Vomiting

• Has potentially lethal effects in the monogastric animal.

Water Loss

• Fluid loss is evident as:
  1. An increased PCV or haematocrit.
  2. An increased total protein concentration.
  3. A prerenal azotaemia.

Gastric Electrolyte Loss

• The main losses are of H⁺ and Cl^-, and also K⁺
• Can potentially cause metabolic alkalosis, although this is only likely with disease which stops at the pylorus, e.g.: pyloric outflow obstruction.
  • In cases where mild alkalosis occurs, homeostatic mechanisms produce a more alkaline urine to restore normal body pH.
  • However, in severe metablolic alkalosis with marked dehydration, acidic urine may be produced- this is termed paradoxical aciduria.
    • Because vomiting induceses hypokalaemia, there is an overriding stimulus in the kidney for Na⁺ (and therefore water) retention.
    • Na+ can only be resorbed in exchange for H+
      • H⁺ is therefore excreted in the urine, causing it to be acidic.
    • Vomiting also induces hypochloraemia, meaning bicarbonate rather than chloride is resorbed with the Na+ to maintain electrical neutrality
      • This perpetuates the alkalosis.
• Vomiting does not occur in the ruminant although abomasal content may reflux into the forestomachs.
  • Sequestration of secretions in the abomasum will have similar effects to pyloric outflow obstruction with vomiting in the monogastric animal.
    • e.g. abomasal torsion
    • Causes dehydration, hypochloraemia, hypokalaemia and metabolic alkalosis.

• Lesions in the small intestine can also lead to vomiting
  • Both gastric acid and pancreatic and intestinal bicarbonate secretions are lost
    • Animal consequently has a normal pH or may even be acidotic.

The Vomit Reflex

The Vomit Reflex - Copyright nabrown RVC

• Emesis is the process of vomiting

• Persistent vomiting can be exhausting and can lead to metabolic alkalosis, dehydration and electrolyte inbalances which may require fluid therapy

• Extreme cases of persistent vomiting can lead to shock

• Retching involves the abdominal and chest walls contracting

• Vomiting includes retching and the action of the diaphragm

• Diaphragm moves caudal to open the cardia

• Gastrointestinal tract have protective stimuli to recognise harmful products ingested. The mechanoreceptors and chemoreceptors respond using viscerent afferent pathways.

• Medulla co-ordinates process

• Chemoreceptive trigger zone in the 4th ventricle responds to blood and CSF

• Inputs also from inner ear and higher centres

• Emetic agents can be used in cases of gastric obstruction and to remove non-corrosive poisons from the stomach (for corrosive poisons charcoal can be used which will help adsorb the substance and decrease its absorbtion into the GIT)

• For more information on emetic and anti-emetic drugs click here

Emetic agents

• Drugs cause emesis by irritating the gastric mucosa
  • Histamine
  • ACh
  • Dopamine
  • Catecholamines
  • 5-hydroxytryptamine
  • Substance P
  • Enkephalins
  • NK1 receptor agonists

Anti-emetic agents

• Anti-emetic agents can be used to treat motion sickness and to treat or prevent vomiting
  • Dopamine (D2) receptor antagonists
  • 5-hydroxytryptamine antagonists
  • NK1 receptor antagonists
  • Muscarinic receptor antagonists
  • Histamine (H1) receptor antagonists
  • Gastroprotective agents