Abstract
The objective of this study was to investigate the effect of anesthesia duration on the quality of recovery in horses. The medical records of horses that were anesthetized and underwent surgery for elective and emergency soft tissue and orthopedic conditions from 2013 to 2019 were reviewed. Horses included in the study (N = 305) fulfilled the following requirements: all had the same premedication/induction protocol and the same balanced anesthesia for maintenance and were anesthetized by the same, experienced Board-certified anesthesiologist. A standardized anesthetic recovery score was completed for all horses to evaluate their recovery and the following interactions were assessed: age, body weight, breed, sex, American Society of Anesthesiologists status, type of surgical procedure, occurrence of hypotension, use of dobutamine, number of additional doses of xylazine/ketamine after isoflurane discontinuation, anesthesia duration, post-anesthetic sedation, and end-tidal isoflurane concentration during maintenance and at the time of transfer to the recovery room. These interactions were assessed based on the quality of recovery score using logistic regression. Duration of anesthesia (P = 0.021) and age (P = 0.003) negatively affected the quality of recovery. The odds of a worse recovery score were increased by 1.20-fold (1.03, 1.41; lower and upper limits) for every additional 30 min of anesthesia duration, while the odds of a worse recovery score were increased by 1.09-fold (1.03, 1.16) for every additional 1 y of age. In conclusion, the results of this retrospective study indicate that increasing the anesthesia duration negatively affects the quality of recovery in horses undergoing routine and emergency surgical procedures.
Résumé
L’objectif de cette étude était d’étudier l’effet de la durée de l’anesthésie sur la qualité de la récupération chez les chevaux. Les dossiers médicaux des chevaux qui ont été anesthésiés et ont subi une intervention chirurgicale élective et d’urgence des tissus mous et orthopédique de 2013 à 2019 ont été examinés. Les chevaux inclus dans l’étude (N = 305) remplissaient les conditions suivantes : tous avaient le même protocole de prémédication/d’induction et la même anesthésie équilibrée pour l’entretien et ont été anesthésiés par le même anesthésiste expérimenté et certifié par le Board. Un score de récupération anesthésique standardisé a été réalisé pour tous les chevaux afin d’évaluer leur récupération et les interactions suivantes ont été évaluées : âge, poids corporel, race, sexe, statut American Society of Anesthesiologists, type d’intervention chirurgicale, survenue d’hypotension, utilisation de dobutamine, nombre de doses supplémentaires de xylazine/kétamine après l’arrêt de l’isoflurane, la durée de l’anesthésie, la sédation post-anesthésique et la concentration d’isoflurane en fin d’expiration pendant l’entretien et au moment du transfert en salle de réveil. Ces interactions ont été évaluées sur la base de la qualité du score de récupération en utilisant la régression logistique. La durée de l’anesthésie (P = 0,021) et l’âge (P = 0,003) ont affecté négativement la qualité de la récupération. Les probabilités d’un pire score de récupération ont été multipliées par 1,20 (1,03, 1,41; limites inférieure et supérieure) pour chaque 30 min supplémentaire de durée d’anesthésie, tandis que les chances d’un pire score de récupération ont été multipliées par 1,09 (1,03, 1,16) pour chaque année d’âge supplémentaire. En conclusion, les résultats de cette étude rétrospective indiquent que l’augmentation de la durée de l’anesthésie affecte négativement la qualité de la récupération chez les chevaux subissant des interventions chirurgicales de routine et d’urgence.
(Traduit par Docteur Serge Messier)
Introduction
General anesthesia in horses is associated with a high risk of morbidity and mortality compared to other species of domestic animals (1–3). Many of the peri-anesthetic mortalities/morbidities in horses are related to the recovery period (4), which is therefore considered a high-risk phase of anesthesia in this species.
Subjective scoring of the quality of recovery in horses and establishing associations with the different confounding factors involved in equine anesthesia, including demographics (age, weight, sex), type and duration of surgery, health status, anesthetic drugs, and type of anesthetic protocol, among others, have allowed several specific risk factors that impact the recovery period to be identified (1,2,4). One of these risk factors is increased anesthesia duration. Its negative effect on the quality of recovery has been demonstrated in several multicenter and single-center studies (1,4,5). In these studies, the different anesthetic protocols and personnel delivering anesthesia add high variability to these confounding risk factors.
The current study reviews recovery scores of horses that underwent elective and emergency soft tissue and orthopedic surgery using the same anesthetic protocol, delivered by the same Board-certified anesthesiologist, in order to correlate anesthesia duration with the quality of recovery. The same anesthetic protocol was used in all horses and was based on the safety and cardiovascular stability induced by this protocol of balanced anesthesia (6) and its clinical application to a variety of cases with different risk status. The aim of this retrospective study was to investigate the effect of anesthesia duration on the quality of recovery. We hypothesized that increased anesthesia duration would significantly affect the quality of recovery in horses in a negative way.
Materials and methods
Horses
Medical records were reviewed of all equine anesthesia carried out by the same Board-certified, experienced anesthesiologist (AV) at the Large Animal Hospital of the University of Guelph, Ontario Veterinary College from 2013 to 2019. Anesthetic episodes with the same premedication, induction protocol, and balanced general anesthesia for maintenance were included in the study. Horses euthanized during anesthesia were excluded.
Data reviewed for these horses included age, body weight, breed, sex, American Society of Anesthesiologists (ASA) classification status, type of surgical procedure, premedication and induction protocol, anesthesia maintenance protocol, occurrence of hypotension, use of dobutamine, number of additional doses of xylazine/ketamine after isoflurane discontinuation, anesthesia duration, post-anesthetic sedation, end-tidal isoflurane concentration during maintenance and during transfer to the recovery room, and recovery scores. Surgical procedures were divided into 4 categories for easier interpretation of data: elective orthopedic surgeries, emergency orthopedic surgeries, elective soft tissue surgeries, and emergency soft tissue surgeries.
Anesthesia
Horses received non-steroidal anti-inflammatory analgesia prior to surgery, at the discretion of the surgeon [phenylbutazone 4.4 mg/kg body weight (BW), IV or flunixin meglumine 1.1 mg/kg BW, IV]. They were premedicated with xylazine and anesthesia was induced with ketamine and diazepam. All horses were mechanically ventilated (approximately 15 mL/kg BW) following endotracheal intubation, with an approximate fresh gas flow of 20 mL/kg BW per minute of 100% oxygen and an isoflurane end-tidal concentration aimed at 1.2%. Direct arterial blood pressure was measured in all horses, using the facial artery, the transverse facial artery, or the metatarsal artery.
The same balanced anesthesia protocol was used for maintenance in all horses: dexmedetomidine IV constant rate infusion (CRI) (2.5 μg/kg BW per hour), which started at the time of draping the horse for surgery and was continued throughout the duration of anesthesia; lidocaine IV boluses (2 mg/kg BW), which started within 10 min from induction, were administered every 60 to 90 min and not given within 30 min before the beginning of recovery; and isoflurane maintenance (1.2% end-tidal concentration). During anesthesia, isotonic crystalloids were administered at 5 to 10 mL/kg BW per hour and dobutamine was administered at 0.25 to 1.0 μg/kg BW per minute to effect if direct mean arterial blood pressure (MAP) was ≤ 65 mmHg and titrated to maintain a MAP > 65 mmHg.
Inhalant anesthesia was discontinued at least 10 min before surgery was deemed to be completed. After isoflurane discontinuation, anesthesia was maintained with injectable IV anesthetics until surgery was completed and the horse was transferred to the recovery room. The IV protocol consisted of intermittent injections every 5 to 10 min of 33 mg xylazine mixed with 66 mg ketamine IV for horses ≥ 400 kg and adjusted down to horses < 400 kg by lowering the dose or prolonging the interval between doses, based on clinical signs of anesthetic depth, including a slow palpebral reflex, eye position, absence of nystagmus, and prevention of movement.
The end-tidal isoflurane concentration was aimed at approximately 0.5% or less at the time of transfer to the recovery room for all horses. The time from discontinuation of isoflurane until transfer to the recovery room was recorded. Anesthesia time was defined and recorded as the time from induction until the time of transfer to the recovery room.
Recovery
All horses were placed in a padded recovery room and received oxygen insufflation at 15 L/min through the endotracheal tube for 15 min. Post-anesthetic sedation with 0.1 or 0.2 mg/kg BW, IV of xylazine was carried out in selected horses once in the recovery room, based on evaluation of pre-anesthetic attitude and temperament and clinical depth appearance at the time of transfer to the recovery room. Horses were extubated when swallowing was present. Selected critical cases, based on prolonged surgeries, major orthopedic surgery, and/or high ASA status, were assisted by head and tail ropes.
Recovery scores for each horse were assigned by the same experienced anesthesiologist using a previously standardized recovery score (7). The recovery score graded 8 events during the recovery period (Table I), which subsequently were added and matched to an overall descriptive recovery score to rank the quality of recovery on a scale from 1 (smooth) to 6 (fatality/accident) (Table II). A score of 6 was given to horses with a fatal event or accident leading to humane euthanasia during recovery. During recovery, time from entering the recovery room until sternal, time until extubation, and time until standing were also recorded.
Table I.
Scoring system for quality of recovery.
| A. Overall attitude |
| 1. Calm |
| 2. Calm/determined |
| 3. Confused/dizzy |
| 4. Frantic |
| B. Move to sternal |
| 1. Smooth/methodical |
| 2. Incomplete attempts, but controlled |
| 3. Crashing, flopping over |
| C. Sternal phase |
| 1. An organized pause |
| 2. Nonexistent |
| 3. Multiple, with struggle |
| D. Move to stand |
| 1. Methodical |
| 2. An organized scramble |
| 3. Used walls for support |
| 4. Ricocheting off walls |
| E. Strength |
| 1. Near full |
| 2. Mildly rubbery |
| 3. Dog-sitting before standing |
| 4. Repeated attempts due to weakness |
| F. Number of attempts to stand |
| Score = n |
| G. Balance and coordination |
| 1. Solid |
| 2. Moderate dancing |
| 3. Reflex saves |
| 4. Careening |
| 5. Falls back down |
| H. Knuckling |
| 1. None |
| 2. Hind limbs only |
| 3. All 4 limbs |
| Total: |
Table II.
Descriptive recovery score.
| 1 | Smooth, calm, and easy recovery in 1 attempt, no ataxia or mild ataxia of less than 5 min duration A (1), B (1), C (1 to 2), D (1), E (1), F (1), G (1), H (1) = 8 to 9 |
| 2 | Calm recovery with some difficulty or weakness, 1 to 3 attempts, mild ataxia of more than 5 but less than 10 min duration A (2), B (1), C (1 to 2), D (1 to 2), E (1 to 2), F (1 to 3), G (2), H (1) = 10 to 15 |
| 3 | Same as 2 but ataxia of more than 10 min duration A (2), B (1), C (1 to 2), D (1 to 3), E (2 to 3), F (1 to 3), G (2), H (1) = 11 to 17 |
| 4 | Uncoordinated recovery with notable difficulty and weakness, more than 3 attempts to stand, marked ataxia, stumbling, ataxia duration of more than 10 but less than 20 min duration A (3), B (1 to 2), C (2 to 3), D (3 to 4), E (3 to 4), F (3 to 5), G (3), H (2) = 20 to 26 |
| 5 | Difficult recovery with disorderly efforts and unable to stand in the first 5 attempts. Marked ataxia that can last up to 20 min A (4), B (2 to 3), C (2 to 3), D (3 to 4), E (4), F ( ± 5), G (3 to 5), H (2 to 3) = 25 to 31 |
| 6 | Fatality/accident |
Statistical analysis
Data were analyzed using logistic regression with Proc Glimmix (SAS 9.4; SAS Institute, Cary, North Carolina, USA). All variables and the quadratic term of continuous variables were included in the model in different combinations to assess their interactions on quality of recovery score. The form of the variable, i.e., linear versus quadratic, that resulted in the best model fit as assessed by the Hosmer-Lemeshow fit test (8) was used in the model. Variables included age, body weight, breed, sex, ASA status, type of surgical procedure, occurrence of hypotension, use of dobutamine, number of IV injections of xylazine/ketamine after isoflurane discontinuation, anesthesia duration, post-anesthetic sedation, and end-tidal isoflurane concentration during maintenance and during transfer to the recovery room. The binary outcome in the logistic regression included considering if anesthesia duration was less than 90 min and if the overall descriptive recovery score was less than 2, to transform the model to binary. A P-value of < 0.05 was considered significant.
Descriptive statistics were carried out in SPSS Statistics for Windows (IBM Corp. Released 2019, Version 26.0. Armonk, New York, USA). The 4 surgical groups were compared with a 1-way analysis of variance (ANOVA) or a Kruskal-Wallis test with Bonferroni post-hoc tests for parametric and non-parametric data, respectively. The interaction effect of post-anesthetic sedation on quality of recovery in the different surgical groups was also assessed with a 2-way ANOVA. Correlations between age or ASA status and anesthesia duration were calculated with a Spearman’s rank correlation.
Results
A total of 305 horses met the inclusion criteria and were included in the study. Their demographics are shown in Table III. Overall, median age and anesthesia duration for all horses was 2.6 y (range: 0.5 to 27 y) and 104 min (range: 32 to 372 min), respectively.
Table III.
Demographics of horses in different surgery groups.
| Surgery group | ||||
|---|---|---|---|---|
|
| ||||
| Orthopedic elective (n = 152) | Orthopedic emergency (n = 15) | Soft tissue elective (n = 108) | Soft tissue emergency (n = 30) | |
| Age (y) | 2.3 (1.3 to 3.4)a | 3.0 (1.8 to 3.4)a,b | 3.4 (2.0 to 6.0)b | 5.3 (2.0 to 10.6)b |
| Weight (kg) | 450 (371 to 500)a | 445 (380 to 470)a,b | 474 (393 to 510)a,b | 505 (426 to 543)b |
| Breed (n) | ||||
| Standardbred | 63 | 5 | 15 | 7 |
| Thoroughbred | 55 | 3 | 24 | 11 |
| Quarter horse | 5 | 0 | 5 | 0 |
| Warmblood | 11 | 5 | 14 | 7 |
| Draft | 2 | 0 | 2 | 0 |
| Miniature | 0 | 1 | 1 | 0 |
| Others | 16 | 1 | 47 | 5 |
| ASA score (n) | 1 (1.0 to 1.75)a | 2 (2.0 to 2.0)b | 1 (1.0 to 2.0)a | 3 (2.0 to 4.0)b |
| I | 114 | 0 | 74 | 1 |
| II | 37 | 15 | 31 | 7 |
| III | 1 | 0 | 3 | 9 |
| IV | 0 | 0 | 0 | 13 |
| V | 0 | 0 | 0 | 0 |
| Anesthesia duration (min) | 105 (80 to 142)a | 104 (77 to 128) a,b | 94 (71 to 113)a | 134 (104 to 163)b |
Values expressed as median [interquartile range (IQR)].
Values for each observation that have no superscript letter in common among different surgery groups are significantly different from each other (P < 0.05).
Comparisons among the surgical groups revealed significant differences for the following variables: age, weight, ASA score, diazepam induction dose, end-tidal isoflurane concentration during anesthesia, anesthesia duration, lidocaine boluses, and time off isoflurane before transfer to the recovery room (Tables III, IV). In general, horses in the soft tissue emergency group were significantly older and heavier, had a higher ASA classification, lower doses of diazepam during induction, lower end-tidal isoflurane concentration during anesthesia, longer anesthesia durations, more lidocaine boluses, and longer time off isoflurane before transfer to the recovery room than the other groups (Tables III, IV). This group also had significantly longer times to extubation and standing during recovery than the other groups (Table V).
Table IV.
Induction and maintenance anesthetic drugs and doses used in surgery groups.
| Surgery group | ||||
|---|---|---|---|---|
|
| ||||
| Orthopedic elective (n = 152) | Orthopedic emergency (n = 15) | Soft tissue elective (n = 108) | Soft tissue emergency (n = 30) | |
| Diazepam during induction (mg/kg BW) | 0.022 (0.20 to 0.27)a | 0.022 (0.21 to 0.26)a,b | 0.021 (0.19 to 0.24)a,b | 0.019 (0.18 to 0.23)b |
| Ketamine during induction (mg/kg BW) | 2.2 (2.0 to 2.4) | 2.2 (2.1 to 2.2) | 2.1 (2.0 to 2.2) | 2.0 (1.9 to 2.3) |
| End-tidal isoflurane during maintenance (%) | 1.20 (1.20 to 1.30)a,b | 1.20 (1.10 to 1.30)a,b | 1.20 (1.20 to 1.30)a | 1.20 (1.10 to 1.20)b |
| Lidocaine (number of boluses; 2 mg/kg BW) | 1.0 (1.0 to 2.0)b | 1.0 (1.0 to 2.0)a,b | 1.0 (1.0 to 1.9)a | 2.0 (1.0 to 2.0)b |
| Total dexmedetomidine dose during total anesthesia duration (μg/kg BW per hour) | 1.53 (1.28 to 1.80) | 1.66 (1.44 to 1.72) | 1.57 (1.38 to 1.81) | 1.70 (1.48 to 1.90) |
| Time off isoflurane before transfer to recovery (min) | 12.0 (9.0 to 15.75)a,b | 15.0 (11.0 to 15.0)a,b | 11 (9.0 to 15.0)a | 15 (10.0 to 20.75)b |
| Number of injections of xylazine (33 mg)/ketamine (66 mg) before moving to recovery | 2.0 (1.4 to 2.3) | 2.0 (2.0 to 3.0) | 2.0 (1.0 to 2.0) | 2.0 (1.8 to 2.0) |
| End-tidal isoflurane before moving to recovery (%) | 0.52 (0.46 to 0.59) | 0.47 (0.43 to 0.52) | 0.51 (0.43 to 0.59) | 0.49 (0.43 to 0.55) |
| Xylazine IV administration during recovery (n) | ||||
| No administration | 77 | 9 | 66 | 19 |
| 0.1 mg/kg BW | 18 | 1 | 7 | 1 |
| 0.2 mg/kg BW | 57 | 5 | 35 | 10 |
Values expressed as median [interquartile range (IQR)].
Values for each observation that have no superscript letter in common among different surgery groups are significantly different from each other (P < 0.05).
Table V.
Times from entering the recovery room until sternal, extubation, and standing for different surgery groups.
| Surgery group | ||||
|---|---|---|---|---|
|
| ||||
| Orthopedic elective (n = 152) | Orthopedic emergency (n = 15) | Soft tissue elective (n = 108) | Soft tissue emergency (n = 30) | |
| Time until sternal (min) | 26 (21 to 38) | 30 (19 to 40) | 28 (21 to 37) | 37 (24 to 49) |
| Time until extubation (min) | 33 (26 to 45)a,b | 34 (23 to 40)a,b | 32 (25 to 43)a | 42 (31 to 63)b |
| Time until standing (min) | 37 (27 to 48)a,b | 38 (23 to 47)a,b | 33 (25 to 45)a | 49 (37 to 65)b |
Values expressed as median [interquartile range (IQR)].
Values for each observation that have no superscript letter in common among different surgery groups are significantly different from each other (P < 0.05).
Assisted recoveries were more common in emergency surgeries than in elective surgeries, 20% versus 9.2% for orthopedic cases and 23.3% versus 1.9% for soft tissue cases. Two horses had a recovery score of 6 as a result of fractures during recovery; one case was a 12.8-year-old horse undergoing a fractured ulna repair that fractured the radius in the same limb during recovery and the second was a 24.5-year-old horse undergoing an enucleation that fractured the humerus. Both horses were assisted with head and tail ropes during recovery. The fractures occurred at the time of move to standing and the horses were euthanized due to poor prognosis.
Post-anesthetic sedation was administered in 49.3% of the horses in the orthopedic elective group, 40% in the orthopedic emergency group, 38.9% in the soft tissue elective group, and 36.7% in the soft tissue emergency group (Table IV). The interaction effect between the use or not of post-anesthetic sedation and surgical group on recovery score was not statistically significant (F = 0.25, P = 0.96).
Episodes of hypotension were observed in 29.8% of horses (n = 91) for less than 25% of the anesthesia duration, in 5.2% of horses (n = 16) for 25% to 49% of the anesthesia duration, in 0.7% of horses (n = 2) for 50% to 75% of the anesthesia duration, and in 0.3% of horses (n = 1) for more than 75% of the anesthesia duration. Dobutamine infusion was used in 6.9% of horses (n = 21) for less than 25% of the anesthesia duration, in 16.4% of horses (n = 50) for 25% to 49% of the anesthesia duration, in 21% of horses (n = 64) for 50% to 75% of the anesthesia duration, and in 17.4% of horses (n = 53) for more than 75% of the anesthesia duration.
In the logistic regression model, no interactions or effect on quality of recovery were detected for surgical group, weight, breed, ASA score, occurrence of hypotension, use of dobutamine, post-anesthetic sedation, end-tidal concentration during maintenance, number of injections of xylazine/ketamine after isoflurane discontinuation, or end-tidal isoflurane concentration at the time of transfer to recovery. Age and ASA status were significant and positively correlated with anesthesia duration, with a Spearman’s correlation of 0.23 (P < 0.001) and 0.32 (P < 0.001) for all horses, respectively. Age (P = 0.003) and anesthesia duration (P = 0.021) significantly affected the quality of recovery (Table VI). The odds of having a problem, defined as having a descriptive recovery score over 2 (Table II), were increased by 1.20-fold (1.03, 1.41; lower and upper limits) for every 30 additional minutes of anesthesia duration and by 1.09-fold (1.03, 1.16) for every additional 1 y of age (Table VI).
Table VI.
Logistic regression model with descriptive quality of recovery as the outcome variable.
| Variable | Estimate | Standard error | 95% CI | P-value | Odds ratio | 95% CI |
|---|---|---|---|---|---|---|
| Intercept | −3.57 | 0.46 | −4.4, −2.67 | |||
| AT | 0.18 | 0.08 | 0.03, 0.34 | 0.021 | 1.20* | 1.03, 1.41 |
| Age | 0.09 | 0.03 | 0.03, 0.15 | 0.003 | 1.09** | 1.03, 1.16 |
AT — Anesthesia time; CI — Confidence interval.
per 30 min.
per 1 y.
Discussion
The results of this study corroborate the findings of previous studies that increased anesthesia duration negatively affects the quality of recovery in horses (1,2,4,5,9). Attempts were made to minimize confounding factors present in other studies by having the same experienced anesthesiologist in charge of all cases and by using the same infrastructure and conditions, the same premedication and induction protocol, the same balanced maintenance technique, and the same recovery score for all horses. While this approach attempted to isolate the effect of the anesthesia duration on the quality of recovery, it was not intended to restrict the type of surgical interventions, since the clinical application of the study was deemed relevant.
Risk factors associated with morbidity and mortality in equine general anesthesia include increased anesthesia duration, age, excitable pre-anesthetic temperament, high ASA score, surgeries involving fracture repairs, hypotension, metabolic disorders, and inadequate positioning/padding on the surgery table (1,2,4,5,10–15). In this study, age was correlated with anesthesia duration and both affected recovery scores. Both variables had odds ratios > 1, indicating that they were risk factors associated with a recovery of less quality. Other studies have also established associations between age (horses > 9 y old) and anesthetic mortality (1,2,4,9) and quality of recovery (4), possibly related to co-morbidities, such as osteoporosis, arthritis, fatigue, and health status (colic, dystocia) (2,4,9). In one study, all horses suffering from fatal fractures or joint luxations during recovery were 14 y of age or older (4), similar to findings in this study where the 2 fatal fractures occurred in a 12.8- and 24.5-year-old horse. One of these horses also had an ophthalmic procedure, which has been associated with recoveries of less quality (16). In the current study, we grouped horses into 4 categories, emergency versus electives for orthopedic and soft tissue injuries, although there were no significant differences among groups in relation to recovery scores.
Anesthesia and/or surgery duration have also been associated with increased risk of morbidity and mortality. Shorter anesthesia durations have been associated with an improved quality of recovery (5), whereas longer duration of anesthesia has been the variable most significantly associated with low quality of recovery, even if age was excluded (4). When time was categorized in periods, anesthetic durations of 30 to 60 and 60 to 90 min had a significantly lower risk of mortality compared to longer periods and the risk increased exponentially with anesthesia durations over 180 min (1). Likewise, a lower mortality rate of 0.24% versus up to 0.9% (1,2,12) was reported in a private practice in which procedures lasted < 60 min and the anesthetic protocol was more consistent in all horses (9).
Increased anesthesia duration leads to accumulation of inhalation anesthetics and increased concentrations in the brain and other tissues, which serve as reservoirs from which the anesthetic is released over time and contributes to ataxia and impacts the quality of recovery (5,11). Although the effect of accumulation of anesthesia on recovery is reportedly more important for lipid-soluble drugs such as halothane, it may also be relevant for the less lipid-soluble drug isoflurane after a long duration of anesthesia (17).
The balanced technique used for maintenance in the current study allowed for similar end-tidal concentrations of isoflurane during surgery and a significant decrease in this concentration upon transfer to recovery, after discontinuing isoflurane administration before the end of surgery. The median end-tidal concentration of isoflurane at time of moving to recovery was 0.47 to 0.52% for all surgical groups. A lower end-tidal anesthetic concentration during recovery should contribute to a faster washout to facilitate a smoother recovery, although prolonged durations of anesthesia and accumulation of inhalation anesthetics in the tissues may offset this process (11). Interestingly, the soft tissue emergency group had the lowest end-tidal concentration during surgery, the longest period of time without isoflurane before transfer to recovery, and similar end-tidal concentration at the time of moving to the recovery room. This group also had the longest anesthesia durations, as well as the longest times to standing and to extubation.
In addition to shorter anesthesia durations, several other factors have been associated with better quality of recovery, including less invasive surgery and the horse’s previous experience with anesthetic events (4,5,18,19). Increased duration of recovery due to a calm horse that only attempts to stand when it is more aware of its surroundings should also improve the recovery. Sedatives are often used during recovery to induce this calm state, especially in horses that have shown nervous behavior during the induction phase (4,5,18,20). All horses were administered intermittent injections of xylazine and ketamine after inhalant anesthesia was discontinued. The need for further xylazine administration during recovery (0.1 to 0.2 mg/kg BW, IV), as assessed by the anesthesiologist, resulted in more than 1/3 of the cases in each group requiring post-anesthetic sedation. This retrospective study was not designed to look at the effect of post-anesthetic sedation on the quality of recovery. No such effects on recovery scores were detected among surgery groups or in any of the horses with the balanced anesthetic protocol implemented.
A weak (r = 32) but significant (P < 0.001) correlation was found for ASA classification and anesthesia duration and the soft tissue emergency group had higher ASA scores and the longest anesthesia duration in general. Higher ASA scores have been associated with increased anesthesia-related morbidity and mortality (1,2,4) and this category of horses has probably required more invasive surgery due to the presenting condition. In this study, however, total recovery scores and descriptive recovery scores of horses in the soft tissue emergency group were not significantly different from those of other groups.
Assisted recoveries were more common in emergency surgeries than in elective surgeries (22.2% versus 6.2%). This could have contributed to a more controlled and better-quality recovery in the high-risk horses, although this remains controversial. For example, in ASA I or II, horses undergoing elective soft tissue or orthopedic surgeries with a balanced technique of isoflurane and triple-drip infusion (guaifenesin, ketamine, and xylazine) and no post-anesthetic sedation, a better quality of recovery, and fewer injuries were more often associated with head- and tail-assisted recoveries than with unassisted recoveries, including fewer attempts to stand (21). There was no difference in recovery scores in horses undergoing emergency abdominal surgery with a balanced technique of isoflurane and a medetomidine infusion, including medetomidine sedation during recovery (22). In both these studies, the number of fatalities was the same between assisted and unassisted groups (21,22). The consensus is that head and tail ropes cannot prevent mishaps during recovery due to the force generated by the horse while moving to stand (5,23). Given the retrospective nature of this study, both assisted and unassisted recoveries were included, although most recoveries were unassisted (91.5%, n = 279/305).
This study did not aim to determine mortality rate, although the 2 deaths in this study included an ASA II and ASA III status before surgery, which resulted in an overall mortality rate of 0.66% (2/305). Interestingly, both these horses were assisted during recovery. Mortality has been defined differently in several studies, as an event occurring within 7 d of induction of anesthesia (1,2,9,11) or only up to and including the recovery phase (4,9) or in the first 24 h post-anesthesia (15), and can include horses euthanized in the intraoperative period (15) or exclude them (1,2,4,12). Overall, mortality rates associated with anesthesia in horses range from 0.68% to 0.9%, including all ASA classes, but mostly represented by ASA I and II horses. When horses with an ASA classification ≥ III were included, such as those undergoing emergency abdominal surgeries, the rate increased to 1.63% to 4.3% (1,2,4,5,12,24).
The risk of morbidity associated with general anesthesia is also associated with the occurrence of hypotension during anesthesia (4,9,14,15). While hypotension occurred in some cases in this study, its duration and magnitude were minimized by administering dobutamine and fluid therapy throughout anesthesia. Hypotension is a common occurrence in anesthetized horses. Up to 55% of horses undergoing elective and emergency procedures required dobutamine to treat hypotension in a retrospective study (25). In the current study, dobutamine was used in 38.4% of the horses for more than 50% of the anesthesia duration and 74.1% of the horses had acceptable arterial blood pressure (defined as > 65 mmHg) for at least 75% of the anesthesia duration. No interactions could be established in this study between hypotension or duration of dobutamine administration and the quality of recovery.
Recovery scoring systems vary among studies and the degree of subjectivity can hinder proper interpretation of the quality of recovery (26,27). The scoring system used in this study involves a composite system with discrete categories, with descriptors assigned to each event of the recovery phase. Based on the best fit, the total score is matched with a numerical rating score to rank the quality of recovery. Several studies have used this recovery score system with good to very good agreement between evaluators (6,7,18) and the combination of the 2 scoring systems allows for better objectivity within a subjective scoring system.
The main limitations of this study include its retrospective nature, the unblinded assessment of quality of recovery, the fact that some horses (8.5%) had assisted recoveries, and that post-anesthetic sedation was subjectively administered in each case. Strengths of the study include the use of one experienced anesthesiologist in all anesthetic events who also scored the quality of all recoveries based on a previously validated recovery score (7) and that all horses were anesthetized with the same protocol throughout the anesthetic duration and in the same facility.
The findings of this study support our hypothesis that increasing anesthesia duration negatively affects the quality of recovery in horses. Improving the quality of recovery should be a priority both for the welfare of equine patients and for the economy of the equine industry, given the high incidence of equine anesthesiarelated morbidities and mortalities. Longer anesthesia duration is a major contributing factor to mishaps during the recovery phase in both multicenter studies and in this single-center study, in which confounding factors were reduced. Limiting surgery durations in horses undergoing general anesthesia would therefore improve the quality of recovery and welfare of our equine patients.
Footnotes
This paper was presented in part at the Association of Veterinary Anaesthetists Autumn meeting, 11–13 September 2019 in Ghent, Belgium.
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