SN4
SHORT NOTE
5M
Write a short note on cyanosis.
Definition
Cyanosis = bluish discolouration of skin and mucous membranes due to excess reduced (deoxygenated) haemoglobin in the superficial capillaries.Appears when reduced Hb exceeds 5 g/dL (SaO2 < 85%).
Types
| Feature | Central | Peripheral |
|---|---|---|
| Mechanism | ↓ arterial saturation | Normal saturation; ↑ extraction due to sluggish flow |
| Sites | Tongue, lips, buccal mucosa and extremities | Extremities only — fingers, toes, nose, ear lobes |
| Temperature of part | Warm | Cold |
| Causes | Cyanotic CHD (right-to-left shunt), severe lung disease, high altitude, methaemoglobinaemia | Cold exposure, heart failure, shock, peripheral vascular disease |
| Effect of O2 | Improves (not if fixed shunt) | No change |
| Effect of warming | No change | Disappears |
Important Points
- Threshold is an absolute amount of reduced Hb, not a percentage
- Severe anaemia → NO cyanosis even in profound hypoxia (Hb 6 g/dL can never give 5 g/dL reduced Hb)
- Polycythaemia → cyanoses easily at mild desaturation
- CO poisoning → NO cyanosis; skin is cherry-red
- Cyanosis is a late and unreliable sign of hypoxia
Effect of Haemoglobin Level
| Hb level | Reduced Hb needed | Result |
|---|---|---|
| Normal (15 g/dL) | 5 g/dL | Cyanosis at SaO2 about 85% |
| Anaemia (6 g/dL) | Can never reach 5 g/dL | No cyanosis even in severe hypoxia |
| Polycythaemia (20 g/dL) | Reached easily | Cyanosis at mild desaturation |
Conditions Simulating Cyanosis
- Methaemoglobinaemia — brownish-blue; does not improve with O2; treated with methylene blue
- Sulphaemoglobinaemia
- Argyria (silver deposition), amiodarone pigmentation
- Deeply pigmented skin — assess the tongue and conjunctiva instead
Applied Aspects
- Use pulse oximetry and ABG — far more sensitive; never wait for cyanosis to appear
- Differential cyanosis (lower limbs blue, upper pink) → PDA with reversed shunt
All questions in Respiratory System
- Q1Describe the mechanics of breathing. Discuss the muscles involved and the pressure changes occurring during one normal respiratory cycle. Add a note on lung compliance. ★
- Q2Describe the transport of oxygen in the blood. Draw and explain the oxygen–haemoglobin dissociation curve and enumerate the factors that shift it. ★
- Q3Describe the nervous and chemical regulation of respiration. Add a note on the Hering–Breuer reflex. ★
- Q4Describe the transport of carbon dioxide in the blood. Explain the chloride shift and the Haldane effect. ★
- Q5Define the ventilation–perfusion ratio. Describe its regional variation in the upright lung and discuss the consequences of V/Q mismatch. ★
- SN1Write a short note on pulmonary surfactant. ★
- SN2Define lung volumes and capacities. Give their normal values and state which of them cannot be measured by spirometry. ★
- SN3Classify hypoxia. Give the causes and effects of each type. ★
- SN4Write a short note on cyanosis.
- SN5Write a short note on Cheyne–Stokes breathing.
- SN6Define dead space. Describe its types and explain its significance in alveolar ventilation. ★
- SN7Describe the physiological changes that occur during acclimatisation to high altitude. ★