PONV prophylaxis strategies vary widely, in part because PONV is a complex and sometimes unpredictable event. There are also numerous 'treatment' options, which further complicates decision-making. Still, researchers have produced some solid evidence that can guide practitioners in making sound decisions when it comes to PONV prophylaxis. Essentially, there are three steps to ensuring the most clinically and cost-effective approach: Target at-risk patients, reduce baseline risk and prescribe the most appropriate antiemetic therapy for the clinical need.

Step One:
Target At-Risk Patients
Outpatient practitioners often err on the side of prevention, and for good reason. PONV can cost outpatient facilities in both dollars and patient satisfaction scores. In office surgical suites in particular, where nursing labor costs are not as fixed as they are in the inpatient environment, a prolonged PACU stay can affect the bottom line. Still, research clearly shows that an "across-the-board" approach to PONV prevention results in overprescribing of prophylactic antiemetics - a practice that is neither clinically nor cost-effective. Antiemetics can cause side effects that hinder recovery.

The first step, then, in defining a PONV prophylaxis strategy is to determine which patients are most likely to develop PONV. This is not always easy because there are numerous considerations - including patient, surgical and anesthetic factors, each which confers a different degree of risk.

Current research shows, however, that the following factors place patients at high risk for PONV: History of PONV and/or motion sickness, preoperative nausea and vomiting, young female, ENT procedures, laparoscopy procedures, major breast surgeries, plastic surgeries, shoulder procedures and strabismus surgeries. Other factors that tend to confer a lesser risk but deserve consideration include: Age less than 50 years, ASA status, nonsmoker, dehydration, anxiety, obesity, pain, gynecologic procedures, intra-abdominal procedures, oral surgery, procedures lasting more than one hour and intra- and post-op opioid use.

Step Two:
Reduce Baseline Risk
For all patients with a high risk, the practitioner can take steps during surgery to minimize PONV. These include:

• Reduce the opioid requirement. NSAIDs can help eliminate or reduce the need for opioids, which are known to cause PONV. In one evaluation of children undergoing strabismus surgery, for example, patients who received rectally administered diclofenac had less PONV, spent less time in the PACU, and had similar pain scores as children who received IV morphine.1 For more painful procedures, like orthopedic and intra-abdominal surgeries, NSAIDs can work synergistically with opioids and thereby reduce the opioid requirement. In one study of knee arthroscopy patients, for example, morphine 3 mg plus ketorolac 30 mg provided significantly better analgesia than morphine alone.2 For such painful procedures, it is important to balance the need for an opioid with the PONV risk, because pain in and of itself can induce PONV. In some cases, optimal analgesia may include an opioid. In one study of abdominal surgery patients, nearly two-thirds experienced nausea relief when their pain was relieved with a postop injection of morphine or ketobemidone.³
• Consider propofol anesthesia. Studies suggest that propofol is an antiemetic, and a propofol-based total intravenous anesthesia (TIVA) regimen can alleviate PONV in high-risk patients. When used as an induction agent only, propofol is not as protective, probably because of its short duration of action. A propofol-based TIVA regimen also eliminates the added risk of early PONV associated with volatile inhalational agents, and may even help offset the emetic potential of short-acting opioids. In a recent study of 100 arthroscopy patients who received a continuous propofol infusion along with remifentanil or alfentanil and 100 percent oxygen, just one patient required postop antiemetic therapy.⁴
• Avoid other emetogenic agents. Although controversial, most studies that isolated the effect of N2O from that of other anesthetic agents show that eliminating N2O from the anesthesia regimen decreases PONV in high-risk patients. Practitioners should also try to avoid the need for higher doses of neuromuscular blockade antagonists (like neostigmine >2.5 mg), which can promote PONV, by using shorter-acting muscle relaxants and using them only when truly needed. Practitioners should also avoid the sedative etonidate, which also causes PONV in high-risk patients.
• Ensure adequate hydration. Since dehydration can contribute to PONV, it makes sense to ensure adequate hydration. Current data suggest that it is reasonable to administer approximately 1 liter of fluid pre- or intraoperatively in patients who have fasted for minor surgical procedures.

Step Three:
Select Appropriate Antiemetic Therapies
For high-risk patients and for patients in whom vomiting may endanger health, prophylaxis therapy is warranted. Current therapies include pharmacotherapy and acupuncture.

• Pharmacotherapy. When selecting antiemetic agents, the guiding principle is to use the most cost-effective agent(s) with the least side effects, and to base the number of agents on the level of risk. For adults with a high-risk factor, a prophylactic antiemetic agent is warranted. For adults with several high-risk factors, a combination of antiemetics that acts on different receptors is appropriate.

In the outpatient setting, a 5-HT3 (serotonin) receptor antagonist (ondansetron, granisetron, or dolasetron) is often the cornerstone of the prophylactic drug regimen. These agents are minimally sedative, have relatively few side effects (headache and constipation are most common), and have a fast onset of action (15 to 20 minutes) when administered intravenously. Although they have different pharmacokinetic properties, chemical structures, effective doses and potencies, the three 5-HT3 receptor antagonists appear to perform very similarly for preventing PONV. A recent literature review showed that, when used prophylactically, 5-HT3 antagonists reduced the incidence of PONV by 46 percent.⁵ Given their similarities, drug cost is an important factor when determining which 5-HT3 antagonist to use.

For high-risk adults who require multimodal drug prophylaxis, single doses of dexamethasone 4 to 8 mg or droperidol >1 mg work well as adjunctive agents. Research indicates that these agents, when used with a 5-HT3 receptor antagonist, can produce response rates of 90 percent or more. Given the recent concern that droperidol may be associated with extra-pyramidal side effects, however, I reserve this agent for PONV treatment when preventive measures fail. It is also wise to limit dexamethasone use to a single, minimally effective dose to avoid the side effects associated with longer-term use (wound healing problems, problems with glucose control, avascular necrosis of the femoral head).

Two other agents - cyclizine (an antihistamine that is not available in the United States) and metoclopramide (a dopamine antagonist in the same class as droperidol) - are effective for PONV prophylaxis. Research suggests that cyclizine 50 mg IV is as effective as ondansetron 4 mg IV for PONV prevention, is significantly less expensive, and may act synergistically with the 5-HT3 receptor antagonists to inhibit PONV better than 5-HT3 receptor antagonist monotherapy. However, it is associated with drowsiness, sedation, blurred vision, and dry mouth - all of which are important considerations in the outpatient setting. Other antihistamines available in the United States include dimenhydrinate and diphehydramine There appears to be little use for metoclopramide, as studies generally indicate that it is ineffective for PONV prophylaxis.

• Acupuncture. There is mounting evidence to suggest that acupuncture can help prevent PONV in high-risk patients and may act synergistically with 5-HT3 antagonists. Typically, practitioners place a band containing small, hard 'seeds' (acupressure) or an electrical stimulation device around the wrist to place pressure on the pericardium 6 (P6) point on the inner wrist, approximately 20 to 30 minutes before induction of anesthesia. This technique has reduced PONV in laparoscopy and endoscopic sinus surgery patients. In one study of plastic surgery patients, the ReliefBand prevented PONV as effectively as ondansetron 4 mg and, when used with ondansetron, further reduced postoperative nausea and vomiting and the need for rescue therapy.6 We are not yet sure if the technique is as effective in children.

The importance of prevention
About 30 percent of surgical patients experience PONV, and patients say they are more compromised by PONV than by postoperative pain. Smart prophylaxis can be both cost- and clinically effective, leading to complete symptom control in over 95 percent of high-risk patients.

The key to success is defining which patients are at risk, reducing their baseline risk, and prescribing effective and safe preventive therapies.

References
1. Wennstrom B, Reinsfelt B. Rectally administered diclofenac (Voltaren) reduces vomiting compared with opioid (morphine) after strabismus surgery in children. Acta Anaesthesiol Scand. 2002;46(4):430-4.
2. Gupta A, Axelsson K, Allvin R, et al. Postoperative pain following knee arthroscopy: The effects of intra-articular ketorolac and/or morphine. Reg Anesth Pain Med. 1999;24(3):225-30.
3. Andersen R, Krohg K. Pain as a major cause of postoperative nausea. Can Anaesth Soc J. 1976;23:366-69.
4. Dershwitz M, Michalowski P, Chang Y, Rosow CE, Conlay LA. Postoperative nausea and vomiting after total intravenous anesthesia with propofol and remifentanil or alfentanil: How important is the opioid? J Clin Anesth. 2002;14(4):275-8.
5. Loewen PS, Marra CA, Zed PJ. 5-HT3 receptor antagonists vs traditional agents for the prophylaxis of postoperative nausea and vomiting. Can J Anaesth. 2000;47(10):1008-18.
6. White PF, Issioui T, Hu J, et al. Comparative efficacy of acustimulation (ReliefBand) versus ondansetron (Zofran) in combination with droperidol for preventing nausea and vomiting. Anesthesiology. 2002;97(5):1075-81.