The four categories of hypoxia
1. Hypoxic hypoxia
2. Anaemic hypoxia
3. Stagnant/ischaemic/hypoperfusion hypoxia
4. Histotoxic hypoxia
1. Hypoxic hypoxia
PO2 of the arterial blood is reduced
i.e
- Congenital cyanotic heart disease (right to left shunt)
- Ventilation-perfusion imbalance
2. Anaemic hypoxia
The arterial PO2 is normal but the amount of Hb available to carry O2 (oxygen carrying capacity) is reduced
i.e.
3. Stagnant/ischemic/hypoperfusion hypoxia
Blood flow to a tissue is so low that adequate O2 is not delivered to the tissues
PO2 and Hb concentration maybe normal
i.e.
4. Histotoxic hypoxia
The amount of O2 delivered to the tissues is adequate but, because of the action of a toxin, the tissue cells cannot use the O2 supplied to them.
i.e.
Why is hypoxia so dangerous?
- Effects on the brain
- PO2 in inspired air at sea level: 160 mmHg
- Sudden drop of inspired PO2 to 20 mmHg:
1. Loss of consciousness in 10-20 sec!
2. Death in 4-5 minutes!
- In generalized forms of hypoxia – brain is affected first
- Effects on the respiration
- Hypoxia stimulates the peripheral chemoreceptors and increases the respiratory rate and depth
- A normal individual is not conscious of respiration – until ventilation is doubled!
- Breathing is not uncomfortable – until ventilation is tripled/quadrupled!
Dyspnoea
- Difficult or laboured breathing in which the subject is conscious of shortness of breath.
- Hypercapnia, and to a lesser extent hypoxia causes dyspnoea
Hyperpnoea
- Increase in the rate or depth of breathing regardless of the patient’s subjective sensation.
Tachypnoea
Cyanosis
- Definition:
- Bluish discolouration of the skin and mucous membrane, appearing when the reduced Hb level in the capillaries is more than 5 g/dL.
- Easily seen in: nail beds, mucous membranes, ear lobes, lips and fingers.
Severity of cyanosis depends on:
1. Total amount of Hb- easily seen in polycythaemia
2. Degree of Hb unsaturation
3. State of the capillary circulation
Cyanosis does not occur in:
- Anaemic hypoxia- low Hb
- CO poisoning -CarbonmonoxyHb is cherry-red
- Histotoxic hypoxia-Blood gas content is normal
High circulating levels of methaemoglobin also shows a bluish discoloration similar to cyanosis.
Other symptoms of hypoxia:
1. Impaired judgement
2. Drowsiness
3. Dulled pain sensibility
4. Excitement
5. Disorientation
7. Anorexia, nausea, vomiting
8. Hypertension
9. Increased rate of ventilation
Effects of hypoxia at cellular level
- Hypoxia causes the production of transcription factors (hypoxia-inducible factors; HIFs).
- These are made up of α and β subunits.
- In normally oxygenated tissues, the α subunits are rapidly destroyed.
- Effects of hypoxia at cellular level cont.
- However, in hypoxic cells, the α subunits dimerize with the β subunits.
- These dimers activate genes that produce angiogenic factors and erythropoietin.