Risk of general anesthesia in horses - A retrospective study on 1.989 cases

Purpose of the study: To determine the incidence of fatalities in horses undergoing general anesthesia for examinations or surgical procedures from 2006 to 2011 at an Equine University Clinic (Munich), to differentiate between fatalities occurring in the time period from induction of anesthesia to recovery (anesthesia-related fatalities) and fatalities occurring after recovery but before discharge from the clinic (anesthesia-associated perioperative fatalities), and to identify risk factors for complications of general anesthesia. Material and Methods: Out of 2,440 anesthesia and medical records 1,989 anesthesia cases could be used for statistical evaluation. The evaluation covered the time period from induction of anesthesia to discharge from the clinic or until death. Based on documented clinical findings in the records, each horse patient was classified according to the American Society of Anesthesiology specialist (ASA-classification). 93.5% were healthy horses and 6.5% were high risk patients. The anesthesia record included 89.2% inhalation anesthesia, and 10.8% injectable anesthesia. 93.2% were elective surgeries. 6.8% were emergency procedures. Results: Ten of 1,989 anesthesia cases had a fatal outcome. Thus, the overall anesthesia related mortality rate was 0.5%. No horse died during maintenance of anesthesia (induction to placement in the recovery box). Four of the 1,989 (0.2%) horses died or had to be euthanatized during recovery. One horse died because of cardiac arrest. Three other horses had to be euthanized due to prolonged recumbency in the recovery stall after colic surgery, myelomalacia and a fracture of the proximal humerus. Six of the 1,985 horses which had survived general anesthesia and recovery died in the postoperative period after recovery but before discharge, the anesthesia-associated perioperative mortality rate being 0.3%. Two of these 6 horses died from colitis x, one had a fatal pleuropneumonia, and another horse died from a large colon torsion. One mare became recumbent because of severe ataxia as a result of an activated severe facet joint arthrosis. The sixth patient had colic surgery and subsequently died of peracute circulatory collapse. By excluding the high risk patients, the overall mortality rate decreased to 0.3% (6/1,859). For horses with a poor general condition (ASA 2-5) the mortality rate was to 3% (4/130). In horses with colic and an ASA 4-5 risk, the mortality rate increased to 3.6% (2/55). A reduced general condition (p = 0.004) and a long anesthesia time (p = 0.002) lead to a high risk for perioperative mortality. Older horses were more frequently affected by harder recoveries (p < 0.001). Horses with a reduced general condition suffered more frequently from intraoperative hypotension (p < 0.001) and harder recoveries (p < 0.001). In addition, there was a significant correlation between a reduced general condition and the development of nerve paralysis (p = 0.009). Soft tissue surgeries also caused intraoperative hypotension (p < 0.001). Surgeries on the head and neck led to the highest proportion of uncontrollable hemorrhage (p < 0.001). Horses in dorsal recumbency significantly more often (p < 0.001) developed a drop in blood pressure, as did horses with a long anesthesia time (p < 0.001). A long duration of anesthesia effected also bradycardia (p < 0.001) as well as nerve paralysis and more difficult recoveries (p < 0.001). The application of acepromazin caused intraoperative hypotension (p = 0.003) and hypoventilation (p < 0.001). Conclusion: The overall anesthetic risk for horses in this study was fairly low (0.5%). It was 0.2% for anesthesia-related fatalities and 0.3% for anesthesia-associated perioperative fatalities. A bad general condition of the individual horse and a longer anesthesia time had a significantly negative influence on the development of anesthetic complications. When comparing anesthetic risks from published studies it is mandatory to specify the indications for anesthesia and to specify the type of surgical procedures that have been performed (elective vs. emergency surgery) as well as to specify the patient population that has been included (younger vs. older horses). Discussing "the risks" of general anesthesia in horses it should be differentiated between anesthesia-related fatalities and anesthesia-associated perioperative fatalities because otherwise the actual anesthetic risk may be overestimated.