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Reproduction: anesthesia

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Podcast: Reproduction: anesthesia

Introduction

Cesarean section

  • The two most important differences between Caesarean section   Uterus: caesarean section  and elective procedures which must taken into account are the effect of the gravid uterus on the mare and the effects of the anesthetic drugs on the foal.
  • The anesthetic considerations for Caesarean section in the mare are similar to those for horses with obstruction of the gastrointestinal tract   Gastrointestinal: anesthesia  , except that the degree of dehydration is usually less severe.
  • In some cases, the mare may be exhausted and slightly dehydrated, but often she is in good condition prior to induction   Reproduction: parturition - induction  .
  • Physiologic changes associated with pregnancy.
    • Increased sensitivity to inhalation anesthetics.
    • Increased minute ventilation.
    • Decreased functional residual capacity (FRC) in lungs.
    • Increased maternal blood volume.
    • Decreased hematocrit and plasma protein concentration.
    • Increased cardiac output.

Indications

  • Indications for Caesarean section in the mare include bicornual pregnancy or transverse presentation   Reproduction: dystocia  , malpresentation accompanied by injury, contraction or infection, and uterine torsion   Uterus: torsion  .
  • Caesarean section in these cases should be carried out as a primary method of delivery, rather than as a last resort, so the mare and foal are as little stressed as possible prior to induction.
  • When the foal is lying in the mare's pelvic canal, it will rapidly suffer fatal anoxia due to dehiscence of the allantochorion. This occurs within 1-2 h of the beginning of second stage labor. It is therefore a priority to carry out the Caesarean section at the soonest opportunity, even if this involves anesthesia in the field with less than optimum conditions, rather than transporting the mare to a distant referral center.
  • It is a good idea to have a second anesthetic machine or other means of ventilating the foal available so the foal can be intubated and given oxygen   Foal: resuscitation and care  .
  • A competent handler, preferably another veterinarian, should also be available in case the foal requires resuscitation   Foal: resuscitation and care  .
  • Although the foal should be kept as near to the mare as possible, it should not be loose with her until she is on her feet and no longer ataxic.

Pre-operative preparation

  • Premedication   Anesthesia: premedication - overview  is not always necessary for Ceasarean section, especially if the foal is known to be alive.
  • If the foal is dead, more time can be taken to assess the mare and to provide supportive therapy, eg fluid therapy   Fluid therapy: overview  before anesthesia.
  • Pre-operative nasal administration of oxygen may prevent hypoxemia immediately after induction.

Induction and maintenance

  • A rapid intravenous induction protocal is indicated, eg xylazine   Xylazine  /ketamine   Ketamine hydrochloride  or thiopental sodium   Thiopental  (10 mg/kg bodyweight)   Anesthesia: induction - overview 
  • Muscle relaxants may be used if suitable facilities for monitoring   Anesthesia: monitoring - overview  are available.
  • Positioning the mare during surgery is of even more importance than in most anesthetics as the weight of the gravid uterus pressing on the diaphragm and the posterior vena cava in dorsal recumbency can cause life-threatening hypoxia   Anesthesia: monitoring - respiratory management  and hypotension   Anesthesia: monitoring - cardiac output and blood pressure  . This also puts the foal at risk. For this reason, dorsal recumbency, slightly tilted to the left, is preferable to reduce the pressure of the gravid uterus on the caudal vena cava.
  • Because of the effects of the uterus pressing on the diaphragm, it is usually beneficial to use mechanical ventilation   Anesthesia: ventilators - overview  .
  • Hypotension during Caesarean section should be treated aggressively using large volumes of polyionic intravenous fluids and possibly dopamine or dobutamine   Dobutamine  , although the effect of these drugs on uterine vascular resistance in the mare is not known.
  • Small pony mares will have fewer problems than larger breeds.

Longer-acting anesthetic agents such as chloral hydrate are best avoided.

  • Although guaifenesin has been shown to pass across the placenta, the combination of guiafenesin   Guaifenesin  and ketamine   Ketamine hydrochloride  for induction has been used with success in equine Caesarean section. Ketamine results in less hypotension than, for example, thiopental   Thiopental   , although its effect on uterine blood flow in the mare is not known.
  • Inhalation anesthetics, although they do pass to the foal, are readily excreted if the foal breathes well immediately after delivery.
  • Isofluorane   Isoflurane  is the inhalation agent of choice for Caesarean section, because its low solubility means it is excreted more rapidly by the lungs than is halothane   Halothane  . It is also excreted virtually unmetabolized.

Pregnancy reduces the requirement for inhalation anesthetics by 25-40%, so anesthetic depth should be carefully monitored throughout the procedure.

  • The longer the anesthetic procedure, the higher the concentration of anesthetic drugs the foal takes up and the greater the degree of cardiopulmonary depression. It is therefore essential to minimize the surgical time.
  • Neonates   Anesthesia: neonate  have a reduced ability to metabolize and excrete anesthetic drugs, so the level of depression will persist much longer than in the adult.
  • The main priority in maintenance of anesthesia is to keep the fetus well oxygenated. This is best achieved by maximal oxygenation of the mare's blood and by preventing development of hypotension so the uterine blood flow remains adequate.
  • After delivery of a live foal, the umbilical cord should be kept intact until the foal has begun to breathe on its own.
  • Resuscitation   Foal: resuscitation and care  of the foal may be required and a second anesthetic machine or oxygen source, should be available where possible, as well as experienced personnel, to administer oxygen via an endotracheal or nasotracheal tube   Eyelid: tarsorrhaphy - temporary  and to provide suction of the lungs where required.

Recovery

  • The foal   Foal: evaluation - neonate  should be kept as close to the mare as possible during the recovery period, but not allowed to be loose with her until she has successfully regained her feet and recovered from any ataxia.

Castration

Standing castration
  • Standing castration   Testis: castration - overview  is usually carried out in the sedated horse   Anesthesia: standing chemical restraint  .
  • Detomidine   Detomidine hydrochloride  , xylazine or romifidine   Romifidine  , are the sedatives of choice in most cases, but combination with an opioid, especially butorphanol   Butorphanol  , both improves sedation (reducing the likelihood of the horse kicking) and provides analgesia.
  • Local anesthetic, usually 30-35 ml of 2% lidocaine   Lidocaine  , is infiltrated into each testicle at a depth of 3-4 cm, followed by infiltration of the scrotal skin.

Advantages

  • Reduced anesthetic risk of sedation compared with general anesthesia.
  • The procedure can be carried out in less time than for castration under general anesthetic.
  • Less equipment is required, as no anesthetic machine is needed.
  • The procedure can be carried out in relative safety with only one veterinarian in attendance, as long as at least one experienced handler is present.
  • In some cases, where a young horse appears on palpation to have an inguinal testicle, administration of detomidine   Detomidine hydrochloride  , xylazine or romifidine   Romifidine  , may cause relaxation of the cremaster muscle and allow the testicle to descend to an operable position. In these cases, it is important to be prepared to administer a full general anesthetic if necessary, as this does not always occur.

Disadvantages

  • Risk of poor accessibility.
  • Difficulty in correcting problems such as excessive bleeding and herniation.
  • All but the most superficial of inguinal cryptorchid castrations would be impossible to carry out in a standing, sedated patient.
  • There is also a slightly greater risk to the surgeon in standing castration, as an apparently well sedated horse can frequently still kick with great accuracy.

Castration under general anesthetic

Advantages

Disadvantages

  • Increased risk due to full general anesthetic   Anesthesia: general - overview  compared with standing sedation.
  • The time taken is greater than for standing castration.
  • At least two trained personnel are required, as a veterinarian or technician must be in charge of monitoring   Anesthesia: monitoring - overview  the anesthetic and topping up intravenous anesthetic if necessary while the surgeon carries out the surgery.

Cryptorchidectomy

  • Cryptorchidectomy   Testis: cryptorchidectomy - inguinal / parainguinal  is an elective procedure.
  • The patient may be an immature adult, or it may be a mature horse, already showing stallion-like characteristics.
  • Handling of animals showing stallion behavior can be more difficult.
  • The patient should be healthy before induction and a thorough preoperative examination should be given.
  • An estimate of surgery time should be made, taking into account the suspected position of the testicle(s) and the experience of the surgeon.
  • Shorter procedures may be best carried out using intravenous anesthesia for induction and maintenance, especially if muscle relaxants such as guaifenesin are used.
  • Administration of oxygen by endotracheal or nasotracheal tube   Eyelid: tarsorrhaphy - temporary  is always advisable.
  • For longer procedures, anesthesia may be maintained   Anesthesia: maintenance - overview  using an inhalation anesthetic, eg halothane   Halothane  or isofluorane   Isoflurane  . In this case, monitoring   Anesthesia: monitoring - overview  is of great importance.
  • Positioning is important in longer procedures, to prevent the development of post-anesthetic myoneuropathy   Muscle: myopathy - postanesthetic  and hypotension as well as to give optimal surgical access to the abdominal cavity.

Ovarian tumors

  • The granulosa cell tumor   Ovary: neoplasia - granulosa / theca cell  is the most commonly encountered ovarian tumor in the mare.
  • They are usually unilateral and may be found in mares of all ages.
  • Some mares with granulosa cell tumors will show aggressive, stallion-like behavior   Behavior: stallion  , which can make handling difficult.
  • As this is an elective procedure, the mare should be in good condition and healthy prior to being anesthetized.
  • These tumors are often large and difficult to access, especially if the ovarian pedicle is short.
  • Isolation and ligation of the ovarian artery and associated plexus can be difficult, resulting in prolonged surgery time and higher risk of hemorrhage than for other types of elective surgery.
  • Traction on the ovarian pedicle can cause hypotension, so blood pressure   Cardiovascular: blood pressure monitoring  should be carefully monitored and the hypotension treated where necessary using intravenous fluids and positive inotropes, eg dopamine   Dopamine hydrochloride  or dobutamine   Dobutamine  .

Pre-operative preparation

  • Premedication   Anesthesia: premedication - overview  should include a reliable sedative, especially in those mares showing aggressive behavior.
  • Xylazine   Xylazine  or detomidine   Detomidine hydrochloride  combined with butorphanol   Butorphanol  give good premedication.
  • A rectal examination   Urogenital: rectal palpation  to determine the size and location of the tumor prior to induction will help the surgeon to decide which laparotomy approach is best for this patient and give the anesthetist an estimate of the surgical time which will be required. Ultrasound examination may also be helpful.

Induction and maintenance

  • Intravenous induction agents   Anesthesia: induction - overview  can be used in combination, eg guaifenesin   Guaifenesin  and ketamine   Ketamine hydrochloride  , where the guaifenesin gives an element of muscle relaxation.
  • Anesthesia can be maintained   Anesthesia: maintenance - overview  using inhalation anesthetics, eg halothane   Halothane  or isoflurane   Isoflurane  .
  • Monitoring   Anesthesia: monitoring - overview  is of great importance due to the prolonged surgery time often experienced.
  • When muscle relaxants are used, intermittent positive pressure ventilation (IPPV) should always be used and this can lead to even greater development of hypotension, which should be detected and treated promptly and aggressively. However, the use of muscle relaxants does seem to be beneficial in that it reduces surgery time by making ligation easier and also reduces the risk of serious hemorrhage by improving accessibility.
  • One way to reduce the hypotension caused by traction on and ligation of the ovarian pedicle, as well as providing some postoperative analgesia, is to inject local anesthetic, eg 2% lidocaine   Lidocaine  ) into the pedicle prior to ligation.

Recovery

  • Many mares which have undergone surgery for removal of a granulosa cell tumor will appear more dull and depressed after surgery than would be expected from experience of similar operations. This may be due to pain, so use of analgesics   Anesthesia: analgesia - overview  such as non-steroidal anti-inflammatory drugs (NSAIDs), eg flunixin   Flunixin meglumine  is advisable.
  • The mare's vital signs should be carefully monitored for signs of internal hemorrhage.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Lin H C, Wallace S S, Robbins R L, Harrison I W & Thurmon J C (1994) A case report on the use of guaifenesin-ketamine-xylazine for equine dystocia. Cornell Vet 84 (1), 61-66 PubMed.

Other sources of information

  • Muir WW, Hubbell J A, Skarda R T & Bednarski RM (1995) Handbook of Veterinary Anesthesia. 2nd edn. Mosby, USA.
  • Hodgson D S & Dunlop C I (1990) General Anesthesia for Horses with Specific Problems. In: The Vet Clin North Am (3), 625-631.
  • Taylor P M (1992) Anesthesia for Specific Procedures. Lecture notes.
  • Freeman D E & Hinrichs K (1990) Granulosa cell tumor. In: Current Pract Equine Surg. Eds: N AWhite & J N Moore. J B Lippincott Company, USA. pp 711-716.
  • Sloane D E (1990) Caesarean Section. In: Current Pract Equine Surg. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 720-722.
  • Adams S B (1990) Cryptorchidectomy. In: Current Pract Equine Surg. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 722-726.
  • Arthur G H, Noakes D E & Pearson H (1989) Veterinary Reproduction and Obstetrics. 6th edn. Balliere Tindall, UK.
  • Hall L W & Clarke K W (1983) Veterinary Anesthesia. 8th edn. Bailliere Tindall, UK.