General and Systematic Veterinary Pharmacology

EMETICS

• Emetics are agents that induce vomition and are classified as
  • centrally acting and
  • reflex acting emetics.
• The main area in the brain responsible for vomiting is the vomiting center. This area is located in the lateral reticular formation of the medulla.
• It receives input from many areas like:
  • Chemoreceptor trigger zone: This is located in the floor of the fourth ventricle and it picks up circulating chemicals/toxins in the blood (e.g. bacterial food poisoning toxins, drugs used for chemotherapy) and cause vomiting.
  • Vestibular apparatus
  • Tractus solitarius - vagal afferents from the gut, heart and testes
  • Direct input from gut (reflex)
• The neurotransmitters involved in the emetic response are
  • On the CTZ: 5HT₃, D₂
  • On the vagal afferents: 5HT₃
  • In the vomiting center: Muscarinic, H₁ receptors

Centrally acting emetics

• This includes:
• Apomorphine is a D₂ agonist that acts on the CTZ to cause vomiting.
  • Apomorphine is contraindicated in cats as it may cause extreme excitement in cat.
  • Apomorphine can be administered subcutaneously or as a lamella (eye tablet) in the conjunctival sac.
  • Since apomorphine depresses the emetic centre, repeated dosing is not recommended when the initial dosing is ieffective.
• Xylazine is used as an emetic in cats.
• Syrup of Ipecacunha is also used as an emetic.
  • It acts both centrally and locally (reflexly) as an emetic.
  • It irritates the gastric mucosa and within 15-30 minutes after administration emesis is observed.
  • It produces a toxic metabolite and hence if emesis does not occur after administering the syrup, it should be removed by administering a gastric lavage.
  • High concentrations of ipecac are cardiotoxic.
  • Animals with ipecac overdose may exhibit arrhythmias, hypotension and myocarditis.
  • Activated charcoal ( a constituent of universal antidote) should not be administered with ipecac as the charcoal adsorbs the syrup of ipecac and prevents it from irritating the gastric mucosa and in turn producing emesis.

Locally acting emetics

• Warm water
• Warm saturated sodium chloride solution
• Sodium chloride crystals placed at the back of the tongue
• Sodium carbonate as crystals
• Copper sulphate 50 ml of a 1% solution
• Zinc sulphate 50 ml as a 1% solution
• Freshly ground mustard paste
• Hydrogen peroxide as a 3% solution

Clinical uses of emetics include

• removal of ingested poison (except when corrosive poisons of petroleum products have been ingested)
• emptying the stomach contents in emergency surgery

• Emetics should not be administered in an unconscious patient, weak and debilitated patient and patients with clinical circumstances like pregnancy, bowel obstructions and hernia.