T2 L8: Compliance and impedance in ventilation

• What are the risks of mechanical ventilation?

  pneumothorax

  lung damage (barotrauma & volutrauma)

  hypotension & reduced cardiac output
• Main difference between mechanical ventilation & CPAP / BiPAP?

  mechanical ventilation: tube from machine directly seals to trachea

  CPAP & BiPAP: mask covers nose & mouth

  
• How can you exhale with CPAP?

  compliance & elastic recoil
• What is BiPAP?

  bi-level positive airway pressure

  also called BPAP / NIV (non-invasive ventilation)

  often used in COPD exacerbation (Type II Respiratory failure with respiratory acidosis)

  reduces work of breathing as:

  • iPAP (inspiratory positive airway pressure) higher
  • ePAP lower

  so air moves in
• What are the forces that oppose the ventilator trying to inflate the lungs?

  low compliance of lung and chest wall (elastic recoil)

  airway resistance and turbulence

  surface tension

  friction sliding pleura
• What is elastance?

  recoil

  high elastic recoil = low compliance = difficult to stretch
• How can lung compliance be altered in lung disease?

  interstitial fibrosis: lowers compliance (stiff)

  emphysema: higher compliance (less stiff)

  
• What is impedance?

  what it takes for a ventilator to push air into lungs
• What is 'work of breathing'?

  Work required to:

  1. stretch elastic tissues of chest wall and lungs (compliance)
  2. move inelastic tissues (eg pleura sliding against chest wall)
  3. push air through tubes (resistance)

  so amount of energy required to produce enough ventilation and respiration to meet metabolic demands of the body
• What pressures are used to measure work?

  Airway pressure

  Trans pulmonary pressure

  trans thoracic pressure

  trans respiratory pressure

  pleural pressure

  abdominal pressire

  alveolar pressure
• What is 'airway pressure'?

  difference in tracheal pressure and atmospheric pressure (W)

  
• What is 'trans-pulmonary pressure'?

  difference in pressure between alveolar space and pleural space

  
• What is the 'trans thoracic pressure'?

  difference in pressure between pleural space and atmospheric pressure

  
• What is 'trans respiratory pressure'?

  difference in pressure between alveolar space and atmosphere
• What is 'trans pulmonary pressure'?

  Pressure difference between alveoli and pleural space.
• What is 'trans-thoracic pressure'?

  pressure difference between alveoli and atmosphere

  should be negative at rest
• How does intrapleural pressure fluctuate during spontaneous tidal breathing?

  negative pressure overall

  gets slightly more negative during inhalation

  returns back to resting during exhalation

  
• How does alveolar pressure fluctuate during spontaneous tidal breathing?

  0 to negative to 0 during inhalation

  0 to positive to 0 during exhalation

  
• Ventilation (movement of air) occurs due to changes in what?

  transrespiratory pressure
• Breathing (expansion of lungs) occurs due to cycling changes in what?

  transpulmonary pressure
• What is 'peak transthoracic pressure'?

  pressure required to overcome both tissue-elastic and airway resistance

  to oppose recoil of lungs pulling the chest inward
• What is the effect of patients laying down (supine) on FRC (force respiratory capacity) and RV (residual volume)?

  smaller

  less volume for breathing, less margin for error

  as guts roll toward head and heart flops down onto lung
• What is the effect of anaesthetics on FRC?

  becomes smaller

  as intercostal muscles relax

  chest wall becomes looser and more compliant

  gravity and lung compliance pull chest wall inward
• At what point does the chest wall like to rest at naturally?

  65% of TLC

  
• What does a 'Static Relaxation-Compliance Curve' show?

  at rest (FRC), chest and lungs pull in opposite directions and are in equilibrium

  
• What is 'dynamic compliance of the lungs'?

  compliance of lung depends on how inflated it is

  lung is less compliant at higher volumes

  due to frictional resistance and turbulence

  
• Why don't inhalation and exhalation PV curves overlay?

  airway resistance

  • delay of volume increase during inhalation: due to pressure needed to pop open collapsed small airways
  • delay of volume decrease during exhalation: due to time needed for air to flow out of small airways