iv. Pulm Gas Volumes – Closing Capacity

Master answer

Closing Capacity

• “Lung volume at which small airways and alveoli in the dependent parts of the lung first begin to close”
• Closing capacity = Closing volume + Residual volume

Pathophysiology and clinical significance of closing capacity

• Airways and alveoli in the dependent parts of the lung are much smaller (cf. nondependent regions), thus with expiration to low lung volumes below FRC these dependent airways and alveoli begin to collapse at “closing capacity” and trap gas distally, thereby causing “atelectasis”
• During atelectasis, a shunt forms (V/Q = 0) as the alveoli affected are not ventilated but remain perfused. This leads to impaired gas exchange that results in arterial hypoxaemia

Factors increasing closing capacity

• Age:
  • CC=FRC at 44yrs in supine position, CC=FRC at 66yrs in upright position
  • FRC depends on position, CC is independent
• Increasing abdominal pressure
• Decreased pulmonary blood flow
• Pulmonary parenchymal diseases which decrease compliance
• Obstructive airway diseases
• LV failure
• Surgery

Measuring closing capacity

Closing volume is determined using the “Single breath N2 test” (similar to Fowler’s method):

• Following a VC breath of 100% O2, the patient slowly exhales and a expired [N2] is measured with a rapid N2 analyser
• A plot of [N2] vs volume of gas expired is made:
  • Phase 1: N2 in anatomical dead space
  • Phase 2: N2 from anatomical dead space and alveolar gas
  • Phase 3: Alveolar plateau is formed by N2 in pure alveolar gas
  • Phase 4: Late in expiration when AW closure starts of occur, expired [N2] begins to rise above the alveolar plateau. The volume expired from the start of this to the end of maximal expiration is the “Closing volume”

• Basis for phase 4:
  • Basal AW closure is indicated by a rise of [N2] from N2-rich apical alveolar gases. Apical alveolar are rich in N2 because:
    • During initial inspiration from RV, the first part of inspired gas (which is the anatomical dead space gas rich in N2) goes mainly into the apical alveoli
    • Apical alveoli are larger and more poorly ventilated (cf. basal alveoli). Thus, the [N2] of apical alveolar gases are less diluted when breathing in 100% O2

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 determined by summating “closing volume” and “residual volume”

Source: Bianca’s notes, anaeskey.com

JC 2019

Answer by JC