Resistance to Breathing:
- ~65% elastic resistance
- ~35% non-elastic resistance
- Airflow ~80%
- Viscous ~20%
- For an elastic body, compliance is the relation between distending force and length
- In the lung, this is the change in volume for a given change in pressure = ΔV/ΔP = liters/cmH2O
- A measure of the elasticity or distensibility of pulmonary or thoracic tissues
- Static compliance - pulmonary compliance during periods without gas flow, such as an inspiratory pause. Calculated using pressure at an inspiratory hold - PEEP.
- Dynamic compliance - pulmonary compliance during periods with gas flow, such as active inspiration. Calculated using peak inspiratory pressure - PEEP. Always less than or equal to static compliance.
- Specific compliance - compliance per unit of lung volume - or (ΔV/ΔP)/V, therefore a neonate's specific compliance is the same as an adults.
Elastic Recoil of the Lungs:
- Tendency of the elastic lung tissue to recoil from the chest wall - resulting in sub-atmospheric intrapleural pressure
- Mean intrapleural pressure ~4-5 cmH2O - sub-atmospheric
- Can be estimated by an oesophageal balloon catheter
- Transrespiratory pressure: PRS = PAlveolar - PAtmospheric
- Transpulmonary pressure: PL = PAlveolar - PIntrapleural
- Transthoracic pressure: PCW = P Intrapleural - PAtmospheric
Static Lung Compliance
- 200ml/cmH2O - normal value for a 70kg adult
- Decreases as lung volume increases due to limitations of the non-elastic components of lung/chest wall, also decreased at low volumes
- Forms a sigmoid curve with varying degrees of hysteresis, with the volume at any given pressure being greater during deflation
- P/V curve never reaches zero due to trapping of gas in small airways. which increases in volume with increasing age and lung disease
- The varying slope of the curve and differential intrapleural pressure down the lung is also partially responsible for the different ventilation of various lung segments
- Supine position decreases FRC and therefore absolute compliance, but won't affect specific compliance
- Measured by the patient taking a breath from a spirometer and holding it until transpulmonary pressure difference becomes stable, and repeated to produce a pressure/volume curve.
- Compliance is the slope of the pressure/volume curve.
Factors Affecting Static Compliance
- FRC is proportional to Compliance - therefore age, body size and posture alters compliance (but age independent of FRC doesn't affect compliance)
- Lung volume is proportional to Compliance - therefore lobar/lung resections, collapse/consolidation and diffuse atelectasis decrease compliance
- Lung elasticity - emphysema increases lung elasticity, pulmonary oedema/congestion/fibrosis decreases lung elasticity
- Emphysema - static compliance is increased but distribution of gas can be very abnormal, therefore dynamic compliance often reduced
- Asthma - P/V curve displace upwards without a change in compliance
Dynamic Lung Compliance
- Always less than or equal to static lung compliance
- This is due to incomplete filling of alveoli in the available time
- True pressure equilibrium between applied and alveolar pressure is not obtained, and lung appears stiffer than it really is
- Time to fill an alveolus depends upon the time constant, τ = airway resistance x compliance
- Higher airway resistance means more time to fill, and slow alveoli will not fill in the time available - especially in emphysema
- Measured using a spirometer - compliance is the slope of the pressure/volume curve. Different in that it is not a series of single readings but a continuous spirometry trace.
- Decreased dynamic compliance is seen especially with increased airways resistance - such as in asthma, chronic bronchitis and emphysema - mainly due to prolonged time constants
- Emphysema increases specific lung compliance in a frequency dependent fashion - dynamic compliance decreases as respiratory frequency increases as alveoli fail to fill
- The time constant is the time taken to reach 63% of final volume, or the time taken to full volume change if the initial rate of change of volume were maintained