Respiratory Failure - Causes & Complications

Question 1

State the main causes of hypoxaemia and tissue hypoxia

Answer 1

• Insufficient ventilation
• Insufficient rate of gas exchange
• Insufficient oxygen carrying capacity
• Insufficient oxygen in the atmosphere

Question 2

Describe type 1 respiratory failure

Answer 2

There is reduced oxygenation (due to altitude, VQ mismatch or pneumonia)
PA oxygen - normal
Pa oxygen - decreased
Pa carbon dioxide - normal
pH - normal
There is an increase PA - Pa oxygen gradient

Question 3

Describe type 2 respiratory failure

Answer 3

There is reduced ventilation (due to asthma, COPD, NMD, drug overdose or apnoea)
PA oxygen - decreased
Pa oxygen - decreased
Pa carbon dioxide - increased
pH - decreased

Question 4

State the equation used to find the alveolar oxygen pressure and some common values that can be inputed

Answer 4

Alveolar oxygen pressure (PAO2) = fraction of oxygen in inspired gas x (barometric pressure - H2O vapour pressure) - (arterial carbon dioxide pressure / respiratory exchange ratio)

Or simply alveolar oxygen pressure = oxygen inspired - oxygen consumed

Fraction of oxygen in inspired gas - 0.21 in ambient air
Barometric pressure - 100 kPa at sea level
H20 vapour pressure - 6 kPa in humidified air
Pa CO2 - measured in patient
RER - assume 0.8 unless told otherwise
PAO2 - calculated

Question 5

How to calculate respiratory exchange ratio?

Answer 5

Respiratory exchange ratio = VCO2 produced / VO2 consumed

Question 6

State the main determinant of RER (respiratory exchange ratio) and some values it can be based on this

Answer 6

Main determinant of RER is the metabolic substrate being used -
Carbohydrate - 1
Protein - 0.9
Fatty acids - 0.7

Question 7

State what can cause hypoventilation via airway obstruction

Answer 7

• Asphyxia/choking
• Obstructive sleep apnoea
• Bronchial obstruction (asthma, bronchitis, cystic fibrosis)

Question 8

State what can cause hypoventilation via an issue with initiation or mechanics of breathing

Answer 8

• Drug overdose
• Neuromuscular disease - MND, muscular dystrophy
• Stroke
• COPD, fibrosis, NRDS, pneumothorax

Question 9

State the reasons why oxygenation may be an issue

Answer 9

• Increased surface area (emphysema)
• Increased basement membrane thickness (fibrosis)
• Increased thickness of fluid layer/oedema
• Alveolar injury
• VQ mismatch - issue with ventilation perfusion coupling

Question 10

Describe the two types of VQ mismatch and what they are caused by

Answer 10

• VQ mismatch -
  Reduced perfusion in lung regions increases the VQ ratio - dead space effect
• Caused by -
  Heart failure (cardiac arrest)
  Blocked vessels (pulmonary embolism)
  Loss/damage of capillaries (emphysema)
• Reduced ventilation -
  Limited diffusion decreases the VQ ratio - shunt effect
• Caused by -
  Pneumonia
  Atelectasis
  Respiratory distress syndrome

Question 11

State some oxygen transport disorders and what causes them

Answer 11

• Oxygen transport disorders -
  Anaemia - reduced RBCs or haemoglobin
  CO poisoning
• Caused by -
  Iron deficiency (reduced production)
  Haemorrhage (increased loss)