Respiratory Failure Flashcards
What is hypoxaemia?
Low O2 in blood
What is hypoxia?
O2 deficiency at tissue level
What is the normal range for O2 sats?
94-98%
What is the normal range for pO2?
9.3-13.3 kPa
What characterises type 1 respiratory failure?
Low pO2 (less than 8 kPa) or O2 sats < 90% pCO2 normal or low
What characterises type 2 respiratory failure?
Low pO2 and high pCO2
What may cause hypoxia?
1) Low inspired pO2
2) Hypoventilation (resp pump failure)
3) V/Q mismatch
4) Diffusion defect
5) Right to left shunt
Where is central cyanosis seen?
In oral mucosa, tongue, lips
Where is peripheral cyanosis seen?
In fingers and toes
What is the difference between ventilation and perfusion in the lung?
Ventilation = AIR that reaches alveoli Perfusion = BLOOD that reaches alveoli
In hypoventilation, what happens to the amount of O2 entering the blood and CO2 entering alveolus per minute?
Remains unchanged as the metabolic rate is the same - perfusion
Why does hypoventilation cause hypoxaemia?
Alveolar pO2 level falls as the alveoli are poorly ventilated, therefore arterial pO2 falls
True or False:
Hypoventilation ALWAYS causes hypercapnia
True
Alveolar + arterial pCO2 increases
Which kind of respiratory failure does hypoventilation cause?
Type 2 respiratory failure
both hypoxia and hypercapnia
What could cause acute hypoventilation?
- Opiate overdose
- Head injury
- V severe acute asthma
What could cause chronic hypoventilation?
Severe COPD
What compensatory mechanisms are seen in chronic hypoxaemia?
- Increased EPO secreted by kidney to raise Hb
- Increased 2,3-DPG (shifts curve to right - favours O2 release at tissues)
What does chronic hypoxic vasoconstriction of pulmonary vessels result in?
Pulmonary hypertension -> right HF -> cor pulmonale
What are the effects of hypercapnia?
Respiratory acidosis
Impaired CNS function
Peripheral vasodilation and cerebral vasodilation (headache)
What are the effects of chronic CO2 retention on central chemoreceptors?
Low CSF pH corrected by choroid plexus cells which secrete HCO3 in to CSF
pH of CSF returns to normal so central chemoreceptors are no longer stimulated but the pCO2 in the blood is still high
In chronic type 2 respiratory failure, why may treatment with O2 worsen hypercapnia?
1) O2 removes stimulus for hypoxic respiratory drive, alveolar ventilation drops worsening hypercapnia
2) Correction of hypoxia removes pulmonary hypoxic vasoconstriction, leading to increased perfusion of poorly ventilated alveoli and diverting blood away from better ventilated alveoli
What is the V/Q ratio of optimal gas exchange?
1
What happens to alveolar pO2 and pCO2 when V/Q ratio is less than 1?
PO2 falls and PCO2 rises
What happens to alveolar pO2 and pCO2 when V/Q ratio is more than 1?
PO2 rises and PCO2 falls
Give an example of when V/Q ratio would be greater than 1
Eg in hyperventilation due to anxiety
When does V/Q mismatch occur?
In disorders where some alveoli are being poorly ventilated eg asthma and pneumonia
Describe V/Q mismatch
If the V/Q ratio drops to <1, i.e. the perfusion is higher than the ventilation, the pO2 falls and the pCO2 in the alveolus will rise.
Hypoxic vasoconstriction can occur, diverting blood to better ventilated parts of the lung. However, these alveoli are unlikely to be able to take up much more O2 than they already are.
As a result, the pO2 remains low, which acts as a stimulus, causing hyperventilation, resulting in either normal or low CO2 levels.
Give an example of where a V/Q mismatch occurs due to reduced perfusion
Pulmonary embolism
