Pathophysiology of respiratory failure Flashcards
Define the oxygen cascade.
The stepwise drop in PO2 from the atmosphere to the mitochondria
In a normal healthy adult, what is the value of the A-a gradient?
<1.5 kPa
Will A-a gradient change in hypoventilation?
Explain this.
No, A-a gradient will be normal in ventilation because both PAO2 and PaO2 drop in parallel.
The alveolar oxygen level (PAO₂) decreases because less oxygen is entering the alveoli, and the arterial oxygen level (PaO₂) decreases because less oxygen is diffusing into the blood.
Which term is used to describe a high V/Q mismatch?
What does it mean in terms of ventilation and perfusion levels?
Dead space
There is adequate ventilation but not perfusion
Which term is used to describe a low V/Q mismatch?
What does it mean in terms of ventilation and perfusion levels?
Pulmonary shunt
There is adequate perfusion but not ventilation
What are the 4 types of hypoxia?
Hypoxaemic hypoxia
Anaemic hypoxia
Stagnant (circulatory) hypoxia
Cytotoxic hypoxia
What is the pathophysiology of hypoxaemic hypoxia?
Low PaO2
What is the pathophysiology of anaemic hypoxia?
O2 carrying capacity reduced
What is the pathophysiology of stagnant (circulatory) hypoxia?
O2 delivery reduced
What is the pathophysiology of cytotoxic hypoxia?
Mitochondria fail to utilize O2 effectively
Low partial pressure of inspired O2, hypoventilation, and a diffusion abnormality, such as fibrosis, cause which type of hypoxia?
Hypoxaemic hypoxia
Carbon monoxide poisoning causes which type of hypoxia?
Anaemic hypoxia
Cardiogenic shock and ischaemia cause which type of hypoxia?
Stagnant (circulatory) hypoxia
Cyanide poisoning causes which type of hypoxia?
Cytotoxic hypoxia
What is the value of severe hypoxaemia found in respiratory failure?
PaO2 <8 kPa
What is the difference between Type 1 and Type 2 respiratory failure?
Type 1 – one gas is wrong, oxygen – hypoxaemia (<8 kPa), PaCO2 is normal or low
Type 2 – two gases are wrong, oxygen and CO2 – hypoxaemia and hypercapnia (PaCO2 >6.5 kPa)
Type 1 respiratory failure is primarily caused by what?
V/Q mismatch
In hypoxaemia, what causes an increase in haemoglobin levels?
Increased erythropoietin production
The expected PaO2 of a patient on oxygen should be approximately how many kPa less than the % inspired concentration? For example, if a patient is on 40% oxygen, what would we expect their PaO2 to be?
10 kPa less than inspired oxygen
The patient would be expected to have PaO2 30 kPa
Between which 2 steps of the oxygen cascade does humidification affect PO2?
Atmosphere + tracheal air
Why does humidification reduce PO2?
Addition of water vapour, which lowers the space available for oxygen
Between which 2 steps of the oxygen cascade does the alveolar gas equation occur?
Tracheal air + alveoli
How is A-a gradient calculated?
PAO2 - PaO2
Which 4 factors are considered when calculating diffusion rate using Fick’s law?
Surface area
Diffusion coefficient
Pressure gradient
Membrane thickness
Sepsis causes which type of hypoxia?
Cytotoxic hypoxia
