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Home » Issue 2 (Summer 2016) » Project Articles » Does inadvertent hypothermia occur in equine patients undergoing general anaesthesia, and if so does this correlate to reduced recovery quality?

Does inadvertent hypothermia occur in equine patients undergoing general anaesthesia, and if so does this correlate to reduced recovery quality?

Author Names: Isabell Riley (BSc (Hons) Equine Veterinary Nursing Science) and Dr Jane Williams



The occurrence and prevalence of anaesthetically induced hypothermia in small animal and human research has been well evidenced, however little comparative research has occurred in equine anaesthesia. Small animal research has identified hypothermia to have a significant impact on pharmacokinetics and recovery in patients. Equine anaesthesia is commonly associated with complications and an unpredictable recovery phase with the anaesthetic fatality rate in equine practice being 0.9%. This study was conducted to investigate the occurrence and influence that inadvertent intra-operative hypothermia has on patient recovery and peri-operative stability.  The objectives of this study were to identify the occurrence of equine hypothermia and analyse whether horses suffer from significant temperature loss peri-operatively. A further objective was to evaluate the post-operative recovery of patients and to establish whether ambient temperature contributes to excessive heat loss during anaesthesia. Prospective convenience sampling of 15 equine patients of varying age (9.9yrs±4.9yrs), gender and breed was used. Patients underwent a range of elective surgical procedures. Ambient and patient temperature were recorded throughout the peri-operative phase and analysed descriptively and statistically. Significant differences (P=0.0001) were identified in patients between morning (37.6˚C ± 0.8˚C) and end of surgery temperature (35.5˚C ± 0.7˚C). No significant correlations were identified between weight, age, surgical length, recovery length, ambient temperature (theatre/ recovery) or recovery score to suggest that these factors contributed to the hypothermia. The occurrence of inadvertent anaesthetic hypothermia in equine patients has been shown in all of the subjects of this study. Continuous monitoring and preventative measures in practice could minimalise this in order to promote patient welfare. Further research into the impact that this has on the physiological status of the horse is required. The development of a universal thermic index is also required to understand the safe ranges for intra-operative patient temperatures.


1.0 Introduction

1.1 Anaesthesia

Anaesthesia is defined as the controlled elimination of sensation via reversible suppression of the central nervous system (CNS), resulting in a reduction of sensitivity to external noxious stimuli and motor neurons (Freeman, 2009). Volatile agents commonly used in equine practice include Halothane, Sevoflurane and most commonly Isoflurane (Brickley and Mody, 2012). These lipophilic agents suppress the CNS via permeation of the blood-brain barrier (Kiyatkin and Sharma, 2009). Inhalation anaesthetics are absorbed via gaseous exchange. Once within systemic circulation, chemical components interact with targeted tissue to maintain unconsciousness (Grimm et al, 2015; Partridge, 2012). Pharmaceuticals such as Ketamine and Acepromazine are also commonly utilised peri-operatively. This enables promotion of adequately maintained intra-operative analgesia and a consistent plane of anaesthesia (Carregaro et al, 2006). Analgesia completes the triad of anaesthesia which consists of muscle relaxant, analgesia and loss of consciousness (Bettschart-Wolfensberger and Larenza, 2007). Sufficient analgesia can allow reduction in volatile agent concentrations, reducing the depression of organs such as the cardiovascular and respiration system (Brickley and Mody, 2012).

During CNS suppression the ability to control regular bodily function is inhibited. Small animal research has found thermoregulation to be one of the many functions affected during anaesthesia (Hubbell and Muir, 2014; Pozos and Danzl, 2006). Intra-operative factors can be complicated to balance and anaesthetists must have adequate knowledge and understanding of pharmacokinetics and pharmacodynamics in order to counteract negative changes which may occur during anaesthesia (Clark- Price, 2015).

Inadvertent intraoperative hypothermia is associated with anaesthesia and research has demonstrated that this can have a profound impact on physiological patient status. Burger and Fitzpatrick (2009) identified significant differences between patients who received thermal maintenance and patients who received no thermal intervention. Their study concluded that anaesthetic induced hypothermia significantly decreased the rate of healing, increased probability of cardiac dysfunction and increased susceptibility to infection (Buttwick et al, 2007; Bonnet and Marret, 2005).

Johnson et al (2002) evaluated the differences between experienced and newly qualified Veterinary Surgeons and their ability to monitor anaesthesia and recovery. Due to the ethicality of using personnel of questionable competency to monitor patients all participants graded the same recorded recovery phase, however this did not evaluate the actions that these individuals may have taken in a real anaesthesia crisis (Kinross and Darzi, 2010). Johnson et al’s (2002) research found no significant correlations. Incorporation of anaesthesia based scenarios may have simulated surgical issues which may have allowed comparison of participant ability to be based on their actions.


1.2 Equine anaesthetic mortalities

Equine anaesthesia is considered to hold the highest rate of complications, mortalities and morbidity within domestic veterinary medicine (Enderle et al, 2008; Proudman et al, 2006). Over the past 20 years considerable developments in medicine and surgical techniques have enabled a greater percentage of surgical interventions to have a positive prognosis (Menzies et al, 2016). However, when compared to developments in the safety of equine anaesthesia, minimum adaptations have been made to decrease the fatality rate (Bidwell et al, 2007; Johnston et al, 1995; Brook and Hildebrank, 1990).

Recent studies have identified the mortality rate as 0.9% in healthy horses undergoing general anaesthesia (Wagner, 2008; Bidwell et al, 2007). In 2002, the overall fatality rate, excluding colic cases, was concluded to be 1.9%. Although the rate of mortality has improved marginally this is not considered to be an adequate reflection of the timeframe (Senior, 2013; Johnston et al, 1995). Multicentre and single centre research has been completed within this field though results may have been influenced by sample size and a standardised practice protocol (Senior, 2013).

When comparing these mortality rates to the mortality rate of 0.17% in canine anaesthesia, equine anaesthesia risk is considered to be significantly elevated (Brodbelt et al, 2008). The differing anatomy of the individual species may contribute to the varying risks associated with fatalities (Carter, 2013). Dogs are anatomically designed to sleep recumbently, whilst the horse is not physiologically adapted to endure periods of recumbence and this therefore may impact notably upon the recovery period (Dugdale et al, 2015). The pressure applied to internal structures during recumbency in horses increases the likelihood of intra-compartmental compressions, predisposing the patient to myopathies as well as exacerbating a reduction in circulatory perfusion (Woodhouse et al, 2013). Small animal research has emphasised the importance of intra-operative thermogenesis in multiple studies, however, temperature is not routinely monitored in equine practice (Clark-Price, 2015; Jago et al, 2015).

Young and Taylor (1993) retrospectively reviewed variables present during anaesthesia. Hypotension, surgical duration, invasiveness and recovery time were identified to significantly impact on recovery success. However emergency patients were excluded, similarly to research carried out by Brodbelt et al (2008). The inclusion of emergency patients can significantly increase the complications seen, in addition to altering the accuracy of statistical analysis (Wohlfender et al, 2014). Utilising a large sample population (n=1,314) increased the probability of observing a range of complications (Denscombe, 2014). Of the population, 69% were thoroughbred or thorough bred crosses which limited the range of horses within the study and therefore limits transferability to the population as metabolism and physiology differs between breeds (Cheung, 2016). Recovery and induction was scored under blind clinical conditions by either veterinary surgeons or nursing staff, however this was not standardised across all data collected and therefore may impact on the accuracy of recovery evaluation (Pereira et al, 2014). By using staff of different clinical grades (Vet/ Nurse) the potential for varying opinions on the actions observed may also have caused discrepancies in data recordings (Perceira et al, 2014). To ensure continuity, future studies should employ standardised protocols including medications, hospitalisation routine, clinical observers of equal grade, recovery methodology and intra-operative inhalation agents in order to produce more valid results (Lockey et al, 2014).


1.3 Anaesthesia related complications

There are many variables which may influence complication rates seen in equine anaesthesia (Nelson et al, 2013). Commonly associated complications include cardiac arrest, fracture, myopathies and neuropathy (Jago et al, 2015). Numerous retrospective and prospective studies have concluded that the physiological status of a patient during surgery significantly impacts on their ability to recover free from complication or injury (Tokushige et al, 2015). Patient parameters such as heart rate (HR), respiratory rate (RR), mean arterial blood pressure (MAP) and temperature are considered almost impossible to measure during recovery due to health and safety (Wagner, 2008). Therefore implementing improvements to increase the likelihood of a smooth recovery must occur during anaesthesia (Shih, 2013). Intraoperative variables which enable equilibrium within the body are routinely monitored, these include HR, RR, MAP, blood gases, eye position, palpable reflexes and oxygen saturation (not exhaustive) (Clarke, 2014; Wagner, 2008). From this, anaesthetists are able to evaluate the depth of anaesthesia and alter the administration of specific pharmaceuticals to provide the desired effect (Rowland, 2013; Shih, 2013; Synder and Wendt-Homickle, 2013). Although commonly considered within small animal research, little evidence suggests that thermoregulation is considered within equine anaesthesia research. In small animal practice, hypothermia not only consistently occurs in the majority of patients but has a significant impact on patient stability (Pottie et al, 2007).

Young and Taylor’s (1993) retrospective study did not include data regarding patient temperature, therefore conclusions drawn did not anticipate the possible impact of this factor (Dencombe et al, 2014). The variables evaluated during data collection could have been a secondary physiological response to temperature reduction (De Vries et al, 2009).


1.4 Hypothermia

Hypothermia is defined as the body’s core temperature reaching dangerously low levels. In horses this is believed to be <37°C (Van-eps et al, 2012; Pottie et al, 2007). Hypothermia occurs during periods when the body is unable to sufficiently generate or maintain heat (also known as thermogenesis). This can impact on the overall metabolism of pharmaceuticals by up to 10% per degree lost (Molnlycke Health Care, 2012; Black, 2009; Kiekkas et al, 2005). Thermoregulation is compromised during anaesthesia via the suppression of the CNS as the hypothalamus (which controls thermoregulation) has an impaired capacity and function during anaesthesia (Kiekkas et al, 2005). This is emphasised by the administration of volatile agents and alpha-2 agonists which increase vasodilation and limit transmission of ascending electrical signals from cold receptors (Saper and Lowell, 2014). Therefore the physiological response to ensue systematic correction such as vasoconstriction and shivering is inhibited (Seamon et al, 2012). Equine anaesthesia is predominately maintained via inhalation and gases utilised are often below room or body temperature predisposing patients to hypothermia (Reynolds et al, 2008; Whitehair et al, 1993). Studies have determined that 22% of body heat lost during anaesthesia is via respiration, therefore provision of warmed air flow could reduce this loss (Diaz and Becker, 2010).


1.4.1 Physical effects of hypothermia

Body temperature is rarely considered within a surgical plan for adult equine patients (Mair et al, 2013; Brodbelt et al, 2008). Hypothermia restricts perfusion to the extremities and blood flow is diverted from the peripheries to support organ function (Urden et al, 2014). The efficacy of antibody and cell-mediated immunity is reduced, therefore increasing susceptibility to infection (Volk et al, 2013; Reynolds et al, 2008; Kiekkas et al, 2005). Wound healing time and pharmacokinetics are also reduced leaving excessive quantities of pharmaceuticals distributed within the body (Clark- Price, 2015). Clinical symptoms of hypothermia displayed by patients may be misinterpreted as being other surgically induced physiological responses such as hypotension or moderate movement (shivering) which could be diagnosed as inadequate anaesthesia depth (Black, 2009; Reynolds et al, 2008; Redondo et al, 2007).

Limited evidence supports the assumption that equine patients experience different complications to small animals. Companion animal research surrounding hypothermia has focused on the body’s inability to metabolise pharmaceuticals, maintain circulatory perfusion and recover with reduced complications (Dugdale, 2010; Polderman, 2009).


1.4.2 Small animal hypothermia

Research regarding small animal anaesthesia has concluded that temperature had a high impact on the physiological ability to metabolise pharmaceuticals. During anaesthesia small animal subjects lose the ability to adequately thermoregulate (Brodbelt, 2009). Susceptibility to poor tissue perfusion during this stage reduces drug distribution and equilibrium. Without mindful monitoring, pharmaceutical administration rates may continue to be infused periodically therefore increasing the probability of overdosing (Bonnet and Marret, 2005).

Pottie et al (2007) evaluated anaesthetic recovery in dogs and found that extended phases of recovery were associated with hypothermia. All participants were undergoing neutering procedures only and patients with a poor health status were excluded. This ensured that the results obtained were not influenced by existing medical conditions and therefore held validity (McGreevy, 2012). Nelson et al (2013) used horses undergoing a variety of non-emergency surgeries to identify the factors which may predispose horses to post-operative colic. Differing procedures, when analysed, were not significant. Pottie et al (2007) used four differing methods to induce patients and some patients received a pre-med of Acepromazine and morphine. This was determined based on the perceived excitability of the patient. Pre-medication has a well-known impact on the recovery phase therefore this should have been grouped (Jai et al, 2015). Statistically this may have negatively affected the analysis of data, however, ethical considerations are relevant to this situation as the administration of pharmaceuticals that do not directly benefit the individual would be unethical (Kilkenney et al, 2014).

Clark- Price (2015) evaluated the coagulation deficit during periods of inadvertent perioperative hypothermia and ambient temperature and body mass were determined to have a positive impact on patient thermoregulation. Ambient temperature within equine theatre is generally perceived to be colder than that of small animals. This is due to the logistics when accommodating hoist equipment (Lane, 2015). In comparison to small animals, this factor, alongside the combination of excessive surface area, may increase the probability of equine patients becoming hypothermic (Soto et al, 2014).

Brodbelt et al (2008), analysed mortality rates in small animal practice. Research evaluated the importance of maintaining adequate organ function and homeostasis. Multicentre data collection allowed researchers to obtain large numbers of subjects. However, the use of differing centres increases the potential variation in surgical protocol, pharmaceuticals and personnel technique and therefore could cause a contrast in results (Cross et al, 2014). By evaluating the large quantity of research available, there is evidence of the significant impact that hypothermia has on delayed recovery and reduced organ function (Andrzejowski et al, 2008). Variables analysed during Brodbelt et al.’s (2008) study included emergencies and patients with systematic vulnerability. This was considered complex and a poor representation of the study population. Some data relating to the circumstances of patient fatality was incomplete when reviewed retrospectively, indicating that the primary cause of death was not clear (Brodbelt et al, 2008). This may negatively impact data analysis. Incomplete data should be excluded from all numerical calculations to ensure that the percentage of patients affected by anaesthesia is measured accurately and a conclusive understanding of the circumstances is considered (Ford et al, 2016). Young and Taylor (1993) reviewed emergency patients as well as patients that underwent elective surgery. As a retrospective approach was employed, the accumulation of emergency patients (which are complicated to forecast) had already occurred and therefore sample size and timeframe were not a limitation (Brodbelt, 2009b). Results were analysed separately in order to produce two differing statistics highlighting the variant values of the study (Field, 2013). This approach allowed the differing risk of the individual patient presentations to be identified. The elective risk therefore was not dramatically altered by the inclusion of emergency patients (Pottie et al, 2007; Young and Taylor, 1993).

The lack of evidence of inadvertent intra-operative hypothermia within equine anaesthesia neglects the relevance of this factor to the overall wellbeing of the patient (George et al, 2010; Farmer et al, 2009). More research is required to demonstrate how the equine physiology interacts with anaesthesia and how this may affect patient recovery (Wakuno et al, 2016).


1.4.3 Hypothermic recovery

Study of the recovery phase in small animals has shown patients with hypothermia to have a significantly poorer recovery, with a decrease in metabolism and excretion impacting on the body’s abilities to regain consciousness (Farmer et al, 2014; Southward et al, 2006). Previous research has highlighted there to be an optimum recovery period. In equine patients, those who attempt to stand within the first 15- 30 minutes often have not regained full motor-neuron control of their peripheries, therefore increasing the likelihood of injuries (Clark, 2014). Patients that exceed the recovery index have a higher occurrence of myopathies or neuropathy due to the prolonged period of time muscle and organ groups have substantial pressure applied (Wiese et al, 2014).

Pottie et al (2007) studied the effects hypothermia had on post-operative recovery and positive correlations between reduced patient temperature and extended recovery were found in dogs. Thermal discomfort can be associated with delayed recovery, therefore routine prevention of hypothermia is carried out in small animal practice (Pottie, et al, 2007; Southward et al, 2006). This is a consequence of the extensive research which has been dedicated to this specific factor of anaesthesia (Brodbelt, 2009a).

Monitoring equine patient temperature during anaesthesia could allow researchers to identify the impact that CNS suppression has on patients’ ability to maintain body heat (Grim et al, 2015). The scope to adapt standard surgical protocols following this research may allow practitioners to reduce the number of variables to which patients are subjected, therefore providing the opportunity for clinicians to minimalise the risks faced by patients during surgery (Dugdale, 2010;Fayyaz et al, 2009). Within small animal nursing, thermoregulatory interventions are implemented throughout the perioperative period to reduce the occurrence of hypothermia (Sun et al, 2015). Active and passive measures are applied including blankets, heat pads and warm forced air flow (Kent et al, 2008). However there are some contraindications to the use of items such as warm blankets during the recovery period. By applying low level heat to the surface area, the cold receptors no longer detect cold stimuli and therefore cease transmission to the hypothalamus to initiate physical responses (Clark-Price, 2015; Sessler, 2014). Therefore this prevents the patient from attempting to regain core body temperature and in turn prolongs hypothermia (Sessler, 2016; Rigotti et al, 2015). Additional study within equine anaesthesia is required to enable interventions to be implemented,and the capacity for further understanding and knowledge to be gained from the application of small animal research methodology is vast. Utilising previous analytical findings may provide evidence to enhance the comparative ability of future research and may aid in potentially decreasing anaesthetic related mortalities in equine practice (Pottie et al, 2007; Bidwell et al, 2007).


1.5 Aims and objectives


The aims of this study were to;

  • Identify if equine patients experience a significant temperature decrease during general anaesthesia and determine how this affects the recovery length and quality.
  • Analyse the factors during surgery that could affect the patient’s temperature intra-operatively.

The following objectives were used to facilitate the successful execution of the aims of the study;

  • Investigate and record how equine temperature changes during peri-operative periods.
  • Determine whether equine patients experience difficulty thermoregulating during general anaesthesia.
  • Evaluate the quality and length of recovery that patients experience during the post-operative phase.
  • Determine if there is a relationship between temperature and the recovery period.
  • Observe and measure ambient temperatures of the theatre and recovery box during surgery and recovery.
  • Discuss potential implications of the results with the equine practice.


1.6 Hypotheses

H1: There is a significant reduction in the core body temperature of patients throughout the perioperative period.

Null H1: There is no significant reduction in the core body temperature of patients throughout the perioperative period.

H2: Patient age and weight will impact upon the reduction of temperature.

Null H2: Patient age and weight will not impact upon the reduction of temperature.

H3: There is a significant correlation between recovery time and patient temperature.

Null H3: There is no correlation between recovery time and patient temperature.

H4: Ambient temperature will have a significant impact on patients’ intra-operative temperature.

Null H4: Ambient temperature will not impact on patients’ intra-operative temperature.

H5: Ambient temperature will have a significant impact on patient recovery.

Null H5: Ambient temperature will not impact on patient recovery.


2.0 Methodology

Quantitative prospective research using convenience sampling was utilised to collect and analyse data, and variables were measured to test the hypotheses via descriptive and statistical testing (Fielding, 2013). This allowed the researcher to gain a greater understanding of the effects that anaesthesia has on thermoregulation and recovery in equine patients.


2.1 Sample selection

Using convenience sampling at a single centre, patients were diagnosed as requiring surgical intervention by a veterinary surgeon (Vettorato et al, 2010; Clark et al, 2007). Convenience sampling allows participants to be included based on accessibility and a genuine need for anaesthesia, therefore negating ethical issues which may arise from experimental research (Gov.UK, 2015; Price and Murnan, 2004). Past studies by Clark et al (2007) and Vettorato et al (2010) utilised similar sample sizes (n=9) to that in this study (n=15) to produce significant results and therefore supports the sample size of this study.


2.1.1 Inclusion and exclusion criteria for participants

Patients included within the study were examined prior to undergoing surgery and were considered to be systematically healthy. 86.6% (n=13) of patients were diagnosed with differing degrees of lameness which was the primary reason for surgery (McIlwraith et al, 2014). Physical health status was further analysed by routine pre-operative blood analysis, including haematology, biochemistry, total protein and pack cell volume (Salem et al, 2016). The information gained via these tests provided an overview of the internal status of organ function and circulatory cell distribution (Rendle et al, 2013).

None of the participants’ conditions were classified as surgical emergencies or defined as a medical emergency for which there was no alternative method of preserving life other than via surgical intervention (Segens Medical Dictionary, 2012; Collins et al, 2011). Presentation of emergency patients differs dramatically to that of elective patients (Dugdale et al,2015). In equine practice gastrointestinal obstruction is the one of the most common emergencies seen (Salem et al, 2016; Dearo et al, 2014). Patients are often stressed, in pain, shocked, dehydrated, hypotensive and are subject to toxaemia, leading to abnormal biochemistry and haematology analyses (Daniels et al, 2015). By excluding emergency patients from data analysis the researcher was able to limit the quantity of variables which may have limited the accuracy of the study (Rajagopalan et al, 2008).

Patients were required to have their temperature taken (in line with practice protocol) during morning, pre-med and evening rounds. Staff also briefly examined the patient, which included HR, RR, gastric noise, mucous membrane, capillary refill time and digital pulses. Dangerous patients who failed to provide baseline recordings were excluded from the study due to patient stress and staff safety (McGreevy, 2013; Coumbe, 2008).

Patients who had previously undergone surgery were excluded from the present study. Past research concluded that patients that experience multiple anaesthetics become habituated to not only the environment, but to the pharmaceuticals used to maintain anaesthesia (Dunkel et al, 2015).

The welfare of patients was considered during the study design. No adaptation to standard hospital protocol was implemented throughout baseline data collection. In order to reduce additional stress, patient parameters which were required for analysis were only obtained during periods of anaesthesia (Wagner, 2010).This ensured that ethical considerations for all subjects participating within the study were at the forefront of concern throughout data collection and no additional stimulation or stress was evoked (Lowes et al, 2012).


2.1.2 Variables

The variables used during data analysis were divided into six main groups. These included temperature readings during the morning (T1), pre-med (T2), start of surgery (at the point of incision) (T3), average GA (T4), end of surgery (T5) and evening (T6). This methodology was based on that of Pottie et al. (2007) which allowed body temperature trends to be compared during the multiple stages of surgery in dogs.

Young and Taylor (1993) recorded participant details for seven years which allowed conclusions regarding specific patient information to be evaluated holistically. This holistic approach was applied to the current study and information regarding patient health status, surgical procedure, age, weight, body score, gender and blood results were obtained. This enabled the results to be descriptively and statistically analysed. The recovery was timed and graded via an ethogram which has been validated via previous research into recovery quality scoring systems (Tokushige et al, 2015; Vettorato et al, 2010).


2.1.3 Data collection

The anaesthetic period for each patient was monitored by suitably trained veterinary surgeons with a range of two- five years’ experience. All veterinary surgeons had undergone training during an internship focusing on internal medicine and anaesthesia in equine practice. Internships were 18 months long and were overseen by senior registered surgeons and an equine internal medicine specialist (White, 2015). These individuals then monitored and graded recovery using blind observational techniques. This ensured that predictions of patient recovery was not altered by knowledge of the patient’s temperature (Rowden et al, 2015). Patient and ambient temperatures were obtained solely by the nursing team to maintain non-biased recovery grading (Holman et al, 2015). This methodology also ensures that the conduction of the study did not impact on the concentration of the anaesthetist and therefore did not compromise patient welfare (Taylor et al, 2015).

Due to the quantity and validity of research carried out within small animal practice regarding the effects of intra-operative hypothermia, a comparative study to ascertain the physiological implications of anaesthesia on the equine ability to thermoregulate is required (Andrzejowski et al, 2008). This supports the rationale for this study to be undertaken as results gained from the research may justify the implementation of nursing interventions to promote thermogenesis within equine surgery (Pottie et al, 2007).


2.2 Experiment protocol

Data were collected from Bushy and Willesley Equine Hospital (B&W) in Breadstone, Gloucestershire from 04/01/16 to 04/03/16. Standard protocols and training was provided for members of staff by the researcher to ensure continuity between differing volunteers. This included a series of pilot study sessions to allow familiarisation with equipment (Cocks and Torgerson, 2013). During the simulated pilot study, testing allowed appropriate changes to the methodology including the start point of temperature recording intraoperatively to promote patient welfare (Cocks and Torgerson, 2013). All patient information was recorded on individual patient record packs and was anonymised. Variables recorded included patient information, temperature records T1-T6, recovery quality scoring system, recovery time records and ambient temperature. Anaesthetic monitoring records were obtained to provide information regarding HR, RR, pharmaceutical, fluids, surgical length and MAP (Basak et al, 2013). Information gathered from B&W was via pre-authorised consent given by the senior surgeon. No clients were approached by the researcher, however all owners signed a general consent form to allow the anonymous release of their animal’s information for the purposes of research (GOV.UK, 2015).


2.2.1 Collection protocol

Documents with the parameters needed for the purpose of the study were provided for volunteers to carry out data collection. Each horse was subject to the same standard collection protocol.

At the point of data collection the ambient temperature was obtained parallel to patient temperature. This was carried out the morning of the surgery and repeated during pre-med, surgical incision, in 15 minute intervals throughout surgery, at the end of surgery and once recovered. The temperature of the theatre and the recovery box were also obtained. During recovery anaesthetists provided additional information where necessary regarding the actions observed which provided a more comprehensive overview of patient status (Vettorato et al, 2010).

Equipment used during data collection included rectal thermometers (Estar (Alfred Cox, SN: ga0087vr). This measuring method has been proven to produce valid results in previous research (Sikoski et al, 2007; Chen and White, 2006). Hygro thermometers (SN- L55AJ) were also used to measure ambient temperature. Both products are accurate to ± 0.1˚C (Basak et al, 2013; Sikoski et al, 2007; Chen and White, 2006). Products were selected based on previous clinical trials which had established the accuracy of each product (Bell, 2015; Chen and White, 2006).

Anaesthetic monitoring equipment was in situ at the practice prior to data collection (Datex- Ohmeda, SN: 4836383/ L-CANE02..00..EN SN:4834177). This equipment was used to monitor the intraoperative variables recorded during anaesthesia (Kulandayan et al, 2012).


2.3 Data analysis

Data were evaluated using a combination of descriptive and statistical analysis. This information was stored on a password protected laptop and patient record forms were stored in a locked filling cabinet (Data protection act, 1998). The following variables were tested;

  • The relationship between body temperature and length of general anaesthesia
  • The difference between patient temperatures during anaesthesia
  • The relationship between surgical length and recovery length
  • The relationship between patient intraoperative temperature and recovery length
  • The relationship between ambient temperature and intraoperative rectal temperature
  • The relationship between patient age/weight and intra-operative body temperature
  • The relationship between patient age/weight and recovery length
  • The difference between varying stages of temperature recording


2.3.1 Descriptive analysis

Mean ± Standard deviation (SD) were calculated for each variable analysed and the duration of surgery and recovery quality were assessed using descriptive qualitative analysis (Steven et al, 2016). Information obtained via general anaesthetic monitoring records (GA sheets) and the ethogram of patient recovery allowed the researcher to critically analyse the differing relationships between multiple variables. Graphs and box plot charts were used to illustrate the data, and comparisons between the temperature readings and the intra-operative variables observed during anaesthesia were included. Trends may be seen between periods of poor thermoregulation and factors seen on the GA sheets. This form of analysis allows evaluation of specific reoccurring relationships between two variables (Neuman, 2012).


2.3.2 Statistical analysis

Data from this study were used to identify whether patients experience a significant decrease in body temperature during general anaesthesia. Variables were then analysed via Kolmogorov- Smirnov tests of normality (SPSS, version 22) to determine whether they were parametric or non-parametric. Data were predicted to be non-parametric due to the small sample size (Field, 2013). Of the 13 variables tested, 9 variables were parametric and the remaining 4 were non-parametric. Due to the distribution of non-parametric data, further analysis was undertaken using Friedman’s test of difference. Friedman analysis was used to test for differences within the six variable groups (T1-T6) (Field, 2013). Bonferroni corrections were applied to adjust the P value (P<0.05/6= P<0.008). A Bonferroni correction may be utilised when statistically analysing single sets of data multiple times simultaneously. This correction reduces the occurrence of false-positive results (Napierala, 2012). Additionally Wilcoxon signed rank (WSR) tests were completed to evaluate the significant differences between two variable groups (Murphy et al, 2014). Non-parametric Spearman rank correlation testing was utilised to investigate relationships between end of surgery temperature, age, weight, recovery length, surgical length, ambient temperature of theatre and recovery, and recovery score. These were analysed to identify factors which may influence patient temperature intra-operatively. These tests were used to measure the correlation coefficient and significance (Murphy and Wolach, 2014).


3.0 Results

3.1 Demographics

The study consisted of fifteen horses, (n=15); nine (60%) geldings, five (33.3%) mares and one (6.7%) stallion. The mean (± SD) weight of patients were 539.67kg ± 100.14kg and their mean age was 9.9yrs ± 4.9yrs. Of the subjects included in the study, 33.3% were Dutch warmbloods (n=5) and 26.7% were thoroughbreds (n=4). Patients underwent a range of elective surgical procedures during data collection (Table 1).


Table 1– Demographics of study population


ID no.

Age/ years Sex Breed Weight/ kg BCS Surgery
1 7 Gelding Dutch Warmblood 548 6 Stifle arthroscopy
2 8 Mare New Forest Pony 516 5 Deep digital flexor Tenoscopy
3 6 Gelding Dutch Warmblood 616 8 Suspensory neurectomy
4 18 Gelding Thorough Bred 536 7 Bilateral neurectomy
5 17 Gelding Arab 445 7 Arthroscopic cyst removal
6 19 Mare Dartmoor 271 8 Deep digit flexor tendon Tenoscopy
7 4 Stallion Dutch Warmblood 632 5 Castrate
8 10 Gelding Connemara 556 6 Calcaneal bursa lavage
9 7 Mare Thorough Bred 518 6 Osteotomy dorsal spinous processes
10 10 Gelding Dutch Warmblood 688 5 Deep digit flexor tendon Tenoscopy
11 14 Gelding Thorough Bred cross 612 5 Metatarsal Arthroscopy
12 7 Gelding Thorough Bred 510 4 Metatarsal Arthroscopy
13 4 Mare Dutch Warmblood 600 6 Bilateral stifle arthroscopy
14 6 Gelding Connemara 488 6 Metatarsal Arthroscopy
15 9 Mare Thoroughbred 520 4 Metacarpophalangeal Arthroscopy


3.2 Temperature

Baseline temperatures taken in the morning (37.6˚C ± 0.8˚C) were compared to five variables taken throughout the peri-operative period (Table 2). All participants consistently experienced heat loss during surgery to 35.5˚C ± 0.7˚C (94.5% ± 2.7%). Of the fifteen participants, 26.7% (n=4) returned to their baseline temperature by evening temperature recording, however 13.3% (n=2) had an increase in temperature (102.5.2% ± 1.9%) during this time. Distribution of patient temperature throughout the six stages measured is displayed in the box plot below (Figure 1).


Figure 1 Riley

Figure 1– Cohort temperature during six peri-operative intervals.


3.2.1 Statistical analysis of temperature changes from T1 to T6:

Friedman’s test of difference found significant differences occurred between patient readings for the points recorded (morning (T1), pre-med (T2), start of surgery (at the point of incision) (T3), average GA (T4), end of surgery (T5) and evening (T6). This test found a strong significant difference ( χ2(5)= 61.882, P < 0.0001). Post hoc analysis using a series of Wilcoxon signed rank tests highlighted significant differences in temperature occurred between the time points where temperature was taken (Table 2).


Table 2: Wilcoxon signed rank test results for T1-T6.

Variable 1 Variable 2 Value Significance

Am temperature (T1)


Pre-med temperature (T2)

         0.906 Not significant

Am temperature (T1)


Incision temperature (T3)




Am temperature (T1)


Average GA temperature (T4)




Am temperature (T1)


End of surgery  temperature (T5)




Am temperature (T1)


Evening temperature (T6)

         0.081 Not significant

Pre-med temperature (T2)


Incision temperature (T3)




Pre-med temperature (T2)


Average GA temperature (T4)




Pre-med temperature (T2)


End of surgery  temperature (T5)




Pre-med temperature (T2)


Evening temperature (T6)




Incision temperature (T3)


Average GA temperature (T4)




Incision temperature (T3)


End of surgery  temperature (T5)




Incision temperature (T3)


Evening temperature (T6)




Average GA temperature (T4)


End of surgery  temperature (T5)




Average GA temperature (T4)


Evening temperature (T6)




End of surgery  temperature (T5)


Evening temperature (T6)




The mean end of surgery temperature (T5) across the cohort was 35.5˚C ± 0.7˚C, compared to a morning temperature (T1) of 37.6˚C ± 0.8˚C. In total the average body heat loss experienced by patients was 5.5% with a range of 11.4%- 1.6% during surgery of differing lengths (71.3 ± 22.6 minutes) (Figure 2).


Figure 2 Riley

Figure 2: Patient temperature T1 (AM temp) and T5 (end of surgery temp)


3.3 Relationships between end of surgery temperature and other variables:

End of surgery temperature (T5) was tested against age, weight, recovery length (RL) and surgical length (SL), ambient temperature-theatre (ATT), recovery score (RS) and ambient temperature- recovery (ATR) using Spearman rank correlation tests. No significant correlation was identified between any of the variables tested (Table 3)


Table 3: Spearman Rank Correlation test results (r= spearman’s correlation coefficient)

Variable 1 Variable 2 r-value P-value
End surgery temperature (patient) Age        0.190 0.497
End surgery temperature (patient) Weight        0.052 0.854
End surgery temperature (patient) Surgery length        -0.132 0.638
Recovery length End surgery temperature (patient)      – 0.254 0.360
Recovery length Surgery length      -0.216 0.440
Recovery length Age      0.268 0.335
Recovery length Weight        -0.045 0.874
Ambient temperature (theatre) End surgery temperature (patient)        -0.041 0.200
Ambient temperature (recovery box) Recovery score        0.052 0.854
Recovery score End of surgery temperature (patient)        -0.081 0.774


3.4 Anaesthesia variables

Horse 15 (H15) was the only patient to be given a recovery score of 1 as MAP and HR throughout the operative phase were unstable.  MAP showed trends such that it declined and increased by approximately 28mmHg during one 25 minute phase (Figure 3). This phrase expressed the largest chronological fluctuation of MAP throughout surgery. HR peaked shortly after this phase to 48BPM from 36BPM (approx). Dobutamine was administered during this time to balance MAP in a normal range (70-80mmHg). RR was maintained via ventilation and therefore had no variation. Levels of isoflurane ranged from 2.75%-3% throughout the duration of surgery and were at their highest concentration during the last 20 minutes before recovery. Horse 15 became cast whilst attempting to stand. Ambient temperatures during recovery were 11˚C (ATR), SL= 90min, T5= 35.7˚C and RL= 45minutes (47.3± 19.6).


Figure 3 Riley

Figure 3: MAP (mmHg) and HR (BPM) of horse 15


Horse 6 experienced staged increases of HR throughout surgery from 32-48BPM (approx.) (Figure 4), all other variables were mostly stable (RL= 28 min, SL=50 and T5=36.4˚C). Horse 6 had a recovery score of 3. During recovery this patient had three failed attempts to stand and became cast before succeeding at standing (ATR=13˚C).  


Figure 4 Riley

Figure 4: Horse 6 HR (BPM) recordings.


4.0 Discussion

This study aimed to investigate the occurrence of hypothermia in equine patients undergoing anaesthesia and subsequently determine the effects that this has on the recovery length and quality.


4.1 Intra-operative hypothermia

All study subjects experienced a reduction in core body temperature to <37˚C. The patients’ temperature decreased to between 34.3˚C- 36.6˚C by the end of surgery. This decrease in patient body temperature is considered to be consistent with current research which states 70%-90% of patients that do not receive preventative treatment develop hypothermia (Sappenfield et al, 2013; Burger and Fitzpatrick 2009). Statistical analysis confirmed that there was a strong significant difference between the multiple sets of variables tested (T1-T6). Therefore the null hypothesis H1 was rejected. The changes seen in patients between T2 (pre-med) and T3 (start of surgery) demonstrate a progressive decrease in temperature following predictions made by the researcher. Pharmacokinetic mediated vasodilation occurs with the admission of opioids and this increased the conduction of body heat loss (McCord et al, 2006).

This outcome is supported by research conducted in small animal medicine and provides evidence for the predicted outcome of the research. Small animal research has shown patients to lose up to 7.5% ± 2.9% of body heat during anaesthesia and that this correlates to increased susceptibility to anaesthesia complications and poor recovery (Redondo et al, 2012). The maximum temperature loss experienced during this study was 11.4% (4.6˚C), however the mean ± SD for the cohort (n=15) was 5.5% ± 2.7%. This overlap theoretically supports the transferability of research between small animal and equine practice(Rendondo et al, 2012; Pottie et al, 2007).

Pottie et al (2007) and Clark-Price (2015) concluded the body to be sensitive to 0.2˚C of change. However during anaesthesia the interthreshold range may tolerate up to 4˚C of core temperature change before hypothalamic responses occur (Saper and Lowell, 2014). Nursing interventions which promote thermogenesis may reduce this by up to 4%, therefore the potential for practice protocol to incorporate this is proven to be a viable solution (Sappenfield et al, 2013; Diaz and Becker 2010). More research to identify acceptable levels of heat loss in equine patients is required to further this understanding and create an applicable methodology to prevent hypothermia (Andrzejowski et al, 2008).

The prevalence of hypothermia requires additional research to determine a thermic index. This will provide a range between hyperthermic and hypothermic temperatures which could determine the adequate patient temperature (Sun et al, 2015). Application of this would allow patient temperature to be recorded on a scale parallel to standard monitoring variables allowing for a more holistic overall assessment of the patient.


4.2 Surgical profile

Subjects underwent differing surgery length (71.3 ± 22.6 minutes). Previous studies concluded that research on subjects that have been diagnosed to require surgery is considered to provide not only more realistic and transferrable results but is also more ethically acceptable (Gov.Uk, 2014; George et al, 2010). The distribution of patient surgical length was normal however no significant correlation was found when analysed against T5 (the temperature at the end of surgery). This conflicts with current research in small animal practice, however this may have been influenced by the relatively small sample size used in this study (n=15) and the surgical lengths of patients (Rowden et al, 2015). Previous research by Young and Taylor (1993) and Bidwell et al (2007) have shown a correlation in equine anaesthesia between surgical length and recovery complications. Pottie et al (2007) also identified this issue, concluding that patients were significantly predisposed to hypothermia when surgical lengths exceeded 30 minutes. This was also associated with a negative correlation to poor recovery in small animals (Seamon et al, 2012). The surface area in equine patients is considerately larger in comparison to companion animals, this would suggest that equine patients have increased susceptibility to heat loss (Van-Eps and Orsini, 2016). However, the proportionate changes identified in small animals has an exacerbated impact on physiological function (Soto et al, 2006).

Subjects within the study population underwent a range of procedures; 60% (n=9) orthopaedic surgery, 33.3% (n=5) soft tissue repair and 6.7% (n=1) castration. Limited variation of surgical procedures, and excluding emergency patients, may have minimalised the physiological stimulation, pain index and level of invasiveness experienced by subjects (Taylor et al, 2015). Previous research by Johnson et al, (1995), determined surgical procedure to be of little significance to the outcome of anaesthesia (with the exception of emergencies). Due to patients having a range of procedures with few instances of identical operations between subjects in this current study, statistical analysis was not undertaken due to the likelihood of skewed data and no applicable significance (Senior, 2013).


4.3 Subject demographic

Previous research has identified patient weight and age to be of relevance to surgical patient temperature. Pottie et al (2007) and Brodbelt et al (2008) identified geriatric and juvenile patients to have a lowered ability to thermoregulate when anaesthetised. The factors analysed during the present study found no significant correlation when tested, therefore the null hypothesis H2 was accepted. This may have been influenced by the variation in subjects’ size and breed. Comparison between ponies and horses indicates weight to have little relevance compared to patient temperature (Clark-Price, 2013).


4.4 Recovery

The relationship between recovery and surgical length was also analysed during this research. Previous research by Bidwell et al (2007), Brodbelt et al (2009a) and Wagner (2008) identified surgical length (equine >1 hour/ small animal >30 minutes) to impact on the recovery, safety and fatality rate of patients. However, multicentre research has been demonstrated to have variable protocols and therefore this may have influenced this rate (Ford et al, 2016). The present research utilised a single practice with integrated surgical protocols and specifically trained anaesthetists. The practice fatality rate is considerably lower than the researched statistic and therefore this may have had an impact on observations seen (Murphy et al, 2014). There is also conflicting research within equine practice by Jimenez et al (2012) who concluded surgical length to have no correlation with recovery length or quality. Of the subjects utilised within this study 66.7% (n=10) underwent surgical procedures >60 minutes and this may have contributed to the lack of significant correlation identified during data analysis.

Age and weight of the subjects were measured and tested against recovery length. Brodbelt et al (2009a) evidenced geriatric feline patients to have a significantly higher risk of fatality peri-operatively. A large sample size (n=73,178) obtained via multicentre data collection increased the validity of this conclusion. However of the subjects utilised during this current study (9.8yrs ± 4.8yrs) only 20% (n=3) were considered to be geriatric, therefore reliable conclusions may not be drawn (Field, 2013).


4.4.1 Anaesthetic profiles

Patient recovery was graded via an ethogram along with the time patients took to stand. Of the population, 46.7% were graded with a recovery score of 4 (n=7), 33.3% were graded with a recovery score of 3 (n=5), 13.3% were graded with a recovery score of 5 (n=2 and, 6.7% were graded with a recovery score of 1 (n=1).

The lowest recovery score observed during this study (Horse 15) was associated with the lowest ATR (11˚C, RS=1). This patient became hypotensive and tachycardic during the operative phase. However the end of surgery temperature of this patient was 36.4˚C (mean= 35.5˚C ± 0.7˚C), therefore although it is unlikely patient temperature was the main influence in the recovery phase the possibility for a combination of unstable MAP and HR and reduced temperature could have predisposed this patient to physiological challenges during recovery. De Vries et al (2009) concluded hypotension to be present in 97.3% of equine patients undergoing surgery when left untreated, therefore systemic circulation is commonly supported by intravenous fluids therapy (IVFT), and Dobutamine infusion (Ringer, 2007; Doherty and Valverde, 2006). Brodbelt (2009a) concluded that IVFT predisposed patients to hypothermia due to the administration of fluids at <37.5˚C, however Horse 15 was only mildly hypothermic regardless of the infusion of increased IVFT indicating the vasoconstrictive properties of treatment may have reduced heat loss (Hoffmann et al, 2012).

Horse 6 also experienced a poor recovery (RS=3, RL=28mins, T5= 36.4˚C, SL= 50mins). Recovery time was below the mean (47.3± 19.6mins) and the patient made three failed attempts to stand and became cast before successfully standing. A lack of significant correlations led to the acceptance of the null hypothesis (H3). HR of Horse 6 continued to increase throughout the surgical period (32- 48BPM). Monitoring then ceased when the patient entered recovery. Due to this there is no evidence to conclude whether HR continued to increase post surgically or whether the cessation of stimulus and volatile agents reduced tachycardia. Previous research (Wagner, 2008; Proudman et al, 2006; Young and Taylor, 1993) concluded short recovery periods (<15-30 mins) and patients with >50BPM (HR) to have a significant reduction in recovery quality. Therefore the HR of the patient has a higher probability of impacting the recovery phase as opposed to temperature alone. This however could not be accurately quantified due to the recording techniques commonly used in practice. In order for significant correlations to be identified, measurements required for statistical analysis should be recorded in numerical format as opposed to approximate symbols (Field, 2013). This would enable accurate assessment of patient parameters to be determined.


4.5 Ambient temperature

Burger and Fitzpatrick (2009) and Kent and Williams (2008) concluded ranges in ambient temperature to have a significant impact on the hypothermic state of patients. Their research concluded that cold environments predisposed patients to loss of body heat via surface area conduction. Uncontrolled ambient temperature of both theatre and the recovery area have been proven in small animal and human medicine to result in delayed and poor recovery due to heat loss. However, during this study ambient temperatures stayed within a close range (ATT=14.2˚C±1.5˚C; ATR=13.5˚C±1.6˚C). This may have contributed to a lack of significant correlations being found between the variables. The null hypotheses H4 and H5 were therefore accepted.


4.6 Industry recommendations

This research concluded that equine patients experience a significant decrease in temperature during the operative phase. Research suggests that the implementation of actively preventing the occurrence of hypothermia in anaesthetised patients increases tissue perfusion, patient comfort and metabolism equilibrium (Langhelle et al, 2010; Andrzejowski et al 2008). Standard protocol in practice should be to monitor patient temperature continuously during the operative and peri-operative phase. This would ensure that patients temperature is controlled within safe parameters (Kent and Williams, 2008). An investigation into the occurrence of hypothermia in emergency abdominal surgery is required to determine whether these patients express severe hypothermia due to the loss of heat expelled via large open cavities (Nelson et al, 2013). Nursing interventions for hypothermic patients such as forced hot air or heat pads could increase the occurrence of optimal recovery length and reduce patient discomfort, discouraging premature attempts to stand. The development of a thermic index would facilitate the measures needed to easily monitor temperature throughout surgery. Considering the size, weight and dimensions of the horse, forced warm air would be the most suitable method to maintain thermogenesis (Sun et al, 2015; Clark-price et al, 2013).


4.7 Limitations

Data collection for this study was completed over a relatively short period of time (8 weeks). During this time the practice experienced a lower surgical case load than in previous months. This affected the quantity of participants available for data collection and therefore led to a reduction in data. Increased numbers of patients would have allowed a greater representation of the population (Rowden et al, 2015). However, previous research utilising comparative samples has collected valid results and therefore the quantity of variables observed throughout the study may have had a larger impact than sample size (Vettorato et al, 2010).

Staff members obtaining the data ensured that the safety of the patient was rightfully prioritised throughout data collection, however this led to incomplete data records and therefore a number of records were excluded. Intra-operative temperature recordings for this study commenced from the point of incision, to ensure that the priority of nurses was to prepare the surgical site, thus shortening anaesthetic lengths. As a result of this, some data collection did not start until 30 minutes after induction, therefore limiting data obtained. In future investigations consideration for additional personnel to be engaged solely in data collection could increase the quantity of data obtained for each patient without compromising patient safety (Wohlfender et al, 2014).


4.8 Further research

This study presented the opportunity for further research to be conducted within the field of equine anaesthesia. The utilisation of a much larger sample population is required to conclude the extent of the effect hypothermia has in equine anaesthesia (Senior, 2013).

Observing parameters of patients during recovery has never been accomplished fully in equine medicine, resulting in an area lacking research (Vettorato et al, 2010). Development of new technology to permit this form of monitoring could allow analysis of recovery data to be completed to provide evidence of the changes patients exhibit during this phase. The danger associated with physical observations in equine recovery make monitoring parameters physically impossible. Therefore future research should employ methods to gain this information via patient fitted monitoring devices (Nelson et al, 2015).

Hypothermia and hyperthermia in equine anaesthesia are relatively unquantified concepts. Further prospective research investigating the application of a patient thermic index (safe range) could be completed on an epidemiological scale to allow the incorporation of thermic homeostasis in line with standard monitoring protocols. This would allow researchers to observe the risk factors and specific patient attributes which may predispose them to hypothermia. Research evaluating a thermic index for equine patients may also incorporate research which has shown specific conditions such as haemorrhage and head injuries to benefit from hypothermia (George et al, 2010; Rigotti and de Vries 2010).


5.0 Conclusion and Applications

The consequences of hypothermia during anaesthesia in equine practice is yet to be fully quantified, therefore this study aimed to evaluate the thermoregulatory ability of the horse during anaesthesia. All subjects within this study experienced a degree of hypothermia during anaesthesia which persisted in recovery, ranging from mild to moderate. Analysis of the data demonstrated subjects to lose between 0.6˚C – 4.6˚C during surgical procedures of differing length and variety. No emergencies were included during data collection and this may have impacted on the severity of hypothermia seen. The null hypothesis H1 was rejected after a significant difference was found to exist between the temperatures taken peri-operatively (T1-T6: P< 0.008).

Further research within the field of equine intra-operative hypothermia is necessary to evaluate the influence that this may have on other parameters such as HR, MAP and RR, as well as the recovery and post-operative period. Research based development of an intra-operative thermic index, ascertained via an epidemiologic study, could enable the design of a defined, optimum temperature range. This could produce a universally accepted monitoring system, which could be implemented intra-operatively along with continuous patient observations to grade thermoregulation in individual patients. Future research is required to evaluate the efficacy of forced warm air when used to prevent equine hypothermia. Although shown to provide suitable cover over large surface areas, the  dimensions of the horse creates issues with traditional warming techniques, such as heat pads and blankets. The provision of this information could provide evidence for an alteration in common practice protocols to enhance the welfare of the horse during surgery.



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