For all the data we have on post-operative nausea and vomiting, surgery's most dreaded side effect continues to confound us. To provide you with more evidence-based guidelines, a group of experts and I reviewed the data published since the last PONV guidelines came out in 2003. Here are the highlights of what we found.

Identifying risk factors
Although many factors have been associated with PONV, we found only a few that occur with enough consistency to be considered independent predictors: a history of PONV or motion sickness, female gender, being a non-smoker and use of opioids. We looked at research indicating other predictors, such as migraines, younger age, anxiety and a low ASA risk classification, but the strength of these factors varied study to study.

The type of anesthesia is a risk factor. Volatile anesthetics, nitrous oxide, and intraoperative and post-op opioids are likely to increase incidences of PONV.

The surgery itself may raise the risk of PONV. Each 30 minutes of surgery seems to raise the risk by 60 percent, so if the patient is at a baseline risk of 10 percent, a half-hour procedure could increase this to 16 percent. Laparoscopic procedures, breast procedures and facelifts are linked to increased risks.

The guidelines offer these independent predictors of PONV risk for children, who often lack the verbal skills to describe nausea:

• surgery that lasts 30 minutes or longer;
• being 3 years old or older;
• strabismus surgery; and
• a history of post-op vomiting or PONV in the patient or his relatives.

Quantifying the risk
The chances of post-operative vomiting increases with each factor present. So with one risk factor present, there's a 10 percent chance of PONV, a 30 percent chance with two, a 55 percent chance with three and a 70 percent chance with four.

Ultimately, you should use antiemetic therapy only if the patient has a moderate or high individual risk from many factors to avoid unnecessary adverse effects from the drugs themselves. But the guidelines say more liberal prophylaxis is appropriate for patients who could be harmed further by vomiting (such as those with gastric or esophageal surgery), who strongly prefer to avoid PONV or when the anesthesia provider decides that there is a need for it.

Once you've assessed the risk factors, it's time to see what you can do to reduce the baseline risk. Since volatile anesthetics and nitrous oxide are linked to PONV, you may want to consider using regional anesthesia or propofol throughout the procedure, when feasible, for high-risk patients. Another option is to reduce the amount of morphine or other opioid medications you administer by adding other analgesic adjuncts such as nonsteroidal anti-inflammatory drugs. Keeping the patient adequately hydrated through the procedure can also help reduce the overall risk.

If you take every reasonable measure and the patient is still at a moderate to high risk of PONV, it's time to consider which antiemetic agents are the most useful — and there can be more than one answer. The research on combination therapy shows that this approach may be more effective if you combine agents that work on different receptors. The possible combinations for adults are:

• droperidol and dexamethasone,
• a 5-HT3 receptor antagonist (such as ondansetron) and dexamethasone,
• a 5-HT3 receptor antagonist and droperidol, and
• a 5-HT3 receptor antagonist, dexamethasone and droperidol.

For pediatric patients, the combinations are:

• ondansetron at 0.05mg/kg and dexamethasone at 0.015mg/kg,
• ondansetron at 0.1mg/kg and droperidol at 0.015mg/kg, and
• tropisetron at 0.1mg/kg and dexamethasone at 0.5mg/kg.

It's especially important to keep the dose limits in mind when administering antiemetics to children, so the guidelines include a chart listing the limits for each agent. The existing data shows that 5-HT3 receptor antagonists are the most effective, so they're recommended as the first-line agent, but so far ondansetron is the only one approved for children younger than 2 years old. Another recommended agent for this population, dolasetron, is indicated only for patients 2 years old or older.

The value of prevention
Although some antiemetics are now available in generic forms, there are still concerns about the cost-effectiveness of prophylactic therapy. But our review of the literature found that there is a value for preventing PONV, because patients that suffer PONV will have to stay in your facility until they have recovered. If they're admitted overnight in the hospital that can easily cost $750 to $1,000 a night.

We also found that patients were willing to pay more to prevent PONV. Patients would be willing to pay $100 to have an effective antiemetic that will prevent PONV and parents would spend around $80 to keep their children free from vomiting.