Introduction
Reptile anesthesia is a challenging branch of medicine. Reptiles have unique anatomic and physiologic differences compared to mammals that must be appreciated in order to perform safe and effective anesthesia. These differences include decreased metabolism, oxygen demands, and blood pressures; lack of a functional diaphragm and respiratory arrest at stage 3 general anesthesia; cardiac and intrapulmonary blood shunting; as well as potential hepatic and renal first-pass effects associated with caudal body drug administration.1–4 The biological differences are huge within the class Reptilia and its over 10,000 species.5 In addition, individual patients often exhibit disease and dysfunction, which further complicate anesthesia.
The primary purpose of this retrospective case study was to thoroughly describe general anesthesia procedures performed in snakes at a veterinary referral hospital. We hypothesized that the inhalant anesthetic standards and protocols used would follow similar advancements made in research but may indicate areas in need of improvement. This description aims to install an appreciation that snake anesthesia be maintained at the same standard of care as that of small animal anesthesia. A secondary purpose was to evaluate the medical records of any patients that experienced anesthesia-related deaths to potentially identify any variables that may predispose to such complications.
Materials and Methods
Case selection criteria
Medical records of snakes that underwent general anesthesia at the University of Georgia’s Veterinary Teaching Hospital from October 2000 through January 2022 were reviewed. Cases were obtained by searching the hospital’s electronic medical record system for snakes that received anesthetic-related charges on their invoice. The patient’s complete medical records, once identified, were screened and data from each anesthetic event documented. Only anesthetic events with detailed anesthesia records were used to assess anesthetic times, patient vitals, and outcomes.
Data set quality assessment
Detailed records underwent an evaluation for overall quality using the statistical program JMP Pro (version 16.0.0; JMP Statistical Discovery LLC). Cases were scored on the basis of general information provided, completeness of anesthesia time points, and number of vitals monitored. Overall quality assessment was based on whether information such as patient ID, date, weight, American Society of Anesthesiologists (ASA) physical status classification score, anesthetics used, anesthesia times, procedure, and vitals were properly and consistently recorded. Records were then ranked as excellent if they included at least 7 of the 8 metrics; good if 5 to 6 were present; lacking if only 3 to 4 were included; and poor if only 0 to 2 parameters were recorded. Quality of anesthesia time intervals were also assessed. Records were scored as excellent if all time points were present, good if only 1 time point was missing, lacking if 2 time points were missing, and poor if > 3 time points were missing. Monitoring of vitals (ie, heart rate, respiration rate, end-tidal partial pressure of carbon dioxide [PETCO2], peripheral oxygen saturation of hemoglobin as measured by pulse oximetry [SpO2], and body temperature [via infrared laser]) was also assessed in a similar manner. Cases were scored as excellent if all parameters were monitored, good if 3 to 4 of the parameters were included, lacking if only 1 to 2 were present, and poor if none were assessed.
Medical records review
Data collected from the records for every anesthetic event included date of visit, length of hospital stay, species, age, sex, type (pet, part of a breeding colony, institutional collection exhibit, or free-ranging wildlife), body weight, body condition score (BCS; scale of 1 to 5; 1 = emaciated, 2 = underweight, 3 = ideal, 4 = overweight, and 5 = grossly obese), medical history, current diagnosis, date of each anesthetic event, ASA score (on a scale of 1 to 5, with 1 = clinically healthy, 2 = mild systemic disease, 3 = severe systemic disease, 4 = severe systemic disease that is a constant threat to life, and 5 = moribund and not expected to survive), premedication agent(s) used and route, duration of time between premedication and induction agent administrations, induction agent(s) used and route, duration of time between induction and maintenance agent administrations, maintenance agent(s) used and route, anesthetic-related intraoperative events, emergency agent(s) used and route, duration of anesthesia, duration of procedure, duration of recovery, recovery agent(s) used and route, reversal agent(s) used and route, anesthesia-related mortality, and the results of any postmortem evaluations, if performed. Also, nadir and peak values were collected for the following: concentration of inhalant, heart rate, respiratory rate (or artificial ventilation rate), body temperature, PETCO2, and SpO2. Times were reported in minutes.
For inhalant maintenance anesthesia, duration of anesthesia was defined as the time between starting and ending inhalant administration. Duration of recovery was defined as the time between discontinuation of inhalant anesthesia to either extubation if intubated or spontaneous movement when intubation was not performed. For injectable maintenance anesthesia, a total duration of anesthesia + recovery was established by determining the time between induction agent administration to the recovery of the reptile defined as either observed spontaneous movement or extubation. Duration of procedure was based on the times logged in the anesthesia record for surgery start and end times.
One author (AMR) assigned retrospective ASA statuses to patients for every anesthetic event. The retrospective ASA status was based on the medical history, physical examination findings, working diagnosis, clinicopathology, and, if applicable, necropsy results. Diagnoses were categorized into general anatomic systems of pathology such as gastrointestinal, musculoskeletal, etc. Nonspecific or nondefinitive diseases for which the origin of pathology was obscure were placed in an “other” category. Procedures were organized into 2 categories: majorly to moderately invasive procedures (requiring a surgical level of anesthesia) versus non- to minimally invasive procedures (requiring deep sedation to light anesthesia). Majorly invasive procedures included entry into the coelom, while moderately invasive procedures included any cutaneous surgery or noncoelomic mass removals. All other procedures were placed into the non- to minimally invasive category including blood collection, imaging, noncoelomic endoscopy, etc. An anesthesia-related death was defined as occurring within 24 hours of an anesthetic event, with any such cases reviewed in detail.
Statistical analysis
Data reported on anesthesia records were used to perform statistical analyses (ie, calculating means and SDs or medians and ranges) for hospital visit length, hospital-associated costs, age, sex, body weight, BCS, retrospective ASA status, injectable anesthetic dose or concentration of inhalant, anesthesia time points (ie, time between premedication and induction agent administrations, time between induction and maintenance agent administrations, duration of inhalant or injectable anesthesia, duration of procedure, duration of recovery, and total anesthesia time), and nadir and peak values of vitals were also reported (ie, heart rate, respiratory [ventilation] rate, body temperature, PETCO2, and SpO2). Patient percentages reported represent the percent of total cases or outcomes. Mean and SD values (determined by a 95% confidence level) were reported as mean ± SD. JMP Pro (version 16.0.0; JMP Statistical Discovery LLC) was used for statistical analysis.
Results
Patient demographics
A total of 139 anesthesia cases were reported over an approximately 22-year period at a referring animal hospital (Supplementary Table 1; summary of anesthesia record numbers used in the study), including data from snakes belonging to 4 families of Serpentes: Boidae (27/139), Colubridae (39/139), Pythonidae (37/139), and Viperidae (32/139). In 4 cases, the genus and species were unknown (Supplementary Table 2; breakdown of cases used in study by genus and species; Supplementary Table 3; summary of patients by demographics). This data set consisted of 110 animals (42 male, 57 female, and 11 sex unknown). The 139 cases were classified as pets (81/139), wildlife (25/139), or those primarily used for educational (31/139) or breeding purposes (2/139; Supplementary Table 4; summary ASA scores and days in hospital broken down by health status). Mean and SD values reported below were calculated from data obtained from all snakes anesthetized (Supplementary Tables 3–5; additional subcategorized values). For all 139 cases, the mean ± SD age of snakes was 9 ± 7 years (range, < 1 year to 28 years; Supplementary Table 3). Body mass also varied among the different snake species (based on overall mean ± SD BCS of 3 ± 1), and most snakes were neither over- nor underweight. However, most patients (79% [110/139] of cases) undergoing anesthesia had a compromised health status (110/139), which could be further classified into categories that included gastrointestinal (11/110), injury related (7/110), ocular (11/110), reproductive (15/110), respiratory (14/110), skin or oral (26/110), systemic multifocal diseases (14/110), or other (12/110; Supplementary Table 4). The most commonly reported procedures performed were sedation for handling (blood collection [91/139] or physical examination [3/139]) or for diagnostic imaging (CT [18/139], endoscopy [9/139], radiography [34/139], ultrasonography [15/139], or MRI [1/139]; Supplementary Table 5; summary of procedures performed). In such cases, the mean ASA scores were higher in compromised animals (2.5 ± 0.8) compared to healthy animals (1 ± 0.2).
Anesthesia data set summary and record quality assessment
One hundred six cases had complete anesthesia records available for review. Of those reports, 27% (29/106) were from individuals that had undergone multiple anesthesia events throughout their lifetime (Supplementary Table 1). Thirty-six reports included premedication as part of their anesthetic plan; however, only 89% (32/36) of these cases had accurately recorded premedication duration times that could be used for further analyses. Other anesthetic duration times that were assessed included the following: general anesthesia (maintenance), procedure, recovery, and sum of total anesthesia time. Duration of anesthesia was accurately reported in 76% (81/106) of cases, while for the 63 cases in which a surgical procedure was conducted, 63% (40/63) accurately reported this time. Only 43% (46/106) of cases perfectly recorded recovery duration, greatly limiting the number of cases for which the sum of anesthesia time could be assessed. Anesthesia vital parameters that were most often assessed in patients included the following: PETCO2, SpO2, heart rate, respiratory rate, and body temperature (Table 1). For anesthesia outcomes, of the 139 cases, 127 animals recovered, 4 failed to recover, 6 were elected for immediate euthanasia, and 2 were euthanatized following receipt of biopsy results.
Data set summary variables for 139 snakes that underwent general anesthesia at the University of Georgia’s Veterinary Teaching Hospital between October 2000 and January 2022.
| Vitals | Boidae | Colubridae | Pythonidae | Viperidae | All snakes | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| N | Mean ± SD | N | Mean ± SD | N | Mean ± SD | N | Mean ± SD | N | Mean ± SD | |
| EtCO2 | ||||||||||
| Nadir | 10 | 14 ± 7 | 16 | 13 ± 4 | 16 | 16 ± 6 | 2 | 10 ± 5 | 44 | 14 ± 6 |
| Peak | 32 ± 13 | 33 ± 16 | 31 ± 14 | 18 ± 5 | 31 ± 14 | |||||
| SpO2 | ||||||||||
| Nadir | 1 | 94 | 5 | 93 ± 13 | 0 | 0 | 6 | 94 ± 12 | ||
| Peak | 99 | 97 ± 3 | 98 ± 3 | |||||||
| Heart rate | ||||||||||
| Nadir | 13 | 34 ± 10 | 25 | 54 ± 11 | 22 | 35 ± 13 | 17 | 46 ± 9 | 77 | 43 ± 14 |
| Peak | 54 ± 13 | 78 ± 14 | 54 ± 16 | 50 ± 11 | 61 ± 18 | |||||
| Respiratory rate | ||||||||||
| Nadir | 10 | 3 ± 1 | 25 | 4 ± 3 | 21 | 3 ± 2 | 8 | 4 ± 2 | 64 | 3 ± 3 |
| Peak | 5 ± 1 | 9 ± 11 | 6 ± 5 | 4 ± 1 | 7 ± 8 | |||||
| Temperatures (°C) | ||||||||||
| First | 7 | 28.9 ± 2.2 | 16 | 29.4 ± 1.7 | 10 | 27.8 ± 1.7 | 6 | 77 ± 2.2 | 39 | 28.3 ± 2.8 |
| Nadir | 27.2 ± 1.7 | 28.9 ± 2.2 | 27.2 ± 2.8 | 76 ± 2.2 | 27.2 ± 2.8 | |||||
| Peak | 30.6 ± 2.2 | 31.1 ± 1.7 | 28.9 ± 2.8 | 77 ± 2.2 | 29.4 ± 2.8 | |||||
| Last | 29.4 ± 2.2 | 30.6 ± 2.2 | 28.3 ± 3.9 | 79 ± 2.2 | 28.9 ± 2.8 | |||||
This table shows the number of cases and respective mean ± SD of nadir and peak values for end-tidal partial pressure of carbon dioxide (mm Hg), peripheral oxygen saturation of hemoglobin (%), heart rate (beats/min), respiration (breaths/min), and body temperatures (˚C) subcategorized by families of snakes.
Before any analyses were undertaken, all available reports underwent a quality assessment to identify any underlying issues (see Materials and Methods for details). Eighty-nine anesthetic records were graded as excellent or good, while 17 were lacking or poor (Supplementary Table 6; summarized quality of data set records). Regarding time intervals recorded, 68 cases were classified as excellent or good, while 38 were labeled as lacking or poor. Only 80 cases monitored vitals. Of those, 46 cases were scored as excellent or good, 34 were lacking, and 26 were poor. Although reporting improved within the last 10 years regarding patient details, no such pattern was evident for anesthesia times or vitals monitored.
Anesthetic agents, doses, and times
After record quality assessment, each available record was further analyzed for the drug used in each category: premedication, induction, maintenance, and recovery. Routes of drugs administered, duration of time (minutes) for each anesthetic category, as well as the average years certain drug combinations or inhalants were used, were also summarized (Supplementary Table 7; full summary of anesthetics reported in this study, including number of cases, anesthesia level, route, anesthesia length, and years used; “n” numbers are low in these groups). To evaluate with more depth and convey general information regarding anesthesia times for individual drug combinations, we assessed each individual injectable or inhalant drug used alone or in combination and reported their respective dose (mg/kg) and anesthesia times (minutes) and discuss here groups that had “n” numbers ≥ 10.
Our data revealed that roughly 40% (56/139) of patients required deep sedation for minor or noninvasive procedures, while 60% (83/139) required a surgical plane of general anesthesia. Anesthesia cases were further categorized by the types of anesthetics used. Although most animals were induced and maintained under anesthesia with just inhalants, over time we did observe a noticeable shift with patients receiving a combination of injectables and inhalants or injectables alone.
For cases in which a premedication was included as part of the anesthetic plan, the drugs alfaxalone, butorphanol, dexmedetomidine, hydromorphone, ketamine, midazolam, and morphine were typically used, either alone or in combination, all of which were injected IM (Supplementary Table 7). With the exception of alfaxalone prior to 2015, a premedication plan typically consisted of a single agent, rather than a combination of drugs, butorphanol being the most popular choice in snakes. After 2015, the most popular preanesthetic plan included one in which alfaxalone was administered alone or in combination with other drugs—typically hydromorphone or midazolam. Drug combinations incorporating dexmedetomidine and ketamine were a close second. The drugs alfaxalone, butorphanol, and hydromorphone were commonly used (Table 2). Alfaxalone was typically dosed between 9 to 15 mg/kg and, when given in combination with other drugs, produced sedation to light anesthesia lasting around 55 ± 33 minutes. This time frame was very similar for hydromorphone at doses ranging between 0.5 to 1 mg/kg. Butorphanol, typically used alone, was administered at doses ranging from 0.9 to 20 mg/kg (2 cases used a high 20-mg/kg dose, and the rest were 10 mg/kg and below).
Data set summary of injectable anesthetics used in snakes in the study described in Table 1.
| Injectable anesthetics | Anesthetic dosages (mg/kg) | Anesthesia times (min) | ||
|---|---|---|---|---|
| N | Mean ± SD | Range | Mean ± SD | |
| Premedication anesthetics | ||||
| Alfaxalone (alone) | 1 | 15.0 | 15.0–15.0 | 19 ± 27 |
| Alfaxalone (combination) | 10 | 13.3 ± 2.5 | 9.0–15.0 | 55 ± 33 |
| Alfaxalone (all cases) | 11 | 13.4 ± 2.5 | 9.0–15.0 | 50 ± 34 |
| Butorphanol (alone) | 10 | 5.0 ± 7.9 | 0.9–20.0 | 15 ± 10 |
| Dexmedetomidine (combination) | 7 | 0.05 ± 0.02 | 0.03–0.08 | 50 ± 46 |
| Hydromorphone (combination) | 12 | 0.7 ± 0.3 | 0.5–1.0 | 53 ± 35 |
| Ketamine (combination) | 8 | 15.5 ± 7.7 | 5.0–25.0 | 58 ± 49 |
| Midazolam (combination) | 7 | 1.1 ± 0.6 | 0.5–2.0 | 34 ± 23 |
| Morphine (alone) | 1 | 1.0 ± xx | 1.0–1.0 | 15 |
| Induction anesthetics | ||||
| Alfaxalone (alone) | 8 | 10.4 ± 4.2 | 5.0–15.0 | 9 ± 19 |
| Alfaxalone (combination) | 4 | 8.7 ± 4.8 | 5.0–15.0 | 46 ± 47 |
| Alfaxalone (all cases) | 12 | 9.8 ± 4.3 | 5.0–15.0 | 17 ± 31 |
| Dexmedetomidine (combination) | 2 | 0.10 ± 0.00 | 0.10–0.10 | 45 ± 51 |
| Hydromorphone (combination) | 1 | 0.5 | 0.5–0.5 | 23 ± 32 |
| Ketamine (combination) | 2 | 10.0 ± 0.1 | 10.0–10.0 | 30 ± 52 |
| Propofol (alone) | 12 | 6.0 ± 2.5 | 2.0–10.0 | 8 ± 9 |
| Propofol (combination) | 2 | 4.4 ± 2.2 | 3.0–6.0 | 93 ± 7 |
| Propofol (all cases) | 14 | 5.8 ± 2.4 | 2.0–10.0 | 19 ± 30 |
| Maintenance anesthetics | ||||
| Alfaxalone CRI (mg/kg/min) | 2 | 0.2 ± 0.1 | 0.1–0.3 | 139 ± 13 |
| Alfaxalone multiple doses | 1 | 5.0 ± 0.0 | 5.0 (2x injections) | 45 |
This table indicates the number of cases available for which dosage information and anesthetic times were both recorded accurately for the following anesthetic stages: premedications, induction, and maintenance. Indication of whether a drug was injected alone or in combination is shown in parentheses. The mean ± SD and range for anesthetic doses are provided; dosages are shown in mg/kg. The mean ± SD for anesthesia times is also shown in minutes.
CRI = Constant rate infusion.
For induction, the majority of cases relied on inhalants, with isoflurane being the most popular choice. After inhalants, alfaxalone and propofol were the next most readily used anesthetics for induction. Frequently, injectable drugs were given IM except for propofol and sometimes alfaxalone, which was given IV or, with the case of propofol, occasionally given intracardially, particularly in larger snakes. Inhalants, on the other hand, were administered to smaller or venomous snakes via an induction chamber (ie, mask, tube, or box). We focused on cases that involved the use of alfaxalone or propofol (Supplementary Table 7). Alfaxalone was typically administered between 5 and 15 mg/kg, with a mean dose of 9.8 mg/kg. Alfaxalone, when administered at higher doses and alone, resulted in a shorter mean induction period, approximately 9 ± 19 minutes. Compared to alfaxalone lower doses in combination with other drugs, the average induction period tended to be closer to 46 ± 47 minutes. In cases that used propofol, the majority used it alone. In such reports, the average dose typically administered was 5.8 mg/kg, with doses ranging between 2 and 10 mg/kg. This resulted in a short induction period, lasting around 8 ± 9 minutes.
For maintenance anesthetics, again many cases depended on the use of inhalants to keep a patient at a surgical plane of anesthesia; 61% (85/139) preferred isoflurane, while 19% (27/139) used sevoflurane (Supplementary Table 7). For 17% (24/139) of cases in which the patient was induced with an injectable anesthetic (generally with alfaxalone), no additional injectable or inhalants were used. We found 39 cases available that implemented the use of isoflurane and 10 cases that used sevoflurane for which anesthesia time points were properly recorded. The mean induction time for isoflurane and sevoflurane, when administered alone, were near identical, approximately 13 ± 19 minutes for isoflurane and 13 ± 11 minutes for sevoflurane. Mean nadir and peak inhalant concentrations and mean anesthesia duration times are shown (Table 3). Inhalants were categorized into 2 groups, one in which they were administered alone and the other in which injectable anesthetics were used for induction. When combined with injectable anesthetics, mean inhalant concentrations were typically lower between the 2 groups. For isoflurane alone, mean nadir and peak inhalant concentrations ranged between 2.8% to 4.3% but fell to around 1.9% to 3.7% when isoflurane was combined with other injectable anesthetics. This difference was less obvious with sevoflurane; however, anesthesia duration between sevoflurane alone and sevoflurane in conjunction with injectables was similar. Lastly, 3 of the remaining cases utilized alfaxalone administered as a constant rate infusion (CRI) or over multiple repeated doses administered IV. These 2 methods of delivery were only recently employed in snakes over the last couple of years, and as such, data is limited.
Data set summary of inhalants used to maintain anesthesia for the snakes described in Table 1.
| Inhalant anesthetics | Anesthetic inhalant nadir % | Anesthetic inhalant peak % | Anesthesia duration (min) | |||
|---|---|---|---|---|---|---|
| N | Mean ± SD | Range | Mean ± SD | Range | Mean ± SD | |
| Maintenance | ||||||
| Isoflurane | 43 | 2.8 ± 1.5 | 1.0–5.0 | 4.3 ± 1.1 | 2.0–5.0 | 72 ± 60 |
| Isoflurane and other anesthetics | 11 | 1.9 ± 0.9 | 1.0–3.5 | 3.7 ± 1.2 | 1.0–5.0 | 79 ± 35 |
| Sevoflurane | 10 | 2.3 ± 0.9 | 1.0–4.0 | 5.0 ± 1.4 | 4.0–8.0 | 125 ± 66 |
| Sevoflurane and other anesthetics | 5 | 1.9 ± 1.0 | 1.0–3.5 | 6.1 ± 2.4 | 2.0–8.0 | 194 ± 88 |
This table indicates the number of cases available for which nadir and peak gas concentrations and anesthetic times were accurately recorded. Inhalants were categorized by type and whether they were used alone or in combination with other anesthetics. The mean ± SD along with range were reported for anesthetic inhalant nadir and peak concentrations as well as anesthesia duration time.
Regarding recovery, most cases did not use any recovery agents, but 25% (26/106) did (Supplementary Table 7). When examining ASA scores, those that received recovery drugs tended to have slightly higher scores (2.4 ± 1.0; range, 1 to 5) than those that did not (2.2 ± 0.9; range, 1 to 4). In these patients, the most common drug chosen was epinephrine, followed by atipamezole, which were frequently administered IM and alone.
Summary of anesthesia times, patient vitals, and anesthesia outcomes
Cases in which a premedication was included as part of the anesthetic plan revealed that the mean time between administration and the onset of induction was 42 ± 34 minutes. For induction, 77 cases were available. The time of onset to maintenance was short, averaging around 16 ± 23 minutes, while maintenance lasted 86 ± 66 minutes among the 81 cases assessed. When evaluating average procedure time, 40 cases were available that showed the average procedural duration was 69 ± 52 minutes. The mean recovery time was 42 ± 36 minutes, and total anesthesia time was 159 ± 102 minutes in 45 cases for which this could be calculated (Supplementary Table 8).
We next examined cases in which an animal died during or within 24 hours of an anesthetic event. The following 2 snakes were euthanatized and not permitted to recover from anesthesia due to extensive pathology identified at the time of surgery: a corn snake (Pantherophis guttatus) with visceral gout and a pine snake (Pituophis melanoleucus) with multifocal mesenteric sarcoma. A female yellow rat snake (Pantherophis obsoleta quadrivittata) failed to recover and was euthanatized 18 hours after anesthesia. This snake presented with reduced body condition (BCS, 2) and hyperuricemia (24 mg/dL). Renal changes had been identified on ultrasonography, and renal biopsy was scheduled following stabilization. The snake (ASA score, 2) was premedicated with butorphanol, intubated, and maintained on isoflurane and oxygen. During anesthesia, heart rate varied between 49 and 62 beats/min, ventilation rate varied between 2 and 5 breaths/min, PETCO2 ranged from 9 to 17 mm Hg, and SpO2 between 70% to 96%. Following renal biopsy and closure, the snake regained spontaneous respiration and was extubated. However, recovery did not progress despite IV fluid therapy, naloxone, doxapram, mannitol therapy, reintubation, and ventilation overnight. Heart rate persisted but spontaneous ventilation never recurred, and the euthanasia was performed the following day (approx 18 hours after cessation of anesthesia). Necropsy revealed severe tubular vacuolar degeneration of the kidney, chronic interstitial nephritis, glomerular sclerosis, periglomerular fibrosis, and necrosis with granulomatous inflammation associated with urate tophi. One ball python (Python regius) died acutely following cardiocentesis and the development of cardiac tamponade. Finally, a corn snake (Pantherophis emoryi and P guttatus cross) died following bronchoalveolar lavage. Although this animal was noted to have clinical respiratory signs (retrospective ASA score of 4), the only pulmonary change on necropsy was locally extensive to diffuse atelectasis. This animal received alfaxalone at 15 mg/kg; induction and maintenance were reported as smooth. After extubation, the animal became apneic and was reintubated. Asystole occurred 45 minutes following reintubation, and intracardiac epinephrine was administered but the animal did not recover. No definitive cause of death was identified.
Discussion
Numerous anesthetic protocols have been utilized for snakes at the referral hospital throughout this 22-year period (2000 to 2022). A thorough review of these anesthetic events was conducted to evaluate parameters and outcomes to improve our understanding of any adverse events.
Our retrospective study also revealed areas that would benefit from improvement. One of the challenges that referral teaching hospitals face is the constant changeover of student physicians and personnel. Because of this repetitive transition of staff, ensuring consistent record keeping is a constant battle. However, with the advancement of technology and electronic records we have an avenue forward to design better systems to ensure accurate record keeping. Within our data set, we found issues in 3 different areas. The first addressed the quality of general information provided about the patient, the second related to accuracy of time points, and, lastly, the third looked at the number of vitals monitored. Because the data set was evaluated as a whole, it’s possible that records that were poorly annotated were representatives from early years and do not reflect the state of records now. To better assess this, we compared the quality of records over the years and found that many records did improve within the last 10 years or so, specifically regarding the amount of patient details provided.
However, this phenomenon was not evident for anesthesia time points or the number of vitals monitored. For anesthesia time points, we speculate that poor record keeping maybe partially be due to ambiguity surrounding start and end points of maintenance and recovery, particularly when injectable anesthetics are used. For instance, for cases in which snakes are not intubated or started on inhalants, determining when they shift from induction to maintenance is rather difficult. This bares out in our data set as only 48% (10/21) of cases, under the category of injectable anesthetics, included both a start and end time point for induction. The shift from maintenance to recovery is also rather vague, and as a result, it is not altogether surprising that this would impact anesthesia duration periods as well, and only 29% (6/21) of records using injectable anesthetics accurately recorded this time frame. Recovery end points were accurately reported in 43% (46/106) of all cases but only for 14% (3/21) of cases in which injectables were used and was less likely to be recorded in wildlife animals and those typically requiring sedation. To address ambiguity regarding anesthesia periods in snakes, specifically for cases in which injectables are used, adjustments are needed regarding delineating start and end anesthesia time points. We recommend the following: loss of a righting response should be used as a metric to characterize the end of induction as well as the start of maintenance, any positive return of reflexes/responses should signify the end of maintenance and start of recovery, and, lastly, the return of a righting response should indicate that the animal is recovered. Using these parameters in snakes would allow for better analyses when injectable anesthetics are used and would facilitate better congruity in data sets involving multiple cases in which inhalants, injectables, or some combination of the 2 are used, thus enhancing retrospective studies.
Regarding monitoring patient vitals, it was not surprising to find that oxygen saturation levels were the least likely parameter to be monitored in snakes. SpO2 probes are notoriously finicky and can be problematic for many reasons. Nor was it unexpected to find that PETCO2 was not frequently monitored, as many snakes (54) were not intubated; patient size and the requirement for light versus surgical anesthesia were likely factors. We did find it startling though that 25% (26/106) of cases failed to record a single vital, the majority of which included cases involving wildlife animals, conditions for which injectables were used, and/or a sedation level of anesthesia was required. It’s possible that underlying components contributing to this poor record outcome are associated with challenges in the field, where monitoring equipment or power sources may not be readily available. In such scenarios, bringing a battery-powered doppler into the field would be extremely beneficial as it is a small, light-weight instrument that is useful in assessing heart rates and identifying arrhythmias in reptiles.
Many factors likely contribute to the development of poor records. Moving forward, having a system in place to audit records would be beneficial to identify and correct any issues as they arise. As hospitals continue to embrace digital systems, developing programs specifically for this purpose is not out of the realm of possibility. Another consideration for improvement is the redesign of anesthesia recording sheets themselves, which are not designed with clinical research in mind. We observed that many anesthesia forms had sections for the anesthesiologist to make comments regarding planes of anesthesia and adequacy of depth, the majority of which were routinely left blank. On occasion information was provided in sections, it varied from patient to patient and overall usefulness for retrospective studies was rather minimal. Taking this into consideration, we recommend redesigning anesthesia sheets in a way that (1) reduces their complexity, thus making them easier to fill out (ie, allows the recorder to circle prefilled options instead of requiring handwritten responses); (2) defines the start and end anesthesia time points using metrics like the loss and return of righting responses, which are more broadly applicable across different taxa; and, lastly, (3) incorporates information that is useful and relevant for anesthesia research. One example is anesthesia depth, which could be limited to the following 3 categories: light, surgical, and excessive, one of which could be circled to provide a useful metric for future retrospective studies.
Regarding anesthetics used, we found that isoflurane was the most popular choice for both inducing and maintaining smaller and venomous snakes under a general plane of anesthesia followed by alfaxalone in medium to larger snakes. A previous study6 of snakes using inhalants determined that the minimum alveolar concentration to prevent movement in 50% of patients for isoflurane is 1.7% and for sevoflurane is 2.4%. Although not directly comparable with our data set because all animals undergoing surgical anesthesia are required to be fully immobilized, lacking any response to noxious stimuli, the values reported in this study are expectedly higher, even for cases in which injectable anesthetics were also used. This finding also was not surprising, as several studies have shown that combining multiple anesthetic agents reduces the amount of each drug required (anesthesia-sparing effect); as a result, this potentially reduces unwanted side effects while boosting desired outcomes.7,8
Review of our data set also revealed that recovery times were considerably longer compared to studies in which inhalants were used alone in mammals (4 to 13 minutes9–11) and birds (6 to 20 minutes12,13) but shorter than other publications involving reptiles, for which the mean recovery time ranged between 46 to 289 minutes.14,15 The finding that recovery is delayed in snakes is not surprising, as it is well-known that compared to many mammals and some birds, reptiles tend to have reduced heart and respiratory rates along with slower metabolisms, which affect drug distribution and elimination.2 Other factors that might contribute to these differences include patient health status or variable understandings and perceptions of recovery in snakes. Studies performed of other reptiles have shown that IM administration of epinephrine reduces recovery time substantially.16,17,18 Although we investigated this in our data set, our analysis was not particularly useful as epinephrine was frequently administered to snakes whose recovery was already prolonged. That being said, the use of epinephrine in snakes should be considered in any anesthesia plan to enhance recovery time.
For cases in which injectable anesthetics were used, in the last several years alfaxalone has steadily risen in popularity in snakes. This is likely due to alfaxalone’s versatility in administration route and, as such, is routinely used as a preanesthetic and/or an induction agent in snakes. Although very limited in number, alfaxalone is also showing promise as a maintenance anesthetic when administered in multiple doses or as a CRI. One of the biggest draws regarding CRIs is the ability to tightly titrate the amount of drugs being administered; however, we suspect that this method is underutilized in snakes because of challenges associated with IV catheter placement, especially in small individuals.
A broader look at our data set reveals that snakes with high ASA statuses, low BCS, and concurrent disease are more likely to develop complications while under anesthesia. Therefore, clinicians are advised to perform a detailed preanesthesia physical examination and determine a BCS and ASA status prior to considering anesthesia.
As with most retrospective studies, limitations include human error, particularly regarding the accuracy or lack of information on medical and/or anesthesia reports. The BCS and ASA statuses were also subjective determinations. Given the low numbers of anesthesia-related deaths, identification of mortality factors was not possible. Outliers in the data were also not excluded, which may impact reported mean values.
In conclusion, this report detailed the largest number of anesthetic events in snakes, provided useful information for clinicians, and highlighted the need for greater vigilance for snakes with low body condition, concurrent disease, and high ASA status. This study also highlighted the need for improvement in areas of record management as well as the importance of auditing to identify and resolve issues following anesthetic events. From our analyses of the records, we also propose that anesthetic records should be tailored specifically with the mindset that such data will be later incorporated in future retrospective studies. Therefore, more effort should be made in designing anesthesia records in such a way that (1) eases the workload of the recorder, which will hopefully have a positive effect on record completeness, and (2) captures valuable information for such studies involving cross-analyses between different anesthetic groups.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org
Acknowledgments
No funding sources were used in the generation of the data or preparation of the manuscript. The authors declare that there were no conflicts of interest.
Heather Morrison and Dr. Rasys contributed equally to the study conception, data collection, analysis, interpretation of results, and draft manuscript preparation. Dr. Quandt contributed to the data collection and draft manuscript preparation. Dr. Divers contributed to study conception, design, and draft manuscript preparation. All authors reviewed the results and approved the final version of the manuscript.
The authors thank Dr. Kellyn Kearney and Melanie Lovette for their initial work on collecting the data.
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