Session 7: Hypoxia & Respiratory Failure Flashcards
Steps of the oxygen supply chain.
Air -> Airways -> Alveolar gas -> Alveolar membrane -> Arterial blood -> Regional arteries -> Capillaries -> Tissues
Define hypoxia.
Reduced oxygen at tissue level. This can be abnormalities of any point of the oxygen supply chain which can lead to hypoxia.
Define hypoxaemia.
Decrease in pO2 in the blood.
Is it possible to be hypoxic without hypoxaemia?
Yes.
Explain how it is possible to be hypoxic without being hypoxaemic.
E.g. severe anaemia where there is reduced oxygen carrying capacity due to low Hb levels. This leads to hypoxia but not reduced pO2 in arteries.
Also poor perfusion which can lead to hypoxia of the affected tissue like in an MI despite having normal gas exchange in lungs and normal pO2 in arteries.
What is respiratory failure defined as?
When arterial pO2 falls below 8kPa or 60 mmHg when breathing air at sea level.
Define Type 1 Respiratory failure
A pO2 below 8kPa
Normal or low pCO2
Define type 2 respiratory failure.
A pO2 below 8kPa
High pCO2 of above 6.7 kPa
Effects of hypoxia.
Impaired CNS function
Central cyanosis
Cardiac arrhythmias
Hypoxic vasoconstriction of the pulmonary vessels
Effects of hypercapnia.
Respiratory acidosis
Impaired CNS functions like drowsiness, confusion, coma and tremors.
Peripheral vasodilation leading to warm hands and bounding pulse.
Cerebral vasodilation leading to headache.
What is needed to maintain arterial pO2 at a normal level?
Normal inspired pO2
Normal alveolar ventilation
Matching V/Q
Normal alveolar capillary membrane (no thickening)
No right to left shunts.
Why is it important to not have a right to left shunt to maintain arterial pO2?
CO from right heart needs to pass through gas exchanging alveoli
Considering the factors needed to maintain normal arterial pO2.
Why might hypoxaemia arise?
Low inspired pO2 at e.g. high altitude.
Hypoventilation
V/Q mismatch
Diffusion impairment
Right to left shunts.
Intra-lung shunt like ARDS
What type of respiratory failure is hypoventilation?
Type 2 resp
What type of respiratory failure is V/Q mismatch?
Type 1 resp failure
What type of resp failure is diffusion impairment?
Type 1 resp failure
Explain hypoxaemia due to high altitude.
High altitude means lower atmospheric pressure. This leads to lower arterial and alveolar pO2.
This leads to stimulation of peripheral chemoreceptors and causes hyperventilation. The end result is low pO2 and low pCO2.
Physiological adaptations to survive in high altitudes for a long time.
Polycythaemia (increased Hb) by EPO
Increased 2-3 DPG
Increased capillary density at tissue level
At what O2 sat and pO2 does tissue damage start to occur
<90%
<8kPa
What does chronic hypoxic vasoconstriction of pulmonary vessels result in?
Pulmonary hypertension
Right heart failure
Cor pulmonale
Give causes of hypoventilation.
Head injury, drug overdose
Resp muscle weakness (damage to muscles or nerves)
Mechanical problems of the chest wall like scoliosis, kyphosis, morbid obesity or rib fractures.
Lung fibrosis
Airway obstructions like in severe asthma or late stages of COPD.
Explain the compensatory effects of chronic hypoxia.
Polycythaemia by increased EPO by kidneys leading to increased Hb levels.
Increased 2,3 DPG to better unload O2
Hypoxia induced vasoconstriction which will lead to pulmonary hypertension and eventually right ventricular hypertrophy and right heart failure.
Effects of chronic hypercapnia.
Respiratory acidosis is compensated by retention of HCO3- by the kidneys to restore CO2/HCO3- ratio.
Central chemoreceptors in persisent hypercapnia leads to choroid plexus secreting HCO3- into CSF to restore the CO2/HCO3- ratio in the CSF. This resets the central chemoreceptors to higher CO2 levels.
Peripheral chemoreceptors lead the hyperventilation after this.
‘Pink puffers’
What is the most common cause of hypoxaemia?
V/Q mismatch
