T2 L8: Compliance and impedance in ventilation

Cards (27)

  • 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