and 50 minutes after DK administration (Figure 1).
Descriptive data for various reflexes and biological responses for 9 adult geckos after IM administration of a combination of dexmedetomidine (0.1 mg/kg [0.045 mg/lb]) and ketamine (10 mg/kg [4.5 mg/lb]) in a forelimb (forelimb treatment) and hind limb (hind limb treatment).
| Variable | Grade | Forelimb treatment | Hind limb treatment |
|---|---|---|---|
| Palpebral reflex | Absent | 2 | 0 |
| Reduced | 5 | 1 | |
| Normal | 2 | 8 | |
| Righting reflex | Absent | 7 | 0 |
| Reduced | 2 | 2 | |
| Normal | 0 | 7 | |
| Superficial pain response | Absent | 3 | 1 |
| Reduced | 5 | 1 | |
| Normal | 1 | 7 | |
| Jaw tone | Absent | 3 | 1 |
| Reduced | 5 | 1 | |
| Normal | 1 | 7 | |
| Escape response | Absent | 7 | 0 |
| Reduced | 2 | 4 | |
| Normal | 0 | 5 |
All geckos received both the forelimb and hind limb treatments, and there was at least a 7-day washout period between treatments. Values represent the number of geckos. Each variable was assessed every 5 minutes for 1 hour after DK administration and was scored on a 3-point scale, where 0 = normal, 1 = reduced, and 2 = absent. A particular reflex or response was recorded as reduced or absent only if that change was observed at 2 consecutive data acquisition points.
Mean ± SEM sedation score (A), heart rate (B), and respiratory rate (C) for 9 adult geckos at predetermined times before (−5 minutes; baseline) and for 1 hour after IM administration of a combination of dexmedetomidine (0.1 mg/kg [0.045 mg/lb]) and ketamine (10 mg/kg [4.5 mg/lb]) in a forelimb (forelimb treatment; gray bars) and hind limb (hind limb treatment; white bars). All geckos received both the forelimb and hind limb treatments, and there was at least a 7-day washout period between treatments. All geckos received atipamezole (1 mg/kg [0.45 mg/lb], SC) 45 minutes after DK administration. The sedation score was a summation of the scores for palpebral reflex, righting reflex, jaw tone, superficial pain response, and escape response, each of which was scored on a 3-point scale, where 0 = normal, 1 = reduced, and 2 = absent. Thus, 10 was the maximum sedation score a gecko could be assigned at any data acquisition point. *Within a time, mean differs significantly (P < 0.01) between forelimb and hind limb treatments.
Citation: Journal of the American Veterinary Medical Association 253, 9; 10.2460/javma.253.9.1146
The mean heart rate for the 9 geckos did not differ between the forelimb and hind limb treatments at baseline. Mean heart rate was significantly (P < 0.001) decreased from baseline at 15, 30, and 45 minutes after DK administration for both the forelimb and hind limb treatments (Figure 1). Also, the mean heart rate for the forelimb treatment was significantly (P < 0.001) less than that for the hind limb treatment at 15, 30, and 45 minutes after DK administration. Following administration of atipamezole, the mean heart rate for both treatments returned to baseline values and did not differ significantly (P = 0.55) at 60 minutes after DK administration.
The mean respiratory rate did not differ from baseline at any time following DK administration for either treatment. The mean respiratory rate for the forelimb treatment was significantly less than that for the hind limb treatment at 5 and 50 minutes after DK administration (Figure 1).
Discussion
In the present study, DK-induced sedation and cardiac depression in geckos was less when the drug combination was administered IM in a hind limb versus a forelimb. Similar results were observed in a studya of red-eared sliders (Trachemys scripta elegans) that received the same combination of DK (dexmedetomidine [0.1 mg/kg] and ketamine [10 mg/kg]), SC, in either the cranial (axillary) or caudal (prefemoral fossa) half of the body. In that study,a a light plane of anesthesia was achieved in all turtles following DK administration in the axillary region, whereas none of the turtles became sedate following DK administration in the prefemoral fossa. Estuarine crocodiles (Crocodylus porosus) that received medetomidine (0.5 mg/kg [0.23 mg/lb], IM) became reliably sedate when the drug was injected into a forelimb but did not become sedate when it was injected into a hind limb or tail, although the heart rate decreased in all crocodiles following medetomidine administration regardless of the injection site.10 Conversely, sedation score, induction time, recovery time, and physiologic and behavioral responses for broad-snouted caimans (Caiman latirostris) did not differ when a combination of ketamine (30 mg/kg [13.6 mg/lb]) and xylazine (1.0 mg/kg) was administered IM in a hind limb versus when it was administered IM in a forelimb.12 However, all 8 caimans of that study lost the righting reflex when the ketaminexylazine combination was administered in a forelimb, whereas only 6 of the 8 caimans lost the righting reflex when the drug combination was administered in a hind limb.12 The inconsistencies observed following forelimb and hind limb administration of sedatives among the geckos of the present study and the turtles,a crocodiles,10 and caians12 of those other studies are likely attributable to differences in study design or species-specific differences in venous anatomy, blood flow characteristics, or drug metabolism.
The venous system of reptiles differs anatomically from that of other vertebrates and varies among different groups of reptiles.4,7,8,13 In most reptiles, blood from the caudal half of the body returns to the heart via a single postcaval vein (ie, caudal vena cava).7 The postcaval vein receives venous blood from the kidneys, gonads, and liver.7 The hepatic portal vein receives venous blood from the gastrointestinal tract and ventral abdominal vein or veins.7 A single ventral abdominal vein is present in most snakes and lizards, but more primitive reptile species (eg, Uromastyx) as well as chelonians and crocodilians have 2 ventral abdominal veins.1,7,13,14 In green iguanas (Iguana iguana), contrast medium administered into an intraosseous femoral catheter enters the central circulation through the ventral abdominal vein, which drains into the liver, whereas venous blood from the tail enters the renal parenchyma before draining into the caudal vena cava.4 In red-eared sliders, venous blood from the hind limbs travels through the femoral veins into the ventral abdominal veins and then directly into the liver and bypasses the renal-portal system.8 Venous drainage from the tail of turtles is distinct from and more variable than that of green iguanas. In turtles, some venous blood from the tail enters the ventral abdominal veins, whereas the rest flows through the renal parenchyma.8
Because many drugs, particularly analgesics and anesthetics, commonly used in reptiles undergo hepatic metabolism and excretion and most venous blood passes through the liver before entering the systemic circulation, hepatic first-pass effects need to be considered.6,8 The 2 anesthetics (dexmedetomidine and ketamine) administered to the geckos of the present study undergo hepatic metabolism before renal excretion.15 The fact that the DK-induced effects on heart rate and sedation were less when the drug combination was administered in a hind limb versus a forelimb was consistent with a hepatic first-pass effect. It was also consistent with results of other studies6,16 in which the distribution of hepatically metabolized drugs in reptiles differed when administered in a hind limb versus a forelimb. In red-eared sliders, IM administration of buprenorphine in a hind limb resulted in substantially lower peak plasma drug concentrations and a 70% decrease in the area under the concentration-time curve, compared with IM administration of the same dose in a forelimb.6 Buprenorphine is primarily cleared by the liver through glucuronidation followed by biliary excretion, and the investigators of that study6 concluded that the decrease in plasma buprenorphine concentrations following IM administration in a hind limb was attributable to a hepatic first-pass effect. In yellow-bellied sliders (Trachemys scripta scripta), IM administration of tramadol (10 mg/kg) in a hind limb resulted in a peak plasma concentration of tramadol that was 75% lower and a peak plasma concentration of O-desmethyltramadol (the primary active metabolite of tramadol) that was 20% higher than when the same dose of the drug was administered in a forelimb.16 Because tramadol is metabolized in the liver, it was assumed that the higher plasma O-desmethyltramadol concentrations following hind limb versus forelimb administration were the result of hepatic first-pass effect.16 O-desmethyltramadol is a more potent analgesic than its parent compound; therefore, it may be clinically desirable to administer tramadol in the hind limb of reptiles. In that study,16 the onset of thermal anti-nociceptive effects of tramadol was 0.5 hours after hind limb administration and 8 hours after forelimb administration, and thermal antinociception was measurable up to 48 hours after both hind limb and forelimb administration.
For drugs that undergo hepatic metabolism or clearance, interpretation of study results when those drugs are administered in the caudal half of the body of reptiles is challenging because the safety and efficacy of those drugs may differ substantially from when they are administered in the cranial half of the body. For example, in a study9 involving Macquarie River turtles (Emydura macquarii), alfaxalone (10 mg/kg) administered IV into a jugular vein induced sedation, whereas IM administration of the same dose into a hind limb did not induce sedation; therefore, it was concluded that alfaxalone was ineffective when administered IM to that species of turtles. However, alfaxalone (10 mg/kg) induces effective chemical restraint following IM administration in a forelimb of other chelonian species.17,18 Alfaxalone undergoes hepatic metabolism followed by hepatic and renal excretion.15 Therefore, it seems plausible that alfaxalone did not induce sedation in Macquarie River turtles following IM administration in a hind limb owing to hepatic first-pass effect. The lesson is that published pharmacokinetic and pharmacodynamic data for drugs, especially those that undergo hepatic metabolism and excretion, in reptiles should be interpreted cautiously and in conjunction with the site of administration.11
Alternatives to hepatic first-pass effect as potential explanations for a decrease in efficacy when drugs are administered in the hind limbs of reptiles include delayed drug absorption owing to differences in the local anatomic features (eg, fat stores) at the injection site and variation in muscle perfusion and blood flow.6,10,16 However, given that, in several reptilian species, venous blood from the hind limbs flows to the ventral abdominal veins then directly into the liver, hepatic first-pass effect appears to be the most plausible explanation for that decrease in efficacy.
Results of the present study indicated that drugs that undergo hepatic metabolism and excretion, such as many anesthetics (including dexmedetomidine and ketamine), opioids, and oxytocin, should not be administered in the caudal half of the body of leopard geckos. These findings were in agreement with results of other studies6,10,16,a involving other reptilian species. In contrast, drugs that do not undergo hepatic metabolism and excretion, such as fluoroquinolones, cephalosporins, and aminoglycosides, can be administered in the caudal half of the body of reptiles without substantial effects on clinical efficacy. However, as a general rule for reptiles, drug administration in the cranial half of the body is preferred to avoid injection site-dependent effects (eg, renal and hepatic first-pass effects) on drug pharmacokinetics and pharmacodynamics.
ABBREVIATIONS
| DK | Dexmedetomidine-ketamine |
Footnotes
Lahner LL, Mans C, Sladky KK. Comparison of anesthetic induction and recovery times after intramuscular, subcutaneous or intranasal dexmedetomidine-ketamine administration in red-eared slider turtles (Trachemys scripta elegans) (abstr), in Proceedings. Am Assoc Zoo Vet Conf 2011;136.
Grub Pie, Repashy Ventures, Oceanside, Calif.
Dexdomitor 0.1, Pfizer Animal Health, New York, NY.
Ketamine hydrochloride injection, Hospira Inc, Lake Forest, Ill.
Research Randomizer, version 4.0, Urbaniak GC, Plous S, Middletown, Conn. Available at: www.randomizer.org. Accessed Jul 11, 2016.
Antisedan, Pfizer Animal Health, New York, NY.
SigmaPlot, version 13, Access Softek, Berkeley, Calif.
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