Materials and Methods

The study was conducted as a retrospective study of female cats undergoing elective OVH at the American Society for the Prevention of Cruelty to Animals Spay Neuter Alliance (ASNA; formerly known as the Humane Alliance) between January 1, 2017, and December 31, 2018. Cats were excluded if they did not undergo surgery owing to poor health or other reasons or did not undergo the OPT procedure (eg, pedicles were ligated with suture because of anatomic anomalies or concerns about tissue integrity) or if the medical record was largely incomplete or missing.

Surgical procedures were performed by ASNA clinic veterinarians, graduate veterinarians undergoing training, and veterinary student externs at 2 clinics at the ASNA campus in Asheville, NC. The veterinarian clinic housed both an HQHVSN clinic for partner rescue organizations and the public and a training program in HQHVSN procedures for practicing veterinarians and their teams. The student clinic was devoted solely to externship training in HQHVSN procedures for fourth-year veterinary students. The training curriculum was specifically designed to address the needs of participants while providing comprehensive supervision of individuals undergoing instruction. Instructors assigned to trainees examined the proficiency of participants and verified competency at each phase of the procedure. The veterinarian training program was a 1-week program and had a trainee-to-instructor ratio of 2:1, with registered veterinary technicians and veterinary assistants monitoring anesthesia for patients. The student externship program was a 2-week program with a student-to-instructor ratio of 4:1; the typical scenario was 2 student surgeons in the operating room with a student monitoring anesthesia for each patient. All trainees received instruction in HQHVSN techniques and concepts, although there was less focus on speed and more guidance and support from instructing veterinarians in the student clinic, because students were expected to be building a good basic knowledge foundation and surgical skills. Also, all trainees observed surgery demonstrations by team instructors and completed a knot-tying lab with their assigned instructor to practice the OPT procedure and surgical knots on models prior to using these skills in surgeries performed at ASNA. In addition to acting as instructors, clinic veterinarians also performed surgeries for high-risk and overflow cases in both clinics.

Patients were a mixture of owned, shelter, rescue, and community cats (unowned, outdoor, free-roaming cats) and were determined to be sufficiently healthy for surgery via physical exam. A visual exam was performed in lieu of a physical exam on community cats. The general protocol for all surgeries followed the ASV guidelines.² OPT was the standard for feline OVH in both the student and veterinarian clinics regardless of age, size, estrus, or pregnancy.

During the physical exam, patients were administered gabapentin solution PO (50 mg to cats weighing ≤ 4.5 kg and 100 mg to cats weighing > 4.5 kg) when such handling was feasible.⁹ If gabapentin administration was not feasible, anesthesia was still induced with the standard induction protocol. Anesthesia was typically induced about 2 hours following the physical exam with a combination of tiletamine-zolazepam, butorphanol, and dexmedetomidine. The combination was prepared by reconstituting tiletamine-zolazepam (Telazol) with 2.5 mL of butorphanol and 2.5 mL of dexmedetomidine as diluents. Each milliliter of the reconstituted solution contained 100 mg of tiletamine-zolazepam, 5 mg of butorphanol, and 250 µg of dexmedetomidine. The combination was administered at a dose of 0.035 mL/kg, IM.¹⁰ All patients received meloxicam (0.1 mg/kg, SC) during surgical preparation, and all patients were evaluated for additional analgesia as needed. Patients undergoing surgery by students were intubated and maintained on isoflurane in oxygen for the duration of the procedure, whereas patients undergoing surgery by veterinarians received 100% oxygen by mask.¹¹ The patient’s surgical site was prepared by clipping the hair and scrubbing the skin with warmed 2% chlorhexidine scrub and solution. All patients were monitored by registered veterinary technicians or veterinary students and with the aid of pulse oximetry for the duration of the surgery.

Surgery was performed via a ventral midline incision, using a spay hook to retrieve the uterus. The ovarian pedicles were autoligated by means of an OPT, except when anatomic anomalies (eg, ectopic ovarian tissue low on the pedicle) prevented the use of the OPT; in these cases, the pedicles were ligated with suture instead. With the OPT, the ovary was first exposed, and the suspensory ligament and ovarian artery were identified. The suspensory ligament was then transected or torn, and an aperture was created in the broad ligament adjacent to the pedicle. A mosquito hemostat held in the closed position was placed parallel to the pedicle, pointing away from the ovary. The tip was passed under the pedicle, and the hemostat was then rotated 180 degrees. The pedicle was clamped with the tip of the hemostat and then transected between the ovary and the hemostat, and the ovary and uterus were laid aside. The surgeon then pushed the knot off the tip of the hemostat. The knot was manually tightened, and the pedicle was inspected for hemorrhage. The uterus was ligated with a single strangle knot, a 2-pass binding knot of the friction knot category, using 3-0 or 2-0 (depending on the size of the patient) polydioxanone. The closure was secured with a single cruciate suture in the body wall (or with additional cruciate sutures for extended incisions) of 3-0 or 2-0 (depending on the size of the patient) polydioxanone. A bupivacaine splash block was then applied, and the subcutaneous and subcuticular tissues were closed with 3-0 or 4-0 (depending on the size of the patient) polydioxanone in a continuous pattern. The skin incision was sealed against contamination with surgical glue, and a standard tattoo was applied with a linear score adjacent to the incision. All patients received warmed fluids SC and were monitored in a designated recovery area with padding, blankets, and warming devices used to prevent hypothermia during the recovery period. Atipamezole was used for reversal at the discretion of the attending veterinarian after 45 minutes. All patients were kept in the clinic overnight prior to release the following morning. Owners, caregivers, and shelter staff were instructed in proper observation and postoperative care and were provided contact information for a 24-hour emergency hotline. The ASNA provided free in-clinic follow-up care directly or via partner clinics, as needed.

Results

A total of 17,493 female cats were presented to ASNA for sterilization during the 2-year study period. Of these, 1,499 did not undergo surgery, primarily because of health concerns or because they were identified as previously having been sterilized and were excluded from the study. An additional 6 cats were excluded owing to an incomplete or missing treatment record, and 21 cats that did not undergo OPT (ie, the ovarian pedicle was ligated with suture instead) owing to anatomical anomalies that resulted in insufficient exposure or tissue quality for autoligation (eg, ectopic tissue, ovarian cysts, adhesions, severely hyperplastic tract, bruised pedicles, extremely friable ovaries, or a dead fetus pushing cranially in the horn) were also excluded. Suture ligation was also performed in another 40 cats, but the reason was not specified in the record. Because complications were not indicated for these cats, it was assumed that the pedicles were ligated with suture for anatomical reasons or as a preventative measure owing to concerns about the integrity of the autoligation. These cats were also excluded. The remaining 15,927 cats were included in the study. Of these, 8,782 were sterilized through the veterinarian clinic and 7,145 were sterilized through the student clinic.

Age of cats included in the study ranged from 1 to 137 months (median, 7 months; Table 1). Mean body weight was 2.49 kg (SD, 0.85 kg). Most cats were privately owned or shelter owned. There were significant differences in cat characteristics between the veterinarian and student clinics. Cats sterilized through the student clinic were younger and weighed less, with a higher percentage of postpartum and lactating cats and cats with fatty and friable ovarian tissue, compared with cats sterilized through the veterinarian clinic. There were considerably more shelter-owned and community cats seen through the student clinic and more privately owned cats seen through the veterinarian clinic. The veterinarian clinic also sterilized more pregnant cats and cats in estrus. Additionally, consistent with the structure of the training clinics, there were no student surgeons in the veterinarian clinic and no veterinarian trainees in the student clinic.

In 49 (0.3%) cats, the pedicle was dropped (8) or torn (41; Table 1) by the surgeon. Most (41/49 [84%]) of these incidents did not result in hemorrhage or other complications and were corrected immediately with suture ligation. Pedicle dropping or tearing was approximately 5 times as common in the student clinic and was significantly more common in the student clinic than in the veterinarian clinic. In 2 cases (both with veterinary students as surgeon in the student clinic), gauze fibers became entangled in the pedicle tie, and the pedicle was ligated with suture without further complications. None of the cat characteristics (age, weight, reproductive status, fatty ovarian tissue, friable ovarian tissue, pyometra or mucometra, pedicle or ovary adhesions, or ownership status) were significantly associated with pedicle dropping or tearing in the veterinarian clinic or the student clinics (Table 2). However, in the student clinic, surgeon experience level (clinic veterinarian vs veterinary student) was significantly associated with pedicle dropping or tearing.

Nineteen (0.12%) cats had hemorrhage involving the pedicle (Table 1), with comparable rates for the veterinarian and student clinics. Four of these instances also involved bleeding from other sources, including the uterus (n = 1), suspensory ligament (1), broad ligament (1), and an unidentified source (1). In all but 1 instance, hemorrhage was identified and addressed intraoperatively by ligating the pedicle with suture; in the remaining instance, hemorrhage was resolved by tightening the autoligation. All cats recovered without further complications, although 1 of these cats was slow to recover from anesthesia. Five additional cats had evidence of hemorrhage, but the source of bleeding could not be determined. If bleeding was assumed to be pedicle involved, the rate of pedicle hemorrhage would have been 0.15% (24/15,927). None of the cat characteristics or surgeon experience level were significantly associated with pedicle hemorrhage (Table 3).

Ten cats died (n = 6) or were euthanized (4) and the cause could not be ruled out as surgery-related; however, all deaths were determined to be unrelated to pedicle hemorrhage. Necropsy reports were available for 7 of the 10 cats. Listed causes of death included uterine stump hemorrhage (n = 3), pneumonia (3), congenital cardiac disease (1), nonspecific respiratory or cardiac distress (2), and nonspecific hemoabdomen (1). The cat with nonspecific hemoabdomen was pregnant and anemic; the pedicle and uterine ligatures were present and appeared to be adequate, as determined during a postmortem exam by an independent pathologist.

Discussion

Results of the present study support previous evidence that the OPT procedure is safe in cats undergoing OVH when performed by experienced surgeons or novice surgeons under supervision. The overall complication rate was low, and development of pedicle-involve hemorrhage was not significantly associated with any of the cat characteristics examined or with surgeon experience level. Also, all identified hemorrhages were detected and corrected intraoperatively, without serious complications or death. Given the reduced anesthetic time associated with the procedure, we suggest that the OPT should be considered an acceptable standard practice in all practice settings.

OVH is the most commonly performed surgical procedure in small animal practice,¹² and hemorrhage is one of the most common complications encountered during OVH.¹³ Clearly, efforts that reduce the incidence of hemorrhage are invaluable in protecting patient safety. Results of the present study support prior studies that show the OPT in cats undergoing OVH is associated with a very low risk of hemorrhage. Pedicle-related hemorrhage occurred in only 19 of 15,927 (0.12%) cats in the present study, a rate comparable to that reported by Miller et al⁶ (0.281%) in a study of OPT performed by experienced surgeons. In the only other study¹⁴ focused on the OPT, 2.7% of cats undergoing the autoligation technique experienced hemorrhage, compared with 5.7% of those undergoing traditional suture ligation, although the difference was not statistically significant. The rate of OPT-related hemorrhage in that study may be higher than rates in our study and that of Miller et al⁶ owing to their sole use of veterinary students as surgeons and their inclusion of all torn pedicles as hemorrhage complications. Other reports of hemorrhage complications in cats are sparse, but 1 study¹² found a rate similar to that reported by Showers et al,¹⁴ with 4% of cats undergoing traditional OVH with student surgeons at a teaching hospital having intraoperative hemorrhage. Differences in hemorrhage rate among studies could be attributable to variations in student experience, program structure, and level of supervision. Similar to our findings, none of the cats experiencing hemorrhage across these studies died as a result, regardless of approach, which is consistent with other reports that intraoperative hemorrhage, while potentially life-threatening and concerning for the surgeon, rarely leads to serious morbidity or mortality.^15,16

The OPT is a readily developed skill in that most veterinary practitioners routinely perform a variation of the technique when neutering male cats and, potentially, male puppies via autoligation of the spermatic cord. The familiarity of hand memory allows for accelerated dexterity in experienced surgeons, but novice surgeons can quickly become adept with practice. Although a previous study⁴ identified no significant differences between veterinary students and veterinarians in minor or major complications of gonadectomy in dogs or cats at shelters, our study did find a higher incidence of pedicle dropping or tearing in the student clinic, compared with the veterinarian clinic, and within the student clinic, a higher incidence among veterinary students, compared with clinic veterinarians. These differences were presumably due to the lesser degree of tissue-handling experience available to the veterinary students, although variations in patient characteristics could have also contributed. The level of student experience may reflect heavily on their ability to successfully use this and other techniques.^4,14,17 Persistent practice of this surgical technique will enhance the surgeon’s skill level and likely markedly reduce the rate of complications. Nevertheless, despite an increased occurrence of pedicle dropping or tearing, there were not statistically significantly more hemorrhage complications among veterinary students. In fact, of all 49 cats (15,927) in which a pedicle drop or tear was encountered, only 8 (16%) reportedly had hemorrhage. Pedicle tears are known to be a leading cause of intraoperative hemorrhage during OVH,¹⁵ although our findings suggest that most pedicle drops or tears do not result in bleeding.

Patient characteristics considered in the present study for potential associations with pedicle-related complications focused primarily on age, weight, and indicators of reproductive status. Owing to increased vascularity, estrus and pregnancy are thought to potentially increase the risk for hemorrhage during OVH, although this is not well researched. Fudge et al¹⁸ found greater blood loss during OVH in pregnant versus nonpregnant cats, although the amount of blood loss was low for both groups and did not result in any significant complications. Additionally, in 2 studies^3,4 of morbidity and mortality in HQHVSN clinics, reported data allowed us to estimate that pregnancy was a co-occurring condition in 25% to 35% of the female cat deaths. Pregnancy and estrus status were not related to hemorrhagic complications in the present study, nor were age and body weight, similar to findings from other OPT studies.^6,14 Our study sample was overall relatively young, which may have influenced the results. Also, given the very low number of cats with complications, the statistical power to detect associations between cat characteristics and complications may have been limited. In light of this and the lack of other available research, potential risk factors for hemorrhage and other complications in OVH deserve further consideration in future studies.

Our findings should be interpreted in the context of certain study limitations. First, as is common with retrospective medical record review studies, information was sometimes missing or unclear. For 40 cats, the reason the pedicles were ligated with suture in place of autoligation was not indicated. Based on the experience of the investigators, these suture ligations were assumed to be a preventative measure related to an anatomic anomaly or concern for knot integrity, because no complications were mentioned, and the cats were omitted from the study. It is possible, however, that for some of these cats, the suture ligation was in response to a pedicle tear or hemorrhage that was not recorded on the treatment sheet. Additionally, the source of the bleeding was not evident for 5 cats with hemorrhage. However, even if hemorrhage was assumed to be pedicle related in all of these cats, the rate of hemorrhage would remain relatively low, and the overall conclusions of the study would not change.

Also, the present study focused on complications occurring prior to discharge, because detailed recheck records were not available for all animals. It is possible that cats had hemorrhage-related complications after they were discharged from the clinic that were not captured. However, when hemorrhage occurs, it most often presents acutely during the intraoperative period,¹⁵ and all cats were kept overnight for observation and examined prior to discharge, which would have likely revealed signs of clinically important bleeding. Postdischarge deaths were recorded and coded, although it is also possible owners and shelters failed to report postdischarge complications or deaths that could have been pedicle related. Also, necropsy reports were unavailable for 3 of the 10 deceased cats. Surgical logs for these cats indicated cardiac or respiratory issues so it is unlikely the cause of death was pedicle or hemorrhage related.

Inherent to the retrospective design, animals were not assigned randomly at the clinic or surgeon experience level. Although data were analyzed separately by clinic to account for differences in animal characteristics across clinics, there were potential differences in the characteristics of animals assigned to clinic veterinarians versus training veterinarians or veterinary students in each clinic. Owing to the small number of cases with pedicle-related complications in this sample, we were unable to fit multivariate models that considered both surgeon and cat characteristics. Often, clinic veterinarians were assigned the higher-risk cases and may have assumed surgery responsibility for other animals that developed a complication early on. This could have resulted in an underestimation of complication rates for the trainees. Additionally, because OPT is standard protocol at ASNA, a comparison of complication rates with the traditional suture approach was not possible in this study.

In summary, study findings support other available evidence that the OPT is a reliable and safe technique for feline OVH. Although used most often among HQHVSN clinic surgeons, the technique should be considered a standard approach in general practice settings as well owing to its efficiency and low complication and mortality rates. Our results suggest the OPT procedure is safe for experienced surgeons new to the technique as well as for novice surgeons under supervision. In light of these findings, we suggest that the OPT procedure should be included routinely in veterinary medical curricula and that private practitioners should feel confident using this approach.