S7 Hypoxaemia And Respiratory Failure Flashcards
Define hypoxaemia and hypoxia
Hypoxaemia - low pO2 in blood
Hypoxia - O2 deficiency at tissue level
What is respiratory failure and its two types?
Impairment in gas exchange causing hypoxia with or without hypercapnia.
Type 1 - hypoxia
Type 2 - hypoxia and hypercapnia
What are the three main causes of respiratory failure?
Hypoventilation - type 2
Ventilation/Perfusion mismatch - type 1
Diffusion defect - type 1
How does hypoventilation cause type 2 respiratory failure?
Lung and alveoli ventilation is reduced
Leads to a fall in alveolar/arterial pO2, and a rise in alveolar/arterial pCO2
Describe some causes of hypoventilation
Acute can be from a head injury or severe acute asthma. Needs urgent treatment.
Chronic can be from severe COPD, better tolerated
What are the effects of hypoxaemia?
Impaired CNS function
Confusion
Cyanosis
Arrhythmias
What is cyanosis?
Bluish discolouration of the skin and mucous membranes due to presence of >50g/litre of unsaturated Hb.
What compensatory mechanisms are there to increase oxygen delivery in chronic hypoxaemia?
Increased EPO secreted by kidney –> raised Hb
Increased 2,3, DPG
What are the effects of hypercapnia?
Respiratory acidosis
Confusion
Headache
Why might treatment of hypoxia worsen hypercapnia?
O2 removes stimulus for the hypoxic respiratory drive, alveolar ventilation drops causing worsening hypercapnia
How does V/Q mismatch cause type 1 respiratory failure?
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
What are some causes of V/Q mismatch?
Pneumonia
Asthma
COPD
RDS of newborn
How does a diffusion defect cause type 1 respiratory failure?
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
What are some causes of a diffusion defect?
Lung fibrosis - increased membrane thickness
Pulmonary oedema - decreases surface area
What causes cor pulmonale?
Right sided heart failure from chronic lung disease.
Chronic hypoxic vasoconstriction of pulmonary vessels results in pulmonary hypertension, right heart failure then cor pulmonale
