Anaesthesia for colic surgery in horses

Horses presenting for emergency exploratory laparotomy under general anaesthesia represent a unique anaesthetic challenge. The pathophysiological changes associated with colic can affect multiple body systems and severely compromise the cardiorespiratory and metabolic stability of a patient. Because the gastrointestinal lesion causing the colic signs can vary in location, duration and severity, the impact on the cardiorespiratory stability in the horse presenting with behavioural signs of colic can also be influenced. Horses with colic caused by a strangulating obstruction, for example where tissue perfusion is severely compromised, represent a high anaesthetic risk. Factors such as hypovolaemia, acid–base and electrolyte disturbances, endotoxaemia, metabolic acidosis and abdominal distension contribute to serious cardiorespiratory compromise (Robertson and Scicluna, 2009). The presence of a distended large intestine can also result in cardiorespiratory compromise as a result of pressure on the diaphragm and reduced venous return (Robertson and Scicluna, 2009).

Preoperative information

A thorough but time efficient preoperative clinical examination is ideal in all horses presenting for colic; however, this may not always be possible or safe in some horses. It is important to first assess whether the horse is in pain and safe to approach. Analgesia and/or sedation may be required to enable safe examination (Dugdale et al, 2020).

Useful information to gather includes:

• Heart rate: this can provide information relating to cardiovascular stability and pain. Intraoperative mortality in horses undergoing colic surgery has been associated with heart rate (Proudman et al, 2006)
• Respiratory rate and effort: respiratory character may be influenced by pain, systemic acid–base balance and abdominal distension. Ventilation may be compromised because of abdominal distension and splinting of the diaphragm, particularly in patients suffering from large colon torsion (Dugdale et al, 2020), large colon displacements or caecal tympany
• Temperature: the presence of a fever or pyrexia may be indicative of conditions such as peritonitis or systemic inflammatory response syndrome
• Gastrointestinal borborygmi: this should be assessed via abdominal auscultation, and intestinal motility can be assessed via ultrasound
• Rectal palpation: this can determine the presence and degree of visceral distension and the location of the distension (small intestinal vs large intestinal). Other abnormalities such as intestinal impactions can also be identified
• Nasogastric reflux: this can indicate either a functional or physical small intestinal obstruction or gastric outflow obstruction. The presence of nasogastric reflux may result in fluid imbalance and electrolyte derangements, depending on the duration and volume of reflux. One retrospective study reported that, at presentation for colic surgery, horses with lower volumes of reflux had better short-term outcomes (Spadari et al, 2023)
• Packed cell volume or haematocrit: this can provide information relating to hydration status. An elevated packed cell volume has been associated with an increased risk of intraoperative mortality in horses undergoing colic surgery (Proudman et al, 2006). Analysis of packed cell volume and total protein can aid decision making for fluid therapy
• Total protein: this may be increased from haemoconcentration, or decreased (for example, as a result of loss through the ischaemic gastrointestinal tract; Boesch, 2013)
• Lactate concentration: this reflects the adequacy of global perfusion (Auckburally et al, 2019). One retrospective study identified that at presentation, horses with blood lactate level <1.2 mmol/litre had a higher probability of a positive outcome compared to horses with a value >6.6 mmol/litre (Spadari et al, 2023)
• Acid–base status: a jugular venous sample can be used for preoperative assessment of acid–base status (Boesch, 2013). Many horses with colic have acid–base abnormalities and horses with any degree of hypovolaemia will have a metabolic acidosis as a result of increased anaerobic metabolism in tissues where perfusion is compromised (Dugdale et al, 2020)
• Blood gas analysis: to evaluate gas exchange in the lung, an arterial sample is required. This is usually collected during general anaesthesia once an arterial catheter has been placed for the purposes of measuring arterial blood pressure. In addition, arterial blood gas analysis can be undertaken preoperatively
• Levels of electrolytes: these are useful to know preoperatively so that preparation can be made to supplement deficiencies. In a retrospective study, 52.5% of horses undergoing colic surgery were hypocalcaemic and 30% were hypokalaemic (Adami et al, 2020).

In addition to these, a nasogastric tube can be placed and secured before induction of general anaesthesia to allow gastric decompression, relieve gastric distension (which may help to reduce the risk of gastric rupture) and reduce associated pain (Robertson and Scicluna, 2009). However, there are two reports of aspiration of gastric contents in horses where a nasogastric tube was left in place and stoppered before induction (Monticelli and Adami, 2019). 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 could be achieved (Monticelli and Adami, 2019). The authors of that case report suggested that preventive measures to prevent aspiration could include intubation in sternal recumbency, emptying of the stomach shortly before anaesthesia and partial withdrawing of the nasogastric tube before induction of anaesthesia (Monticelli and Adami, 2019). Caecal or colonic trocharisation is occasionally required to decompress the abdomen (Dugdale et al, 2020). A blood sample should also be obtained and analysed before administering intravenous fluids where possible. The minimum level of information required for general anaesthesia may include (Boesch, 2013):

• Complete blood count or packed cell volume
• Total protein level
• Acid–base status including lactate, creatinine and urea levels
• Electrolyte levels (sodium, potassium, chloride, calcium)
• Blood glucose level.

Hypovolaemic horses may present with elevated creatinine levels, making the distinction between pre-renal or renal causes of azotaemia may be harder to achieve before fluid therapy. Evaluation of plasma creatinine level, urine specific gravity and urine output can help to identify horses with renal dysfunction (Barton and Hassel, 2023).

It is important to consider that the management of a large animal with severe and unrelenting pain is challenging, and the urgency of the surgical procedure needs to be balanced against the need to restore circulating volume and correct electrolyte and acid–base disturbances (Auckburally et al, 2019).

Pre-anaesthetic medication

The aim of pre-anaesthetic medication is to provide sedation, analgesia and muscle relaxation. Provision of effective analgesia is the first consideration in most instances (Robertson and Scicluna, 2009). Horses presenting with colic may have multiple sources of pain, including distension of the intestine with fluid and gas, mesenteric stretch or traction, local hypermotility, early ischaemia of the intestinal wall, intestinal inflammation or peritonitis (Dugdale et al, 2020). Several classes of drugs may be used for pre-anaesthetic medication in colic cases, including alpha-2 adrenoreceptor agonists (xylazine, detomidine and romifidine), opioids (butorphanol and buprenorphine in particular), spasmolytics (butylscopolamine) and non-steroidal anti-inflammatory drugs (Duz, 2019). These drugs also play a pivotal role in providing pre-anaesthetic medication.

In the UK, several non-steroidal anti-inflammatory drugs are licensed for the management of abdominal discomfort in horses, including flunixin meglumine, metamizole, meloxicam and ketoprofen (Duz, 2019). Non-steroidal anti-inflammatory drugs may play an important role in equine analgesia (McFadzean and Love, 2018); however, it is important to consider that most colic patients have received a non-steroidal anti-inflammatory drug before arriving at a surgical facility (Robertson and Scicluna, 2009). Therefore, it is important to obtain an accurate medication history from the referring veterinarian before administering further medication.

The administration of opioids to horses with colic remains controversial. Although it is known that opioids decrease intestinal motility, there are still limited data on the analgesic efficacy and clinical gastrointestinal effects of opioids in horses with naturally occurring colic (Boesch, 2013). The optimal dose of systemically administered morphine for analgesic in clinical cases has not yet been established (McFadzean and Love, 2018). There is some evidence in the experimental setting that methadone produces analgesia with less reduction in gastrointestinal motility compared to morphine (Carregaro et al, 2014), but further work is required to establish if this is the case in horses with colic. The author administers morphine or metha-done (0.1–0.2 mg/kg intravenously or intramuscularly) preoperatively to horses undergoing colic surgery.

Sedation

An alpha-2-adrenoreceptor agonist (such as xylazine) with or without an opioid is commonly used for premedication. Alpha-2-adrenoreceptor agonists provide sedation, analgesia and muscle relaxation but also cause bradycardia and bradyarrhythmias, diuresis, sweating, ataxia and reduced gastrointestinal motility (Dugdale et al, 2020). In systemically compromised horses such as those presenting for colic surgery, the cardiovascular effects may not be well tolerated, necessitating careful titration of doses to effect. Some horses' pain is refractory to any analgesic drugs and continuing to give boluses of alpha-2-adrenoreceptor agonists to these horses will only further depress cardiovascular function (Boesch, 2013).

Antimicrobial administration

Antimicrobial administration should be performed on an individual patient basis, taking into account factors such as bowel trocharisation, paracentesis, leakage of bacteria from areas of devitalised intestine, intestinal resection and anastomosis, enterotomy and needle decompression of gas-distended bowel which may all result in peritoneal contamination (Robertson and Scicluna, 2009).

Preoperative fluid therapy

The aim of preoperative fluid administration is to rapidly correct volume deficits and normalise electrolyte and acid–base disturbances (Robertson and Scicluna, 2009). In patients with colic, fluid deficits are common and can affect one or more body fluid compartments. Dehydration arises when there is a total body water deficit which may occur when water loss exceeds water in-take (Auckburally et al, 2019). Hypovolaemia arises when there is a decreased circulating blood volume and can be immediately life-threatening such as severe haemorrhage (Auckburally et al, 2019). The horse should be stabilised as much as possible before general anaesthesia, but this period is often time-pressured. Initiation of correction of fluid, acid–base and electrolyte derangements should begin as soon as is practical in the pre-anaesthetic window (Boesch, 2013).

However, the management of an equine patient with severe and unrelenting pain is difficult, and the urgency of the procedure needs to be balanced against the need to restore circulating volume and correct electrolyte and acid–base disturbances (Auckburally et al, 2019). The selection of one type of fluid over another for fluid resuscitation in horses remains controversial and is influenced by individual clinician preference. Isotonic crystalloid solution, hypertonic saline and colloid solutions are the three types of fluids most commonly used for fluid therapy in horses.

Isotonic crystalloid solution

• Rapid administration of boluses of isotonic crystalloids alone may be inadequate to restore circulating volume in horses presenting with colic
• Administration of large volumes of crystalloid solution may require the placement of an additional intravenous cannula to allow effective administration
• Only 25% of the infused volume will remain in the intravascular space after 60 minutes
• Rapid administration or large volumes of crystalloid fluids may lead to endothelial damage, adverse effects on the microcirculation and tissue oedema (Auckburally et al, 2019; Barton and Hassel, 2023).

Hypertonic saline (Box 1) and colloids (Box 2) are effective in providing fluid volume resuscitation in horses, but it is still unclear whether they are superior to isotonic crystalloids alone (Arden et al, 1991). In the past decade, a critical re-evaluation of resuscitative fluid therapy practices has ensued, initiated by data acquired in the human medical field (Barton and Hassel, 2023). Clinicians may use a combination of types of fluids during volume resuscitation rather than isotonic crystalloid solution alone.

Induction of general anaesthesia

A combination of a benzodiazepine and ketamine can be administered intravenously to induce general anaesthesia (Boesch, 2013). A benzodiazepine is commonly included as the co-induction agent alongside ketamine to reduce the increased muscle tone associated with ketamine and to provide better conditions for tracheal intubation and surgery (Dugdale et al, 2020). Guaifenesin is a centrally-acting muscle relaxant and may be used in combination with thiopentone to produce induction of general anaesthesia. Guaifenesin can be intravenously infused to effect followed by thiopentone (5 mg/kg intravenously). Guaifenisin/thiopentone induction of general anaesthesia may be characterised by a more dramatic onset of unconsciousness and abrupt falling to the ground. This may be less desirable in the presence of distended abdominal viscera which may risk rupture during an abrupt descent to the floor (Dugdale et al, 2020).

Box 1.Hypertonic saline

• Can improve cardiac output, tissue perfusion and microvascular flow as a result of erythrocyte shrinkage (Auckburally et al, 2019)
• Results in the plasma volume expansion by 3–4 litres for every 1 litre of hypertonic saline administered (Schmall et al, 1990; Auckburally et al, 2019)
• Causes a significant increase in serum sodium and serum chloride as well as osmolality (Schmall et al, 1990)
• Has short-lived effects, so it must be administered immediately before induction of general anaesthesia (Auckburally et al, 2019)
• Hypertonic saline will not correct underlying fluid deficits, so additional fluids must be administered (Snyder and Wendt-Hornickle, 2013)
• Is contraindicated in cases of severe dehydration because it will cause further dehydration of the interstitial and intracellular fluid compartments (Auckburally et al, 2019). Therefore, it is important to ensure that fluid resuscitation using a balanced crystalloid solution is co-administered or administered before administration of hypertonic saline.
• Must be followed by isotonic crystalloid administration to replenish the extravascular space (Auckburally et al, 2019)

Box 2.Synthetic colloids

• May be used for plasma volume expansion in the event of a hypovolaemic crisis and hypoproteinaemia
• Can provide volume of expansion of 75–100% of the infused volume, and the effect is longer lasting compared to hypertonic saline
• Can maintain colloid osmotic pressure in the intravascular space for longer periods compared to hypertonic saline
• Synthetic colloids should not be administered in volumes exceeding 10 ml/kg because of the risk of coagulopathy (Auckburally et al, 2019)
• Use in human and small animal critical care medicine has been controversial, but there is no evidence that colloid use in horses has a negative impact on survival (Barton and Hassel, 2023)
• The risk of coagulopathy may differ depending on the specific type of colloid used. This value represents a conservative value. For specific synthetic colloid molecules, there is evidence to indicate that the administration of 40 ml/kg bodyweight tetrastarch (130/0.4) in horses is more likely to induce changes in coagulation as measured by thromboelastography (Viljoen et al, 2014)

The trachea must be promptly intubated with a cuffed endotracheal tube (usually 24–28 mm in the average 450–550 kg horse) immediately after the horse becomes recumbent (Boesch, 2013). In preparation for the placement of the endotracheal tube, the cuff should be lightly coated with sterile water-based lubricant; this permits a better seal with the trachea at a lower cuff inflation pressure (Boesch, 2013).

Maintenance

Inhalational agent anaesthesia

Isoflurane (licensed) and sevoflurane (not licensed for use in horses in the UK) are the two most common inhalational anaesthetic agents used to maintain general anaesthesia in horses. Inhalant anaesthetic agents have several negative physiological effects including:

• Dose-related cardiovascular depression affecting cardiac output, blood pressure and muscle perfusion (Steffey and Howland, 1980)
• Reduction in cardiac output (Risberg et al, 2016) and arterial blood pressure
• Hypoventilation (Aida et al, 1996).

Therefore, the cardiorespiratory function of horses maintained using these agents must be closely monitored, particularly for systemically compromised patients.

Partial intravenous anaesthesia is a technique which involves the intravenous administration of an anaesthetic or sedative drug alongside inhalational agent anaesthesia. This technique reduces inhalational agent requirements, and for horses undergoing colic surgery, partial intravenous anaesthesia using lidocaine could be considered.

Partial intravenous anaesthesia using a lidocaine continuous rate infusion can reduce inhalational agent requirements (Valverde et al, 2010) and provide analgesia (Murrell et al, 2005). Since the metabolism of lidocaine may be affected by physiological derangements such as hypovolaemia, it is important to ensure correct dose administration and use an infusion pump to deliver intravenous lidocaine. Furthermore, it is worth considering that the effect of severe gastrointestinal disease on the pharmacokinetics of lidocaine in horses under general anaesthesia remains unclear (Nannarone et al, 2015) and therefore care should be taken to ensure correct dose administration. Some authors have described the use of a preloading lidocaine dose before starting a continuous rate infusion of 50 µg/kg/min intravenously, although a prospective randomised clinical study demonstrated that preloading with a lidocaine bolus before lidocaine continuous rate infusion did not influence isoflurane requirements, cardiopulmonary effects or recovery compared to no preloading bolus (Nannarone et al, 2015). Lidocaine administration should be ceased at least 30 minutes before recovery as it has been shown that lidocaine infusion for the duration of surgery resulted in a significantly higher degree of ataxia in recovery and tendency to have a poorer recovery quality (Valverde et al, 2005).

Partial intravenous anaesthesia using alpha 2-adrenoreceptor agonists reduces volatile agent requirements, most likely by providing extra sedation and analgesia, with a more stable maintenance of anaesthesia and better recovery qualities (Gozalo-Marcilla et al, 2015). However, the impact of alpha 2-adrenoreceptor agonists on the cardiovascular system should be considered and so far, the evidence relating to the use of alpha 2-adrenoreceptor agonists involves healthy horses (Gozalo-Marcilla et al, 2015). Their use in systemically compromised horses such as those with colic remains controversial (Gozalo-Marcilla et al, 2015).

Mechanical ventilation

Controlled mechanical ventilation may be used during inhalational agent anaesthesia to provide a tailored approach to assisted ventilation. Mechanical ventilators may be volume-controlled, pressurecontrolled or time-cycled. The design of the ventilator dictates how the settings can be adjusted to deliver the desired frequency, peak inspiratory pressure and tidal volume. Horses may hypoventilate and become hypercapnic during anaesthesia and can therefore develop respiratory acidosis which may compound a previously existing metabolic acidosis, resulting in a very low blood pH (Dugdale et al, 2020). Mechanical ventilation can be used to maintain effective ventilation and produce normocapnia but the effects of controlled mechanical ventilation on the cardiovascular system should be considered. The generation of positive airway pressure during controlled mechanical ventilation reduces venous return and can have a detrimental effect on the cardiovascular system; this may be more prominent in horses with cardiovascular instability. One retrospective study showed that 85% of horses recovering from colic surgery recovering from anesthesia had received controlled mechanical ventilation intraoperatively. The authors discussed that close monitoring of the effects of controlled mechanical ventilation on the cardiovascular system is recommended and cardiovascular support should be administered where necessary (Proudman et al, 2006).

Positive end expiratory pressure may be used to improve arterial oxygenation in horses during general anaesthesia. However, application of positive end expiratory pressure has been shown to decrease cardiac index and gastrointestinal oxygenation and perfusion at positive end expiratory pressure values >15 cm H₂O (Hopster et al, 2017). If mechanical ventilation is used during anaesthesia, the positive intrathoracic pressure can impede preload and ventricular ejection. This can be appreciated by observing the effect on the arterial pressure waveform or the plethysmogram generated from the pulse oximeter probe. If depression of these waveforms is seen during the inspiratory phase of ventilation, this may suggest that more fluid volume is required (Auckburally et al, 2019).

Monitoring

Electrocardiogram

This provides information relating to heart rate, heart rhythm and cardiac complex morphology. Intraoperative tachycardia (>55 beats per minute) was associated with an increased risk of peri-anaesthetic death in horses undergoing colic surgery in a retrospective study (Adami et al, 2020).

Direct arterial blood pressure monitoring

Arterial cannulation is required for direct arterial blood pressure measurement (Dagnall et al, 2021), with the facial artery a common site for cannulation in horses. The arterial waveform represents vascular distension in response to ejection of the stroke volume (Dagnall et al, 2021) and provides useful information relating to vascular tone or distensibility, volume depletion and myocardial systolic function. As cardiac output and tissue perfusion are not measured clinically, mean arterial pressure values >70 mmHg should be used as a goal for tissue perfusion (Auckburally et al, 2019).

Non-invasive blood pressure devices have been compared to direct arterial blood pressure monitoring, which is considered the gold standard for measuring arterial blood pressure (Dagnall et al, 2021). Oscillometric non-invasive blood pressure measurement has been found to be more variable (Hatz et al, 2015; Rousseau-Blass et al, 2020) than invasive blood pressure measurement, often overestimating mean arterial pressure (Hatz et al, 2015). Oscillometric measurement of non-invasive blood pressure in one study showed that 21.7% of all measurements failed to obtain a reading (Hatz et al, 2015). Since horses experiencing colic may experience cardiovascular instability and systemic vascular tone derangements, it is recommended that blood pressure is measured directly or invasively where possible. In one retrospective study, 88.3% of horses undergoing colic surgery experienced hypotension (mean arterial pressure <70 mmHg) intraoperatively (Adami et al, 2020).

Pulse oximetry

Pulse oximetry is a non-invasive method of measuring oxygen saturation of arterial blood. In one retrospective study, the prevalence of hypoxaemia in horses undergoing exploratory laparotomy alone was 23.07% (Hovda et al, 2022). Pulse oximetry indicates the presence of pulsatile flow to tissues and displays the pulse rate (Dagnall et al, 2021). The gold standard of arterial oxygenation measurement is arterial blood gas analysis, but real-time continuous monitoring with pulse oximetry and the generation of the waveform or plethysmogram also provides important information which can be very easily and rapidly accessed.

Capnography

This produces a continuous waveform of carbon dioxide partial pressure from the gases in the breathing system sampled close to the patient (Dagnall et al, 2021). Capnography can help the anaesthetist tailor mechanical ventilation to maintain normocapnia, provide assurance for correct endotracheal tube placement and breathing system integrity and displays respiratory frequency.

Blood gas analysis

Blood gas analysis enables the anaesthetist to tailor the ventilatory assistance for the horse.

Acid–base balance

The acid–base balance can be measured intraoperatively at regular intervals. Metabolic acidosis is identified on blood gas analysis by a decreased blood pH (<7.35) and a base deficit (or negative base excess) (Auckburally et al, 2019). One retrospective study demonstrated a significant association between acidosis (pH <7.34) and perioperative mortality in horses undergoing colic surgery (Adami et al, 2020). A variety of acid–base disturbances can exist in horses presenting for, or undergoing colic surgery. In horses diagnosed with intestinal ischaemia, 42% had mixed disturbances including respiratory acidosis and metabolic alkalosis or meta-bolic acidosis and respiratory alkalosis. In horses diagnosed with enteritis, 73% experienced metabolic acidosis with 16% of those also showing respiratory acidosis (Navarro et al, 2005). Abdominal distension may compromise ventilation both preoperatively and intraoperatively. A retrospective study identified that horses presenting for colic surgery had a significantly lower partial pressure of arterial oxygen compared to healthy horses (Viterbo et al, 2023).

Electrolyte concentrations

Electrolyte concentrations should also be managed during anaesthesia, and calcium and potassium should be supplemented as necessary (Auckburally et al, 2019). Hypocalcaemia and hypokalaemia were common electrolyte abnormalities in one retrospective study of horses undergoing colic surgery (Adami et al, 2020).

Packed cell volume and total plasma protein

These can be measured intraoperatively to indicate trends in fluid and osmotic balance. Postoperative total plasma protein <4.5 g/dl or a decrease of >2.2 g/dl perioperatively has been associated with a worse short-term outcome in horses undergoing colic surgery (Spadari et al, 2023).

Fluid therapy

Careful consideration of fluid therapy both in the intraoperative and postoperative phase is warranted. Intraoperative fluid therapy usually involves administration of a balanced isotonic crystalloid solution, which may be supplemented with electrolytes as necessary. Colloids (including natural colloids such as plasma) may be useful to preserve plasma oncotic pressure when hypoproteinaemia is present.

Intraoperative complications

Hypotension

Ideally, mean arterial pressure must be >70 mmHg under general anaesthesia to adequately perfuse not only the vital organs, but the large skeletal muscle groups, to decrease the risk of postanaesthetic myoneuropathy (Boesch, 2013). Many horses anaesthetised for colic surgery are hypotensive, so the use of positive inotropes, vasopressors and fluid therapy is often necessary to support peripheral arterial blood pressure (Dugdale et al, 2020). Mean arterial pressure is influenced by stroke volume, systemic vascular resistance and heart rate, indicating that all these parameters should be taken into account when addressing hypotension. Reduction in anaesthetic depth where possible, fluid therapy and administration of positive inotropes or vasopressors may be useful in the management of hypotension.

Dobutamine is the most commonly used inotropic drug because of its efficacy, titratability and rapid onset of action (Boesch, 2013). Dobutamine can be administered to effect if or once the heart rate is within or near the normal range (Dugdale et al, 2020). Careful administration of noradrenaline or phenylephrine solution to effect alongside fluid therapy can be useful when diastolic pressure is low (<40 mmHg) such that coronary perfusion is compromised and tachycardia accompanies hypovolaemia (Dugdale et al, 2020). It must be considered that some vasopressors may inhibit tissue perfusion. Phenylephrine reduces microvascular intestinal blood flow as a result of the strong vasopressor effect (Dancker et al, 2018). The same study reported that noradrenaline led to no significant changes in local microvascular blood flow (Dancker et al, 2018). In horses undergoing colic surgery with compromised intestinal tissue, it is important to be aware of the potential for reperfusion injury. The re-establishment of perfusion to previously compromised intestine can result in the elaboration of anaerobic metabolism products into the circulation and production of reactive oxygen species (Dugdale et al, 2020) which can cause abrupt hypotension and, in some cases, cardiovascular collapse.

Hypoxaemia

Hypoxaemia is defined as a decrease in the arterial oxygen partial pressure with a consequent decrease in haemoglobin arterial oxygen saturation (Hovda et al, 2022). Clinically this is defined as an arterial oxygen partial pressure of <80 mmHg, or 60 mmHg in severe cases (Hovda et al, 2022). Causes of hypoxaemia are multifactorial, but ventilation perfusion mismatch can contribute significantly and may arise when there is compromised ventilation as a result of compression atelectasis or compromised perfusion because of hypotension. Compression atelectasis is more pronounced in horses positioned in dorsal recumbency because of the cranial displacement of the diaphragm and the gastrointestinal tract (Nyman, 1989). In horses with colic and distended abdominal visci or increased abdominal content volume, the compressive effect on the diaphragm may be even greater. A retrospective study identified that large intestinal lesions were associated with three times the likelihood of hypoxaemia than small intestinal lesions in horses undergoing emergency exploratory laparotomy (Hovda et al, 2022). The same study also reported that horses undergoing emergency laparotomy weighing >550 kg were almost five times more likely to experience hypoxaemia than horses weighing <450 kg.

Manipulation of the abdominal contents may also influence oxygenation. McCoy et al (2011) found that arterial oxygen partial pressure significantly increased in horses undergoing colic surgery after the ascending colon's exteriorisation. Controlled mechanical ventilation can be used to promote improved oxygenation if tidal volume or respiratory frequency are not sufficient to maintain normocapnia. However, it must be considered that controlled mechanical ventilation does not always improve hypoxaemia. Aerosolised salbutamol improves arterial oxygen partial pressure in anaesthetised horses (Robertson and Bailey, 2002). Aerosolised salbutamol administration in anaesthetised horses has been occasionally associated with unwanted cardiovascular side effects (Casoni et al, 2014) and hypokalaemia (Adami et al, 2020).

Hypercapnia

Hypercapnia (arterial carbon dioxide tension of >70 mmHg) is usually the result of hypoventilation in horses undergoing colic surgery. Abdominal distension, dorsal recumbency and anaesthesia may all have a detrimental effect on spontaneous ventilation and ventilation efficiency leading to hypoventilation and hypercapnia. Mild hypercapnia (arterial carbon dioxide tension of 40–70 mmHg) results in increased cardiac output, decreased systemic vascular resistance and improved tissue perfusion. Arterial carbon dioxide tension values of >70 mmHg can have detrimental effects including acidaemia, myocardial depression, central nervous system depression and cardiac arrhythmias (Kerr and McDonell, 2009). Generally, ventilatory support such as controlled mechanical ventilation should be considered when arterial carbon dioxide tension reaches 70 mmHg, or when blood pH is <7.25–7.20 (Kerr and McDonell, 2009). In horses undergoing colic surgery with degrees of metabolic acidosis, the additional effect of hypercapnia on decreasing blood pH may encourage earlier ventilatory support.

Cardiac arrhythmias

The presence of intraoperative tachycardia may reflect cardiovascular compromise, and one retrospective study found an association between intraoperative tachycardia and perioperative mortality in horses undergoing colic surgery (Adami et al, 2020). Ventricular tachycardia, atrial and ventricular premature complexes, atrial fibrillation and accelerated idioventricular rhythm may occur during anaesthesia in horses associated with the administration of anaesthetic drugs (Schwarzwald et al, 2009) and may be more common in horses with cardiovascular instability.

Recovery

Horses subjected to surgical treatment for colic are haemodynamically unstable and unhealthy compared to animals anaesthetised for elective procedures (Santiago-Llorente et al, 2021). Recovery is a critical time for horses after colic surgery and may be made more challenging as a result of muscle fatigue, cardiovascular and electrolyte derangements. A large single centre retrospective study identified several risk factors for poor quality recovery after colic surgery including increasing duration of general anaesthesia and the use of higher intraoperative dosages of ketamine and/or thiopental (Louro et al, 2022).

Another retrospective case series identified that high dobutamine requirements, poor premedication quality and a prolonged time to stand (>70 minutes) are risk factors for unsatisfactory recovery quality in horses undergoing anaesthesia for colic surgery (Santiago-Llorente et al, 2021). Horses requiring higher doses of dobutamine may be less haemodynamically stable and this may be reflected in this study – it found that horses that experienced poor recoveries also presented higher packed cell volume values, high heart rates and blood lactate levels as well as lower mean arterial pressure values (Santiago-Llorente et al, 2021). In order to promote safer recoveries in horses after colic surgery, interventions such as head and tail rope assistance and sedation should be considered.

Sedation

One approach to address the potential for horses to try and stand before regaining normal cognition, is to provide sedation in recovery (Bettschart-Wolfensberger, 2021).

The administration of alpha 2-adrenoreceptor agonists during recovery from equine anaesthesia has a positive effect upon overall recovery quality. Providing recovery sedation not only provides additional analgesia but also prevents premature attempts to rise, the combination of which may improve recovery quality (Santos et al, 2003; Woodhouse et al, 2013). It is important to note that most of the evidence for recovery sedation involves healthy horses, so the effects of alpha 2-adrenoreceptor agonists on the cardiovascular system in systemically compromised horses should be considered. However, one study reported that when horses were not given romifidine in recovery after colic surgery, their recovery quality was unsatisfactory (Santiago-Llorente et al, 2021).

Head and tail rope assistance

Two studies have demonstrated the beneficial effect of head and tail rope assistance on recovery quality in horses after colic surgery (Kock Nicolaisen et al, 2020; Louro et al, 2022). Another study did not find a benefit of using head and tail rope assisted recovery in horses after colic surgery vs unassisted recovery (Rüegg et al, 2016). However, it is likely that further research is needed to accurately assess the influence of head and tail rope assistance on recovery quality. Results of a systematic review of factors influencing recovery quality in horses concluded that further prospective, multi-centre studies involving a larger number of horses are required to assess the influence of recovery assistance in terms of recovery quality and recovery-associated mortality (Loomes and Louro, 2022). Currently, the lack of a recovery scoring system validated for use with head and tail ropes and scarcity of multi-centre studies with large study populations may limit current ability to accurately assess this intervention.

Postoperative care

Postoperative care usually encompasses analgesic provision guided by pain scoring, a fluid therapy regimen and additional therapeutic measures depending on requirements (for example, antimicrobials and prokinetics). When developing a fluid therapy plan, it is essential to consider ongoing losses which occur as a result of the primary gastrointestinal disease in addition to pre-existing dehydration and maintenance needs (Barton and Hassel, 2023). Ongoing diagnostics may be required for monitoring progress such as abdominal ultrasonography to assess intestinal motility, for example.

Postoperative analgesia is of paramount importance in critically ill patients (Boesch, 2013). Non-steroidal anti-inflammatory drugs are commonly used postoperatively. In horses recovering after surgery for small intestinal strangulating obstructions, treatment with firocoxib reduced a biomarker of endotoxaemia when compared with flunixin meglumine, while continuing to provide similar levels of pain control (Ziegler et al, 2019). Another study compared flunixin and meloxicam in horses after surgery for small intestinal strangulating lesions and found that more horses receiving meloxicam showed gross signs of pain than those treated with flunxin (Naylor et al, 2014). Other options for analgesia include infusions of lidocaine, ketamine or α2-agonists. The α2-agonists decrease gastrointestinal motility and should be used judiciously in these patients (Boesch, 2013). Equine pain scales for use after colic surgery should be employed where possible – these include but are not limited to the post-abdominal surgery pain assessment scale in horses (Graubner et al, 2011) and the composite pain scale (Bussières et al, 2008) which has been used to assess visceral pain in horses after colic surgery (van Loon, 2014).

Conclusions

Horses undergoing emergency exploratory laparotomy can present a unique and unpredictable challenge for the equine anaesthetist. Considering the potential implications of systemic physiological instability on the anaesthetic management and analgesic provision for the horse may improve overall patient care. Consideration of the entire perioperative duration is important from provision of pre-anaesthetic analgesia, performance of safe anaesthetic induction, delivery of intraoperative fluid therapy, through to decisions regarding recovery assistance and the promotion of safe recovery. Furthermore, the anaesthetist must be prepared to react to the often rapidly changing physical status of the equine patient undergoing emergency colic surgery, be able to interpret the information from monitoring equipment and intervene where necessary.

KEY POINTS

• The anaesthetic and analgesic care of equine colic patients can be complex and challenging, and requires adequate knowledge of the complications which may be encountered.
• Horses may present with a variety of preoperative clinical examination findings which may significantly impact cardiorespiratory stability.
• Perioperative fluid therapy choices should be made based on an individual patient basis.
• Analgesic provision is a priority in horses suffering from colic and several options can be used to facilitate a balanced approach.