HYPOXIA

Hypoxia is deficiency of oxygen at the tissue level.

Hypoxaemia is low partial pressure of oxygen in the arterial blood.

Oxygen saturation < 88%

Oxygen chain

Oxygen in air (20.84%) – action of chest wall and diaphragm – patent respiratory passages – elastic alveoli – surfactant – wall of alveoli and capillary – RBC – haemoglobin – adequate venous and arterial circulation – capillary walls – tissue fluid – tissue cells - mitochondria

Causes of hypoxia

1. Extrinsic hypoxia

Inadequate oxygenation of lungs for extrinsic reasons, (extrinsic hypoxia).

Low oxygen in atmosphere (e.g. altitude)

Hypoventilation (e.g. neuromuscular disorder)

AMS - Acute Mountain Sickness

HACE HAPE

Immediate descent High altitude - anything over 8 200 feet

Maximum assent of 1 000 feet per day over 10 000 feet and a rest day every 3 days

9% suffer at 10 000 feet 53% at 15 000 feet 66% at over 16 400 feet

Adaptation to altitude

More RBC production Mitochondria increase.

Hypoventilation

Myasthenia gravis Guillain-Barre syndrome

Elapid toxicity Neuromuscular blockade

Opiate overdose, (apnoea may occur with small doses in the presence of chronic hypoxia)

2. Pulmonary hypoxia

Increased airway resistance, (e.g. asthma) Alveolar Problems, (e.g. emphysema)

Reduced membrane transport of oxygen, (e.g. RDS)

Increased airway resistance

Diseases causing bronchospasm e.g. asthma, allergic reactions.

Diseases causing airway obstruction.

Upper airway – angioedema Choking/Asphyxia (e.g. drowning)

Lower airway e.g. bronchitis, cystic fibrosis

Alveolar Problems

Emphysema - loss of alveolar surface area. Atelectasis - collapse of alveoli.

Reduced membrane transport of oxygen

Role of surfactant. Pneumothorax, haemothorax

Infections e.g. tuberculosis, pneumonia

3. Stagnant hypoxia and shunts

Venous - to - arterial shunts

Atrial septal defect Patent ductus arteriosus.

Sluggish circulation, stagnant hypoxia

Congestive cardiac failure Shock

Localised circulatory deficiency

Ischaemia and infarct e.g. intermittent Claudication.

Tissue oedema

Oedema - increases diffusion distances.

4. Anaemic hypoxia

Anaemia

Iron, B12, foliate deficiency Chronic haemorrhage Sickle cell

5. Histotic hypoxia

Poisoning of cellular enzymes (e.g. cyanide)

Reduced cellular metabolic activity, (e.g. toxicity, vitamin deficiency such as beri-beri)

Glucose + Oxygen ------ Energy + Water + Carbon dioxide

Oxidative phosphorylation ADP ------- ATP

ATP ----- ADP + P + Energy

Physiological response to hypoxia

Chemoreceptors communicate with the medulla oblongata via the glossopharyngeal and vagus nerves - induces increased sympathetic outflow – vasoconstriction, increased cardiac output, increased stimulation of diaphragm and intercostals muscles

Oxygen delivery to tissues

Oxygen flux = cardiac output X arterial oxygen saturation X haemoglobin concentration x 1.39 (the volume of oxygen in mls carried by 1g of haemoglobin)

Cardiovascular response

Increase in heart rate and stroke volume - tachycardia

Peripheral vasoconstriction - pallor

Increase in BP - hypertension

Increased sympathetic activity – possible sweating

In other words there is an attempted cardiovascular compensation

Cyanosis is a late sign indicating a low PaO₂, (caused by reduced oxyhaemoglobin in the capillary blood < 5g/dl)

Respiratory response

Tachypnoea – attempt to increase oxygen intake so increase SaO₂

Increased respiratory volumes

Any CO₂ increase will massively stimulate the respiratory centre in the medulla

Red cell response

Erythropoietin mechanism

More severe hypoxia

Myocardial hypoxia – bradycardia - reduction in BP – reduced myocardial perfusion

Accumulation of respiratory waste products in myocardium - dysrhythmias such as PVCs

Failure of compensatory increase in blood circulation

Cerebral hypoxia - depressed mental function, impaired, judgement, reduced

level of consciousness, irritability, confusion, lethargy and eventually coma, drowsiness,

excitement, disorientation, loss of time sense, headache, nausea, vomiting, dulled pain

sensitivity, possible convulsions.

Skeletal muscle hypoxia - reduced work capacity of muscle

`Oxygen lack not only stops the machine but wrecks the machinery` JS. Haldane (1860-1936)

Time to tissue damage

Brain - 1 minute Myocardium - 5 minutes Kidney and liver - 10 minutes

Skeletal muscle - 2 hours

Sudden drop in pO2 say to < 20 mm Hg (16,000 m) causes loss of consciousness in about 20 seconds, death in 4-5 minutes.

Treatment of hypoxia

Treatment of underlying cause

Oxygen therapy

Oxygen saturations

ECG

Cardiac and respiratory stimulants

Aids to ventilation

Blood gas analysis

Respiratory drive

Hypoxic drive

Hypercapnia drive