Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Interpret low oxygen measurements, cyanosis and finger clubbing as distinct clinical signals, confirm artefact or dyshemoglobinaemia, identify the underlying gas-exchange mechanism, and avoid treating signs without investigating their cause.
Oxygen measurements describe different parts of delivery. PaO2 is dissolved arterial oxygen tension; saturation is the proportion of haemoglobin binding sites carrying oxygen; content depends strongly on haemoglobin concentration; tissue delivery also depends on cardiac output. A patient with profound anaemia may have normal PaO2 and SpO2 but poor oxygen delivery, while carbon monoxide poisoning may produce a reassuring standard oximeter reading despite impaired carriage. Treat the patient and the mechanism, not one number.
Hypoxaemia commonly results from ventilation-perfusion mismatch in airway disease, pneumonia, oedema and pulmonary embolism. Hypoventilation raises carbon dioxide unless another process coexists. Diffusion limitation becomes more apparent on exertion in interstitial or pulmonary vascular disease. A true right-to-left shunt responds less completely to oxygen. These principles determine which tests and supports are useful; they do not justify withholding oxygen in a critically unwell patient.
Airway disease, pneumonia, oedema and pulmonary embolism create regions where ventilation and blood flow are poorly matched, the commonest mechanism of hypoxaemia.
Intracardiac or intrapulmonary shunt, thickened alveolar membranes and altitude each lower arterial oxygen through distinct physiological routes.
Central depression, neuromuscular weakness, obesity and severe airway obstruction reduce alveolar ventilation, causing hypoxaemia with carbon dioxide retention.
Lung cancer, bronchiectasis, fibrotic lung disease, cyanotic heart disease and selected gastrointestinal disorders drive clubbing independently of current oxygen saturation.
Impaired inspired delivery, ventilation, perfusion matching, diffusion or shunt lowers oxygen tension and haemoglobin saturation, thereby altering ventilation, gas transfer or respiratory mechanics.
Increased ventilation, heart rate and red-cell production attempt to preserve delivery, while severe or rapid hypoxaemia causes organ dysfunction.
A sufficient concentration of deoxygenated haemoglobin or abnormal pigment changes skin and mucosal colour, so appearance depends partly on haemoglobin and perfusion.
Circulating megakaryocytes and growth mediators reaching distal digits are thought to promote vascular and connective-tissue proliferation around nail beds.
Low saturation with tachypnoea, work of breathing, confusion, new oxygen need or haemodynamic instability signals acute respiratory failure. Pneumonia, oedema, embolism, pneumothorax and severe airway disease are prioritised by history and examination.
Drowsiness, headache, asterixis, shallow breathing or exhaustion with hypercapnia suggests inadequate alveolar ventilation from COPD, obesity hypoventilation, neuromuscular weakness, chest-wall disease or depressant medicines. Normal saturation on oxygen can conceal this.
Headache, confusion or collapse after smoke exposure with a misleadingly normal PaO2 suggests carbon monoxide; cyanosis with chocolate-coloured blood and saturation that does not respond as expected suggests methaemoglobinaemia. Standard pulse oximetry is unreliable in both.
Loss of the normal nail-fold angle, increased nail-bed fluctuation and distal phalangeal expansion support clubbing. Compare all digits and previous appearance; isolated nail curvature, osteoarthritis and swelling can mimic it.
Clubbing with painful swollen wrists or ankles, periostosis and joint symptoms suggests hypertrophic pulmonary osteoarthropathy and strengthens the need to search urgently for intrathoracic malignancy or chronic suppurative disease.
Motion, poor perfusion, nail products and device bias can produce false readings; waveform review, repeated sites and arterial sampling resolve discordance.
Cold or low-flow extremities may appear blue while central mucosa and arterial oxygen remain relatively preserved, unlike central cyanosis.
Carbon monoxide or methaemoglobin disrupts oxygen carriage and oximetry interpretation; exposure history and co-oximetry are discriminating.
Nail curvature, osteoarthritis or local vascular disease may resemble clubbing without loss of the nail-fold angle or true nail-bed fluctuation.
Severe anaemia can produce poor oxygen delivery without obvious cyanosis because little deoxygenated haemoglobin is available to change colour.
Blue-grey tongue or oral mucosa supports central cyanosis, while cool blue extremities with a pink tongue suggests peripheral slow flow. Lighting and skin tone alter appearance, so confirm with oximetry, blood gas or co-oximetry as appropriate.
Severe or sustained hypoxaemia causes confusion, seizure, coma and irreversible brain damage, adding morbidity beyond the initial pulmonary disorder.
Reduced oxygen delivery and sympathetic stress can destabilise myocardium, particularly in coronary disease, and increasing the burden of otherwise local respiratory disease.
Chronic alveolar hypoxia causes pulmonary vasoconstriction and vascular remodelling, eventually straining the right heart, and increasing the burden of otherwise local respiratory disease.
Sustained hypoxaemia increases erythropoietin and red-cell mass, raising blood viscosity and complicating symptoms, adding morbidity beyond the initial pulmonary disorder.
Treating a saturation or sign without finding its cause can postpone recognition of embolism, malignancy, shunt or progressive lung disease.
Dissolved oxygen contributes little to total carriage. Haemoglobin concentration and saturation dominate content, while cardiac output determines delivery; all four matter in a shocked or anaemic patient.
The concentration of deoxygenated haemoglobin, skin pigmentation, lighting and perfusion alter visibility. Severe anaemia may prevent obvious cyanosis despite dangerous hypoxaemia.
Pulse oximeters can overestimate saturation in some darker-skinned patients, particularly near clinically important thresholds. A discordant or deteriorating patient warrants blood-gas confirmation and clinical escalation.
A person with COPD can also develop lung cancer, bronchiectasis or fibrosis. New clubbing should trigger investigation rather than being incorporated into the existing COPD label.
A preserved or raised transfer coefficient with low alveolar volume does not automatically normalise the overall gas-transfer defect; volume, haemoglobin and disease mechanism still require integrated interpretation.
Using pulse oximetry as a test of ventilation and missing hypercapnia.
Withholding oxygen from a critically hypoxaemic COPD patient rather than titrating and checking gases.
Accepting a standard saturation as reassuring after smoke exposure without co-oximetry.
Diagnosing central cyanosis from fingertips alone in a cold, poorly perfused patient.
Attributing true new clubbing to uncomplicated COPD.
Prescribing long-term oxygen from one acute-illness saturation without formal stable assessment.
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A patient has headache and confusion after an enclosed-space fire. PaO2 is normal and the standard pulse oximeter reads 98%. Which investigation is most important for the suspected oxygen-carrying problem?
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
Oxygen is prescribed to a target and does not correct inadequate ventilation. Record device and flow, repeat blood gases in hypercapnic risk, and treat carbon monoxide exposure with its specific emergency pathway.
Do not rely on standard pulse oximetry or PaO2 to grade carbon monoxide poisoning. Antidotal decisions depend on exposure, symptoms, pregnancy, concentration and contraindications.