Anaesthetic emergencies

Opioid toxicity and postoperative respiratory depression

Overview and approach

Opioids, sedatives, residual anaesthesia and neuromuscular block can impair ventilation.

Recognise opioid-induced ventilatory impairment mainly by excessive sedation, not respiratory rate or SpO2 alone; a sedation score of 2 is early and 3 is severe (1)(2).
Sedation scale used on Australian ward charts: score 2 = constantly drowsy; score 3 = difficult to rouse or unresponsive. Score 3, or score 2 with a low respiratory rate (≤5 in this protocol), is the trigger for naloxone and an emergency call (4).
Higher-risk patients: long-acting or slow-release opioids, intraoperative neuraxial morphine, elderly or frail patients, and renal or hepatic impairment (4)(5).
Aim: restore adequate breathing without fully reversing analgesia or causing withdrawal. Use small titrated doses, not a full overdose dose (4)(5).

Assessment and clinical features

Assess consciousness, respiratory rate/effort, oxygenation, capnography if available and recent drugs; consider OSA and renal impairment.

Check drug charts for PCA, epidural or intrathecal opioid, slow-release opioids, and co-sedatives such as benzodiazepines and gabapentinoids. Check pupils and look for other causes.
If naloxone does not produce the expected response, consider other diagnoses such as hypoglycaemia (4).

Differentials and investigations

ABCDE and immediate monitored assessment; oxygen alone can conceal hypoventilation.

Differentials: other sedatives, residual neuromuscular block, hypercapnia, hypoglycaemia (4), stroke, sepsis and electrolyte disturbance. Check BGL. Consider a blood gas if hypercapnia is suspected.
If hypoxia persists despite a normal respiratory rate, seek senior advice (5).

Management and follow-up

Call emergency/anaesthetic help, support ventilation and use reversal treatment under protocol with ongoing observation.

Withhold opioids, assess airway and ventilation, stimulate, give oxygen and escalate. For sedation score 3, or score 2 with a very low respiratory rate, give naloxone titrated to arousal and respiratory rate; repeat, because naloxone acts for a shorter time than many opioids (1)(2). Monitor sedation, respiratory rate and SpO2 closely until stable, then regularly for hours after immediate-release opioids (longer for slow-release) (1)(2).
Immediate steps: stop opioid infusions or remove the PCA button, recovery position, oxygen, full set of vital signs, and stay with the patient. Call MET/code blue for sedation score 3 or respiratory rate ≤5 (4).
Postoperative or therapeutic opioid (example ward standing order): naloxone 100 microgram IV (400 microgram diluted to 4 mL = 100 microgram/mL), every 2-3 minutes, up to 400 microgram. If there is no IV access, give 400 microgram IM or subcut as a single dose (4). protocols differ on increment size, interval and respiratory-rate threshold. Follow the local standing order.
Overdose or toxicology setting: start low and titrate. Dilute 400 microgram to 10 mL (40 microgram/mL) and give 1, 2, 3 then 4 mL IV every 60 seconds. Further 200-400 microgram increments can be given up to about 2 mg, then consider other diagnoses. Larger initial doses (400 microgram every 60 seconds) are safe if the patient is not opioid dependent (5).
In opioid-dependent patients, aim for a protected airway and SpO2 above 92% on room air rather than full reversal. Withdrawal can cause vomiting and aspiration, and catecholamine surge with arrhythmia or pulmonary oedema (5).
Re-sedation and infusion: onset is 1-2 minutes IV, but duration is only about 20-90 minutes (5). If the patient re-sedates, repeat the total bolus dose that restored breathing, then start an infusion at two-thirds of that dose per hour in HDU (5). One ward protocol uses 400-800 microgram/hour (2 mg in 500 mL = 4 microgram/mL) titrated to response (4).
Observation after naloxone: check RR, sedation and SpO2 every 1-2 minutes until the patient is more alert and RR is ≥10. Then record observations hourly until stable and 4-hourly for 24 hours (4). Observe for re-sedation for at least 2 hours after the last bolus and 4-6 hours after stopping an infusion (5).
Review the analgesic plan afterwards: lower dose, a different agent, or multimodal analgesia. Do not resume the same regimen unchanged.

Red flags and clinical pearls

Re-sedation can occur after reversal. Do not leave a drowsy hypoventilating patient unmonitored.

Sedation precedes respiratory depression. A normal SpO2 on supplemental oxygen does not exclude hypoventilation (1).
Slow-release, methadone, buprenorphine and neuraxial morphine carry the highest risk of recurrence. Buprenorphine response to naloxone is less predictable (5).
An agitated patient does not need naloxone (5).

References

Last updated from Sen’s notes on 9 October 2026.