2019 Second Sitting Paper Question 07

2019-2-07 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). 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: o CC=FRC at 44yrs in supine position, CC=FRC at 66yrs in upright position o 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

Changes in lung volumes with aging (erect position) Zaugg M, Lucchinetti E. Respiratory function in the elderly. Anesthesiol Clin North America 2000; 18: 47–58 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: o Phase 1: N2 in anatomical dead space o Phase 2: N2 from anatomical dead space and alveolar gas o Phase 3: Alveolar plateau is formed by N2 in pure alveolar gas

JC 2019 2019 Second Sitting Paper Question 07

o 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”

Source: pftforum.com • Basis for phase 4: o 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”

Examiner Comments: 49% of candidates passed this question. 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.

Source: Bianca’s notes, anaeskey.com

JC 2019