The confidential enquiry into perioperative equine fatalities (CEPEF): mortality results of Phases 1 and 2

Objectives To document the equine perioperative mortality rate and to highlight any factor associated with an increased risk of death up to 7 days after anaesthesia. Study design A prospective observational epidemiological multicentre study. Methods Data were recorded from all equidae undergoing general anaesthesia in 62 clinics. Power calculations indicated that 45 000 cases were required to detect the significance of important variables. Details of each horse, operation, anaesthetic agents and clinic personnel were recorded. Outcome at 7 days was recorded as: alive, put to sleep (PTS) or dead. Data were analysed by a standard multilevel logistic regression approach, considering the effects of clustering at the level of clinic. Results Data were collected from 41 824 cases over 6 years. A total of 39 025 (93.3%) were alive on day 7 and 785 were dead giving an overall death rate of 1.9% (95% CI: 1.8–2.0) and 2014 (4.8%) were PTS. About 5846 horses undergoing emergency abdominal surgeries (‘colics’) were excluded from subsequent analyses. A total of 35 107 ‘noncolic’ horses were alive at 7 days and 328 dead giving a death rate for noncolics of 0.9% (95% CI: 0.8–1.0). Five hundred and forty‐three (1.5%) noncolic horses classified PTS were excluded from further analyses. There were 109 (33%) deaths from cardiac arrest or post‐operative cardiovascular collapse, with 107 (32%) from fractures and myopathies. Fracture repair, out of hours surgery, and age below 1 month was associated with increased risk of dying whereas the use of acepromazine and intravenous anaesthetic agent maintenance of anaesthesia was associated with reduced risk. Conclusions A number of potential contributors to the high risk of anaesthetic‐related mortality have been identified. Further investigation of the underlying mechanism for their apparent harmful effects and development of alternative techniques is merited.     

Clinical application of continuous spirometry with a pitot‐based flow meter during equine anaesthesia

This report documents the feasibility and clinical information provided by a new method for spirometric monitoring adapted for equine anaesthesia. Monitoring of ventilatory function was done with continuous spirometry during general anaesthesia of client‐owned horses presented for various diagnostic and surgical procedures. An anaesthetic monitor with a spirometry unit for human anaesthesia was used. To allow the measurement of large tidal volumes, a remodelled larger version of the pitot tube‐ based flow sensor was used. This technology provided reliable spirometric data even during prolonged anaesthesia when water condensation accumulated in the anaesthetic circuit and the sensor. In addition to flow and volume measurement and respiratory gas analysis, the continuous display of flow‐volume and pressure‐volume loops offered visually recognisable information about compliance, airway resistance and integrity of the circuit. Continuous spirometry with this monitoring system was helpful in evaluating the efficacy of spontaneous ventilation, in adjusting intermittent positive pressure ventilation and detecting technical faults in the anaesthetic apparatus and connection with the patient. This adapted spirometry method represents a practical and reliable measuring system for use during equine anaesthesia. The variety of information provides an opportunity to optimise anaesthetic management in this species.     

Equine perioperative fatalities associated with general anaesthesia at a private practice--a retrospective case series

OBJECTIVE: To determine the fatality rate of horses undergoing general anaesthesia at a private equine referral practice using a limited number of anaesthetic protocols. METHODS: A retrospective analysis of records (n = 17 961) from all horses undergoing general anaesthesia for surgical procedures from 1997 to 2001 at Rood and Riddle Equine Hospital, Lexington, Kentucky, USA. Results were reported as percentage of the population, and as crude mortality rates for each procedure (deaths per 1000). RESULTS: The prevalence of equine fatalities directly related to anaesthesia was 0.12% (n = 21) and this rose to 0.24% (n = 42) with the inclusion of horses killed or dying within 7 days post general anaesthesia. Causes of death directly related to anaesthesia were cardiac arrest (n = 10), fracture in recovery stall (n = 8), neuropathy and myopathy necessitating euthanasia (n = 3). Crude mortality rates per procedure were < or =7 deaths per 1000 cases, except arthrodesis/osteotomy cases were 66.7 deaths per 1000 cases. CONCLUSIONS: The anaesthetic fatality rate at this practice is lower than has been reported previously. CLINICAL RELEVANCE: Familiarity with an anaesthetic protocol in combination with reduced anaesthetic time, emergencies of shorter duration between diagnosis and surgery, and adequate preoperative examination appear to minimize the risks associated with general anaesthesia in horses.     

Controlled mechanical ventilation in equine anaesthesia: Physiological background and basic considerations (Part 1)

Controlled mechanical ventilation (CMV) is routinely used in equine anaesthesia, with many different options available to mechanically deliver breaths. The complexity of respiratory pathophysiology in anaesthetised horses and the wide range of devices available is described in this scoping review. The first part of the review outlines basic equine respiratory physiology and pathophysiology during anaesthesia to illustrate what makes horses prone to inefficient gas exchange and ventilation when they are recumbent. The difference between spontaneous ventilation and CMV is reviewed and basic considerations of CMV are explored in more detail.     

Immunomodulation during and after castration under inhalation anaesthetic without genotoxic effects on equine lymphocytes

Genotoxic DNA damage due to inhalation anaesthesia has been demonstrated in human lymphocytes. In order to evaluate anaesthesia-associated changes in cell-mediated immunity on the basis of a potential DNA damage as a health risk in horses, single cell gel electrophoresis and lymphocyte proliferation assay were performed on equine lymphocytes which were obtained before, during and after regular castration under inhalation anaesthetic. No significant lymphocytic DNA damage due to isoflurane anaesthesia was observed, whereas lymphocyte proliferative reactivity and lymphocyte counts decreased significantly (p≤0.05) during and after anaesthesia. The present study thus indicates that the combined anaesthesia does not result in significant DNA damage, which hence cannot be held responsible for the observed changes in the immune response of equine lymphocytes. However, the recognized compromises of immune function ought to be considered especially in immunologically challenged animals.     

Anaesthesia in horses using halothane and intravenous ketamine–guaiphenesin: a clinical study

Objective The aim of this study was to define and evaluate a combined inhalation?intravenous anaesthetic protocol for use in equine anaesthesia. Study design Prospective, randomized clinical trial. Animals Twenty‐eight horses (body mass 522 ± 82; 330–700 kg [mean ± SD; range]) with a mean age of 6 ± 4 years (range: 2–18 years) presented to the university hospital for various surgical procedures requiring general anaesthesia. Materials and methods Animals were randomly allocated to one of two treatment groups. Anaesthesia was maintained in 14 horses with halothane alone (H group). The mean end‐tidal halothane concentration was 1.24%. In the second group (n = 14) anaesthesia was maintained with both halothane (end‐tidal concentration 0.61%) and a continuous infusion of a ketamine–guaiphenesin mixture (HKG group). The two techniques were compared in terms of qualitative differences and cardiopulmonary effects. Results The stability of anaesthesia was significantly greater in group HKG and the need for dobutamine to maintain blood pressure was significantly less. Recovery times and quality were acceptable in all cases. There were no significant differences between the groups. Conclusions The infusion of ketamine and guaiphenesin in horses receiving low inspired concentrations of halothane provides suitable surgical anaesthesia and lowers the risk of hypotension. Clinical relevance The anaesthetic technique described in this study is a useful and practical alternative to inhalation anaesthesia using halothane alone.     

Effects of a demand valve on pulmonary ventilation in spontaneously breathing, anaesthetised horses

The pressure flow characteristics of a demand valve which has been suggested to be suitable for use in anaesthetised horses were determined at a range of flow rates commonly encountered in equine anaesthesia. The resistance of the valve was found to be very much greater than the resistance of normal large animal anaesthetic apparatus or the equine lower respiratory tract. The effects of the valve on pulmonary ventilation were investigated in seven anaesthetised, intubated horses. Respiratory rate and dynamic compliance were unaffected by connection of the valve but mean tidal and minute volumes and peak flow rates were substantially reduced. The change in transpulmonary pressure over the respiratory cycle was doubled and indices of work of breathing increased by a factor of three. It was concluded that the resistance offered by the valve was too great for its use in spontaneously breathing horses to be recommended.     

Pre-operative and anaesthesia-related risk factors for mortality in equine colic cases

Mortality rates for horses that have undergone emergency abdominal surgery are higher than for other procedures. Here, multivariable modelling of data from 774 surgical colic cases is used to identify pre-operative and anaesthesia-related variables associated with intra- and post-operative mortality. Intra-operative mortality was significantly (P<0.05), and positively associated with heart rate and packed cell volume (PCV) at admission, and negatively associated with the severity of pain. Post-operative mortality increased with increasing age and PCV at admission. Draught horses, Thoroughbreds and Thoroughbred-cross horses carried a significantly worse prognosis. We detected a small but significant variability in the risk of intra-operative death amongst referring veterinary surgeons. Different anaesthetic induction agents, inhalation maintenance agents and the use, or not, of intermittent positive pressure ventilation had no significant effect on risk of death. We conclude that cardiovascular compromise, level of pain, age, and breed are all associated with the risk of mortality in equine surgical colic cases.     

Is isoflurane safer than halothane in equine anaesthesia? Results from a prospective multicentre randomised controlled trial

REASONS FOR PERFORMING STUDY: Approximately 1 in 100 horses suffer unexpectedly from anaesthetic-related death. Identification and use of the safest anaesthetic drugs should support this aim. Experimental evidence has suggested that isoflurane should be a safer maintenance agent in equine anaesthesia than halothane. HYPOTHESIS: The death rate would be reduced in horses being maintained with isoflurane compared to halothane. METHODS: A multicentre randomised controlled trial was undertaken to compare the effects of isoflurane and halothane for maintenance of equine anaesthesia for all types of operation. Data were analysed from 8242 horses in which anaesthesia was maintained with either halothane or isoflurane using mixed effects logistic regression models. RESULTS: No overall benefit of either drug was detected. However, although not part of the primary hypothesis, data showed that the overall death rate was significantly reduced in horses age 2-5 years with isoflurane and that death from cardiac arrest was also reduced with isoflurane, particularly in high risk cases. CONCLUSIONS AND POTENTIAL RELEVANCE: Halothane remains an acceptable anaesthetic for maintenance of anaesthesia in horses, but isoflurane may be safer in the young horse and in high risk cases.     

Development of equine post anaesthetic myelopathy: Thirty cases (1979–2010)

Post anaesthetic myelopathy is a rare but devastating complication of equine anaesthesia and there is a need for better understanding of the condition and to raise veterinary awareness. The aim of this study was to collect data on unreported cases and combine those with reported cases to better identify possible risk factors and early clinical signs and outcomes of horses developing post anaesthetic myelopathy (PAM). A survey was conducted to identify cases of equine PAM. Records were also requested via the American College of Veterinary Anaesthesiologists listserve. Additional cases of PAM were located through review of the literature. Eighteen new cases and 12 published cases were identified. Large‐framed breeds between 6–24 months of age were more commonly affected (23/30). The majority of horses were positioned in dorsal recumbency during anaesthesia. Lack of movement and deep pain of the rear limbs, loss of anal tone and lack of panniculus response from the mid to caudal thorax distally were the most common clinical findings. Reported treatments resulted in no improvement of clinical signs and all horses died or were subjected to euthanasia from a few hours to 8 days post operatively. Poliomyelomalacia of the caudal spinal cord is the most common histopathological finding. Although a rare complication it is a catastrophic risk that can be eliminated by performing a standing operation. Hopefully in the future better understanding of this condition will lead to prevention and treatment strategies.     

Aspiration pneumonitis (Mendelson's syndrome) as perianaesthetic complication occurring in two horses: A case report

No published reports on the occurrence of Mendelson's syndrome (pneumonitis caused by aspiration during anaesthesia) in horses were found in the literature. Although the peculiar anatomy of the equine stomach makes horses less prone than other species to regurgitate, gastric reflux may still occur in horses with colic under certain circumstances. The colic horses in this report had in common stomach impaction, abdominal distention and preanaesthetic placement of a nasogastric tube, which was not withdrawn prior to induction. In both cases, a significant volume of gastric reflux was noted pouring from the endotracheal tube during general anaesthesia for exploratory laparotomy. It was hypothesised that the cause of gastric reflux was the combination of increased intra-abdominal pressure and patency of the cardia, and that inhalation of gastric content occurred at induction, before tracheal intubation. Treatment, which failed to improve oxygenation, consisted of repositioning of the horses to facilitate passive drainage of gastric content from the airways, active suction through the endotracheal tubes, ventilation strategies, improvement of haemodynamics to increase the pulmonary perfusion, and administration of bronchodilators. One horse was subjected to euthanasia owing to poor prognosis. Aspiration pneumonitis should be regarded as a life-threatening, although rare, perianaesthetic complication in equine colic cases. Patency of the cardia and increased intra-abdominal pressure are possible predisposing factors. Partial or even total withdrawal of the nasogastric tube prior to anaesthetic induction and tracheal intubation performed with the horse positioned in sternal recumbency may be undertaken as preventive measures in patients at high risk of developing Mendelson's syndrome.     

Endotracheal intubation in horses – are complications truly rare?

Endotracheal intubation is an essential component of general anaesthesia in horses to facilitate delivery of inhalation anaesthetic agent and oxygen, artificial ventilation, and prevent pulmonary aspiration of blood or gastric reflux. Experimental studies have identified a high incidence of tracheal mucosal injury after intubation resulting from direct trauma or local ischaemia from the pressure of the inflated cuff. Recommendations to minimise injury include gentle intubation, disconnection from the anaesthesia machine when moving the horse, and monitoring the endotracheal tube cuff pressure. New studies are needed to evaluate trachea and cuff pressure interactions under current practice conditions, including specialised ventilation modalities such as positive end‐expiratory pressure and continuous airway pressure.     

The use of sedatives,analgesic and anaesthetic drugs in the horse: An electronic survey of members of the American Association of Equine Practitioners (AAEP)

Reasons for performing study: To determine the sedative, analgesic and anaesthetic drugs and techniques that are used by equine veterinarians. Hypothesis or objectives: To provide equine veterinarians with information concerning veterinary use of anaesthetic techniques, a reflection of the collective experiences of the profession. Methods: A survey was conducted of those members of the American Association of Equine Practitioners (AAEP) with an electronic mail address on file with the organisation using proprietary, web‐based software. The survey was comprised of 30 questions divided into 8 sections: nonsteroidal anti‐inflammatory drugs; local anaesthesia; alternative techniques; standing chemical restraint; epidural anaesthesia; short‐term anaesthesia; long‐term anaesthesia; and a place for the respondent to make comments. Results: The response rate was 13.8% (952/6911) AAEP member veterinarians primarily use phenylbutazone and flunixin as anti‐inflammatory drugs, and lidocaine and mepivacaine for local anaesthesia. Combinations of drugs are preferred for standing chemical restraint. While many veterinarians frequently utilise short‐term anaesthesia, longer anaesthesia is less frequently performed. Conclusions: Most AAEP member veterinarians use sedatives in combination to provide standing chemical restraint. Extra‐label use of drugs is a core component of current equine sedation and anaesthetic practice. Potential relevance: Equine veterinarians can compare their choices of anaesthetic drugs with others practising equine medicine and surgery and may be stimulated to investigate alternative methods of providing comfort to horses.     

Rethinking equine anaesthetic risk: Development of a novel Combined Horse Anaesthetic Risk Identification and Optimisation tool (CHARIOT)

The most widely used method of assessing the physical status of humans presented for anaesthesia is the American Society of Anaesthesiologists Physical Status Classification System (ASA-PS). The ASA-PS has been applied to animals including horses, but its use is problematic. The authors virtually convened a group of internationally recognised equine veterinary anaesthesiologists with the goal of developing a risk assessment tool for the evaluation of healthy and diseased horses presented for anaesthesia. With this survey and analysis, the authors aim to develop a simple rubric system to assess equine anaesthetic risk that augments the ASA-PS system. The goal is to identify both diseased and healthy horses at increased anaesthetic risk prior to induction of anaesthesia so that measures might be implemented to potentially reduce morbidity and mortality.     

Aerosol therapy in the equine species

Inhalation therapy plays an increasing role in the management of equine respiratory disorders. Thisalternative to systematic treatment permits a high concentration of medication to act locally while minimizing side effects and residues. In human medicine, literature in this field is prolific and continuously renewed, whereas in veterinary medicine, applications of aerosol therapy are less extensive. This review considers the principles of action of the different types of devices used for inhalation, i.e., nebulization, metered-dose inhalation and dry powder inhalation, describes the technical and practical requirements for their use in the equine species and considers the advantages and disadvantages of each inhalation device. The pharmacological agents currently administered to horses by inhalation are also discussed. Perspectives of aerosol therapy in the equine species, including aerosols already used in human medicine and their potential applications for horses are described.     

Hypoxaemia during anaesthesia in seven horses with colic

Anaesthetic records of horses with colic anaesthetised between June 1987 and May 1989 were reviewed. pH and blood gas analyses were performed during 157 operations from which the horses were allowed to recover. A PaO₂ of 8.0 kPa or less was measured during anaesthesia in seven of these horses. The horses were of different breeds, ages and sexes. Anaesthesia was induced with xylazine, guaifenesin and ketamine in four horses and with xylazine, guaifenesin and thiobarbiturate in three horses. Anaesthesia was maintained with inhalation anaesthetic agent and oxygen: isoflurane in five horses, halothane in one horse, and initially halothane but later isoflurane in one horse. Systolic arterial pressures during anaesthesia ranged from 80 to 150 mmHg, diastolic arterial pressures were between 60 and 128 mmHg, and heart rates were between 28 and 44 beats /min. Controlled ventilation was initiated at the start of anaesthesia. PaCO₂ exceeded 6.7 kPa in three horses but was subsequently decreased by adjustment of the ventilator. PaO₂ of 8.0 kPa or less was measured during early anaesthesia, with one exception, and persisted for the duration of anaesthesia. The horses' inspired air was supplemented with oxygen during recovery from anaesthesia, at which time measurement of blood gases in three horses revealed no increase in PaO₂. Recovery from anaesthesia was uneventful. The surgical problems involved primarily the large intestine in five horses and the small intestine in two horses. Six horses were discharged from the hospital alive; one horse was reanaesthetised later the same day and destroyed without regaining consciousness. We concluded that none of the objective values recorded during the pre-anaesthetic evaluation could have been used to predict the complication of intraoperative hypoxaemia. We observed that once hypoxaemia developed it persisted for the duration of anaesthesia and even into the recovery period when the horses were in lateral recumbency and regaining consciousness. We assume that the altered metabolism from anaesthetic agents and hypothermia combined with adequate peripheral perfusion contributed to the lack of adverse consequences in six of the horses. The contribution of hypoxaemia to the deteriorating condition of the seventh horse is speculative.     

The outcomes of epidural anaesthesia in horses with perineal and tail melanomas: Complications associated with ataxia and the risks of rope recovery

Melanoma is a common neoplasm in horses, with a high rate of incidence in the perineum and ventral tail. Surgical excision is often recommended to remove such masses and can be achieved in the standing horse either via local infiltration of local anaesthetic, or via epidural administration of a local anaesthetic and/or an adrenergic α₂ receptor agonist. The clinical records of all horses with tail and/or perineal melanomas that received epidural anaesthesia at a single equine hospital, over a 7-year period, were reviewed to determine the drugs administered and complications associated with the epidural injection. Twenty-five cases were identified, five of which received epidural anaesthesia on two occasions. Epidural anaesthesia was insufficient to allow surgical melanoma excision in two horses; repeated epidural injection allowed sufficient loss of sensation in one of these horses and additional regional infiltration of local anaesthetic allowed completion of surgery in the other. Three horses developed significant ataxia associated with epidural anaesthesia, two of which became recumbent. Both recumbent horses were placed under general anaesthesia, and in one of these, rope-assisted anaesthetic recovery resulted in additional complications associated with facial nerve paresis, fractured ribs and a fracture of the first coccygeal vertebra, with associated neurological dysfunction of the rectum, anus, tail and surrounding skin.     

Tracheal trauma and pneumonia secondary to endotracheal intubation in a horse undergoing general anaesthesia,computerised tomography and myelography

Endotracheal intubation is commonly performed in horses undergoing general anaesthesia to avoid fluid aspiration and provide mechanical ventilation and inhalational anaesthetic agents. Secondary laryngeal and tracheal trauma following intubation is not rare. This case report describes the successful treatment of a horse with laryngeal and tracheal trauma secondary to intubation during myelography. Based on other clinical reports and clinical experience, movement of the endotracheal tube during myelography was considered the most likely cause of the damage. This case underscores the importance of monitoring horses for development of respiratory signs after general anaesthesia for computerised tomography and myelography.     

Anaesthesia and analgesia of the donkey and the mule

The number of donkeys and mules throughout the world is stable, and awareness of their use and concern for welfare, pain recognition and treatment are receiving increasing veterinary interest. Therefore, accurate information about anaesthesia and analgesia in donkeys and mules is important to ever more equine practitioners. Since donkeys are physiologically and pharmacologically different from horses, knowledge on species specific aspects of anaesthesia and analgesia are very important. Mules combine elements from both donkey and horse backgrounds, leading to great diversity in size, temperament and body type. Physiologically, they seem to resemble horses more than donkeys. This review highlights the current knowledge on various anaesthetic and analgesic approaches in donkeys and mules. There is still much information that is not available about donkeys; in many circumstances, the clinician must use available equine information to treat the patient, while monitoring carefully to observe for differences in response to therapy compared to the horse.     

Depth of anaesthesia

One hundred and fifty years after the first general anaesthetic in 1846 our knowledge about the mechanisms of general anaesthetics is still very sparse. The concept ‘depth of anaesthesia’ was introduced by John Snow (1847). He described ‘5° of narcotism’. Because one single agent had to provide all the components of general anaesthesia, the main problem for the anaesthetist was to avoid morbidity and mortality associated with excessively deep anaesthesia. The introduction of curare in 1942 allowed muscle relaxation required for surgery during a lighter level of anaesthesia, but also changed the emphasis from the problem of too deep anaesthesia and death, to too light anaesthesia and litigation. The problem of awareness during general anaesthesia with muscle relaxants provided the main impetus for monitoring depth of anaesthesia. During daily clinical practice the anaesthesiologist relies on clinical signs to evaluate anaesthetic depth, although several studies have shown a poor correlation between the 2 (Cullen et al. 1972; Evans and Davies 1984; Russell 1993). Different methods have been used in attempts to measure anaesthetic depth (Evans and Davies 1984; Stanski 1994), but none have been developed to a state where they can be used routinely in the operating theatre. This review will cover some of the parameters used to evaluate anaesthetic depth.