General considerations for anaesthesia in animals
General considerations for anaesthesia in animals should be carried out for successful recovery of the animal after surgery.
Anaesthetic drugs depress the respirations, cardio vascular and nervous system at therapeutic doses. During anaesthesia, the patients oxygenation, ventilation, circulation and temperature shall be continually evaluated.
Oxygenation(arterial O2 saturation SaO2SPO2) is measured employed a pulse oximeter.
Normal SPO2 is greater than 90%- 95%.
Ventilation status is assessed by capnometry, that is measurement of end tidal co2.
Normal end tidal co2 is 35-45mmHg.
Hypoxia may develop during anaesthesia due to apnea and airway obstruction during induction.
Brachycephalic breeds(pug)are prone to airway obstruction due to the redundant (unnecessary)pharyngeal tissue they have.
Endotracheal intubations prevents aspiration facilitates 02 and anaesthetic administration and controls ventilation.
Airway implement involves the nasal passage and hence oral pathway must be kept open during anaesthetic if Endotracheal intubations is not performed in dogs.(horses are obligate nasal breathers).
Anticholinergics and dissociative anaesthetics cause tachycardia.
Atropine is effective in controlling salivation.
Atropine can be used in the management of bradycardia caused by halothane, narcotics and neuroleptanalgesics.
Glycopyrolate do not cross blood brain barrier unlike atropine.
Elimination of anaesthetics depends upon metabolic rate.
Small animals have higher BMR/unit surface area and require larger dose per unit of body weight.
Animals with large quantity of fat required less anaesthetic than lean muscular animals in good condition.
New born animals have low BMR require lesser anaesthetics. Narcotics have minimal effect on myocardium.
Acepromazine produce minimal direct myocardial depression but decreases peripheral vascular existence causing arterial hypotension.
Acepromazine produces minimal respiratory depression.
Alpha 2 agonists produce dysarrhythmics, that is sinus bradycardia, SA and AV nodal block and are avoided in patients with impaired cardiac output. Ketamine, tiletamine and nitrous oxide cause catecholamine release.
Halothane increases the sensitization of myocardium to catecholamine. Barbiturates cause spleenic congestion.
Xylazine inhibits small intestinal and caecal motility in horses and ponies for upto two hours after a dose of 1.1 mg/kg body weight.
Nitrous oxide should not be used in patients with pneumothorax. Acepromazine has antiemitic antiarrhythmic and antihistamine property.
Acepromazine is avoided in animals that are to allergic testing, it lowers the seizure threshold and should not be used in patients with hepatic encephalitis.
Acepromazine alters the metabolism of procaine and succinylcholine.
Acepromazine may cause paraphimosis in stallions and may be avoided.
Opioides have little or no effects on liver. Barbiturates are used with caution in patients with significant hepatic diseases.
Ketamine is metabolites in liver of dog, whereas it is excreted unchanged form in urine of cats.
Diazepam is highly protein bound and is used with caution in hyperglobulinemic patients. It absorbs to plastics and other photosensitive.
Diazepam is synergistic with thiobarbiturates, Opioides and possibly alpha agonists. Midazolam is water soluble, more potent than diazepam but has shorter duration of action.
Elimination of injectable anaesthetics depends upon redistribution within the tissues, detoxification principally in the liver and excretion via the kidneys.
Morphine is eliminated via the G.I. tract.
Haemorrhage during surgery, significantly increase the sleeping time.
Animals show a period of excitement during induction always require more anaesthetic, hence pre anaesthetics sedation is advised.
Dissociative anaesthetics do not induce ocular sign of increase CNS depression.
Higher doses of general anaesthesia is to be avoided in patients with renal failure. Maintenance of potent airways via Endotracheal intubations is mandatory in brachycephalic breeds of dogs during anaesthesia.
Emergence struggling is usually more of a problem in equine anaesthesia.
Ruminants have a different temperament than horse and seldom make premature attempts to rise after anaesthesia.
Decompensated heart disease is a contraindication for general anaesthesia. Mild state of diuersis should be established with i/v fluids in hephaitis patients prior to the administration of anaesthetic drugs.
Opioides are incorporated in the anaesthetic protocol of ear surgeries as they are painful.
Hypotension and seizures are the contraindications for acepromazine.
Dobutamine or dopamine(2-10mg/kg.i/v )may be given during surgery for ionotrophic support. Hyperventilation may cause a significant decrease in portal blood flow. Halothane and isoflurane decreases the portal blood flow. Halothane may cause post anaesthetic hepatic dysfunction(halothane hepatitis).