What is the most appropriate way to prescribe
ondansetron for ordinary or post op nausea?
Postoperative nausea and vomiting (PONV) occurs
in 30% of surgical patients and is a major concern for patients as they contemplate
surgery. Risk factors for development of PONV include female gender, history of motion
sickness or PONV, type and duration of surgery and use of postoperative opioids. Development of PONV
can increase discomfort, dissatisfaction, length of stay and overall health care costs for
Ondansetron is indicated in the United States for the
prevention of postoperative nausea and vomiting (PONV) in adults and children. Suggested oral dosing for adults is a single 16 mg dose one hour before anesthesia with no
recommendations for the pediatric population. Injectable ondansetron may be given as a 4 mg dose
infused over 2 -5 minutes for patients older than one month and a 1 mg/kg dose for newborns to
infants one month old.
Ondansetron is a selective 5-HT3-receptor
antagonist that blocks serotonin peripherally on vagal nerve terminals and centrally in the
chemoreceptor trigger zone. The onset of action is about 30 minutes with metabolism occurring in the
liver. Bioavailability is around 56% to 71% via the oral route. Ondansetron’s half-life is 3
to 6 hours at the maximum dose in chemotherapy patients of 32 mg daily.
One study looked at PONV after cardiac surgery
where 4 mg of ondansetron IV was given at the end of surgery and 12 mg was added to the
patients’ PCA pump which had a pre-set volume for all patients. Ondansetron significantly reduced PONV
when compared to placebo in these patients. Another study looked at ondansetron 4 mg
IV given to shoulder surgery patients 30 minutes before the end of their surgery. This study also showed a significant
reduction in PONV when compared to placebo.
According to consensus guidelines for managing
PONV, IV ondansetron is most effective when given at the end of surgery and shows more of an
anti-vomiting effect than anti-nausea.[1,6] The number needed to
treat (NNT) for IV ondansetron 4 mg is approximately 7 for the prevention of nausea, indicating 7
patients would need to be treated with ondansetron 4mg IV to prevent one patient’s
nausea. The NNT to prevent one case of postoperative vomiting is 6. The 8 mg dose has a
NNT of 6 for the prevention of nausea and a NNT of 5 for the prevention of vomiting. The guidelines
suggest that if PONV occurs within 6 hours after surgery, patients should not receive a repeat dose
of ondansetron if it was used prophylatically. For episodes occurring
more than 6 hours after surgery, patients can be treated with a repeat dose of IV ondansetron 4-8
Ondansetron has a favorable side effect
number needed to harm (NNH) with a single dose of ondansetron is 36 for headache indicating one case
of headache for each 36 patients treated with ondansetron. The NNH is 31 for increased liver enzymes
and 23 for constipation. According to the
International Anesthesia Research Society guidelines on PONV, if prophylaxis of ondansetron fails
<6 hours after surgery, a different class of antiemetic should be given, however a dose of
ondansetron can be repeated if it has been >6 hours after the end of surgery. If the repeat dose
fails to adequately control nausea and vomiting, another antiemetic may be chosen to alleviate
further nausea and vomiting.
Literature pertaining to treatment of general
nausea and vomiting with ondansetron is limited. Ondansetron is not specifically indicated for
general nausea and vomiting. Several studies have shown some utility of ondansetron when used in a
pediatric population to treat vomiting in acute gastroenteritis. The mechanism of action of
ondansetron suggests it would have a possible role in treating general nausea and vomiting as it
acts on the chemoreceptor trigger zone that controls vomiting.. One of the primary sites of action of the chemoreceptor trigger zone is
mediated by the 5-HT receptor. Ondansetron may play a role in inhibiting the chemoreceptor trigger
zone, which may help to control general N/V in patients.
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Chen, YF, WN Yeh, KH Lee, et al. "Intravenous Ondansetron as Antiemetic Prophylaxis for Postoperative
Nausea and Vomiting after Shoulder Arthroscopy." Journal of Chang Gung Medicine.
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Michelle L. Dehoorne-Smith, and Pramodini B. Kale-Pradhan. "Prevention of Postoperative Nausea
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