EMETICS

The current presentation include mechanism involved in emesis and pharmacology of different emetics used clinically.
Reference: Essentials of Medical Pharmacology, Sixth Edition, K D Tripathi.

1. EMETICS Anusha Shaji , B.Pharm , M.Pharm Assistant Professor Department of Pharmacology Nirmala College of Pharmacy, Muvattupuzha , Ernakulam

2. Emesis Vomiting occurs due to stimulation of the emetic (vomiting) centre situated in the medulla oblongata. Although nausea and vomiting may occur in a variety of conditions For example: motion sickness, pregnancy, and hepatitis And are always unpleasant for the patient The nausea and vomiting produced by many chemotherapeutic agents demands especially effective management. Nearly 70 to 80 percent of all patients who undergo chemotherapy experience nausea or vomiting.

3. Several factors influence the incidence and severity of chemotherapy-induced emesis (figure 28.9 ) Emesis not only affects the quality of life B ut can also lead to rejection of potentially curative antineoplastic treatment . In addition, uncontrolled vomiting can produce dehydration, profound metabolic imbalances, and nutrient depletion.

4. Mechanisms that trigger vomiting Two brainstem sites have key roles in the vomiting reflex pathway. The chemoreceptor trigger zone (CTZ) is located in the area postrema and the vomiting center CTZ is outside the blood-brain barrier, thus, it can respond directly to chemical stimuli in the blood or cerebrospinal fluid. The vomiting center, which is located in the lateral reticular formation of the medulla, coordinates the motor mechanisms of vomiting. The vomiting center also responds to afferent input from the vestibular system, the periphery (pharynx and gastrointestinal tract), and higher brainstem and cortical structures .

5. Cytotoxic drugs, radiation and other g.i . irritants release 5-HT from enterochromaffin cells → acts on 5-HT3 receptors present on extrinsic primary afferent neurones (PAN) of the enteric nervous system (ENS) ↓ Which connect with vagal and spinal visceral afferents to send impulses to NTS and CTZ. Released in large quantity, 5-HT may also spill into circulation and reach CTZ. It may as well be released from platelets by inflammatory mediators. However, 5-HT is not the only mediator of such signals: many peptides and other messengers are also involved.

6. Mechanisms that trigger vomiting

7. Emetics These are drugs used to evoke vomiting. 1. Act on CTZ : Apomorphine 2. Act reflexly and on CTZ : Ipecacuanha Vomiting needs to be induced only when an undesirable substance (poison) has been ingested. Powdered mustard suspension or strong salts solution may be used in emergency . They act reflexly by irritating the stomach.

8. Apomorphine It is a semisynthetic derivative of morphine Acts as a dopaminergic agonist on the CTZ. Injected i.m ./ s.c . in a dose of 6 mg, it promptly ( within 5 min) induces vomiting . It should not be used if respiration is depressed, because it has inherent respiratory and CNS depressant actions. Oral use of apomorphine is not recommended because the emetic dose is larger , slow to act and rather inconsistent in action. Apomorphine has a therapeutic effect in parkinsonism, but is not used due to side effects.

9. Ipecacuanha The dried root of Cephaelis ipecacuanha contains emetine It is used as syrup ipecac (15–30 ml in adults, 10–15 ml in children, 5 ml in infants) for inducing vomiting. It should be available in every household for emergency use. It is less dependable than parenteral apomorphine and takes 15 min or more for the effect, but is safer. It acts by irritating gastric mucosa as well as through CTZ.

10. All emetics are contraindicated in: (a) Corrosive (acid, alkali) poisoning: risk of perforation and more injury to esophageal mucosa . (b) CNS stimulant drug poisoning: convulsions may be precipitated. (c) Kerosine (petroleum) poisoning: chances of aspiration of the liquid (due to low viscosity) and chemical pneumonia are high . (d) Unconscious patient: may aspirate the vomitus , because laryngeal reflex is likely to be impaired. (e) Morphine or phenothiazine poisoning: emetics are ineffective.