Postoperative Nausea and Vomiting
MECHANISMS OF NAUSEA AND VOMITING
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.
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.
ADVERSE EFFECTS
Some clinicians dismiss PONV as an inevitable consequence of surgery but from the patients perspective PONV is almost always associated with distress and dissatisfaction. Importantly, if nausea or vomiting persist there is a risk of rare, but potentially serious, perioperative morbidity. These complications are summarized in Table 42.1.
TABLE 42.1
Patient distress and dissatisfaction
Pulmonary aspiration
Postoperative pain
Wound dehiscence/haemorrhage
Eyes
Head and neck
Oesophageal
Abdominal
Dehydration, electrolyte disturbance and/or requirement for intravenous fluids
Delayed oral input
Drugs
Nutrition
Fluids
Delayed mobilization
Delayed discharge