Physiology

The vomiting center of the brain, in the reticular formation, receives input from the chemotactic trigger zone, gastrointestinal tract, vestibular portion of the eighth cranial nerve, and pharynx. Important neurotransmitter receptor sites documented, or suspected, to be associated with PONV include serotonin, dopamine, histamine, neurokinin-1, opioid, acetylcholine, and muscarinic receptor sites (see Chapter 98).

Risk factors

Identification of independent risk factors for PONV is complicated because of limitations in study designs and the wide array of variables influencing PONV, including patient-related, surgery-related, and anesthesia-related factors. Some risk factors appear to be well established (Box 109-2). Other factors may be associated with PONV, but confirmation of these factors will require additional investigation. Factors that are unlikely to increase the incidence of PONV include obesity, higher intraoperative inspired fraction of O₂ (50%-80% vs. 30%), and early stage of the menstrual cycle (Figure 109-1).

Several risk-scoring systems have been developed; they combine various established risk factors in an attempt to simplify and predict the likelihood of PONV occurring (see Box 109-2). Identification of high-risk patients will allow a more effective and cost-efficient prophylactic treatment program to be established, whereas low-risk patients (i.e., most general surgical patients) would be spared the added expense and possible side effects of treatment.

Treatment

The first step in treating PONV is to prevent it. Some risk factors are fixed (e.g., sex, surgical procedure); however, some factors are variable and may be influenced by the anesthesiologist. A lower incidence of PONV is associated with (1) the use of regional anesthesia as compared with general anesthesia, (2) using propofol for induction or maintenance of anesthesia, (3) avoiding the use of nitrous oxide, (4) using opioid-sparing drugs (e.g., nonsteroidal anti-inflammatory drugs and cyclooxygenase-2 inhibitors), and (5) providing adequate hydration.

The next steps in preventing PONV are to identify high-risk patients and administer an effective prophylactic program to them. Next, any underlying causes of hypotension or cerebral hypoxia should be identified and corrected. For patients who develop PONV, treatment options include nonpharmacologic measures, such as acupuncture, acupressure, and transcutaneous electrical nerve stimulation, and the administration of antiemetic agents, either individually or in combination from the various classes (see Chapter 98).

Conclusion

Further investigation is required to ultimately eliminate the problem of PONV; however, advances have been made to significantly lower the incidence of PONV through preoperatively identifying high-risk patients and providing appropriate treatment.