A 58-year-old man with type 2 diabetes and hypertension undergoes open abdominal aortic aneurysm repair under general anesthesia. Intraoperatively, he receives nitrous oxide, isoflurane, and fentanyl. He is extubated in the operating room. Within 30 minutes of arrival in the recovery room, he develops nausea and vomiting. His blood glucose is 280 mg/dL, blood pressure is 165/95 mmHg, and heart rate is 110 bpm. Which of the following is the MOST important modifiable intraoperative factor that should be avoided in future cases to reduce PONV risk in this patient?

Explanation

## Nitrous Oxide and PONV Risk **Key Point:** Nitrous oxide is one of the most emetogenic components of general anesthesia and is an independent, modifiable risk factor for PONV. Its elimination significantly reduces PONV incidence, particularly in high-risk patients. ### Mechanism of Nitrous Oxide Emetogenicity Nitrous oxide contributes to PONV through multiple pathways: 1. **Direct chemoreceptor trigger zone stimulation** — activates the vomiting center 2. **Increased intragastric pressure** — distension of the stomach 3. **Diffusion hypoxia** — transient hypoxemia during emergence 4. **Expansion of closed gas spaces** — increases bowel distension **High-Yield:** Nitrous oxide is one of the few emetogenic factors that can be completely eliminated without compromising anesthetic efficacy. Removing it from the anesthetic technique is a simple, high-impact intervention. ### Evidence for N₂O Avoidance | Intervention | PONV Reduction | |---|---| | Avoidance of N₂O | 30–50% reduction | | TIVA vs. volatile anesthetics | 20–30% reduction | | Opioid reduction | 10–20% reduction | | Prophylactic antiemetics | 30–40% reduction | **Clinical Pearl:** In this diabetic, hypertensive patient undergoing major vascular surgery (high baseline PONV risk), the combination of volatile anesthetic + nitrous oxide is particularly emetogenic. Eliminating N₂O is the single most impactful intraoperative modification. ### Why This Patient Is High-Risk - Major surgery (AAA repair) — prolonged operative time - Volatile anesthetic (isoflurane) — emetogenic - **Nitrous oxide** — highly emetogenic (avoidable) - Opioid use (fentanyl) — mild emetogenic effect - Male sex with comorbidities — moderate baseline risk - Postoperative pain and hyperglycemia — additional risk factors ## Recommended Modification for Future Cases **Optimal anesthetic technique:** - Eliminate nitrous oxide entirely - Consider TIVA with propofol infusion (less emetogenic than volatile anesthetics) - Maintain adequate analgesia with opioids (reducing opioids excessively increases pain-related PONV) - Prophylactic multimodal antiemetics (dexamethasone + ondansetron) - Maintain euglycemia and normothermia [cite:Miller's Anesthesia 8e Ch 42; Apfel CC et al. A Simplified Risk Score for Predicting Postoperative Nausea and Vomiting. Anesth Analg. 1999;88(6):1362–1369.]