Charlotte Marquis, BVM&S (Hons.) DACVAA MRCVS
Horses are large, powerful prey animals with an engrained flight response. Ponies are smarter, smaller horses. Both frequently require sedation, general anesthesia and analgesia to facilitate surgical and diagnostic procedures. The risks associated with equine general anesthesia are well established, and horses have the highest perioperative mortality rate of domestic species. The first enquiries into equine perioperative mortality were made over 20 years ago, but the findings of those studies remain relevant today. In 2002, the overall death rate for horses at 7 days post-operatively was cited as 1.9% while the non-colic death rate was cited as 0.9%. It’s important to remember that equine mortality data alone underestimates the impact of nonlethal injuries that occur in recovery, with fractures, myopathies and neuropathies being some of the biggest villains. Many of us are looking forward to updating current practices through the newest phase of equine mortality research, CEPEF4, and I’m hoping these new findings will help us optimize outcomes for both our horses and ponies.
Standing sedation offers a safe but effective alternative to general anesthesia for performing certain procedures in horses and ponies. Advantages of standing sedation compared to general anesthesia in horses and ponies may include preserved cardiopulmonary status (e.g., decreased incidence of right to left shunting, hypoxemia, hypotension), improved muscle perfusion and possible earlier return to function. The advantages of standing sedation in horses and ponies should always be weighed against the potential risks. Inadequate sedation may lead to excitement and reactivity while excessive sedation leads to ataxia or falls. The preparation and planning are paramount to having a boring (i.e., safe, successful) day on the equine floor.
Plan in advance! A pre-sedation physical exam should always be performed prior to the start of the standing sedation. Don’t limit yourself to just a physical exam, remember to “size up the scene” too. What is the horse’s temperament? Is he or she painful? How do the jugulars look for venous access? Performing both a subjective and objective evaluation of your patient will help you make the most of your time and prevent problems before they start. My patient is mean! I’ll have to give it IM sedation before I place an IVC. In the meantime, I’ll set up supplies for my case. Yes, the horse has a chronic hindlimb lameness, I’ll need to be patient and walk it slowly to the stocks. The left jugular looks good, I’ll try that side for my IVC…Preliminary diagnostics like bloodwork and lameness/neurological evaluation, when indicated, evaluation can help identify other potential snags. Get as much information as you can as early as you can.
Plan for the worst! Standing sedation procedures are frequently converted to general anesthesia in the event of equipment failure, inadequate surgical field visualization, lack of patient compliance and fall of the horse/pony. Horses or ponies receiving epidural anesthesia with local anesthetics are especially at risk of falling, and conversion to general anesthesia should always be anticipated in these patients. Have extra sedative & induction drugs calculated and on-hand, always. Know where your ropes, toboggan and endotracheal tubes are. Sometimes, the way we provide care is as important as the type of care we provide. Make sure the room is free of clutter and all surgical & anesthetic equipment is set up prior to getting the patient out. Eliminate excess noise from the surrounding area and make sure the stocks open & close. Grab a head rest, towels, blindfold and earplugs to keep your horse’s head comfortable and quiet. Sedation doesn’t guarantee immobility, and using some basic devices to block out the sound can really add to your standing technique.
Now for the fun part! Place an IV catheter in your patient and wash out their mouth prior to intravenous sedation. If you have to premedicate IM, plan to use 2x as much drug (i.e., 10mg acepromazine and 5mg detomidine becomes 20mg and 10mg respectively). Pick your drugs deliberately. Using drugs from different pharmacologic classes allows more conservative dosing and minimizes individual drug class side effects. Incorporating systemic and locoregional analgesic techniques will provide better analgesia compared to single-drug therapy. Below are some examples of drugs commonly used for standing sedation in horses and ponies.
ACEPROMAZINE:
Acepromazine is a phenothiazine drug that provides light sedation, tranquilization & anxiolysis, but no analgesia. Horses and ponies that receive acepromazine tend to be calmer and less reactive to their surroundings. Acepromazine tends not to be sufficient for sedation in horses when used as a sole agent, but is very useful for decreasing the excitatory & locomotor effects of opioids and reducing a2 agonist requirements. Acepromazine is MAC-sparing, increases arterial blood oxygenation and decreases risk of perioperative mortality in horses when used on its own. Acepromazine has several mechanisms of action as an D2 dopaminergic, 5HT1 & 2 serotonergic, a1 adrenergic, H1 histamine and acetylcholine muscarinic receptor antagonist. A1 receptor blockade leads to splenic engorgement, vasodilation, decreased mean arterial blood pressure and penile prolapse in male horses. Acepromazine may be contraindicated in standing laparoscopic abdominal surgeries as drug-induced splenic enlargement sometimes challenges surgical field visualization. Long-lasting penile prolapse in male horses can lead to paraphimosis, and rarely a permanent loss of reproductive function. However, a 2011 retrospective study and veterinary anesthesiologist opinion poll showed that 80% of surveyed anesthesiologists used acepromazine in stallions. The authors discussed that the risk of permanent penile dysfunction in male horses after receiving acepromazine was extremely low (less than 1 in 10,000 cases), and that more restricted use of acepromazine in stallions versus geldings and mares was not justified. I’ve used acepromazine in many stallions and geldings, and I’d encourage you to try it in healthy patients for longer procedures. That being said, if I had a horse whose value was inextricably linked to his reproductive function (i.e., American Pharoah, Totilas, Big Star) I’d probably limit my use of acepromazine. Some horses are exceptionally quiet and may not need it, too. Just think about your patient population carefully.
A 0.01-0.05 mg/kg IM or IV dose of acepromazine lasts 6-8 hours in horses and ponies. Because acepromazine is not reversible and causes a decrease in blood pressure, it’s best suited for healthy, normovolemic horses. After IV administration of acepromazine, some sedation is usually achieved within 15 minutes and a peak effect occurs at 45 minutes. Peak sedation occurs approximately 45 minutes after IM administration. Acepromazine’s long duration of action makes it beneficial for long procedures because it usually lasts through surgery and recovery. If you choose to use acepromazine in your standing sedation protocol, make sure to give it far enough in advance to reap the benefits.
ALPHA-2 AGONISTS (A-2):
A-2 agonists are the heroes of standing sedation in horses. They provide reversible profound sedation, muscle relaxation, visceral and somatic analgesia, although the sedative effects of A-2s tend to outlast the analgesic effects. A-2s xylazine, detomidine, dexmedetomidine and romifidine are used most frequently clinically. After a single bolus, duration of sedation is longest for romifidine followed by detomidine, dexmedetomidine and xylazine. For standing procedures, sedation is maintained adding your A-2 of choice to a crystalloid fluid bag. The A-2 spiked fluids can be administered at a calculated dose/drip rate, or by titrating the drip rate to effect. Remember that A-2s have a ceiling effect- additional drug doses increase duration but not always depth of sedation. I find I can halve my A-2 CRI drip rate by adding an opioid and acepromazine to my horse’s protocol.
As with any drug, patient selection is so important. Administration of A-2 agonists in horses results in baroreceptor mediated bradycardia, biphasic hypertension followed by hypotension, sweating, hyperglycemia, diuresis, sweating, decreased gastrointestinal motility and increased uterine tone and contractility. A lot of these effects tend not to be dose dependent. A-2s increase urinary flow within 30-60 minutes after administration and the diuresis can be impressive! Have a bucket and shavings on hand for short procedures. Placing a urinary catheter is extremely helpful for longer procedures or where urine splashing would be really frustrating, like lower limb orthopedic surgery. Detomidine is the preferred A-2 for use in pregnant mares because it affects uterine contractility less than xylazine.
EPIDURAL ANESTHESIA & ANALGESIA:
Epidural opioids, local anesthetics, A-2 agonists and ketamine provide robust analgesia for standing surgical procedures in horses and ponies. A single caudal or intercoccygeal injection can be performed, or alternatively an epidural catheter can be placed. Depending on the drug and volume administered, you can provide systemic and/or regional analgesia. Preservative free (PF) morphine is hydrophilic and as a result tends not to be absorbed systemically. Diluting epidural morphine to a high volume enables cranial drug spread, allowing analgesic coverage of the viscera, trunk and front limbs. Epidural lidocaine provides loss of sensation to the tail, all structures under the tail (e.g., perineum, rectum, vulva, vagina, urethra) and bladder. By using a low volume and injection speed for epidural local anesthetics, you can limit cranial drug spread and lower the likelihood of hindlimb ataxia.
LIDOCAINE:
Lidocaine is a Class 1B antiarrhythmic drug that blocks voltage gated Na+ channels. This is likely how lidocaine provides MAC-sparing and analgesia, although the exact mechanisms which it does so have not been elucidated. Intravenous lidocaine in horses provides sedation, visceral analgesia, anti-inflammatory and endotoxemic effects, making lidocaine a popular choice for management of colic pain. Although the in vivo effects of lidocaine in horses have been studied extensively, there is little data regarding lidocaine administration and postoperative colic surgery survival. The CHARIOT study is an ongoing research project investigating the effects of lidocaine infusion on equine intestinal function and survival after colic surgery. The more information we have, the better we can do for our patients!
Both lidocaine and its metabolites can result in some undesirable effects, mainly myoclonus and ataxia. Very high doses can result in seizures. For these reasons, I load lidocaine at 2mg/kg IV over 2-5 minutes. If I put a horse on a 50 mcg/kg/min lidocaine CRI, I’ll halve it after 2-3 hours. If I’m running a horse on lidocaine during a surgical procedure, I’ll stop it at least half an hour before the end of surgery to optimize recovery.
OPIOIDS:
Opioids provide somatic and visceral analgesia by working peripherally and centrally. Opioids work at a variety of receptors (Mu, Kappa, Delta), and depending on their pharmacology, exert various analgesic effects and durations of action. Opioids have a synergistic sedative effect when paired with A-2 agonists and decrease overall A-2 requirements in horses and ponies. The analgesic and gastrointestinal effects of opioids in horses and ponies have been studied extensively, and the use of opioids in these species is a heavily debated topic! Colic after opioid administration is a major fear amongst equine practitioners. Several studies have evaluated the effects of full Mu opioids on the incidence of post-anesthetic colic, and it has been shown that administration of both morphine and hydromorphone in horses do not increase the risk of post-anesthetic colic development. It’s important to remember that many things- fear, anxiety, distress, trailering, showing, fasting, A-2 agonist administration amongst others- slow gastrointestinal motility too. Anytime a scalpel is going to skin, I’d encourage you to use opioids in horses and ponies. Especially when opioids are used in a multimodal analgesic protocol, a little goes a long way. Based on both the literature we have available and my clinical experience, I believe the benefits of opioid use in horses outweigh the risks.
Increased locomotor activity and excitement are other potential side effects of opioid use in horses. I tend to see less excitement when I give a low dose opioid slowly, after I’ve already given an A-2. Full Mu opioids such as morphine tend to reduce fecal weight and moisture while prolonging fecal transit time. Butorphanol can result in some head shaking and twitching. Most opioids, but not methadone, cause some degree of histamine release. This process may lead to hives, urticaria, hypotension and collapse in horses and ponies. When giving full Mu agonists to horses and ponies, I’d recommend giving IV doses slowly over 5 minutes to limit excitement.
KETAMINE:
Ketamine is an NMDA antagonist and dissociative anesthetic agent. This sympathomimetic drug provides anesthesia and analgesia but poor-quality muscle relaxation. Ketamine can be given at sub-anesthetic doses to provide modulation of chronic pain, profound somatic analgesia and additional sedation in cases where horses are refractory to A-2s and opioids. Ketamine also has anti-inflammatory properties and has been shown to down regulate the production of inflammatory cytokines. In my clinical experience, ketamine is really useful for overcoming the ceiling effects of A-2s, and those awful chronic wound and burn cases. A 0.1-0.5 mg/kg IV bolus of ketamine takes about 60 seconds to start working and sedates horses for 15-30 minutes.
Here’s how I usually start a standing procedure. For healthy horses getting painful surgery, I’ll do an acepromazine +/- A-2 premed in the horse’s box 40 minuteS before the procedure starts. I’ll lead the horse over to the stocks, give an A-2 bolus and start the A-2 CRI. After the horse is sufficiently sedate, I’ll give an opioid bolus and start that CRI. While I work on that, I ask someone to clip, scrub and block the caudal epidural site plus limbs if we’re doing lower limb orthopedic surgery. I usually put lidocaine, xylazine and morphine in caudal epidurals for surgical procedures. These are just a couple of drug combinations you can try!
Individual drugs for standing sedation in horses and ponies
DRUG | CLASS | DOSE | COMMENTS |
Acepromazine | Phenothiazine | 0.01-0.05 mg/kg IV | I give 10-15mg IV to most 450kg horses. |
Detomidine | A-2 | Bolus: 5-10 mcg/kg IV CRI: 10-40 mcg/kg/hr Make it: 25mg detomidine into 500ml saline bag. 1dr/1s on a 10dr/ml set provides an 80 min infusion at 40mcg/kg/hr for a 450kg horse | I give 3-5mg IV to most 450kg horses. Can cut rate in half when using opioid bolus/CRI. |
Dexmedetomidine | A-2 | Bolus: 2-5 mcg/kg IV CRI: 5mcg/kg/hr Make it: 3.5 mg dexmed in 500ml saline bag. 1dr/1s on a 10dr/ml set provides an 80 min infusion at 5mcg/kg/hr | See detomidine. |
Romifidine | A-2 | Bolus: 0.1mg/kg IV CRI: 30 mcg/kg/hr Make it: 20mg romif in 500ml saline bag. 1dr/1s on a 10dr/ml set provides an 80-min infusion at 0.03mg/kg/hr for a 450kg horse | See detomidine. |
Xylazine | A-2 | Bolus: 0.5-1 mg/kg IV CRI: 0.5-0.65 mg/kg/hr Make it: 400mg xylazine into 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-minute infusion at 0.65mg/kg/hr for a 450kg horse | See detomidine. |
Butorphanol | Opioid- K agonist, Mu antagonist | Bolus: 0.02-0.1mg/kg IV CRI: 0.02-0.05mg/kg/hr Make it: 15mg butorphanol into 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-minute infusion at 0.02mg/kg/hr for a 450kg horse | Same deal as the A-2 CRIs. |
Hydromorphone | Opioid- Full Mu agonist, NMDA antagonist | Bolus: 0.15mg/kg IV CRI: 0.05mg/kg/hr Make it: 30mg methadone into 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-min infusion at 0.05mg/kg/hr. | Same deal as the A-2 CRIs. |
Methadone | Opioid- Full Mu agonist, NMDA antagonist | Bolus: 0.15mg/kg IV CRI: 0.05mg/kg/hr Make it: 30mg methadone into 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-min infusion at 0.05mg/kg/hr. | Same deal as the A-2 CRIs. |
Morphine | Opioid- Full Mu agonist, K agonist | Bolus: 0.05-0.1mg/kg IV CRI: 0.03-0.05mg/kg/hr Make it: 30mg morphine into 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-min infusion at 0.05mg/kg/hr. | Same deal as the A-2 CRIs. |
Lidocaine | Local anesthetic | Bolus: 1.3-2 mg/kg IV CRI: 25-50 mcg/kg/min Make it: Deliver undiluted on a syringe pump, or mix 2000mg lidocaine in 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-minute infusion at 0.05mg/kg/hr. | Halve rate 2 hours in and discontinue 30 minutes from the end of surgery to avoid ataxia/myoclonus in recovery. |
Ketamine | Dissociative anesthetic- NMDA antagonist | Bolus: 0.1-0.2 mg/kg IV CRI: 0.3-0.6mg/kg/hr Make it: 200mg ketamine into 500ml saline. 1dr/1s on a 10dr/ml set provides an 80-minute infusion at 0.3mg/kg/hr | |
Epidural drugs & drug combinations for standing procedures in horses and ponies
DRUG | CLASS | DOSE OR TOTAL VOLUME FOR A 450KG HORSE | COMMENTS |
Lidocaine | Short acting local anesthetic | 0.2 mg/kg | Lasts 1hr. Limit volume to <5mls to avoid hindlimb ataxia. |
Mepivicaine | Longer acting local anesthetic | 0.2 mg/kg | Lasts 2-3 hrs. See lidocaine. |
Bupivicaine | Longer acting local anesthetic | 0.05 mg/kg | Lasts 6-8 hrs. See lidocaine. |
Xylazine | A-2 | Bolus: 0.1mg/kg IV 0.1mg/kg Q/S to 10ml w/ saline | Some systemic absorption will occur and result in sedation. |
Detomidine | A-2 | 0.02mg/kg Q/S to 10ml w/ saline | See xylazine. |
Morphine | Full Mu agonist, K agonist | 0.1mg/kg Q/S to 20ml w/ saline | Q/S to 60ml w/ saline to get coverage of the front limbs. |
Ketamine | Dissociative anesthetic, NMDA antagonist | 1mg/kg Q/S to 20ml w/ saline | See xylazine. |
Lidocaine + Xylazine | As before | See lidocaine. | Limit combined drug volume to 10mls to avoid hindlimb ataxia. |
Lidocaine + Morphine | As before | See lidocaine. | See lidocaine + xylazine. |
Bupivicaine + Morphine | As before | See lidocaine. | See lidocaine + xylazine. |
Xylazine + Morphine | As before | See xylazine, morphine. | See xylazine, morphine. |
Detomidine + Morphine | As before | See detomidine, morphine. | See detomidine, morphine. |
Relevant citations:
Anesthesia & analgesia for standing equine surgery:
Pharmacokinetics and pharmacodynamics of hydromorphone in healthy horses:
How to maximize standing chemical restraint:
Opioid analgesia in horses (Clutton):
Confidential enquiry into perioperative equine fatalities: CEPEF 4 (Bettschart, Johnston):
The confidential enquiry into perioperative equine fatalities (CEPEF): mortality result of Phase 1 and 2 (Johnston, Taylor, Eastment, Wood):
How to place an epidural catheter easily and maximize its clinical effects (Hustace, Schlipf):
Contemporary use of acepromazine in the anaesthetic management of male horses and ponies: retrospective study and opinion poll (Driessen, Zarucco, Kalir, Bertolotti):
CHARIOT Lidocaine study- Improving outcomes in colic horses: a randomised controlled trial of lidocaine (Archer):
A retrospective evaluation of the effect of perianesthetic hydromorphone administration on the incidence of postanesthetic signs of colic in horses (Reed, Skrzypczak, Barletta, Quandt, Sakai):