T2 L4: Resistance and Obstruction in Ventilation

• What is hypoxaemia?

  low oxygen levels in arterial blood

  mostly reflects function of lungs & pulmonary vasculature
• What is the oxygenation of blood measured by?

  ABG
• What is hypoxia?

  low levels of oxygen in one or many tissues

  typical saturation <94%

  reflect both hypoxaemia and peripheral ischaemia
• What is blood oxygen saturation measured by?

  pulse oximetry
• Does supplemental oxygen treat breathlessness?

  no

  breathlessness often about work of breathing

  treats hypoxaemia
• What are the characteristics of a simple face mask?

  supplies both nose and mouth with O2

  has holes in side of mask ot allow exhaled air to escape

  patient inhales some exhaled air still in mask

  safer if O2 supply fails: simply breathes atmospheric air through exhalation ports

  
• What are the characteristics of the nasal cannula as an oxygen delivery device?

  modest O2 delivery

  low O2 rates - depleted at fast respiratory rates

  least intrusive

  used when: hypoxaemia is modest and natural breathing is almost sufficient

  
• What are the characteristics of venturi valves & mask as an oxygen delivery device?

  controlled O2 delivery

  mixes high flow O2 with room air

  at fast respiratory rates still delivers pre-determined O2

  different colours deliver different conc of FiO2

  Used in COPD when CO2 retention is occurring

  
• What are the characteristics of a non-rebreather mask as an oxygen delivery device?

  maximum O2 delivery

  reservoir mask

  at fast respiratory rates still delivers max O2

  sudden rapid inspiration draws from reservoir

  used for all other serious hypoxaemia EXCEPT when positive pressure is needed

  
• How does O2 delivery change alveolar O2?

  increases FiO2
• How is alveolar gas different from inspired air?

  much more CO2

  less O2 (varies during breathing cycle)

  warmer

  more H2O
• How does exhalation occur with non-rebreather mask on?

  2 sets of one-way valves to maximise O2 delivery conc:

  1. For inhalation from bag
  2. For exhalation

  
• What are the risks of a non-rebreather mask?

  one-way valves can occasionally fail

  which could kill an unconscious patient

  must monitor O2 delivery & O2 saturation
• What does inhaling cause in the oxygen delivery devices?

  inhaling causes mixing of O2 in mask with air

  faster the O2 supply, the less mixing
• What are the target O2 saturations of different patients?

  Acutely unwell individual: 94-98%

  • may change due to concern of effects of ROS causing inflammation

  COPD or known risk of hypercapnoeic respiratory failure: 88-92%

  COVID: >92%
• When is clinical assessment indicated?

  when blood O2 saturation falls by 3% from the target range
• Oxygen is a medication and should be prescribed before provided.
• What is the effect of high altitude on oxygen?

  FiO2 remains same

  PAO2 is lower (as air pressure lower)

  pO2 is lower:

  • O2 diffusing into blood depends on partial pressure of O2 in air fraction
  • BP & HR increases up a mountain

  So a climber's SaO2 (O2 saturation) is much lower as Hb sat curve is the same.
• What is airway resistance due to?

  friction between a gas molecule moving faster than its neighboring molecule
• What is airway friction reduced by?

  more distance between a given gas molecule and walls

  fewer neighbors or less stickiness between them

  slower movement of gas
• What is the relationship between airway radius and laminar flow?

  resistance is inversely proportional to the 4th power of radius

  flow is inversely proportional to viscosity of fluid
• What is impedance?

  forces that resist air from being pumped in by a ventilator

  effects of: air resistance + tissue stiffness

  preventing inflation
• Why is it more difficult to pump air into lungs at first?

  build up of pressure in 'early' airways:

  airway resistance slows movement of air to alveoli

  also slows down chest wall expansion

  later this extra pressure disappears when pressure deep in lungs and in mouth reach equilibrium
• High airway pressure does NOT necessarily equal high alveolar pressure.
• How is the air resistance like in small vs larger airways?

  individually each small airway at bottom has higher resistance than larger airways

  however, in total small airways are lower resistance

  
• What is a fixed obstruction?

  one big blockage in a large airway

  examples: vocal cords do not relax, tumour in throat

  result: maximum flow rate (bronchi) is reduced but slower flow rates (smaller airways) are almost unaffected

  
• How can airways collapse?

  during forced exhalation thoracic pressure outside lungs exceeds pressure inside airways

  some small airways or alveoli collapse

  prevents gas behind collapse from leaving

  wall structure holds some airways open
• Why do patients with COPD/emphysema have collapsed airways?

  wall structure holds airways open

  COPD/emphysema breaks down wall structure
• How are collapsed airways re-opened?

  needs extra pressure inside airways during inspiration

  pops airways open, giving extra resistance
• What do airflow vs volume graphs show?

  1. Early expiration (large airways)
  2. Late expiration (small/mid airways)
  3. Early inspiration (large airways)
  4. late inspiration (Small/mid airways)

  
• How does variable resistance due to large obstructions in airway affect the breathing cycle?

  only affect one half: either inflation OR deflation
• How does an extra-thoracic obstruction (eg in throat) affect the breathing cycle?

  blocks during inflation

  during inhalation: inside airway has low pressure while pressure in mouth atmospheric
• How does an intra-thoracic obstruction (eg in bronchus) affect the breathing cycle?

  blocks during deflation

  during exhalation: inside airway pressure is lower than pleural pressure generated by chest wall compression

  because pressure transmission is slow and incomplete
• How is resistance affected when multiple airways are added together?

  added in parallel: resistance always gets smaller

  added in series: always gets bigger
• What type of patients need help ventilating?

  patients with type 2 respiratory failure
• What are the characteristics of BiPAP / NIV?

  provides high FiO2

  at higher-than-atmospheric pressures

  forehead strap to maintain pressure seal

  
• What are the potential oxygen delivery devices for patients with Type 2 respiratory failure?

  Nasal cannulae

  BiPAP/NIV

  With these devices, each breath is still determined spontaneously by patient.

  BiPAP adjusts its air pressure to “follow” the patient’s natural breathing.
• How is 'stenting' to hold open small airways / alveoli done?

  PEEP (positive end expiratory pressure) - small amount of air still remains in alveoli after expiration

  CPAP, BiPAP/NIV, mechanical ventilaiton

  counteract lung shrinkage driven by elastic recoil
• What is CPAP?

  continuous positive airway pressure

  for stenting or splinting small airways (avoid collapse)
• What is Obstructive Sleep Apnoea (OSA)?

  recurrent episodes of partial or complete upper airways collapse during sleep

  resulting in alveolar hypoventilation & subsequent hypoxaemia and hypercapnia

  event terminated by arousals (wake up and use voluntary muscles to open pharynx)