Cyanosis

💙 Cyanosis is a bluish discoloration of the skin, lips, or mucous membranes due to increased levels of deoxygenated haemoglobin (>5 g/dL) in the capillaries. It signals underlying hypoxaemia or abnormal haemoglobin and must be assessed urgently in the clinical context.

📖 Definition & Pathophysiology

• 🔵 Central cyanosis: Bluish tongue, lips, mucous membranes — caused by systemic arterial desaturation (e.g. congenital heart disease, severe lung disease).
• 🖐️ Peripheral cyanosis: Bluish fingers, toes, nail beds — due to reduced peripheral blood flow (e.g. cold exposure, shock, heart failure).
• 🧪 Pathophysiology: Visible once deoxygenated Hb exceeds ~5 g/dL; anaemia may mask cyanosis, while polycythaemia makes it more obvious.

🔎 Clinical Assessment

• 👀 Inspection: Tongue/lips (central), extremities (peripheral), nail beds, conjunctivae.
• 🩺 Associated signs: Clubbing (chronic hypoxia), oedema (heart failure), murmurs, crackles, respiratory distress.
• 🧑‍⚕️ History: Onset (sudden vs chronic), breathlessness, chest pain, cardiac or respiratory history, drug exposure (e.g. dapsone causing methaemoglobinaemia).

⚡ Causes of Cyanosis

🔬 Investigations

• 🩸 Arterial blood gas (ABG): PaO₂, SaO₂, lactate.
• 📟 Pulse oximetry: quick screening (beware of methaemoglobinaemia causing falsely low values).
• 🫁 CXR: pneumonia, pulmonary oedema, lung collapse.
• ❤️ ECG & Echocardiogram: arrhythmia, cardiac failure, congenital heart disease.
• 🧪 Special tests: MetHb levels, carboxyhaemoglobin if CO exposure suspected.

💊 Management Principles

• ⚡ Immediate: Give high-flow oxygen, monitor sats and vitals, establish IV access.
• 🫁 Respiratory causes: Treat underlying disease (antibiotics, bronchodilators, anticoagulation for PE, diuretics for pulmonary oedema).
• ❤️ Cardiac causes: Resuscitate, manage arrhythmias, treat heart failure, urgent cardiology/surgery for congenital shunts.
• 🧪 Haematological causes: Stop offending drug, give methylene blue for methaemoglobinaemia, venesection if polycythaemia.
• ❄️ Peripheral cyanosis: Warm patient, restore circulation, treat shock.

🚩 Red Flags

• Sudden-onset central cyanosis in a previously well patient.
• Cyanosis not improving with oxygen (suggests cardiac shunt or abnormal haemoglobin).
• Associated chest pain, collapse, or altered mental state.
• Paediatric cyanosis — always consider congenital heart disease.

🧠 Key Teaching Points

• Cyanosis depends on absolute, not relative, desaturation — so anaemia can hide it, and polycythaemia exaggerates it.
• Differentiate central vs peripheral by checking mucous membranes.
• Always correlate with oxygen saturations and ABG — don’t rely on visual inspection alone.
• In congenital heart disease, cyanosis may be present since birth and worsen with exertion.

Cases — Cyanosis

• Case 1 — Congenital Heart Disease (Tetralogy of Fallot): A 4-year-old boy presents with bluish lips and clubbing. He squats after exercise to relieve breathlessness. Exam: ejection systolic murmur at left upper sternal edge. O₂ sats 85% on room air. Diagnosis: Central cyanosis due to right-to-left shunt in Tetralogy of Fallot.
• Case 2 — Severe COPD Exacerbation: A 68-year-old man with a 40-pack-year smoking history presents with dyspnoea, wheeze, and confusion. He has peripheral cyanosis with oxygen saturations of 80% on air. ABG: hypoxaemia with hypercapnia. Diagnosis: Central cyanosis due to severe type II respiratory failure in COPD.
• Case 3 — Methemoglobinaemia (Drug-Induced): A 30-year-old woman develops sudden-onset cyanosis after taking dapsone for dermatitis herpetiformis. O₂ saturation remains low despite high-flow oxygen. Blood appears chocolate-brown. Diagnosis: Cyanosis from methemoglobinaemia.
• Case 4 — Peripheral Cyanosis (Low Cardiac Output): A 72-year-old man with advanced heart failure presents with cold extremities and bluish fingers. O₂ saturation normal (95%), but capillary refill prolonged. Diagnosis: Peripheral cyanosis due to poor peripheral perfusion in low-output heart failure.

Teaching Commentary 🔵

Cyanosis is the bluish discolouration of skin/mucous membranes due to increased deoxygenated Hb (>5 g/dL). - Central cyanosis: hypoxaemia or abnormal Hb (affects lips/tongue) — causes include congenital heart disease, severe lung