S7 Hypoxaemia And Respiratory Failure

Question 1

Define hypoxaemia and hypoxia

Answer 1

Hypoxaemia - low pO2 in blood

Hypoxia - O2 deficiency at tissue level

Question 2

What is respiratory failure and its two types?

Answer 2

Impairment in gas exchange causing hypoxia with or without hypercapnia.
Type 1 - hypoxia
Type 2 - hypoxia and hypercapnia

Question 3

What are the three main causes of respiratory failure?

Answer 3

Hypoventilation - type 2
Ventilation/Perfusion mismatch - type 1
Diffusion defect - type 1

Question 4

How does hypoventilation cause type 2 respiratory failure?

Answer 4

Lung and alveoli ventilation is reduced

Leads to a fall in alveolar/arterial pO2, and a rise in alveolar/arterial pCO2

Question 5

Describe some causes of hypoventilation

Answer 5

Acute can be from a head injury or severe acute asthma. Needs urgent treatment.
Chronic can be from severe COPD, better tolerated

Question 6

What are the effects of hypoxaemia?

Answer 6

Impaired CNS function
Confusion
Cyanosis
Arrhythmias

Question 8

What is cyanosis?

Answer 8

Bluish discolouration of the skin and mucous membranes due to presence of >50g/litre of unsaturated Hb.

Question 9

What compensatory mechanisms are there to increase oxygen delivery in chronic hypoxaemia?

Answer 9

Increased EPO secreted by kidney –> raised Hb

Increased 2,3, DPG

Question 10

What are the effects of hypercapnia?

Answer 10

Respiratory acidosis
Confusion
Headache

Question 11

Why might treatment of hypoxia worsen hypercapnia?

Answer 11

O2 removes stimulus for the hypoxic respiratory drive, alveolar ventilation drops causing worsening hypercapnia

Question 12

How does V/Q mismatch cause type 1 respiratory failure?

Answer 12

When V/Q < 1, pO2 is low so central chemoreceptors stimulated hyperventilation.
However after hyperventilation the affected alveoli are still poorly ventilated, but the normal alveoli aren’t and so shall have increased perfusion and V/Q > 1.
This can lower pCO2 but shall only have a little effect on pO2 as Hb shall be saturated.
So low pO2 with normal (or low) pCO2 - Type 1

Question 13

What are some causes of V/Q mismatch?

Answer 13

Pneumonia
Asthma
COPD
RDS of newborn

Question 14

How does a diffusion defect cause type 1 respiratory failure?

Answer 14

Poor diffusion of oxygen across alveolar membrane, CO2 is unaffected as it is more soluble.
Hypoxia stimulates peripheral chemoreceptors, hyperventilation, further increasing CO2 removal.
So pO2 low and pCO2 normal or low - Type 1

Question 15

What are some causes of a diffusion defect?

Answer 15

Lung fibrosis - increased membrane thickness

Pulmonary oedema - decreases surface area

Question 16

What causes cor pulmonale?

Answer 16

Right sided heart failure from chronic lung disease.
Chronic hypoxic vasoconstriction of pulmonary vessels results in pulmonary hypertension, right heart failure then cor pulmonale