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

Abstract

Purpose of the study: To determine the incidence of fatalities in horsesundergoing general anesthesia for examinations or surgical proceduresfrom 2006 to 2011 at an Equine University Clinic (Munich), todifferentiate between fatalities occurring in the time period frominduction of anesthesia to recovery (anesthesia-related fatalities) andfatalities occurring after recovery but before discharge from the clinic(anesthesia-associated perioperative fatalities), and to identify riskfactors for complications of general anesthesia. Material and Methods:Out of 2,440 anesthesia and medical records 1,989 anesthesia cases couldbe used for statistical evaluation. The evaluation covered the timeperiod from induction of anesthesia to discharge from the clinic oruntil death. Based on documented clinical findings in the records, eachhorse patient was classified according to the American Society ofAnesthesiology specialist (ASA-classification). 93.5% were healthyhorses and 6.5% were high risk patients. The anesthesia record included89.2% inhalation anesthesia, and 10.8% injectable anesthesia. 93.2%were elective surgeries. 6.8% were emergency procedures. Results: Tenof 1,989 anesthesia cases had a fatal outcome. Thus, the overallanesthesia related mortality rate was 0.5%. No horse died duringmaintenance of anesthesia (induction to placement in the recovery box).Four of the 1,989 (0.2%) horses died or had to be euthanatized duringrecovery. One horse died because of cardiac arrest. Three other horseshad to be euthanized due to prolonged recumbency in the recovery stallafter colic surgery, myelomalacia and a fracture of the proximalhumerus. Six of the 1,985 horses which had survived general anesthesiaand recovery died in the postoperative period after recovery but beforedischarge, the anesthesia-associated perioperative mortality rate being0.3%. Two of these 6 horses died from colitis x, one had a fatalpleuropneumonia, and another horse died from a large colon torsion. Onemare became recumbent because of severe ataxia as a result of anactivated severe facet joint arthrosis. The sixth patient had colicsurgery and subsequently died of peracute circulatory collapse. Byexcluding the high risk patients, the overall mortality rate decreasedto 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 ASA4-5 risk, the mortality rate increased to 3.6% (2/55). A reducedgeneral condition (p = 0.004) and a long anesthesia time (p = 0.002)lead to a high risk for perioperative mortality. Older horses were morefrequently affected by harder recoveries (p < 0.001). Horses with areduced general condition suffered more frequently from intraoperativehypotension (p < 0.001) and harder recoveries (p < 0.001). In addition,there was a significant correlation between a reduced general conditionand the development of nerve paralysis (p = 0.009). Soft tissuesurgeries also caused intraoperative hypotension (p < 0.001). Surgerieson the head and neck led to the highest proportion of uncontrollablehemorrhage (p < 0.001). Horses in dorsal recumbency significantly moreoften (p < 0.001) developed a drop in blood pressure, as did horses witha long anesthesia time (p < 0.001). A long duration of anesthesiaeffected also bradycardia (p < 0.001) as well as nerve paralysis andmore difficult recoveries (p < 0.001). The application of acepromazincaused intraoperative hypotension (p = 0.003) and hypoventilation (p