Pathophysiology of respiratory failure Flashcards
Describe what the oxygen cascade is.
The progressive decline in oxygen partial pressure (PO2) as oxygen moves from the atmosphere through various steps to the mitochondria
List 4 factors which affect the oxygen cascade.
Humidification
Alveolar gas equation
Diffusion
Physiological shunts
What increases the PAO2 (alveolar O2) more than hyperventilation?
An increase in FiO2 (fraction of inspired oxygen) – for example, oxygen therapy
What is the alveolar-arterial (A-a) gradient?
The difference between the mean calculated alveolar PO2 (PAO2) from the alveolar gas equation and the systemic arterial PO2 (PaO2) measured using an arterial blood gas
In a normal healthy adult, what is the value of the A-a gradient?
<1.5 kPa
What will the A-a gradient be like in hypoventilation?
Normal
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
List 5 causes of hypoxaemic hypoxia.
Low partial pressure of inspired O2 e.g. due to altitude
Hypoventilation
V/Q mismatch
Right to left cardiac shunts
Diffusion abnormality e.g. fibrosis
List 2 causes of anaemic hypoxia.
Anaemia
Carbon monoxide poisoning
List 2 causes of stagnant (circulatory) hypoxia.
Cardiogenic shock – reduced cardiac output
Ischaemia due to local lack of perfusion
List 2 causes of cytotoxic hypoxia.
Cyanide poisoning
Sepsis
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
List 6 causes of Type 2 respiratory failure.
Opiates
Chest wall deformity
Obesity
Neurological impairment
COPD
Near-fatal asthma
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
