4. Hypoxia

Question 1

What is hypoxia?

Answer 1

O2 deficiency at tissue level.

Question 2

What is hypoxiaemia?

Answer 2

O2 deficiency in the blood.

Question 3

What is the transport chain of O2 from the air to the tissues?

Answer 3

Airways, alveoli, pulmonary capillary, alveolar capillary membrane, bound to Hb, pulmonary veins, left heart, cardiac output, aorta, regional arteries, capillary blood.

Question 4

What causes poor regional perfusion?

Answer 4

Arterial narrowing or occlusion due to atheromas.

Question 5

What are the symptoms of poor regional perfusion?

Answer 5

Claudication, angina, neurological deficit.

Question 6

What is the presentation of claudication?

Answer 6

Pain on exercise, relieved by rest, weak/absent peripheral pulses.

Question 7

What is the cause of global poor perfusion?

Answer 7

Shock.

Question 8

What are the types of shock?

Answer 8

Hypovolaemic, cardiogenic, mechanical, distributive.

Question 9

What is the pathophysiology of hypovolaemic shock?

Answer 9

Loss of volume -> low venous return -> low preload -> low CO -> fall in arterial pressure -> baroreceptor mediated sympathetic reflexes -> venoconstriction and autotransfusion.

Question 10

What is the pathophysiology of cardiogenic shock?

Answer 10

Damage to myocardium -> heart pumps too little -> arterial pressure falls -> baroreceptors stimulate sympathetic outflow -> reduced perfusion -> skin is cold, pale and clammy, tachycardia.

Question 11

What is the pathophysiology of septicaemia?

Answer 11

Endotoxins released by bacteria -> overwhelming vasodilatation -> dramatic fall in TPR -> CO can’t keep up.

Question 12

What is the pathophysiology of anaphylactic shock?

Answer 12

Release of histamine from mast cells -> overwhelming vasodilatation -> dramatic fall in TPR -> drop in blood pressure.

Question 13

What are the consequences of poor perfusion in shock?

Answer 13

Tissue damage - lactic acid produced from anaerobic metabolism, tissues begin to die, release vasodilator mediators, circulatory collapse, multiorgan failure.

Question 14

What are the causes of poor oxygen carrying capacity?

Answer 14

Anaemia, CO poisoning.

Question 15

How can blood oxygenation be measured?

Answer 15

Pulse oximeter, arterial blood gas analysis.

Question 16

Which type of respiratory failure does ventilatory failure cause?

Answer 16

Type 2.

Question 17

How does ventilatory failure cause type 2 respiratory failure?

Answer 17

Not enough O2 enters alveoli, not enough CO2 leaves so pO2 is low and pCO2 is high.

Question 18

What are the causes of ventilatory failure?

Answer 18

Poor respiratory effort - respiratory centre depression, muscle weakness; chest wall problems - scoliosis/kyphosis, trauma, pneumothorax; stiff lungs - severe fibrosis; hard to ventilate lungs - high airway resistance in late stages of COPD and severe asthma.

Question 19

How does the body respond to chronic CO2 retention?

Answer 19

CSF acidity corrected by choroid plexus and central chemoreceptors ‘reset’ to higher CO2 levels, persisting hypoxia.

Question 20

What are the effects of hypoxia on pulmonary arterioles?

Answer 20

Pulmonary hypertension, right heart failure, cor pulmonale.

Question 21

Which type of respiratory failure does poor diffusion across the alveolar membrane cause and why?

Answer 21

Type 1 - O2 diffusion affected by CO2 is so soluble that its diffusion isn’t affected.

Question 22

What can cause poor diffusion across the alveolar membrane?

Answer 22

Fibrotic lung disease, pulmonary oedema, emphysema.

Question 23

What are the causes of diffuse lung fibrosis?

Answer 23

Fibrosing alveolitis, asbestosis, extrinsic allergic alveolitis, pneumoconiosis.

Question 24

Which type of respiratory failure does ventilation perfusion mismatch lead to?

Answer 24

Type 1.

Question 25

What can cause V/Q mismatch?

Answer 25

Pneumonia, acute asthma, RDS of newborn; pulmonary embolism.

Question 26

How do pulmonary embolisms cause V/Q mismatch?

Answer 26

Blood is redirected to other parts of the lung, ventilation of affected parts is wasted, resulting hypoxia.