Synopsis
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.
- Hypoxaemia means low arterial oxygen; hypoxia means inadequate tissue oxygen availability. Anaemia or circulatory failure can produce tissue hypoxia despite a normal PaO2.
- Pulse oximetry estimates saturation, not ventilation or oxygen content. It cannot detect hypercapnia and is affected by poor perfusion, motion, nail products, dyshemoglobinaemia and device bias.
- Central cyanosis involves tongue and oral mucosa and suggests arterial desaturation or dyshemoglobinaemia; peripheral cyanosis can arise from slow flow and vasoconstriction with normal arterial oxygenation.
- Cyanosis may be difficult to detect in darker skin and can be absent in severe anaemia; rely on objective physiology rather than visual reassurance.
- Mechanisms of hypoxaemia are low inspired oxygen, hypoventilation, ventilation-perfusion mismatch, diffusion limitation and right-to-left shunt; the carbon-dioxide pattern and response to oxygen help separate them.
- For most acutely ill adults prescribe oxygen to 94-98%; use 88-92% pending gases when hypercapnic respiratory failure risk is recognised, unless an individual target applies.
- True finger clubbing reflects expansion of the distal phalanx and loss of the nail-fold angle. COPD alone does not explain clubbing and should not stop investigation for cancer, bronchiectasis or interstitial disease.
- New clubbing requires a targeted cardiopulmonary and systemic assessment even when saturation is normal; chronic cyanotic heart disease, liver disease and inflammatory bowel disease are important non-pulmonary associations.
Key red flags
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.
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.
Investigation priorities
Screen and trend arterial saturation while assessing signal reliability.
Management branches
Low saturation with acute symptoms, physiological compromise or increasing oxygen requirement.
- Use ABCDE, verify the oximeter signal, prescribe oxygen to the appropriate target and call senior help when distress, altered consciousness or haemodynamic compromise is present.
- Obtain an arterial blood gas when severe hypoxaemia, hypercapnia risk or ventilatory failure is possible; support ventilation rather than escalating oxygen alone when carbon dioxide and pH show failure.
- Treat the mechanism in parallel: airway obstruction, pneumonia or sepsis, pulmonary oedema, embolism, pneumothorax, hypoventilation or dyshemoglobinaemia according to the relevant emergency pathway.
- Reassess after each intervention, tracking device, flow, saturation, work of breathing, mental state and gas trend; a rising oxygen requirement is deterioration even if saturation remains in range.