Introduction to respiratory function and failure

Question 1

What is the function of the respiratory system?

Answer 1

Gas exchange to ensure blood is oxygenated and waste products such as CO2 can be removed

Question 2

How does oxygen get from the atmosphere to the tissues?

Answer 2

• O2 inhaled from atmosphere into alveoli within lungs
• O2 diffuses from alveoli into blood within pulmonary capillaries
• O2 transported in blood (binds to haemoglobin forming oxyhaemoglobin complexes)
• O2 diffuses into cells/tissues for use in aerobic respiration
• CO2 diffuses from respiring tissues to blood
• CO2 diffuses from alveoli, then expired into atmosphere

Question 3

How do changes in respiratory function enable blood gas homeostasis?

Answer 3

• Rate at which gases diffuse between alveoli and blood is proportional to the difference in partial pressure between the 2 areas (I.e Increase difference = increase rate of diffusion)
• If level of O2 or CO2 changes in alveoli, rate of exchange changes accordingly
• The body modulates alveolar PO2 and PCO2 by altering the rate of ventilation. Generally, an increase in ventilation = increase in PaO2 and a decrease in PaCO2 (Pa is the alveolar partial pressure
• By changing the rate of ventilation, the body modulates the rate of gas exchange and the level of O2 and CO2 in the blood.

Question 4

Describe the key functional requirements for efficient supply of oxygen from the atmosphere to tissues

Answer 4

• Adequate rate of ventilation - sufficient O2 provided to achieve effective rate of gas exchange and full oxygenation of Hb
• Gas exchange surface conducive to effective diffusion of O2 and CO2 (small diffusion distance, high surface area, high permeability)
• Efficient coupling of ventilation and perfusion within individual alveolar units
• Appropriate level of cardiac output - blood being pumped between lungs + tissues

If any of these impaired or insufficient for body’s O2 demands, blood O2 levels will decrease

Question 5

What is respiratory failure?
What does this lead to?

Answer 5

• Inadequate oxygenation of blood being facilitated by lungs relative to metabolic demands of the body)
• Clinical definition is <8kPa
• Inadequate O2 supply leads to organ dysfunction and injury. Insufficient removal of CO2 leads to acidosis as it reacts with H2O to form carbonic acid
• Depending on nature of pathology/dysfunction, CO2 removal may or may not be affected

Question 6

What types of pathologies can cause respiratory failure?

Answer 6

• Pneumonia - causes pleural effusion (fluid in pleural cavity), decreasing the area of expansion available to the lungs
• COPD
• Cystic fibrosis
• Anaemia
• Asthma
• Stroke
• Motor neurone disease
• Atelectasis

Question 7

What are the causes of respiratory failure?

Answer 7

• Insufficient ventilation
  
  • Obstruction of airways (chocking, asthma, COPD)
  • Failure to breath adequately (stroke, motor neurone disease)
• Insufficient gas exchange
  
  • Exchange surface dysfunction (pulmonary fibrosis)
  • VQ mismatch (atelectasis, pneumonia)
• Insufficient oxygen carrying capacity (anaemia)
• Insufficient oxygen in atmosphere (altitude)

Question 8

Describe type 1 respiratory failure

Answer 8

• • Caused by Decreased oxygenation of blood due to decreased diffusion of oxygen from alveoli to blood
  • Decreased PaO2 (hypoxaemia)
  • PaCO2 is normal, not affected
  • Clinical examples are pneumonia, ARDS, pulmonary embolism

Question 9

Describe type 2 respiratory failure

Answer 9

• Caused by Decreased Ventilation due to the decreased movement of air between the atmosphere and alveoli
• This causes a decrease in PaO2 (hypoxaemia)
• There is an increase in PaCO2 (hypercapnia)
• Clinical examples are asthma, COPD, cystic fibrosis

Question 10

Describe how asthma leads to type 2 respiratory failure

Answer 10

• Reduced rate of ventilation due to constricted airways
• Inadequate ventilation, alveolar CO2 increases, O2 decreases, impacting pressure gradients between alveoli and blood
• Reduced gas exchange between alveoli and blood

Question 11

Describe how atelectasis leads to type 1 respiratory failure

Answer 11

• One lung must ventilate for both lungs due to one lung having collapsed
• This leads to an increase in ventilation, but oxygen diffusion doesn’t increase due to the blood perfusing the dysfunctional lung being poorly oxygenated
• By the time blood perfusing different lung mixes together again, hypoxaemia remains

Question 12

What are the ways respiratory function can be clinically assessed, and what do they measure?

What are the normal ranges and how are they measured?

Answer 12

• Blood O2 saturation - Measures proportion of available Hb molecules that are currently saturated. Normal range is 95%+ and measured using pulse oximetry or ABG
• PaO2- Partial pressure of O2 in arterial blood - Normal range = 10.3 -13.3 kPa and measured using ABG from radial artery
• PaCO2 - Partial pressure of CO2 in arterial blood - Normal range 4.6-6 kPa - Measured using ABG