Emetics

Emetics


 

Vomiting (Emesis)— it is a type of protective reflex.

           The act of emesis is controlled by the vomiting centre in the medulla and close to it is other visceral centres in the medulla oblongata, like the respiration, salivation and vascular control centres.

     

       The vomiting centre does not initiate but it rather co-ordinates the act of emesis and receiving stimuli from various sources, mainly—

1.      The chemoreceptor trigger zone (CTZ) is nearby and is extremely sensitive to the action of drugs and other chemicals.

2.      The vestibular system (when there is loss of equilibrium).

3.      The periphery—distension or irritation of the GIT, Myocardial infarction, biliary or renal stone.

4.      Cortical centre (higher centre)

The vomiting centre and the nucleus tractus solitarius contain many muscarinic, cholinergic and histamine (H₁) receptors and the CTZ is rich in Dopamine receptors.

Drugs that block these receptors are effective anti-emetics.

Classification of anti-emetics—

 Dopamin (D₂) receptor agonist—Domperidone (acts on CTZ and Gut)

                                                           Metoclopromide (acts on CTZ and Gut) 

                                                            Phenothiazines (acts on vomiting centre and CTZ)

5HT₃ receptor antagonist (serotonin receptor antagonist)—

                                                                                               Ondansetron (acts on CTZ and Gut)

                                                                                                 Granisetron

                                                                                                   Tropisetron

Antimuscarinics and Antihistamines— Cyclizine and Promethazine (Antihistamines)

                                                                         Scopolamine and Hyoscine (Antimuscarinics)         

                               —Both act on the vomiting centre and Gut

 

Other agents—   Lorazepam

                              Corticosteroid

                              Cannabinoids

—in case of vomiting due to cytotoxic drugs

Metoclopromide—

    It acts centrally by blocking D2 receptor of CTZ.

    It also acts peripherally by enhancing the action of ACH at muscarinic nerve endings in the gut.

    It raises tone of the of lower oesophageal sphincter, releases pyloric antrum and increases peristalsis to emptying upper gut.

    

Uses—

1. Vomiting associated with GIT disorders.

2. Vomiting associated with cytotoxic drugs.

3.Vomiting associated with radiotherapy.

Adverse effects—

1. Dystonia—extrapyramidal disorder (basal ganglion)

2.Oculogyric crisis

3. Gynaecomastia

4.Lactation (↑ prolactin secretion)

Ondansetron—

    A selective 5HT3 receptor antagonist.

    It is a highly effective anti-emetic for vomiting associated with cytotoxic drugs.

    Anticancer drugs release serotonin from the extrachromaffin cells in the gut mucosa which then activates specific receptors in the gut and the CNS. The action of Ondansetron is partly central and partly peripheral.

 
 

Adverse effects—  Headache

                                 Constipation

                             Flushing of the face

Types of vomiting and the drugs used for them—

1. Vomiting due to cytotoxic drugs—Ondansetron + Dexamethasone (corticosteroid). Given IV and is a good combination.

2.Vomiting after general anaesthesia (to avoid aspiration pneumonia)—Metoclopromide + Ondansetron.

3. In pregnancy—usually no drug in the first trimester, if necessary then Prothiazines like Promethazine with caution of low dose. If there is increased vomiting there may be abortion. This condition is called Hyperemetic Gravidarum.

4. Vomiting due to vertigo—Cyclizine or Prochlorperazine (antihistamine drugs)

5.Motion sickness—Hyoscine group of drugs, Promethazine, Cyclizine. Medicine is taken before the journey.