Rachel Hector and Dean Hendrickson Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Science, Colorado State University, 300 West Drake Road, Fort Collins, CO, 80523, USA Due to benefits such as decreased pain, reduced tissue damage, and quicker return to performance, laparoscopic and thoracoscopic procedures are increasing in popularity. In the horse, these may be done under standing sedation or general anesthesia. For standing procedures, stocks that allow access for surgeons while also providing restraint for the horse are highly recommended. For procedures performed in the recumbent horse, the ability of the surgical table to be tilted (e.g. to allow the surgeon access to the caudal abdominal organs) is a key feature. However, the anesthetist must also consider the impact to physiological parameters and accuracy of measurements as the horse is moved. These are in addition to the alterations resulting from insufflation of gas to facilitate surgical visualization. Effects on cardiovascular parameters, oxygenation, ventilation, and intracranial pressure (ICP) are summarized in the following text. In addition to the dose‐dependent effects of inhaled anesthetics on cardiovascular function, changes in positioning (tilt) and abdominal gas insufflation can alter cardiac output and blood pressure in horses. For example, cardiac output and mean arterial blood pressure (typically measured in the facial artery) improve in Trendelenburg position as the tilting angle becomes steeper. This is thought to be related to gravitationally increased venous return from the caudal vena cava and resulting larger stroke volume. There is likely to be some additional effect via the sympathetic nervous system resulting from concurrent increases in arterial carbon dioxide (Hofmeister et al. 2008). Abdominal insufflation alone has also been reported to improve blood pressure in horses undergoing laparoscopy (Donaldson et al. 1998), though it should be noted that this may be a result of increased vasomotor tone rather than cardiac output (Grabowski and Talamini 2009). Pressures in the hind portion of the body have not been well quantitated, and concerns for myopathy and neuropathy with long periods of Trendelenburg remain unknown. In reverse Trendelenburg, arterial blood pressure and cardiac output decrease, and patients may require additional inotropic support to maintain blood pressure. However, rarely are horses anesthetized with such a steep head up angle that this has a clinically significant effect on cardiac output and oxygen delivery to tissues (Binetti et al. 2018; Schauvliege et al. 2018). Hemodynamic changes related to positioning and insufflation are generally transient, and blood pressure and cardiac output return to baseline when horses are returned to horizontal positioning and a normal intra‐abdominal pressure (Binetti et al. 2018; Hofmeister et al. 2008). If the thorax is insufflated during thoracoscopy, it has been shown that as pleural pressure increases cardiac output decreases in dorsally recumbent horses (likely because of compression of venous return to the heart). The decrease in cardiac output is similar across a range of pleural pressures, but the lowest possible pleural pressure is recommended to maintain both cardiovascular and pulmonary function (less than 2 mmHg). As with abdominal de‐sufflation, cardiac output normalizes with pleural de‐sufflation (Bohaychuk‐Preuss et al. 2017). During thoracoscopy, additional considerations include the potential for ventricular arrhythmias (e.g. ventricular premature contractions, ventricular tachycardia) that may be elicited if the heart is directly manipulated (Hardy et al. 1992). These can also be seen in the absence of cardiac manipulation in systemically ill horses (Díaz et al. 2014). Figure 9.1 Horse placed in Trendelenburg to facilitate visualization of caudal abdominal organs during laparoscopy. Source: Courtesy of Dr. Dean Hendrickson. Insufflation of gas into the abdomen facilitates visualization of abdominal organs but negatively affects the ability of the lungs to expand. Additionally, during laparoscopy for caudal abdominal procedures as for example removal of a cryptorchid testicle, horses are often placed in Trendelenburg position causing cranial displacement of the viscera (Figure 9.1). This further decreases lung volumes and compliance and compounds the negative effects of abdominal insufflation on pulmonary function. Abdominally insufflated horses in Trendelenburg have significantly lower arterial oxygen values compared to horizontal positioning. Though oxygenation improves, it does not return to baseline with return to horizontal positioning and subsequent abdominal de‐sufflation (Hofmeister et al. 2008). In one study without abdominal insufflation, when head down horses were subsequently converted to a head up (reverse Trendelenburg) position, the arterial oxygenation remained low compared to horses that had been positioned in the opposite order due to early development of atelectasis. This information also suggests there may be some oxygenation benefit to providing a period of reverse Trendelenburg at the beginning of anesthesia when Trendelenburg position is required later in the procedure (Binetti et al. 2018). Considering the effect on lung volumes and compliance, it is not surprising that Trendelenburg positioning and abdominal insufflation also lead to increases in arterial carbon dioxide concentration, especially if ventilation is pressure‐limited. The use of carbon dioxide as a common surgical insufflation gas also causes hypercapnia because carbon dioxide diffuses readily into the bloodstream (Hofmeister et al. 2008; Binetti et al. 2018). Historically, many gases including air, helium, and nitrous oxide have been used for insufflation. Today, carbon dioxide is almost exclusively used because of reduced risk of fatal complications such as combustion or gas embolism. While gas embolism is still possible with carbon dioxide, its diffusibility in blood mitigates the risk (Neuhaus et al. 2001; Ikechebelu et al. 2005). While not all thoracoscopic procedures require insufflation of gas, the lung does passively collapse and oxygenation is negatively affected. In horses that are insufflated with carbon dioxide, insufflation pressures of 2 mmHg or less appear to preserve acceptable pulmonary function. When higher thoracic insufflation pressures are used (over 5 mmHg), oxygenation is significantly affected and horses are likely to be hypoxemic. Interestingly, subsequent de‐sufflation does not restore baseline pulmonary function (Bohaychuk‐Preuss et al. 2017). A unique consideration for horses undergoing thoracoscopy is the potential need for one lung ventilation (OLV). This is used to provide optimal visualization and surgical access. In human and small animal surgery, OLV is most commonly achieved using a commercially produced double‐lumen endotracheal tube or endobronchial blocking device. Less commonly, a long endotracheal tube can be used to perform a purposeful endobronchial intubation (Mayhew et al. 2012). Commercial devices sized to facilitate OLV are not readily available for horses but techniques to isolate lung fields have been described, including the fabrication of a double‐lumen endotracheal tube (Elliott et al. 1991) or endobronchial blocker. One endobronchial blocking technique slides a 10 mm cuffed endotracheal tube in the lumen of a 26 mm endotracheal tube through a hole placed at the distal end of the larger tube. The “tube in tube” is then guided such that the small tube sits in the desired bronchus, blocking it as the cuff is inflated. Guidance is provided by video endoscopy. This technique, however, requires a tracheostomy because the length of the commercially available tubes is not sufficient to reach the bronchus from the oral cavity (Bauquier et al. 2010). An alternate technique allowing for orotracheal intubation and bronchial blockade is described using a 26 mm endotracheal tube with a long broncho‐alveolar‐lavage catheter (with balloon) inserted in a similar fashion and used as an endobronchial blocker when inflated (Gozalo‐Marcilla et al. 2012). Because of the potential for displacement of the blocker and occlusion of the entire airway at the carina, blockers should be placed, and position confirmed when the horse is appropriately positioned on the surgical table and no longer needs to be moved. Capnography is a useful tool during OLV to ensure the bronchial blocker has not slipped out of place: occlusion of the entire trachea will cause the capnogram to acutely disappear. Should this occur, the bronchial blocker balloon should be deflated immediately, and re‐positioned. Management of OLV has been well described in human and small animal medicine (Mayhew and Friedberg 2008; Mayhew et al. 2012; Schisler and Lohser 2019) but there is limited experience with it in horses. OLV effectively creates a large physiologic shunt, as the collapsed lung is perfused, but not ventilated and extensive venous admixture occurs. Therefore, large alveolar to arterial oxygen tension gradients and hypoxemia are expected. In theory, hypoxic pulmonary vasoconstriction (HPV) occurs to direct more blood flow to the ventilated lung to maintain ventilation‐perfusion matching. However, horses do not have as robust an HPV response compared to other species (MacEachern et al. 2004; Elliott et al. 1991) and inhalant anesthetics also dose dependently blunt this response. Though not studied in horses, total intravenous anesthesia is a beneficial management strategy in humans (Cho et al. 2017). In human medicine, a major concern is balancing the potential for lung injury with strategies used to treat and prevent resultant hypoxemia. Ventilator‐induced lung injury is a possibility in any mechanically ventilated human patient, and OLV predisposes patients to acute lung injury (ALI) (Lohser and Slinger 2015). Protective ventilation strategies using low tidal volumes, low‐level positive end‐expiratory pressure (PEEP), PEEP titration, and permissive hypercapnia are now routine practice, though the pathology of OLV associated ALI is multifactorial (Schisler and Lohser 2019). These strategies have not been investigated in horses. Though often overlooked in equine anesthesia, horses in a head down position in dorsal recumbency have dramatic increases in ICP leading to significant intracranial hypertension. Both inhalant anesthesia and hypercapnia cause cerebral vasodilation and altered autoregulation of cerebral blood flow. This is compounded by hydrostatic pressure changes associated with positioning (Brosnan et al. 2002) and likely abdominal insufflation. Whether this is of clinical importance in horses without risk factors for increased ICP is unknown. However, clinical signs of increased ICP in humans include headaches, nausea, blurred vision, and confusion (Dunn 2002; Leinonen et al. 2018), which have the potential to influence the recovery phase from anesthesia. Additionally, facial and nasal swelling is more likely to develop in a head down position, particularly in horses that are hypoproteinemic or volume overloaded. Maintaining airway patency in recovery becomes an important consideration due to the resulting nasal edema. The patient management team should be prepared to manage this prior to extubation if it occurs. Surgical complications can also occur, especially associated with cannula placement for laparoscopy. These include inadvertent vessel or organ puncture (Figure 9.2). Hemorrhage is rarely a significant problem in the horse, but bowel damage can be critical. Reducing the amount of feed in the colon is beneficial for mitigating this risk: pre‐anesthetic fasting of at least 24 hours is recommended (Hendrickson 2008). Figure 9.2 Inadvertent bowel puncture during placement of the cannula. Source: Courtesy of Dr. Dean Hendrickson. Many procedures can be performed successfully in horses using standing sedation, including ovariectomy and cryptorchidectomy, cystotomy, intestinal and pulmonary biopsies, nephrosplenic space ablation, and diaphragmatic hernia repair (Hendrickson and Wilson 1997; Hanson and Galuppo 1999; Epstein and Parente 2006; Schambourg and Marcoux 2006; Relave et al. 2008; Lund et al. 2013; Gialletti et al. 2018). This section will briefly review sedation and locoregional anesthetic techniques used for standing surgery. Considerations for general anesthesia are provided later in the chapter. Several drugs or drug combinations can be used for sedation, but α2‐adrenoreceptor agonists (α2‐agonists) form the foundation of many protocols. Acepromazine and opioids can be used in conjunction with α2‐agonists to augment sedation or analgesia. Table 9.1 provides a summary of selected information relevant to dosing and attributes of these drugs. Additionally, sample sedation protocols are outlined in Table 9.2. Several excellent sources on the use of different α2‐agonists in horses exist (England and Clarke 1996; Valverde 2010; Gozalo‐Marcilla et al. 2015). The qualities of each α2‐agonist can be used advantageously depending on the chosen sedative technique. For example, a short‐acting drug such as xylazine or dex/medetomidine is ideal for titration by infusion or as a bolus when only a limited amount of additional sedation time is required. Long‐acting drugs such as detomidine and romifidine are well suited for bolus dosing for longer procedures. There is no one protocol that will work for every horse, and combinations of the different α2‐agonists can allow utilization of each drug’s individual benefits. Acepromazine is a long‐acting phenothiazine tranquilizer that typically results in dose‐dependent, mild‐to‐moderate anxiolytic effect when given alone (Ballard et al. 1982; Knych et al. 2018). It has been used with opioids and α2‐agonists as an adjunct to sedation for standing procedures and is thought to be particularly beneficial when given to an undisturbed horse before it is brought into the surgical area. Hypotension can be a significant side effect in a volume depleted animal. Avoiding the use of acepromazine in stallions is advocated by some, but evidence suggests that this complication is likely dose dependent (Ballard et al. 1982) and uncommon (Driessen et al. 2011). Opioids, such as butorphanol, morphine, hydromorphone, or methadone, typically improve the quality of sedation provided by α2‐agonists; when given in the absence of sedatives or tranquilizers to healthy, non‐painful horses, opioids tend to cause central excitement and increased locomotor activity (Clutton 2010). Experimental and clinical studies conflict with respect to the analgesic efficacy of opioids in horses, and controversy exists in part due to the risk of undesirable side effects (Clutton 2010; Figueiredo et al. 2012). These include reducing gastrointestinal motility and fecal retention (Boscan et al. 2006). However, appropriately used clinically relevant doses of opioids do not appear to predispose horses to post‐operative colic (Andersen et al. 2006; Skrzypczak et al. 2020). Sedation can be administered by varying routes but is most commonly given IV in bolus dosing or as an infusion, generally through an IV catheter. Alternate routes for drug administration are also included. Table 9.1 Drugs used for standing sedation in horses and suggested dose ranges. IV = intravenous; IV CRI = intravenous constant rate infusion; IM = intramuscular. Doses should be adjusted to the patient and used in conjunction with appropriate local/regional anesthesia. Table 9.2 Sample sedation protocols for a 500 kg horse undergoing standing surgery. Protocols are author‐provided examples only. Actual sedation should be individualized to the patient and used in conjunction with appropriate local/regional anesthesia. Administration of single boluses of sedative drugs throughout the procedure is a common technique. An IV catheter is not required but is strongly recommended. Most procedures will require some amount of redosing, and a short‐term IV catheter minimizes the trauma and hassle associated with multiple off‐the‐needle injections. This improves both patient compliance and surgical working conditions, in addition to reducing the risk of inadvertent intracarotid injection. While IV bolus dosing is easy, it can create over‐sedation during some parts of the procedure while providing insufficient sedation at other times. A continuous infusion of sedative drugs in a bag of IV fluids provides an alternative and allows for adjustment of the drip rate in accordance with the horse’s level of sedation. This technique is simple, and it does not require specialized equipment. However, if a combination of drugs is used in the bag, one drug cannot be delivered without also administering the other. Administration of individual drugs using syringe or fluid pumps allow for more precise control. Epidural administration of α2‐agonists and opioids can provide analgesia for standing laparoscopic procedures. Some also are readily absorbed from the epidural space (Skarda 1994). For example, detomidine administered epidurally to mares undergoing ovariectomy provided similar surgical conditions to an intravenous detomidine infusion (Virgin et al. 2010). Because of analgesia and/or enhanced systemic sedation, horses may require less total systemic drug when receiving an epidural (Van Hoogmoed and Galuppo 2005). Epidural injections are typically performed at the first intercoccygeal joint (Co1‐Co2) or the sacrococcygeal junction; compared to a lumbosacral epidural injection, it is technically easier to perform at this location and there is no risk of entering the subarachnoid space. For additional information on how to perform an epidural injection, please refer to Michou and Leece 2012. Epidural drugs are diluted to a volume intended to reach the desired sensory innervation for the surgical site. The paralumbar fossa receives sensory innervation from branches of spinal nerves T18, L1 and L2; therefore, a volume of approximately 15–20 ml is required for cranial migration from the intercoccygeal injection site. Slow injection of this volume is advised as administration may be uncomfortable for the horse (Hendrickson et al. 1998; Natalini and Linardi 2006). Because this dermatome is cranial to both the sensory and motor innervation to the hind limbs (L4‐S2) (Singh 2017), local anesthetics should not be used in this scenario. High doses of certain alpha‐2 agonists can also cause hind limb weakness and occasionally recumbency (Wittern et al. 1998) (Figure 9.3). If long‐term epidural analgesia is required, an indwelling epidural catheter (Figure 9.4) can be placed (Robinson and Natalini 2002). Figure 9.3 Recumbency observed following epidural drug administration prior to planned laparoscopic procedure. Source: Courtesy of Dr. Dean Hendrickson. Desensitization of the surgical site with a local anesthetic is frequently needed for compliance and improved analgesia in standing surgery. Lidocaine, mepivacaine, bupivacaine, and ropivacaine are all local anesthetics that are used in used in horses. Additional information specific to their similarities and differences is available elsewhere. (Campoy and Read 2013) An extended duration, liposomal encapsulated bupivacaine formulation (Nocita®) is now available in veterinary medicine. Though it is not licensed for horses, it is becoming popular for a variety of purposes in equine surgery. It is meant to be used by infiltration in the surgical incision and provides local anesthesia of the surgical site for approximately 72 hours in dogs, which could significantly reduce the need for systemic analgesics (e.g. non‐steroidal anti‐inflammatory drugs) post‐operatively. Recommendations for horses are primarily anecdotal but research is emerging (Knych et al. 2019; Griffenhagen et al. 2019; McCracken et al. 2020). Caution should be exercised when administered in the vicinity of motor nerves due to potential risk of prolonged motor paralysis.
9
Anesthetic Management for Laparoscopic and Thoracoscopic Procedures
Considerations for Laparoscopy and Thoracoscopy
Cardiovascular Effects
Respiratory Effects
Other Considerations
Standing Sedation and Local Anesthesia
Sedative Drugs
α2‐Adrenoreceptor Agonists
Acepromazine
Opioids
Intravenous and Adjunctive Administration Techniques
Drug
Suggested doses
Notes
Tranquilizer
Acepromazine
IV: 0.005–0.03 mg/kg
IM: 0.02–0.04 mg/kg
Several minutes time for onset even if given IV.
α2‐agonists
Dexmedetomidine
IV: 0.003–0.005 mg/kg
IV CRI: 0.001–0.008 mg/kg/h
Epidural: 0.005 mg/kg
Short duration of action (<20 minutes): CRI route ideal for longer procedures. Epidural dosing not extensively studied in equids.
Medetomidine
IV: 0.007–0.01 mg/kg
IV CRI: 0.003–0.005 mg/kg
Xylazine
IV: 0.5–1 mg/kg
IV CRI: 0.3–1 mg/kg/h
Epidural: 0.2 mg/kg
Detomidine
IV: 0.005–0.02 mg/kg
IV CRI: 0.005–0.02 mg/kg/h
Epidural: 0.015–0.03 mg/kg
Romifidine
IV: 0.02–0.08 mg/kg
IV CRI: 0.03–0.05 mg/kg/h
Epidural: 0.03–0.06 mg/kg
Less ataxia compared to other α2‐agonists. Reported poor efficacy by epidural route.
Opioids
Butorphanol
IV: 0.01–0.04 mg/kg
IV CRI: 0.01–0.02 mg/kg/h
Nalbuphine
IV: 0.03–0.06 mg/kg
Buprenorphine
IV: 0.003–0.005
Epidural: 0.004
Several hour duration of effect when given systemically. Large doses can lead to opioid‐induced excitement in the absence of long‐lasting sedation. Acepromazine administration may help decrease associated locomotor activity.
Morphine
IV: 0.05–0.2
IV CRI: 0.03–0.1 mg/kg/h
Epidural: 0.1–0.2 mg/kg
Methadone
IV: 0.1–0.2 mg/kg
IV CRI: 0.05 mg/kg/h (Gozalla‐Marcillo et al. 2019)
Epidural: 0.1 mg/kg
Hydromorphone
IV: 0.01–0.04 mg/kg
Epidural: 0.04 mg/kg
CRI dosing has not been established but could be extrapolated from bolus dose data.
Meperidine
IV: 1 mg/kg
Epidural: 0.3–0.8 mg/kg
Initial sedation
Infusion
Top‐up sedation
Protocol 1
Acepromazine 5 mg IV in stall 15 minutes prior to additional sedation.
Detomidine 8 mg IV in surgical area.
Detomidine 2–5 mg/h and dexmedetomidine 1 mg/h on infusion pumps.
If needed, detomidine 1–2 mg IV bolus or dexmedetomidine 0.25–0.5 mg IV bolus if briefer sedation required.
Protocol 2
Xylazine 300 mg and methadone 25 mg IV in surgical area.
800–1600 mg in 0.5–1 l IV fluid dripped to effect.
Example: 0.75 mg/kg/h is 1 drop/second of 1.6 mg/mlxylazine using a 15 drop/mldrip set: rate can be adjusted up or down as needed.
If needed, xylazine 80–160 ml bolus from bag (50–100 mg IV).
Methadone 25 mg IV every two hours.
Protocol 3
Romifidine 30 mg and morphine 25 mg IV in surgical area.
None.
Romifidine 20–30 mg IV and morphine 10–20 mg IV bolus every 45–90 minutes.
Dexmedetomidine 0.25–0.5 mg IV bolus if briefer sedation required.
Protocol 4
Detomidine 5 mg and butorphanol 5 mg IV in surgical area.
10–20 mg in 0.5–1 l IV fluid dripped to effect.
Example: 0.01 mg/kg/h is 1 drop/second of 0.02 mg/ml detomidine using a 15 drop/ml drip set: rate can be adjusted up or down as needed.
If needed, detomidine 50–150 ml bolus from bag (1–3 mg IV).
Dexmedetomidine 0.25–0.5 mg IV bolus if briefer sedation required.
Butorphanol 5 mg IV every 45 minutes.
Intravenous Bolus
Intravenous Infusion
Epidurals
Local Anesthetic Techniques
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