ISSN 2398-2977      

Anesthesia: general - overview

pequis

Synonym(s): General anesthesia


Introduction

  • Can be defined as a reversible state of unconsciousness, produced by a controlled intoxication of the nervous system.
  • There is a decreased sensitivity to environmental stimuli and reduced motor response to stimuli.
  • Anesthetic agents may be delivered via a variety of routes, but are most commonly administered by intramuscular or intravenous injection and/or by inhalation.
  • Starvation for up to 18 h pre-operatively is routine to reduce the risk of hypoxemia developing due to the effect of a full intestine on the diaphragm of an animal which is recumbent for a prolonged period.
  • A thorough clinical examination should be carried out prior to anesthetic administration; an accurate weight should be determined if possible.
  • Equine anesthesia is frequently carried out in the field, sometimes under adverse conditions.This means that precautions must be taken to protect both the patient and the operatives from environmental effects and from injury, particularly during the induction and recovery phases.
  • Anesthesia in the horse is more complex than in many other species and it is important to prevent anesthetic emergencies and predict potential anesthetic problems with each individual case.

Print off the Owner factsheet All about anesthesia  All about anaesthesia to give to your clients.

The stages of anesthesia

Premedication
  • The aim of premedication Anesthesia: premedication - overview is to provide good patient co-operation, reduced anxiety, smooth induction, provide analgesia and muscle relaxation, without excessive adverse cardiopulmonary effects. Many of the drugs used for premedication also reduce the requirements for intravenous and inhalational anesthetics.
  • A combination of sedatives, tranquilizers and analgesics Anesthesia: analgesia - overview may be used alone or in combination.
Induction
  • Induction Anesthesia: induction - overview is usually by administration of an intravenous agent. Drugs used for induction have a rapid onset and bypass the excitement phases of anesthesia.
  • Historically, foals were given inhalational inductions but this method may be less favorable than intravenous inductions Anesthesia: neonate. However, in very sick foals it is the technique that is often employed, particularly since the introduction of volatile agents with low blood solubilities, eg sevoflurane Sevoflurane.
  • The aim of induction is a safe, smooth descent into unconsciousness with minimum movement by the horse.
  • The placement of an intravenous catheter Intravenous catheterization into the jugular vein will make administration of an intravenous drug easier and safer, especially in a restless patient, and also facilitates the administration of further anesthetics, analgesics and supportive drugs.
Sites for induction

Clinic or purpose-built induction box

  • Padded walls (at least 2 m high) and floor with some method of restraining the horse are the essentials.
  • Other design features will depend on the availability of separate rooms for surgery and for recovery Anesthesia: recovery - overview.

Indoor arena/school

  • Usually a good choice, because the ground is soft and even and there should be no sharp objects, plenty of space and environmental effects are reduced.
  • A disadvantage can be the dustiness of many indoor arenas from which the horse's eyes will need to be protected and can make a poor surgical environment.

Stable

  • Far from ideal.
  • In addition to removing protruding objects such as mangers etc, from the area, hazards such as walls which do not reach high enough or gaps under stable doors must also be noted and dealt with.
  • Clean straw bedding (left long enough for any dust to settle) is usually the best choice.
  • The horse should be stood against a wall with its hindquarters in a corner for induction. The side to be operated on should be closest to the wall and only two handlers should be present, because of the confined space.

Ensure that the stable is strong enough for a horse to recover from anesthetic in it.

Outdoors

  • A sheltered area, protected from the prevailing wind and shaded from bright sunlight in summer should be sought.
  • A layer of straw on the ground should help to insulate the patient from the cold and a clean blanket may be used to cover the animal during the surgery.
  • Care should be taken to clear the surrounding area of any sharp or hard objects and any noise should be kept to a minimum.
  • A strong, secure headcollar and soft, thick, cotton lead-rope will help the handler to control the horse more effectively. Extra handlers are also of use, as they may be able to help control the place where the horse lands and which side it lands on. For example, turning the horse's head to the left just before it begins to go down will encourage the horse into right lateral recumbency.
Maintenance
  • Maintenance Anesthesia: maintenance - overview of anesthesia may be by giving increments or variable rate infusions of intravenous anesthetic drugs, but more commonly by use of gaseous agents.
Hypoxemia
  • This is a major problem in equine anesthesia Anesthesia: monitoring - respiratory management.
  • Pressure of abdominal organs on the diaphragm → decreased lung volume compared with that of the conscious animal. Pre-operative starvation to decrease the volume of gastrointestinal contents is routine.
  • Anesthetic-induced depression of respiration Anesthesia: monitoring - cardiac output and blood pressure → decreased arterial oxygen tension.
  • Obtundation of hypoxic pulmonary vasoconstriction (HPV) which serves to shunt blood away from underventilated areas of the lung in the conscious horse, occurs when volatile anesthetics are administered.
  • HPV is somewhat preserved during IV anesthesia and, as such, oxygen levels may be improved. However, the use of inhaled anesthetics require an anesthetic machine and an oxygen supply, and therefore a higher inspired fraction of oxygen can be provided.
  • Horses undergoing total intravenous anesthesia (TIVA) are often breathing room air.

Monitoring anesthesia

  • This takes on a great importance in equine anesthesia   Anesthesia: monitoring - overview  .
  • Quality of respiration (rate, depth and rhythm) should be continually monitored   Anesthesia: monitoring - respiratory management  .
  • Digital palpation of the pulse should be regularly carried out, note being taken of both the rate and quality   Anesthesia: monitoring - heart  .
  • The capillary refill time and color of mucus membranes can give a good guide to the state of peripheral tissue perfusion.
  • Ocular signs, such as the position of the eye, palpebral reflex, sponteneous blinking, and the presence or absence of nystagmus can all be useful, but some variation is seen with different anesthetic agents and at different stages of the procedure.

The palpebral reflex can fatigue if too frequently tested, eg when the horse is in lateral recumbency the upper eye may have reduced response to stimulation, whereas the lower (unstimulated) eye will respond vigorously.

  • Anal reflex can be monitored, but is somewhat unreliable as an indicator of depth.
  • Response to stimulation can be observed.
  • Blood pressure may be monitored either directly (by catheterizing the facial, transverse facial or greater metatarsal arteries and using an aneroid manometer and pressure veil, or more sophisticated transducer/amplifier/recorder systems), or indirectly (using an inflatable cuff and Doppler ultrasound, or oscillometric monitors, eg Critikon-Dinamap)   Anesthesia: monitoring - cardiac output and blood pressure  .
  • Electrocardiography (ECG)   Cardiovascular: ECG (electrocardiography)  .
  • Blood gas analysis   Blood: gas analysis  .
  • End-tidal carbon dioxide levels (capnography).
  • Pulse oximetry.

Recovery

  • For further information on recovery see the overview Anesthesia: recovery - overview.
  • A well-protected area should be provided, similar to those described under sites for induction.
  • Oxygen supplementation during the recovery period may be of some value, especially in those individuals suffering from nasal edema and partial upper airway obstruction, where a nasotracheal or nasopharyngeal tube may be used to administer oxygen. It can be taped in place for the period of recovery.
  • Alternatively, the endotracheal tube can be secured to the interdental space and left in situ until the horse is standing, and oxygen can be insufflated through it. However, the insufflation of oxygen at the rates possible from flowmeters, fails to match peak inspiratory flow in an adult horse; the benefits are therefore contentious.

Provide a minimum of 15 l/min for an adult horse by insufflation.

  • Recovery from anesthesia can take 20-60 min on average (but this is very dependent on the duration of anesthesia) and can occasionally involve some violence if the animal tries to stand while it is still severely ataxic.
  • The nature of the recovery varies according to the drugs administered, the duration of anesthesia and the temperament and breed of the horse, and many other factors not yet fully understood.

Premedication

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Induction

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Maintenance

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Inhalation anesthesia

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Monitoring

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Herholz C (2010) Clinical application of continuous spirometry during equine anaesthesia and in spontaneous breathing, awake horses. Equine Vet Educ 22 (7), 361-363.
  • Hubbell J A E, Saville W J A & Bednarski R M (2010) The use of sedatives, analgesic and anaesthetic drugs in the horse: An electronic survey of members of the Americal Association of Equine Practitioners (AAEP). Equine Vet J 42 (6), 487-493 PubMed.
  • Moens Y P S (2010) Clinical application of continuous spirometry with a pitot-based flow meter during equine anaesthesia. Equine Vet Educ 22 (7), 354-360.
  • Muir W W, Lerche P & Erichson D (2009) Anaesthetic and cardiorespiratory effects of propofol and 10% for induction and 1% for maintenance of anaesthesia in horses. Equine Vet J 41 (6), 578-585 PubMed.
  • Andersen M A, Clark L, Dyson S J & Newton J R (2006) Risk factors for colic in horses after general anesthesia for MRI or non-abdominal surgery: absence of evidence of effect from peri-anesthetic morphine. Equine Vet J 38 (4), 368-374 PubMed.
  • Marntell S, Nyman G & Hedenstierna G (2005) High inspired oxygen concentrations increase intrapulmonary shunt in anaesthetised horses. Vet Anaesth Analg 32 (6), 338-347 PubMed.
  • Mayerhofer I, Scherzer S, Gabler C & van den Hoven R (2005) Hypothermia in horses induced by general anesthesia and limiting measures. Equine Vet Educ 17 (1), 53-56.
  • Hubbell J A E & Muir W W (2004) Use of the alpha-2 agonists xylazine and detomidine in the perianaesthetic period in the horse. Equine Vet Educ 16 (6), 326-332.
  • Muir W W (1998) Anesthesia and pain management in horses. Equine Vet Educ 10 (6), 335-340.
  • Moens Y P S (1994) The reliability of modern monitoring in veterinary anaesthesia. J Vet Anaesth 21, 94-98.
  • Benson G J & Thurmon J C (1990) Intravenous anesthesia. Vet Clin North Am 6 (3), 519-525 PubMed.
  • Brunson D B (1990) Use of halothane and isoflurane in the horse. Vet Clin North Am 6 (3), 529-540 PubMed.

Other sources of information

  • Holland M (1990) Preanesthetic Medication and Chemical Restraint. In: Current Practice of Equine Surgery. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 59-64.
  • McDonell W N & Dyson D H (1990) Monitoring the Anesthetized Horse. In: Current Practice of Equine Surgery. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 87-93.
  • Pascoe P J (1990) Induction and Recovery Techniques. In: Current Practice of Equine Surgery. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 64-69.
  • Steffey E P (1990) Inhalation Anesthesia. In: Current Practice of Equine Surgery. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 77-83.
  • Trim C M (1990) Intravenous Anesthesia - Induction and Maintenance. In: Current Practice of Equine Surgery. Eds: N A White & J N Moore. J B Lippincott Company, USA. pp 69-77.
  • Hall L W & Clarke K W (1983) Veterinary Anesthesia. 8th edn. Bailliere Tindall, UK.
  • Brander G C, Pugh G M & Bywater R J (1982) Veterinary Applied Pharmacology and Therapeutics. 4th edn. Bailliere Tindall, UK.

Related Images

RELATED CONTENT

Abdomen: laparoscopy

Abdomen: laparotomy

Abdomen: lipoma - pedunculated

Abdomen: pain - adult

Abdomen: penetrating wounds

Abdomen: surgical approaches

Abdomen: ventral midline hernioplasty

Accessory ligament DDFT: desmotomy

Accessory ligament SDFT: desmotomy

Anesthesia: analgesia - overview

Anesthesia: circuits - overview

Anesthesia: epidural

Anesthesia: induction - overview

Anesthesia: inhalational

Anesthesia: intubation

Anesthesia: machines - overview

Anesthesia: maintenance - overview

Anesthesia: monitoring - cardiac output and blood pressure

Anesthesia: monitoring - heart

Anesthesia: monitoring - overview

Anesthesia: monitoring - respiratory management

Anesthesia: neonate

Anesthesia: premedication - overview

Anesthesia: recovery - overview

Annular ligament: constriction

Annular ligament: transection - palmar / plantar

Behavior: headshaking

Bladder: cystoplasty

Bladder: cystorrhaphy

Bladder: cystotomy

Blood: gas analysis

Bone: osteitis - septic

Bone: slab fracture repair - lag screw

Butorphanol

Canker: hypertrophic pododermatitis - chronic

Cardiovascular: balloon-tipped intravascular occlusion

Cardiovascular: blood pressure monitoring

Cardiovascular: ECG (electrocardiography)

Carotid artery: ligation

Cecum: typhlectomy

Cecum: typhlotomy

Cervix: trauma - repair

Colon: resection

Colostomy

Conjunctiva: graft - pedicle flap

Conjunctivitis: foreign body

Cornea / sclera: suturing

Cornea: debridement

Cornea: foreign body

Cornea: superficial keratectomy

Coxofemoral joint: disease - overview

Critical care: monitoring

CSF: collection

CSF: collection - ultrasound-guided antlato-occipital

Cyclosporine

Deep digital flexor tendon: tenotomy

Desflurane

Detomidine hydrochloride

Diazepam

Diprenorphine

Distal phalanx: fracture repair

Epiglottis: entrapment - repair

Epiglottis: persistent frenulum

Esophagus: esophagostomy

Esophagus: esophagotomy

Esophagus: resection and anastomosis

Esophagus: surgical approaches

Ethmoid: hematoma

Exposure keratopathy

Eye: dacryorhinocystography

Eye: dermoid excision

Eye: drug administration 02 - topical

Eyeball: enucleation - transconjunctival

Eyeball: enucleation - transpalpebral

Eyelid: canthotomy

Eyelid: resection

Eyelid: tarsorrhaphy - temporary

Eyelid: trauma

Eyelid: trauma - repair

Farriery: shoe removal

Femur: diaphysis - fracture repair

Femur: physeal fracture

Femur: physeal fracture - repair

Flexor tendon: trauma

Foal: evaluation - neonate

Foot: heel and hoof - avulsion injuries

Foot: hoof wall - crack repair

Foot: keratoma - resection

Foot: wall resection

Gastrointestinal: anesthesia

Gastrointestinal: enterotomy

Gastrointestinal: neoplasia

Guaifenesin

Guttural pouch: surgical approaches

Halothane

Hard / soft palate: cleft - surgery

Head: radiography

Head: trephination

Heart: atrial fibrillation

Heart: atrioventricular block - 2nd degree

Infraorbital nerve: perineural anesthesia

Intravenous catheterization

Iris: prolapse management

Isoflurane

Jejunocolic / ileocolic anastomosis

Jejunum: jejunocecostomy

Jejunum: jejunojejunostomy

Joint: arthroscopy - overview

Joint: septic arthritis - foal

Keratitis: mycotic

Ketamine hydrochloride

Larynx: arytenoidectomy

Larynx: hemiplegia

Larynx: laryngoplasty

Larynx: ventral laryngotomy

Larynx: ventriculocordectomy

Lidocaine

Lung: pleuropneumonia - bacterial (pleuritis)

Magnetic resonance imaging

MC / MT 2 and 4: fracture repair

MC / MT 2 and 4: partial removal

MC / MT 3: condylar fracture repair

MC / MT 4: removal

MC/MT 3: fracture

MCP / MTP joint: arthrodesis

MCP/MTP joint: luxation

Medial patellar ligament: desmotomy

Methoxyflurane

Midazolam

Morphine

Mouth: trauma lip - repair

Musculoskeletal: anesthesia

Musculoskeletal: back pain

Musculoskeletal: external fixation - casts

Myelography

Nasolacrimal duct: nasal puncta opening

Nasolacrimal duct: obstruction - acquired

Navicular bone: suspensory desmotomy

Orbit: fracture repair

Ovary: ovariectomy - laparotomy approach

Palmar carpal retinaculum: section

Palmar digital nerve: neurectomy

Paranasal sinus: bone flap technique

Paranasal sinus: drainage ostia - enlargement

Paranasal sinus: lavage

Pelvis: fracture

Pelvis: radiography

Penis: paralysis / priapism

Penis: phallectomy - Scott's technique

Penis: phallectomy - Vinscot technique

Penis: phallectomy - William technique

Penis: phallopexy

Pentazocine

Pharynx: cyst - resection

Pharynx: ventral pharyngotomy

PIP joint: arthrodesis

Post-castration epididymal sperm extraction

Prepuce: resection

Proximal phalanx: fracture

Proximal sesamoid bone: fracture repair - lag screw

Proximal sesamoid: fracture repair - wire

Radius: fracture repair

Rectum: trauma - management and repair

Reproduction: anesthesia

Reproduction: dystocia

Respiratory: exploratory surgery

Respiratory: neonatal respiratory distress syndrome

Restraint methods

Rib: fracture

Romifidine

Sarcoid

Scapula: fracture

Scapula: fracture repair

Sevoflurane

Sodium chloride

Soft palate: palatoplasty

Soft palate: sternothyrohyoid / omohyoid - myectomy

Soft palate: trimming

Sternum: bone graft collection

Tarsus: arthrodesis

Tarsus: distal tarsal fixation

Tarsus: luxation

Teeth: apical infection

Teeth: extraction

Teeth: repulsion

Temporohyoid joint: osteoarthropathy

Temporomandibular joint: fracture

Testis: castration - overview

Testis: castration - post-operative complications

Testis: cryptorchidectomy - flank

Testis: cryptorchidectomy - inguinal / parainguinal

Testis: cryptorchidectomy - suprapubic paramedian

Testis: cryptorchidectomy - ventral midline approach

Tetanus

Therapeutics: cardiovascular system

Therapeutics: skin

Thiopental

Third eyelid: excision

Third eyelid: flap

Third eyelid: trauma - repair

Thorax: thoracoscopy

Thorax: thoracotomy

Tibia: bone graft collection

Tongue: trauma - repair

Trachea: resection and anastomosis

Trachea: tracheostomy - permanent

Trachea: tracheotomy

Transport

Transvenous cardioversion

Tuber coxae: bone graft collection

Ulna: fracture repair

Ulnaris lateralis: tenotomy - flexural deformity

Umbilicus: hernia

Umbilicus: herniorrhaphy

Ureter: neoureterostomy

Urethra: urethroplasty

Uterus: caesarean section

Wound: healing - second intention

Wound: immobilization

Wound: primary closure

Wound: secondary closure

Xylazine

RELATED FACTSHEETS

All about anaesthesia

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!