Postoperative Nausea and Vomiting

It is estimated that up to 80% of patients experience postoperative nausea and/or vomiting (PONV) within the first 24 h after surgery. For most, PONV is easily manageable but for a smaller, ‘high risk’ cohort, symptoms can be distressing and disabling and, in some cases, have been described as worse than the pain of surgery. It is a common misconception that general anaesthetics are solely responsible for PONV because it is often forgotten that surgical procedures under anaesthesia involve not only a pharmacological but also a physical ‘assault’ on the patient. Fortunately, the multifactorial aetiology of PONV lends itself to interventions using a variety of different treatment options. Prevention, along with prompt and effective treatment of PONV, decreases the risk of adverse effects, limits the length of inpatient stay and reduces hospital costs. More importantly, successful prevention of PONV greatly improves patient satisfaction.

DEFINITIONS

Nausea is derived from the Greek word naus denoting ‘ship’ and was used originally to describe the feeling of seasickness. Nausea is an unpleasant sensation referred to the upper gastrointestinal tract and pharynx. It is associated with dizziness and a strong urge to vomit. Assessment of nausea is extremely difficult because symptoms are subjective, entirely patient-dependent and often difficult to quantify. For this reason, the incidence of postoperative nausea is frequently underestimated. Retching is the involuntary process of ‘unproductive vomiting’. It is characterized by the synchronous contraction of diaphragmatic and abdominal muscles against a closed mouth and glottis. Retching is extremely distressing and is usually accompanied by feelings of intense nausea. Vomiting represents the final common pathway of a highly coordinated sequence of events involving gastrointestinal, abdominal, respiratory and pharyngeal muscles which results in the active and rapid expulsion of contents from the stomach and upper intestine. In contrast to the rather subjective assessment of nausea, vomiting is easily identifiable and measurable!

MECHANISMS OF NAUSEA AND VOMITING

Vomiting Centre

The vomiting reflex probably developed as an evolutionary protective mechanism against ingestion of harmful substances or toxins. However, vomiting also occurs in response to a wide range of pathological and environmental triggers including sight, smell, motion and gastrointestinal disturbances. Afferent signals mediated by the vagus, vestibular and higher cortical nerves are carried to discrete areas within the brainstem collectively known as the ‘vomiting centre’ (Fig. 42.1). Traditionally, the vomiting centre was thought to be a single anatomical entity but there is increasing evidence that it is made up of a disparate group of interconnected cells and nuclei located in the lateral reticular formation of the medulla and the nucleus tractus solitarius (NTS). All information entering the vomiting centre is processed and co-ordinated (via autonomic and motor nerves) into a highly complex series of neuronal signals to initiate the three phases of the vomiting reflex.

FIGURE 42.1 Schematic diagram of the vomiting reflex. See text for details.

Chemoreceptor Trigger Zone (CTZ)

The so-called ‘chemoreceptor trigger zone’ consists of several nuclei found within the area postrema at the caudal end of the fourth ventricle. Although the CTZ is anatomically located within the central nervous system, its unusual pattern of endothelial fenestrations allows it to ‘sense’ chemicals not only within cerebrospinal fluid, but also within circulating peripheral blood. In simple terms, the highly vascularized CTZ utilizes its defective blood brain barrier to detect potentially harmful substances present within the circulation. The CTZ contains an abundance of cholinergic, dopaminergic, histaminergic, serotinergic (5-HT) and opioid receptors which send afferent projections to the vomiting centre. Stimulation of the CTZ contributes significantly to the nausea and vomiting experienced by surgical patients, and pharmacological antagonism of receptor signalling within this important area represents a key strategy in the prevention and treatment of PONV.

Stimulation of the Vomiting Centre

Figure 42.1 outlines the various triggers and neural connections involved in the initiation of the vomiting reflex. The vomiting centre acts as the final processor for all sensory information received from central and peripheral receptors. Although the majority of afferent information from the periphery is relayed through the CTZ, other important causes of emesis are described below.