Anesthesiology APPLIED Exam › Pharmacology, equipment and physics

Opioids and postoperative respiratory depression

Pharmacology, equipment and physics·~5 min read· Physician-reviewed✓ Sourced · 4

Stem

A 67-year-old man, 112 kg, with obstructive sleep apnea who does not use his CPAP, is on the ward the night after a total knee arthroplasty. He takes extended-release oxycodone at home and has a hydromorphone PCA with a basal infusion. The nurse calls: he is hard to rouse, his respiratory rate is 8, and his SpO2 is 84% on 4 L nasal cannula.

Question

You are called to the ward. What do you do?

Ideal answer

HeadlineOpioid-induced respiratory depression until proven otherwise: airway and breathing first. I stimulate him, give oxygen, jaw thrust, bag-mask if he is not breathing enough, and stop the PCA.

Not rousing: naloxone in small 40 mcg increments, every 2–3 minutes, below the label's 0.1–0.2 mg postoperative dose because he takes oxycodone at home, titrated to respiratory rate and arousal, not full wakefulness.¹
Glucose, every sedating drug reviewed, a blood gas, because the opioid is not the only candidate.
Naloxone infusion, because a long-acting opioid is involved, as it is here.
Monitored bed with continuous pulse oximetry and capnography, because naloxone can wear off before his long-acting opioid.
Prevention: multimodal analgesia, risk screening and continuous monitoring for the high-risk patient.

Watch for: over-reversal: pain, hypertension, vomiting, circulatory stress, and at the extreme pulmonary edema.¹

Breaking down the answer

Sedation precedes apnea — somnolence and sedation are the most common precursors of opioid-induced ventilatory impairment, before the respiratory rate falls;² I use a sedation scale (POSS) and hold opioids at level 3, frequently drowsy and drifting off mid-conversation.
Risk factors I name — obstructive sleep apnea, obesity, age over 65, opioid naivety, renal impairment, concurrent benzodiazepines or gabapentinoids, the first 24 hours after surgery, continuous basal PCA infusions. In renal failure morphine-6-glucuronide accumulates, so hydromorphone or fentanyl.
Why capnography on oxygen — supplemental oxygen delays detection of hypoventilation on pulse oximetry,² so high-risk patients get continuous oximetry with central monitoring plus capnography.
No basal infusion in opioid-naive patients — it adds respiratory depression without improving analgesia. Acetaminophen, an NSAID, regional anesthesia or ketamine reduce opioid need.
Naloxone in an arrest — 0.4–2 mg boluses every 2–3 minutes; no response to 10 mg, reconsider the diagnosis.¹ Its 20–90 minutes is shorter than many opioids, far shorter than methadone: infusion at two-thirds of the effective dose per hour.³
Neuraxial morphine — depression can appear many hours after intrathecal or epidural morphine: the ASA/ASRA guideline monitors at least hourly for 12 hours, then at least every 2 hours to 24 hours; after single-shot fentanyl, a minimum of 2 hours.⁴

Follow-up question

You give naloxone 80 mcg and he wakes. Forty minutes later the nurse finds him apneic again, and after the next dose he develops pink frothy sputum and hypoxemia. What is happening, and what do you do?

Follow-up answer

HeadlineRenarcotization: the naloxone has worn off his extended-release oxycodone. I tell the nurse and call for help.

I ventilate, give another titrated dose, start a naloxone infusion, and move him to the ICU or step-down unit. The frothy sputum after reversal is negative-pressure or catecholamine-mediated pulmonary edema; I support with CPAP or reintubation, and a diuretic if volume overload contributes. I reassess with a blood gas and chest film, then file a safety report, because a ward arrest on opioids is a system failure as well as a clinical one.

Pitfalls that cost points

Giving 0.4 mg naloxone to a sleepy opioid-tolerant patient with a respiratory rate of 10, causing withdrawal and uncontrolled pain.
Missing the other causes: residual block, CO2 narcosis, stroke.
Relying on SpO2 while the patient is on 4 L nasal cannula.

Rapid-fire

Q: Why does supplemental oxygen hide hypoventilation? A: It keeps saturation normal while PaCO2 climbs, so SpO2 falls only when he is severely hypercarbic; the neuraxial guideline warns routine oxygen can prolong apneic episodes and mask hypoventilation.⁴

Q: Why are patients with sleep apnea at extra risk? A: Opioids blunt the arousal response that ends each obstruction, and REM rebound on nights two and three makes obstruction worse.

Q: Why does sedation predict depression better than respiratory rate? A: The opioid blunts the ventilatory response to CO2 and the patient drifts deeper before the rate falls.

Q: Why does buprenorphine respond poorly to naloxone? A: It binds the mu receptor with high affinity; I need larger doses of naloxone and often ventilatory support.

Numbers to know

Item	Value
Naloxone titration	0.1–0.2 mg IV every 2–3 min (label); 40 mcg in the tolerant¹
Naloxone arrest dose	0.4–2 mg IV, repeat every 2–3 min¹
Naloxone duration	20–90 min³
Naloxone infusion	2/3 of effective dose per hour
Intrathecal morphine monitoring	At least hourly × 12 h, then q2h to 24 h⁴

Sources

1.Naloxone hydrochloride injection: dosage and administration · FDA label (Pfizer) · pfizermedical.com
2.Monitoring for Opioid-Induced Respiratory Depression · APSF, 2018 · apsf.orgGupta and Edwards, APSF Newsletter, February 2018.
3.Clinical Pharmacology and Toxicology Pearl of the Week: Naloxone · University of Calgary Cumming School of Medicine, 2025 · cumming.ucalgary.caReviewed February 2025; duration of action 20–90 min, infusion at two-thirds of the effective dose per hour.
4.Practice Guidelines for the Prevention, Detection, and Management of Respiratory Depression Associated with Neuraxial Opioid Administration · ASA/ASRA, 2016 · asahq.org

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.