Respiratory Failure

2 key processes ■ Ventilation ■ Diffusion

Ventilation ■ Process of moving air in and out of the lungs ■ Moves oxygen towards the alveoli and carbon dioxide away from them ■ Adequacy determined by the rate and depth of breathing

Ventilation

Think ■ What will happen to the PaO2 and PaCO2 in hypoventilation?

Diffusion ■ Transfer of oxygen and carbon dioxide across the alveolar membrane ■ Dependent upon Surface area of alveolar membrane Thickness of membrane Perfusion of alveoli

Diffusion ■ Carbon dioxide diffuses very easily ■ Oxygen diffusion is very sensitive to any changes in the membrane/perfusion

Hypoxia ■ 2 causes ■ Hypoventilation – amount of oxygen being transferred to alveoli is not enough to meet metabolic demand ■ V/Q mismatch – diffusion of oxygen is impaired

V/Q mismatch

Think… ■ What conditions can affect the surface area or thickness of the membrane?

Response to oxygen therapy ■ If the alveolus is partially obstructed, oxygenation can be improved by increasing the amount of inspired oxygen

Response to oxygen therapy ■ If the alveolus is completely obstructed then shunting occurs – i.e. blood bypasses the alveolus without being oxygenated. Oxygen will not improve SpO2

Response to increasing rate/depth of breathing ■ When hypoxia is caused by hypoventilation, increasing the rate/depth of breathing will improve oxygenation by increasing oxygen delivery to the alveoli ■ If diffusion is impaired then oxygenation does not improve with increased respiratory effort

Think… ■ When might perfusion of the alveolus be impaired?

Contact time ■ Blood has to stay in the alveolus long enough for diffusion to take place ■ Increased cardiac output shortens the time that blood stays in the alveolus ■ Normally this isn’t a problem – diffusion is fast enough

Think… ■ Why does a patient with pulmonary fibrosis cope with day-to-day life but become very hypoxic when they develop an acute illness?

Type 1 respiratory failure ■ PaO2 < 8kPa ■ PaCO2 < 6kPa ■ Whilst breathing room air ■ At sea level

Type 1 respiratory failure ■ Occurs when V/Q mismatch impairs oxygen diffusion ■ But why does the CO2 stay normal???

Hypercapnia ■ Remember that carbon dioxide diffuses easily ■ Even when diffusion is impaired in acute illness, this is easily compensated for with increased respiratory rate

Hypercapnia ■ So… ■ In order for CO2 to accumulate, ventilation (i.e. moving air in and out of the lungs) MUST be impaired

Ventilation

Hypoventilation ■ Results in… ■ Hypoxia ■ Hypercapnia ■ Can occur with or without underlying V/Q mismatch

Without V/Q mismatch… ■ Opiate overdose Opiates depress the central respiratory centre causing hypoventilation Hypoxia and hypercapnia No impairment of diffusion Give supplemental oxygen Naloxone to increase rate/depth of breathing

With V/Q mismatch… ■ Asthma Initially causes type I respiratory failure Airway inflammation and oedema impairs oxygen diffusion The patient gets tired – respiratory muscles weaken Hypoventilation occurs resulting in hypercapnia

Type II Respiratory Failure ■ PaO2 < 8kPa ■ PaCO2 > 6kPa ■ Whilst breathing room air ■ At sea level

Any Questions?

Oxygen therapy ■ Nasal ■ Fixed performance (venturi) ■ Non-rebreathe

Nasal ■ Usually well tolerated ■ Does not interfere with speaking/eating etc ■ FiO2 is dependent on flow rate – 1-4L/min Maximum around 32-36% O2 ■ Can dry out the nasal muscoa

Venturi ■ Air is entrained through the holes in the valve thus diluting the oxygen mix ■ Must be set at the correct flow rate

Non-rebreathe mask ■ Oxygen flows directly into the reservoir and the patient breathes from this ■ Avoids entrainment of room air and dilution of the oxygen mix

Non-invasive ventilation ■ 2 main types Continuous positive airway pressure (CPAP) Bi-level positive airway pressure (BiPAP)

CPAP ■ Provides high-flow pressurised oxygen ■ Continuous baseline pressure ■ Patient breathes on top of this ■ Splints open collapsed airways ■ Reduces work of breathing ■ Cannot augment tidal volume/rate

What is CPAP useful for?

BiPAP ■ Alternates between a baseline level of pressure and a higher level ■ Can ‘force’ air into the lungs to augment tidal volume ■ Can also increase respiratory rate using mandatory breaths

What is BiPAP useful for

Any questions?

An example ■ 24 year-old male athlete comes to the emergency department with shortness of breath and left sided chest pain that came on suddenly whilst running

A: Patent B: RR 24 SpO2 92%. Hyper-resonant to percussion left apex with reduced breath sounds over that area. Trachea central C: P80 reg BP 124/60. Normal heart sounds D: Alert E: Nil else to find

Chest x-ray

Pneumothorax ■ Primary No underlying lung pathology ■ Secondary Occurs in the context of chronic lung disease

Management ■ BTS algorithm

Any questions?