Question 7

College Answer

This is a core area of physiology that relates to everyday Intensive Care practice, thus it was 
expected that more than the observed number of candidates would have scored well. Candidates 
performed poorly purely from a lack of sufficient knowledge. Easy marks were to be gained 
purely by mentioning that compliance is defined by ∆V/∆P, the ∆P being the gradient from 
alveolar – intrapleural, normal values, static and dynamic compliance. Good answers would then 
include a mention of how static and dynamic compliance is measured (specifically how volume 
and pressures are measured.

Discussion

What is lung compliance?

• Respiratory compliance is defined as the change in lung volume per unit change in transmural pressure gradient. It is usually about 100ml/cm H₂O.
• Static compliance is defined as the change in lung volume per unit change in pressure in the absence of flow. It is composed of:
  • Chest wall compliance (usually 200ml/cm H₂O.
  • Lung tissue compliance (also usually cm H₂O.)
• Dynamic compliance is defined as the change in volume divided by change in pressure, measured during normal breathing, between points of apparent zero flow at the beginning and end of inspiration. Its components are:
  • Chest wall compliance
  • Lung tissue compliance
  • Airway resistance (which makes it frequency-dependent)
• Specific compliance is compliance that is normalized by a lung volume, usually FRC. It is used to compare compliance between lungs of different volumes (eg. child and adult)

How is lung compliance measured?

• Supersyringe method:
  • Static compliance is measured by inflating the lung in volume increments, usually 100ml
  • Time  (~23-3 seconds) is allowed for gas pressure to equilibrate between units with different time constants
  • This is the gold standard for measuring static compliance
  • The disadvantage is the time it takes to perform (minutes) and the need to disconnect the patient from the ventilator
• Constant flow method:
  • A low inspiratory flow (as low as 1.7L/min) is administered over 10-15 seconds
  • A low expiratory flow is then controlled to observe the expiratory pressure change
  • Because the flow is low, airway resistance is said to contribute minimally
  • This method has a tendency to underestimate inspiratory compliance and overestimate expiratory compliance
  • The advantage is that one does not need to disconnect the patient from the ventilator
• Multiple occlusions methods
  • During normal ventilator function, breath occlusions are repeated at different volumes, with normal breaths in between.
  • The advantage is that there is no need to discontinue normal ventilation, and that the process can easily be automated.
• Limitations of all methods of measuring static compliance:
  • All methods usually require the patient to be sedated and paralysed
  • There is the possible escape of gas into the pulmonary circulation, which gradually decreases the lung volume during measurement
  • Changes in gas pressure associated with increased humidity and temperature are ignored
• Measurement of dynamic compliance
  • Occurs during normal ventilator function, and makes no attempt to correct for pressure produced by airway resistance
  • Usually automated and integrated into modern ventilator function
• Measurement of volume
  •  Volume in modern ventilators is measured by measuring flow recordings over time, and reconstructing volume from these
  • Volume can also be measured directly by the supersyringe method
  • Some older ventilators (eg. piston models) measured volume directly as a part of their normal function.
• Measurement of pressure
  • ​​​​​​​Pressure in modern ventilators is measured using integrated silicon waver transducers
  • Alternatively, an aneroid manometer may be used to measure pressure during supersyringe inflation and deflation

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