Post-op Nausea and Vomiting Guidelines

By Tom Heaton

For some time now I have been meaning to have a good look at what the evidence is around the management of post-op nausea and vomiting (PONV). As a particularly unpleasant side effect of what we do, it also seems to me to be a bit more of an unpredictable one. Different clinicians I have worked with have put different weight on the numerous risk factors and adjusted the prophylactic antiemetic choices accordingly, with practice ranging from zero to triple agent prophylaxis. Whilst aware of the different risk factors, I hadn't heard much about calculating the contribution that these factors make, and thus how to use them to make a more supported decision. The recent publication of the ‘Consensus Guidelines for the Management of Postoperative Nausea and Vomiting’ was therefore just what I was looking for. Published in Anesthesia & Analgesia in Jan 2014, the guidance was put together by the Society for Ambulatory Anesthesiology and is well worth a read if you haven’t yet had the chance. The aim of this post was to have a look at a few of the keys points in the document and try and summarise them.

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Given the length of the document (and my own short attention span) I thought breaking down the review into some bite sized chunks would be more palatable, and as such I thought we’d start with a look at the assessment of risk factors for PONV that had been my main area of questioning at the start.

Key Initial point

• Postoperative vomiting occurs in about 30% of patients, with nausea in 50%.
  
• As well as unpleasant it can prolong the stay of day-case patients, with cost implications.

Identifying Risk Factors

There are a number of well established risk factors for PONV, which for ease of memory can be classified as Patient, Surgical, or Anaesthetic factors (my personal way of categorising). The authors have tried to ascribe an appropriate power to these different factors, as well as flag up some factors that probably don’t impact on PONV risk. They have done this using the calculated odds ratio for each factor alongside the corresponding 95% confidence intervals.

Patient Factors:

• Age under 50 (Odds ratio (OR) with 95% confidence intervals (CI) = 1.79 (1.39 - 2.30))
  
• Age per decade (OR 0.88, CI 0.84 - 0.92)
  
• Female gender (OR 2.57, CI 2.32 - 2.84)
  
• History of PONV (OR 2.09, CI 1.90 - 2.29)
  
• Non smoker (OR 1.82, CI 1.68 - 1.98)
  
• History of motion sickness (1.77, CI 1.55 - 2.04)
  

Surgical Factors:

• Cholecystectomy (OR 1.90, CI 1.36 - 2.68)
  
• Gynaecological surgery (OR 1.24, CI 1.02 - 1.52)
  
• Laparoscopic surgery (OR 1.37, CI 1.07 - 1.77)
  

Anaesthetic Factors:

• Use of volatile anaesthetic agent (OR 1.82, CI 1.56 - 2.13)
  
• Duration of anaesthetic (OR 1.46 h-1, CI 1.30 - 1.63)
  
• Postoperative opioid use (OR 1.47, CI 1.31 - 1.65) - irrespective of type of drug in dose dependent manner
  
• Nitrous oxide use (OR 1.45, CI 1.06 - 1.98)
  

Several factors that have previously been described as contributing to PONV risk have been disproven or shown to be of little clinical relevance, including: anxiety, patient BMI, use of supplemental oxygen, and use of a nasogastric tube. Others have got conflicting evidence on their relevance, including: ASA status, and use of muscle relaxant reversal agents. One point I found particularly interesting was the weak link between intraoperative opioids and PONV, instead the link purely being with postoperative use.

Risk Scoring

This is all useful information but, as the authors point out, these factors indepedently are not sufficient to predict risk and instead need considering along with co-existing factors. As such they recommend the use of scoring systems to change clinical information into a appropriate plan. This is supported by the evidence that this works to reduce the incidence of PONV as well as preventing blanket use of antiemetic medications that are not without their own side effects and costs.

The two common scoring systems that the authors highlight are the Apfel score and the Koivuranta score. These attribute points to some of the above mentioned risk factors in order to stratify patients as low, medium or high risk of PONV, and advise on anti-emetic therapy as appropriate. Whilst these are recognised as being useful, their sensitivity and specificity is quoted as being around 65-70%, thus requiring some interpretation of scores alongside other factors that aren’t covered.

As well as routine adult PONV risk assessment, the authors briefly look at the problems of PONV in children and post-discharge nausea and vomiting in day-case patients, providing similar scoring systems with which to approach these patients. Indeed I think the post-discharge risk assessment is particularly relevant given the ever increasing amount of day-case surgery we are undertaking and the lack of easy relief of PONV when these patients have left hospital.

Final Thoughts

Since reading this review, assessing for PONV risk has become a more central point of my pre-op assessment compared to something of an afterthought when the end of surgery is approaching. Whilst scoring systems and odds ratios don’t provide you with definite percentages for the patient in front of you, I personally find them reassuring as they provide a numerical dimension to the factors; something I feel more comfortable with. 

That just about wraps up a quick look at the first part of the guidelines, but I hope to have a quick review of some of the other interesting component in future posts. As always thanks for reading and please post any comments, links or ideas on the topic.

Tom

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Useful Links:

Consensus Guidelines for the Management of PONV - The Anesthesia & Analgesia article
Medscape PONV - The Medscape review of the subject
Anaesthesia UK - Review of PONV
AAFP - A review of the risk scoring with the Apfel and Koivuranta scores

Image courtesy of David Castillo Dominici/Freedigitalphotos.net