This study failed to observe any benefit of supplemental oxygen on PONV after thyroidectomy and confirmed the antiemetic efficacy of droperidol.
Gut manipulation during colon surgery and the associated splanchnic vessel dissection surely results in at least some intestinal ischemia.12
Indeed, the stress of major surgery per se
stimulates the sympathetic nervous system and can substantially reduce splanchnic blood flow. Even slight intestinal ischemia triggers substantial release of 5-hydroxytryptamine and other emetogenic factors.13
Supplemental perioperative oxygen presumably reduces the amount of ischemic intestinal tissue and thus, the risk of nausea and vomiting.
Pneumoperitoneum, which is required for laparoscopic procedures, also induces significant hemodynamic changes and release of vasoactive hormones14 15
that can reduce intestinal perfusion.15–17
However, the extent to which laparoscopy impairs intestinal perfusion may depend on subtle details of surgical technique — perhaps explaining why supplemental oxygen was beneficial in one study of laparoscopic gynaecologic surgery7
but not in another.8
Thyroid surgery differs from abdominal surgery in that intestinal ischemia is unlikely because the surgical procedure does not directly involve the splanchnic circulation. Furthermore, the risk of significant bleeding is low. Hypotension, and consequent intestinal hypoperfusion, is thus rare during thyroid surgery. That supplemental oxygen did not reduce the risk of nausea and vomiting is consistent with the theory that oxygen prevents PONV by ameliorating subtle intestinal ischemia rather than by acting directly at the chemotactic trigger zone.
The difficulty with this theory is a recent study in which prophylactic supplemental oxygen was found to reduce the incidence of nausea and vomiting in victims of minor trauma who developed motion sickness during ambulance transport.18
It is unlikely that these patients had intestinal ischemia, suggesting that the benefits of supplemental oxygen were mediated by another mechanism. The most likely is that reduction of nausea by oxygen depends on dopamine release from the carotid bodies. The carotid bodies tonically release dopamine in amounts that are inversely related to arterial oxygen tension.19
The importance of this observation is that the chemotactic trigger zone is sensitive to dopamine as well as serotonin.20
Hyperoxia per se
may thus reduce nausea and vomiting via
a dopamine-dependent mechanism.
That then leaves the question of why oxygen proved ineffective in patients recovering from thyroid surgery. The most likely explanation is that nausea and vomiting in these patients was mediated by yet another mechanism. There are several possibilities. For example, intraoperative dissection of the recurrent laryngeal nerve(s) may contribute because this nerve is a branch of the vagal nerve that is highly involved in emetic reflexes. PONV after thyroidectomy may also result from nociceptive reflexes originating from the pharynx and larynx.3
(Pharyngeal pain, exacerbated by swallowing, is typical after thyroid surgery.) To the extent that PONV results from throat pain, it is unlikely to be affected by hyperoxia. Consistent with this theory, pain scores and analgesic consumption was similar in each of our three treatment groups. Postoperative nausea and vomiting has numerous aetiologies and many factors are known to influence the incidence of this complication. It is therefore unsurprising that a treatment effective for some aetiologies would fail in other circumstances.
We found that droperidol significantly reduced the incidence of PONV. This study thus confirms that droperidol prevents PONV.21–23
For example, low-dose droperidol (0.625 mg) compares favourably with ondansetron (4 mg) 22–23
— although droperidol is considerably less expensive than the 5-HT3
antagonists. The incidence of side effects are reportedly similar with each class of drugs.24–26
These data suggested that the least expensive was probably preferable for routine use. However, the Food and Drug Administration has recently added a “black box” warning to the label for droperidol because the drug has been associated with life-threatening QT prolongation. However, there is considerable question in the anaesthesia community about the justification for this warning.27
Patients were allowed to drink 4 h after surgery, and most did at that time or shortly thereafter. However, emetic episodes were triggered by drinking water in the 30% and 80% oxygen groups. This then delayed their first meal. As a result, droperidol not only reduced the risk of PONV, but also allowed patients to tolerate solid food significantly more quickly.
In summary, 80% perioperative oxygen was ineffective in preventing PONV after thyroidectomy. A difference in the aetiology of PONV after thyroidectomy and abdominal surgery may explain the discrepancy between our results and those reported after abdominal surgery. We also observed that droperidol 0.625 mg, a drug that is inexpensive but possibly associated with serious side effects, significantly reduced the incidence of PONV and shortened the time for first meal.