First move
Look at the patient and the airway before the number. Open the airway, sit the patient up, and give oxygen while you work out why.
Then ask the question this card exists for: is anything monitoring ventilation? On supplemental oxygen the pulse oximeter is not a ventilation monitor. In the phase 1 study, hypoventilation to half of normal minute ventilation produced no fall in saturation at all at an inspired oxygen fraction of 0.25 or 0.30, while the room-air group desaturated fu-2004. On the ward, patients on supplemental oxygen had 2.7 times the rate of opioid-induced respiratory depression episodes of patients on room air while their desaturation events did not differ doufas-2023.
Work the differential
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Hypoventilation. Opioid, residual anesthetic, residual block. Look at rate, depth and level of consciousness; use capnography if you have it. The saturation will be reassuring until it is not fu-2004.
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Obstruction. Tongue, laryngospasm, secretions, blood. See the laryngospasm card.
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Atelectasis and shunt. The commonest reason a postoperative saturation sits low despite adequate ventilation. Sit up, encourage deep breaths, consider positive pressure.
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Aspiration. New wheeze or crackles after vomiting or a difficult airway.
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Low inspired oxygen or a disconnected circuit. Check the source before the patient.
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Methemoglobinemia. Suspect it when the saturation sits low and will not move with oxygen. In dogs the oximeter overestimated true saturation in proportion to methemoglobin until about 35%, then plateaued at 84 to 86% and fell no further barker-1989. Across 242 published episodes related to local anesthetics, a saturation of 90% or below with an arterial oxygen tension of 70 mmHg or more was present in 91.8%, and a single spray of benzocaine was enough to cause it guay-2009.
If it is methemoglobinemia
Send co-oximetry, not another pulse oximeter reading guay-2009.
Methylene blue 1 mg/kg intravenously over 5 to 30 min, repeated once after an hour if the level stays above 30% or symptoms persist. It is contraindicated in G6PD deficiency, where it risks hemolysis and may not work, and it carries a boxed warning for serotonin syndrome. It can itself lower the oximeter reading fda-methylene-blue-label-2025.
Rebound has been described up to 18 hours after treatment when the trigger was mucosal benzocaine, so keep watching guay-2009.
Before you settle
The guideline records that assessment of respiratory function during recovery is associated with early detection of hypoxemia, and asks for airway patency, respiratory rate and saturation to be assessed periodically during emergence and recovery. It is an oximetry-based recommendation and does not mandate a ventilation monitor asa-pacu-2013-d3.
A saturation restored by turning up the oxygen has been treated. A saturation restored by turning up the oxygen in a patient who is hypoventilating has been hidden.
Sources
[[fu-2004]] randomized trial 2004 n: Phase 1 n=45 (OR, patient-controlled: FiO2 0.21 n=25, 0.25 n=10, 0.30 n=10); phase 2 n=288 randomised in the PACU (room air 155, supplemental O2 133) paywalled era-limited
Fu ES, Downs JB, Schweiger JW, Miguel RV, Smith RA. Supplemental oxygen impairs detection of hypoventilation by pulse oximetry. Chest. 2004 Nov;126(5):1552-1558.
Two-phase study in surgical patients, not volunteers. Phase 1 (OR, n=45): hypoventilation defined as a 50% reduction in minute ventilation; SpO2 declined only in the room-air group, with no decline at all at FiO2 0.25 or 0.30. Phase 2 (PACU, n=288, randomised room air vs supplemental O2, SpO2 every minute for up to 40 min): desaturation below SpO2 90% in 9.0% on room air vs 2.3% on supplemental O2, P=0.02. The direction is the teaching trap: supplemental oxygen removes the oximeter's hypoventilation signal.
[[doufas-2023]] cohort 2023 n: 202 open
Doufas AG, et al. Incidence of postoperative opioid-induced respiratory depression episodes in patients on room air or supplemental oxygen: a post-hoc analysis of the PRODIGY trial. BMC Anesthesiol. 2023 Oct 4;23(1):332.
Surgical WARD patients (not PACU) with blinded continuous oximetry and capnography: 74 always on room air, 88 intermittent, 40 continuous supplemental oxygen. On supplemental oxygen vs room air: all OIRD episodes IRR 2.7 (95% CI 1.4-5.1), apnea IRR 2.8 (1.5-5.2), bradypnea IRR 3.0 (1.2-7.9); high PRODIGY score IRR 4.5 (2.2-9.6). 'Despite oxygen desaturation events not differing between SO and RA, SO may clinically promote OIRD.' Answers the Day 3 placeholder only by analogy (ward, not PACU).
[[guay-2009]] case series 2009 n: 242 published episodes paywalled
Guay J. Methemoglobinemia related to local anesthetics: a summary of 242 episodes. Anesth Analg. 2009 Mar;108(3):837-845.
Systematic summary of 242 published episodes of local anaesthetic-related methaemoglobinaemia: SpO2 <=90% with PaO2 >=70 mmHg discrepancy in 91.8%; SpO2 minus co-oximetry difference -6.2% to 44.7%; a single spray of benzocaine may induce methaemoglobinaemia; coma reported at MetHb 32.2% (children) and 29.1% (adults); rebound MetHb up to 59.9% up to 18 h after methylene blue with mucosal benzocaine; complications include hypoxic encephalopathy, MI and death; conclusion 'Benzocaine should no longer be used'. Prilocaine adult limit 5.0 mg/kg. PubMed pubtype: Meta-Analysis.
[[barker-1989]] cohort 1989 n: 5 anesthetized dogs paywalled
Barker SJ, Tremper KK, Hyatt J. Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry. Anesthesiology. 1989 Jan;70(1):112-117.
ANIMAL study (design vocabulary has no term; 'cohort' is a placeholder - director to decide). MetHb induced up to 60% by intratracheal benzocaine on FiO2 1.0: SpO2 overestimated SaO2 proportionally until MetHb ~35%, then plateaued at 84-86% and did not fall further; with further desaturation SpO2 changed far less than SaO2 (slopes 0.16-0.32). This is the primary source for the '~85% plateau'.
[[fda-methylene-blue-label-2025]] advisory 2025 n: NA open
Methylene Blue Injection, USP 0.5% [prescribing information]. Amneal Pharmaceuticals LLC. Revised 11/2025. DailyMed set ID 67da2a69-8166-46c7-921c-d6ec47bfeac8.
FDA label: indicated for acquired methemoglobinemia (pediatric and adult); 1 mg/kg IV over 5-30 min, repeat 1 mg/kg after 1 h if MetHb remains >30% or symptoms persist; CONTRAINDICATED in G6PD deficiency (risk of hemolytic anemia) and 'may not be effective in patients with G6PD deficiency'; boxed warning for serotonin syndrome with serotonergic drugs and opioids; 'The presence of methylene blue in the blood may result in an underestimation of the oxygen saturation reading by pulse oximetry.'
[[asa-pacu-2013-d3]] guideline 2013 n: NA open
American Society of Anesthesiologists Task Force on Postanesthetic Care. Practice Guidelines for Postanesthetic Care: an updated report. Anesthesiology. 2013 Feb;118(2):291-307.
States that the original literature indicated assessment and monitoring of respiratory function during recovery is associated with early detection of hypoxaemia (Category A2-B evidence) and that new literature is insufficient to further evaluate that finding; consultants and ASA members agree airway patency, respiratory rate and SpO2 should be assessed periodically during emergence and recovery. Note this is oximetry-based; the guideline does not mandate a ventilation monitor.