Describe the factors that affect lung compliance. (03A14, 1990)
Static compliance is the compliance measured when there is no gas flow into or out of the lung.
Compliance is made up of lung compliance and chest compliance
=> 1/Ct = 1/Cl + 1/Ccw
(Ct = total compliance, Cl = lung compliance, Ccw = chest wall compliance)
Surface tension accounts for 70% of the elastic recoil
The slope of the P-V curve is not constant across different lung volumes.
At high lung volumes, compliance is reduced because more pressure is required to stretch the already stretched elastic tissues further.
At very low volumes, compliance is reduced because of closed airway and collapsed alveoli
=> increased surface tension
=> increased pressure is needed to re-open the airway/alveoli
=> reduced compliance
At the base of the lung, at very low volumes, compliance is even more reduced because of positive intrapleural pressure
Posture affects compliance by affecting the lung volume.
Restriction of chest expansion also affect lung volmen and chest wall compliance.
Contributes to stiffness of the lung, especially in the case of pulmonary congestion
Prolonged periods of hypoventilation without periodic deep breath may lead to reduced compliance.
Maybe related to atelectasis.
In animal studies, increased bronchocontriction can lower time constant and reduced dynamic compliance.
Static compliance is probably not affected.
In diseased lungs, where time constant for the alveolis are different, units with higher time constants are slow to fill and empty.
With higher respiratory rate, the problem worsens:
=> units with high time constant hypoventilates
=> less lung units participate in volume changes
=> dynamic compliance reduced.
With collapsed alveoli
=> greatly increased surface tension
=> very high pressure is required to re-open airway/alveoli
(see West p97)
NB: 1/compliance = elastance
Recoil pressure of the lung (transpulmonary) = alveolar pressure - intrapleural pressure
Recoil pressure of the chest = intrapleural pressure - atmospheric pressure
Measurement of compliance see Nunn p54
Elastic forces - lung vs chest wall (20050121-10)
? is compliance highest at FRC
? diagram of pressure volume loop