Introduction
Postanesthetic colic (PAC) accounts for over 65% of complications associated with general anesthesia in horses, with a reported incidence of between 3% and 11%.1–3 Since colic represents a mortality risk and incurs additional cost to the client, recommendations for perioperative feeding practices that reduce its incidence are needed. It has long been considered the standard of care to withhold hay prior to anesthetic induction, with previous studies citing fasting times of between 6 and 12 hours.1–5 The intent of fasting is to reduce the occurrence of PAC and intraoperative anesthetic complications such as impaired ventilation and oxygenation secondary to colonic pressure on the diaphragm.
Commonly used sedative, anesthetic, analgesic, and antimicrobial drugs have been implicated in reductions in gastrointestinal tract (GIT) motility and risk of PAC.6–8 Fasting horses as a means to overcome complications associated with reduced GIT motility as a result of anesthesia has no firm basis in published literature, and sudden feed restriction has proven to have negative effects such as reduced water intake, decreased gastric pH, and colic.9–11 A recent retrospective review3 of nonfasted horses undergoing general anesthesia for elective procedures identified a PAC incidence rate of 2.5%, which is lower than what has previously been reported for fasted horses. However, there are currently no prospective, randomized, controlled studies to compare specific perioperative feeding protocols in horses.
The primary objective of this study was to compare manure output, selected anesthetic outcomes (hypoxemia and hypotension), water intake, and development of PAC in horses that were not fasted prior to or following elective general anesthesia as compared to horses that were fasted before and slowly refed after anesthesia. A secondary objective was to determine whether the presence of a muzzle in fasted horses negatively affected water intake and manure production. We hypothesized that perioperative fasting would be associated with decreased manure output, increased time to pass manure following recovery, and increased incidence of PAC without concurrent decrease in anesthetic complications. Secondarily, we hypothesized that fasted and muzzled horses would have decreased water intake and manure production compared to both nonfasted and fasted nonmuzzled horses.
Methods
Animals
All procedures followed a protocol reviewed and accepted by the IACUC at Colorado State University (Protocol No. 3149). Informed consent was obtained from all clients prior to enrollment into the study. The study had 45 horses enrolled, and they were divided equally into 3 treatment groups. Sample size was generated assuming a clinically significant outcome difference in manure output between groups to be approximately 40%. Estimating a probable population SD in manure output of 30%, a sample size of 12 horses/group was calculated (90% power; α = 0.05). This was increased to 15 horses/group to account for the lack of previously published data on this topic.
Mares or geldings of any breed, with the exception of ponies and heavy draft horses, aged between 1 and 20 years old and presenting to the Colorado State University Veterinary Teaching Hospital for elective orthopedic procedures with an anesthesia time of at least 30 minutes but no longer than 250 minutes were eligible for the study. Horses were considered only if they were deemed to have an American Society of Anesthesiologists physical status of I or II, had no previous history of colic, and were systemically healthy as evidenced by a normal physical examination including cardiopulmonary and gastrointestinal auscultation, CBC, and serum chemistry panel. Enrollment was based on informed owner consent.
Horses were housed in box stalls deeply bedded with straw and had ad libitum water access. They were required to remain in the hospital for a minimum of 16 hours preoperatively and 16 hours postoperatively to be eligible for the study.
Experimental protocol
All horses were weighed the evening prior to surgery to ensure accurate drug dosing and data calculation. Horses were then randomly assigned to 1 of 3 treatment groups with an online randomizer (GraphPad Software). The 3 treatment groups were as follows: not fasted (NF), fasted muzzled (FM), or fasted not muzzled (FNM). Group NF horses had continuous access to dry grass hay or grass/alfalfa mix hay from the time they entered the stall the day before surgery. Initially, 2.2 kg of hay was placed in the stall and hourly checks were performed, at which point a further 0.9 kg of hay was added if the horse did not have hay left in the stall. Hay was not added within 1 hour of the planned induction time, but no hay was ever removed from the stall. Thirty minutes after returning to the stall postoperatively, hay was provided continuously in the same fashion until discharge. Horses in this group were not muzzled at any time.
Group FM horses were provided continuous hay as above until 10 hours prior to anesthetic induction, at which point hay was removed and a muzzle was placed. Two hours after returning to the stall postoperatively, they were provided 0.23 kg of hay every 3 hours in a slow refeeding scheme. Between feedings, the muzzle was placed to prevent ingestion of straw bedding. After 12 hours of slow refeeding, continuous hay provision was resumed until discharge. Group FNM horses followed the same feeding protocol as group FM but were not muzzled at any time point. If FNM horses were seen to be eating straw, it was recorded.
At the start of each hour, horses were checked and passage of a pile of manure was recorded. Manure was collected in a bucket with the straw bedding removed. The consistency of the manure was noted, and the subjective size of the manure pile was noted as small, normal, or large. The collected manure was left in a bucket for 12 hours prior to weighing. Manure output was recorded in kilograms of manure per kilogram of body weight of horse per hour in the stall, as well as number of manure piles per hour in the stall both preoperatively and postoperatively. Time to first passage of manure postoperatively was also recorded on the basis of the hour at which the manure was first noted.
Water was provided ad libitum in 3 buckets. Each contained 14 L of plain water and one contained an added cup of sweet feed (corn, oat, barley mix), which is standard practice at this institution. When water was added to buckets, the amount remaining in each bucket was tabulated. Then, water intake was calculated in liters per kilogram of body weight of horse per hour both pre- and postoperatively.
Pain scores were performed once prior to surgery and then every 8 hours postoperatively by a single investigator (CKB) using both of 2 scales: the Composite Orthopedic Pain Scale12 and Horse Grimace Scale.13 At the same time points, a physical examination including heart rate (HR), respiratory rate (RR), rectal temperature, and gastrointestinal auscultation was performed.
The day of surgery, horses were administered 4.4 mg of phenylbutazone/kg IV (phenylbutazone 20%) alone or in combination with 11 mg of cefazolin/kg IV, cefazolin and 6.6 mg of gentamicin/kg IV (gentamicin sulfate), or gentamicin and 22,000 IU of potassium penicillin/kg IV (penicillin G potassium). Antimicrobials were administered on the basis of the attending surgeon’s clinical preference and the procedure to be performed. All horses received a single preoperative dose of the chosen antimicrobial 30 minutes prior to anesthetic induction. Horses were then premedicated with 0.01 mg of acepromazine/kg IV, followed 10 minutes later by 0.005 mg of detomidine/kg IV (Dormosedan) in their stall. Horses were subsequently walked to the anesthesia induction box and administered up to 1 mg of xylazine/kg IV (Rompun), incrementally, to effect for heavy sedation, prior to anesthetic induction with 2.5 mg of ketamine/kg IV (Zetamine) and 0.7 mg of propofol/kg IV (Propoflo).
Horses were orotracheally intubated and maintained under general anesthesia with isoflurane in 100% oxygen with a large animal anesthetic machine (Tafonius Junior; Hallowell EMC). All were mechanically ventilated with a tidal volume of 15 mL/kg and target PaCO2 of 45 to 55 mm Hg and provided crystalloid fluids IV (pHyLyte) at 5 mL/kg/h. An arterial catheter was placed percutaneously in a facial or dorsal metatarsal artery for continuous monitoring of systolic, diastolic, and mean arterial blood pressure (MAP). A continuous infusion of dobutamine was administered as needed to maintain an MAP > 70 mm Hg. Hypotension was considered at MAP < 70 mm Hg, and the duration of time a horse spent below this value was recorded.
An arterial blood gas was performed after 15 minutes of anesthesia and PaO2 was recorded. Immediately prior to recovery, 0.1 mg of morphine/kg was administered IM. If the procedure involved a joint or tendon sheath, an additional 0.05 mg of morphine/kg was injected intrasynovially (up to 0.1 mg/kg total dose if > 1 joint). In the recovery box, 1 µg of dexmedetomidine/kg IV (Dexdomitor) was administered immediately following the horse’s first breath. Time to standing was recorded and recovery quality with a visual analog scale score (0 to 100 mm, worst to best) was graded by a single observer (RCH). Horses were extubated either at swallow, movement, or standing on the basis of the attending anesthesiologist’s preference.
Any horse displaying colic signs (eg, pawing, inappropriate recumbency, rolling, flank watching) or horses that had not yet passed manure 12 hours postoperatively underwent a transrectal palpation. The number of horses in each group with no manure passage within 12 hours of surgery or showing signs of colic was recorded. In these horses, nasogastric intubation was performed and enteral fluids were administered if indicated, and further diagnostics such as abdominal ultrasound, abdominocentesis, and IV fluids were performed depending on the clinical course of the horse and wishes of the owner. Regardless of treatment group, feed was subsequently withheld. All treatments were recorded, and manure output was continued to be monitored for the duration of the horse’s stay in the hospital; however, data after this time point were not included in any subsequent statistical analysis due to deviation from the study protocol. If any treatment group reached 4 horses showing signs of colic, the group would be terminated, with no further cases enrolled.
Horses were discharged a minimum of 16 hours postoperatively. Follow up on postoperative colic, if not seen in hospital, was made via phone consultation with the owner 24 hours after discharge.
Statistical analysis
Statistical analysis was performed with SAS version 9.4 (SAS Institute Inc), and summary statistics (mean, SD, SE, minimum, median, and maximum) were calculated for each variable at each time point. Model assumptions of normality and equal variance were evaluated with residual diagnostic plots. Primary response variables were considered manure output (kg/kg of body weight/h and piles/h), time to first passage of manure postoperatively, and incidence of colic. For colic, a Fisher exact test was used to compare proportions between treatments.
A mixed model was fit separately for multiple time point response variables (HR, RR, rectal temperature, manure output, water intake). Treatment (NF, FM, and FNM), time (preoperatively or postoperatively), and treatment X time interaction were included as fixed effects. Horse was included as a random effect to account for repeated measures. Tukey adjusted pairwise comparisons were used to compare the mean response between treatments, separately for each time point. Pairwise comparisons between mean response postoperatively versus preoperatively were also performed. A Bonferroni adjustment was used to control for multiple testing.
For single time point response variables (age, body weight, anesthesia time, duration of hypotension, PaO2 at 15 minutes of anesthesia, time to standing in recovery, recovery score, time to first passage of manure), a 1-way ANOVA model was fit, including treatment. A square root transformation was used to satisfy model assumptions. Tukey adjusted pairwise comparisons were used to compare mean response between treatments.
Multiple regression models were also fit using postoperative manure output and time to first passage of manure (square root transformed) as the response. Potential predictors included treatment, anesthesia time, duration of hypotension, antimicrobial choice, xylazine dose, and weight. For manure output, the preoperative values were also included as potential predictors. Model selection was done with backward elimination such that variables were removed from the model sequentially until P values for all remaining variables achieved P < .05.
Spearman correlations were calculated to examine associations between manure output postoperatively and pain scores (Composite Orthopedic Pain Scale and Horse Grimace Scale at baseline and then 8 hours postoperatively).
Results
A total of 53 horses were consented to enroll in the study. Eight horses were excluded from the study for the following reasons: delayed or canceled surgical procedure, excessive anesthesia time, bloodwork abnormalities, drugs or fluids administered outside the study protocol, or incomplete data collection. If a horse was excluded, an additional horse was enrolled in its place to generate a total of 15 complete cases/group. An outline of the cases enrolled and exclusion from the study is shown in Figure 1.
Summary of case enrollment and exclusion criteria for horses during the study period.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.24.04.0235
Demographics
Of the 45 horses included in the statistical analysis, there were 29 (64%) Quarter Horses, 8 (19%) warmbloods, 3 (7%) Arabians, 2 (4%) Thoroughbreds, 2 (4%) Percherons, and 1 (2%) Morgan. Mean ± SD horse age was 8 ± 5.5 years, and mean ± SD weight was 511 ± 81 kg. There was no evidence of a statistical difference between groups for horse age, weight, or physical exam parameters (HR, RR, rectal temperature) at any point in the experiment.
Surgical procedure
All horses presented for an elective orthopedic procedure. In all cases, the presenting clinical sign was lameness that was graded as either a 2 or 3 out of 5 on the American Association of Equine Practitioners’ lameness scale.14 Thirty-six (80%) horses presented for an arthroscopic or tenoscopic procedure, of which 9 (25%) involved the tarsocrural joint, 8 (22%) the femorotibial joints, 7 (19%) the radiocarpal or middle carpal joint, 7 (19%) the metatarso/metacarpophalangeal joint, and 2 (5%) the distal interphalangeal joint. Three (8%) horses underwent tenoscopy of the digital flexor tendon sheath. Of the horses not undergoing arthroscopy, 5 (11%) underwent a splint bone fracture removal and 4 (9%) a check ligament desmotomy. All horses were positioned in dorsal recumbency. There was no evidence of a difference between groups in either the pre- or postoperative pain scores, which are summarized in Supplementary Table S1.
All horses received phenylbutazone. Thirty-four (75%) horses also received cefazolin, and the remainder received either no antimicrobial, cefazolin plus gentamicin, or gentamicin plus potassium penicillin. Thirty-six (80%) horses were administered intrasynovial morphine in addition to their IM dose. The mean ± SD total intrasynovial dose of morphine was similar between groups: 0.058 ± 0.019 mg/kg in group NF, 0.065 ± 0.024 mg/kg in group FM, and 0.057 ± 0.019 mg/kg in group FNM. Of the 9 horses that only received an IM dose of morphine, 2 came from group NF, 3 from group FM, and 4 from group FNM.
Anesthetic variables
Anesthetic data and comparisons between groups can be found in Table 1.
Mean ± SD of anesthetic data in 45 horses undergoing surgery using a standardized general anesthetic protocol with continuous access to hay perioperatively (not fasted; n = 15), a 10-hour preoperative fast and slow refeeding with muzzle application (fasted muzzled; 15), or the same feed restriction but without muzzle application (fasted not muzzled; 15).
| Variable | Not fasted | Fasted muzzled | Fasted not muzzled | Between-group comparison (mixed model) |
| Xylazine dose (mg/kg) | 0.76 ± 0.17 | 0.79 ± 0.17 | 0.76 ± 0.20 | P = .9 |
| Anesthesia time (min) | 86 ± 36 | 100 ± 53 | 81 ± 28 | P = .4 |
| Duration of hypotension (mean arterial pressure < 70 mm Hg; min) | 18 ± 17 | 14 ± 14 | 11 ± 11 | P = .4 |
| PaO2 at 15 min of anesthesia (mm Hg) | 289 ± 55 | 311 ± 72 | 286 ± 72 | P = .5 |
| Time to standing in recovery (min) | 35 ± 12 | 46 ± 20 | 42 ± 16 | P = .2 |
| Recovery score (visual analog scale, 0–100 mm, worst to best) | 85 ± 10 | 84 ± 18 | 89 ± 10 | P = .6 |
Manure output, water intake, and colic
Pre- and postoperative water intake and manure output as well as time to first passage of manure postoperatively are shown in Figure 2. Manure output was significantly reduced in all groups postoperatively, but postoperative manure output was significantly lower in both fasted groups compared to group NF. Time to first passage of manure was significantly longer in the fasted groups. Most manure piles subjectively appeared normal in size, and no horses in any group were noted to be eating straw bedding.
Mean ± SD water intake and manure output data in 45 horses undergoing surgery using a standardized general anesthetic protocol with continuous access to hay perioperatively (not fasted; n = 15), a 10-hour preoperative fast and slow refeeding with muzzle application (fasted muzzled; 15), or the same feed restriction but without muzzle application (fasted not muzzled; 15). bwt = Body weight. *A significant difference from preoperative values. ƗA significant difference between groups.
Citation: Journal of the American Veterinary Medical Association 262, 9; 10.2460/javma.24.04.0235
There was evidence of a positive association between higher horse weight and longer time to first passage of manure (P = .005) as well as higher preoperative manure production to higher postoperative manure production in both weight and number of piles (P = .004; P < .001). No evidence of association between preoperative xylazine dose, pain score 8 hours postoperatively, or antimicrobial selection and manure output was found. All horses passed manure within 12 hours of surgery. Signs of colic were noted in no horses in group NF, 2 of 15 horses in group FM, and 2 of 15 horses in group FNM. There was no evidence of a difference in the proportion of horses displaying signs of colic between groups (P = .5). Specific details on horses with colic are presented in Table 2. No additional complications were observed in any of the horses associated with the group’s feeding protocol. All manure passed was subjectively considered to be of normal consistency.
Case summaries for 4 horses showing colic signs after surgery using a standardized general anesthetic protocol with a 10-hour preoperative fast and slow refeeding with muzzle application (group FM) or the same feed restriction but without muzzle application (group FNM).
| Case details | Treatment group | Diagnosis | Outcome |
| 5-y-old 560-kg Thoroughbred gelding, left tarsal arthroscopy (95 min) | FM | Large colon impaction, 24 h after discharge | Resolved with medical management |
| 7-y-old 610-kg Oldenburg gelding, left stifle arthroscopy (65 min) | FNM | Large colon impaction, < 6 h after discharge | Resolved with medical management |
| 20-y-old 598-kg Dutch warmblood gelding, right stifle arthroscopy (70 min) | FNM | Cecal impaction, prior to discharge | Resolved with medical management |
| 15-y-old 603-kg warmblood gelding, right carpal arthroscopy (105 min) | FM | No specific diagnosis made, colic signs prior to discharge | Resolved with medical management |
Discussion
The results of this study confirmed the hypothesis that horses with access to free-choice hay prior to and following recovery from general anesthesia would pass significantly more manure and would pass manure significantly sooner after surgery than their fasted counterparts without detrimental effect on anesthetic parameters. Although incidence of PAC was highest in both fasted groups, this difference did not reach statistical significance. We also did not find evidence of a difference with respect to water intake. Contrary to our second hypothesis, we found no evidence that muzzle placement affected manure output or water intake.
Previous studies investigating PAC are retrospective in nature and have important differences between fasting times (varying from 6 to 18 hours1–5,15), antimicrobial selection, surgical procedures, anesthetic protocols, and geographical location (including travel history of horses prior to surgery). In addition, definitions of GIT dysfunction or PAC differ between studies, from reduced fecal output to signs of abdominal discomfort. One study1 examining horses that underwent general anesthesia, excluding those undergoing emergent abdominal procedures, found an incidence of PAC (as defined by active signs of colic) of 10.5%, with the majority occurring the day of or day after surgery. In this study, horses having elective anesthesia were fasted 12 hours, but fasting in emergent cases was not reported.1 A study3 analyzing horses undergoing inhalation anesthesia for elective orthopedic procedures without prior fasting reported a PAC incidence, as defined by horses requiring treatment for signs of abdominal discomfort, of only 2.5%. However, a direct comparison between studies is impossible.
In our study, the occurrence of PAC as defined by clinical signs of colic was 0% in group NF compared to 13% in both fasted groups, with an overall rate of 8.8%. A previous retrospective study2 of clinical cases in our hospital reported an 8.7% incidence of PAC or “decreased fecal output requiring treatment” in horses fasted on average 6 hours and having varying postoperative feeding practices. A direct comparison is again difficult to make because of the relative variety of confounding factors, changes in anesthesia practices over the years since the last study, lack of horses in the prior study with continuous access to hay prior to anesthesia, and lack of accurate measurements of manure output.
At least 2 prior studies4,5 have identified that horses with delayed passage of manure postoperatively subsequently had an increased risk of developing active signs of colic, which may imply an overall benefit to early manure production. When considering manure output and time to first passage of manure, maintaining a fed state in horses perioperatively in our study showed clear advantages. This could be due to the fact that allowing horses to fill their stomach with feed positively influenced colonic motility through the gastrocolic reflex16 or even because chewing alone has positive effects on gastrointestinal transit times in horses.17 Feed restriction, even for short periods of time and especially in the presence of conspecifics allowed access to feed, represents a major stressor in domesticated horses.18 In other species, stress has been shown to delay gastric emptying through activation of corticotropin-releasing factor receptors, though colonic motility is concurrently enhanced.19 It is not clear what role stress played in our results. However, subjectively, horses in group NF were overall more relaxed and content in hospital.
The fasting duration in the current study was decided on the basis of the average fasting time cited in previous papers and to closely mimic a conglomeration of fasting and refeeding protocols between clinicians in our hospital. Initial recommendations to fast horses were extrapolated from the human and small animal literature, where a perioperative fast is required to reduce the risk of regurgitation and therefore aspiration pneumonia.3,20,21 Due to the normal gastric anatomy of the horse, regurgitation is rarely a problem.22 Regurgitation or clinical signs of aspiration pneumonia were not a complication experienced in the current study or previous studies involving fed horses.3 All horses in the current study remained afebrile following general anesthesia, with no clinical signs associated with respiratory compromise. However, very subtle signs of aspiration pneumonia may have gone undetected.
Equine fasting recommendations are further based on the premise that emptying the large colon decreases the pressure placed on the diaphragm, particularly in the dorsally recumbent horse, consequently decreasing the risk of hypoxemia from compression atelectasis and hypotension from increased intrathoracic pressure. All treatment groups had similar arterial oxygen tensions at 15 minutes of anesthesia that were typical of mechanically ventilated horses anesthetized at our regional barometric pressure. In ruminants, it has been found that a 48-hour fast does not decrease the size of the rumen sufficiently as to improve hypotension.23 In healthy horses provided free-choice hay and administered nondigestible beads via nasogastric tube, mean ± SD time to recovery of 75% of the beads in manure was reported to be 42.2 ± 11 hours, suggesting that a typical 10- to 12-hour fast is also unlikely to significantly alter colonic contents in horses.24
Despite a significant feed reduction in fasted groups, water intake did not vary between groups, even with the application of a muzzle. Prolonged feed deprivation has been shown to reduce water intake in horses, but the duration of fasting in our study was likely insufficient to produce this effect.10,11,25 It is also possible that water consumption measurement was inaccurate, as group FM were often noted to be playing in their water buckets (particularly to access the handful of grain added to one of the buckets). The muzzle may have displaced a significant amount of water and made our final measurements less precise. Alternately, horses that were less occupied by eating hay may have played in their water or consumed water from boredom or hunger.
Interpreting the results of clinical studies of this nature must be performed while acknowledging its inherent limitations. Although our physical examinations of all horses included auscultation of GIT borborygmi, this method of assessing motility both is subjective and yields inconsistent results in research settings.26,27 Therefore, we did not include that information in our results. We were unable to use a more objective measure of motility or transit time, such as abdominal ultrasound or nondigestible bead recovery,17 but doing so could potentially have provided useful information about the physiologic differences occurring between treatment groups. The fecal water content was also not directly measured, and doing so would have provided further information allowing us to determine whether the differences in manure weight postoperatively between groups were related to relative water content or bulk feed content. Additionally, while all horses remained normothermic postoperatively and no clinical signs of respiratory compromise were noted, we did not directly assess horses for signs of aspiration pneumonia in the period after surgery using rebreathing examinations and thoracic ultrasonography. Therefore, we may have missed potential cases of aspiration pneumonia that were not clinical during the period in which horses were hospitalized.
The horses included in this study were undergoing elective orthopedic procedures. It has previously been established that orthopedic pain is a risk factor for the development of PAC, particularly cecal impactions.28 The pain scores of horses in our study were low and comparable between groups. Pain scores also had no relation to manure output, possibly because horses were largely comfortable pre- and postoperatively, allowing other factors to be of greater influence in manure production. Therefore, it is possible that the findings of this study cannot be extrapolated to more painful procedures that carry an increased risk of PAC. Additionally, while we found a clear difference in manure output in the early postoperative period, we were unable to demonstrate a significant difference in PAC. This could be because our study was underpowered to determine this difference, leading to a type 2 statistical error.
In conclusion, this was the first prospective, randomized, controlled study assessing perioperative feeding protocols in horses presenting for elective procedures. Our findings suggest that offering horses free-choice hay perioperatively improves early postoperative manure output without detrimental impact on anesthetic variables. Further research into this topic that involves the objective measurement of gastrointestinal motility in fasted compared to not fasted horses may help to further define the potential benefits of allowing horses access to food prior to general anesthesia. On the basis of our findings, horses that were not fasted prior to anesthesia passed more manure and passed manure sooner after surgery compared to fasted horses. Therefore, current recommendations in our hospital are to enable horses continuous access to feed perioperatively.
Supplementary Materials
Supplementary materials are posted online at the journal website: avmajournals.avma.org.
Acknowledgments
None reported.
Disclosures
The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.
Funding
The authors have nothing to disclose.
ORCID
C. Barton https://orcid.org/0000-0003-4493-8888
L. Goodrich https://orcid.org/0000-0002-8010-4429
C. Kawcak https://orcid.org/0000-0003-2401-9710
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