Antiemetics and related drugs

Antiemetics and related drugs

Introduction

Nausea and vomiting have many causes including drugs, motion sickness, fear, pregnancy, vestibular disease, and migraine. In previous decades anesthesia was almost synonymous with vomiting, but with the advent of new anesthetic agents and more aggressive treatment, the incidence of vomiting has decreased. However, even the latest agents have failed to eradicate this troublesome symptom encountered in the peri-operative period.

Physiology

The vomiting center (VC) coordinates vomiting. It has no discrete anatomical site but may be considered as a collection of effector neurons situated in the medulla. This collection projects to the vagus and phrenic nerves and also to the spinal motor neurons supplying the abdominal muscles, which when acting together bring about the vomiting reflex.

The VC has important input from the chemoreceptor trigger zone (CTZ), which lies in the area postrema on the floor of the fourth ventricle but is functionally outside the blood-brain barrier. The CTZ is rich in dopamine (D2) receptors and also serotonin (5-HT) receptors. Acetylcholine (ACh) is important in neural transmission from the vestibular apparatus.

Another input is summarized in Figure 18.1. The treatment of nausea and vomiting is aimed at reducing the afferent supply to the VC. While the administration of antiemetics forms a vital part of treatment, attention should also be given to minimizing the administration of opioids by the use of non-steroidal anti-inflammatory drugs and avoiding unnecessary anticholinesterase administration.

When propofol is used to maintain anesthesia for minor surgery, where the use of opioids is limited, it may reduce the incidence of postoperative nausea and vomiting (PONV).

The following types of agents have been used:

  •  Dopamine antagonists
  •  Anticholinergics
  •  Antihistamines
  •   5-HT3 antagonists
  •   Miscellaneous

Phenothiazines

Phenothiazines are the main group of antipsychotic drugs (neuroleptics) and have only a limited role in the treatment of vomiting. They are divided into three groups on the basis of structure, which confers typical pharmacological characteristics

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Chlorpromazine

Chlorpromazine is used in schizophrenia for its sedative properties and to correct altered thought. Its effects on central neural pathways are complicated but are thought to involve isolating the reticular activating system from its afferent connections. This results in sedation, disregard of external stimuli, and a reduction in motor activity (neuroleptic). It is sometimes used to control vomiting or pain in terminal care where other agents have been unsuccessful. It has also been shown to be effective in preventing PONV. It is occasionally used to treat hiccup

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