Define closing capacity (10% of marks). Describe the factors that alter it (30% of marks), its clinical significance (30% of marks) and one method of measuring it (30% of marks).
Many candidates confused the factors that affect closing capacity (CC) with factors which affect functional residual capacity (FRC). Some candidates confused airway closure with expiratory flow limitation secondary to dynamic airway compression.
A good answer would have included the following:
Small airway closure occurs because the elastic recoil of the lung overcomes the negative intrapleural pressure keeping the airway open. Thus, airway closure is more likely to occur in dependant parts of the lung where airways are smaller. Normally closing capacity is less than FRC in young adults but increases with age. Closing capacity becomes equal to FRC at age 44 in the supine position and equal to FRC at age 66 in the erect position. Closing capacity is increased in neonates because of their highly compliant chest wall and reduced ability to maintain negative intrathoracic pressures. In addition, neonates have lower lung compliance which favours alveolar closure. Closing capacity is also increased in subjects with peripheral airways disease due to the loss of radial traction keeping small airways open.
The consequences of airway closure during tidal breathing include shunt and hypoxaemia, gas trapping and reduced lung compliance. In addition, cyclic closure and opening of peripheral airways may result in injury to both alveoli and bronchioles. Closing volume (CV) may be measured by the single breath nitrogen washout test or by analysis of a tracer gas such as xenon during a slow exhaled vital capacity breath to residual volume. Residual volume (RV) cannot be measured directly but is calculated as follows: the FRC is measured using one of three methods: helium dilution, nitrogen washout or body plethysmography. The expiratory reserve volume (ERV) may be measured using standard spirometry. Using the measured FRC and ERV we may calculate RV from the equation:
RV = FRC – ERV. Then CC = RV + CV..
- Closing capacity is the maximal lung volume at which airway closure can be detected in the dependent parts of the lungs
- It can also be defined as the volume at which transition from Phase III to Phase IV occurs during an inert gas washout measurement.
- Closing capacity is composed of residual volume (RV) and closing volume.
- Closing capacity is altered by:
- Expiratory air flow: (higher flow = higher CC)
- Expiratory effort (more effort = higher CC)
- Small airways disease, eg. asthma or COPD
- Increased pulmonary blood volume, eg in CCF
- Decreased pulmonary surfactant
- Parenchymal lung disease, eg. emphysema
- Age (increasing age = increased closing capacity)
- At age 44, supine FRC is lower than closing capacity
- At age 66, erect FRC is lower than closing capacity
- Closing capacity can be measured by:
- Gas bolus measurement, where a subject inhales a small bolus of tracer gas, starting at RV
- Resident gas method, where a subject inhales a TLC of oxygen, starting from RV
- Both methods produce a graph of gas concentration over volume, which has four distinct phases.
- The significance of closing capacity is:
- Higher CC decreases the effect of pre-anaesthetic preoxygenation
- Higher CC increases dependent atelectasis
- It is responsible for the age-related decrease in oxygenation, because of shunt
- It aggravates lung injury through cyclic atelectasis
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