SN3
SHORT NOTE
5M
★ High-yield
Classify hypoxia. Give the causes and effects of each type.
Definition
Hypoxia = inadequate supply of oxygen to the tissues to meet their metabolic requirements.
Classification
| Type | Mechanism | Causes | PaO2 | Response to O2 |
|---|---|---|---|---|
| Hypoxic | ↓ PaO2 → Hb inadequately saturated | High altitude, hypoventilation, V/Q mismatch, shunt, diffusion defect | ↓ | Good (except true shunt) |
| Anaemic | Normal PaO2, ↓ O2 carrying capacity | Anaemia, CO poisoning, methaemoglobinaemia | Normal | Poor |
| Stagnant (ischaemic) | ↓ blood flow to tissue | Heart failure, shock, arterial or venous obstruction | Normal | Poor |
| Histotoxic | Tissue cannot utilise O2 | Cyanide, sulphide (inhibit cytochrome oxidase) | Normal | Poor |
Effects
Acute
- CNS — restlessness, impaired judgement, euphoria → confusion → drowsiness → convulsions → coma
- CVS — tachycardia, ↑ cardiac output
- RS — hyperventilation (only if PaO2 < 60 mmHg)
- Cerebral cortex most sensitive — survives only 4–6 min of complete anoxia
Chronic
- Polycythaemia, clubbing
- Pulmonary hypertension → cor pulmonale
Anoxia, Hypoxaemia and Asphyxia
| Term | Meaning |
|---|---|
| Hypoxia | Inadequate O2 at tissue level |
| Hypoxaemia | ↓ PaO2 in arterial blood |
| Anoxia | Complete absence of O2 |
| Asphyxia | Hypoxia with hypercapnia |
Oxygen Therapy
- Hyperbaric O2 — O2 at 2–3 atmospheres → large rise in dissolved O2 → used in CO poisoning, gas gangrene, decompression sickness
- Oxygen toxicity — prolonged high FiO2 → free radical injury → substernal pain, convulsions (Paul Bert effect), and retinopathy of prematurity in newborns
Applied Aspects
- Only hypoxic hypoxia responds well to O2 therapy
- Histotoxic hypoxia → venous blood bright red; arteriovenous O2 difference reduced
- Cyanide poisoning → treated with sodium nitrite + sodium thiosulphate, or hydroxocobalamin
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. ★