Functional residual capacity

• Volume of air in lungs at the end of normal tidal expiration
• Occurs at equilibrium point between the recoil of chest/diaphragm and lungs to collapse inwards
• Sum of residual volume and expiratory reserve volume
• Normal FRC 30ml/kg (approx 2.2l in 70kg male)

Function of FRC

• Oxygen reservoir
  • Normally 270mls O2 → can be expanded to 1.8l if breathing FiO2 100%
• PaO2 buffer
  • Ensures no fluctuation in PaO2 between inspiration and expiration
• Prevents atelectasis
  • Normally FRC above closing capacity
  • Decreases V/Q mismatch
• Minimizes work of breathing
  • Minimizes airway collapse → resistance decreased
  • Minimizes alveolar atelectasis → decreased work required to re-inflate alveoli
• Lung kept at most compliant segment of compliance curve
• Decreases pulmonary vascular resistance
  • Extra-alveolar and alveolar capillaries kept open
  • Minimizes hypoxic vasoconstriction

Factors Affecting FRC

Sakurai / Gladwin / JC 2020

2020B 02: 79% of candidates passed this question.
This question was in two parts with the percentage of marks allocated an indication of the relevant time or detail expected per part. The second part of the question also contained two distinct headings which should have been used in the answer. As an outline question, dot points with a brief explanation of each point were expected. Most candidates drew diagrams, few of which added value. For a diagram to add value it should be accurate, have labelled axes, a scale with numerical values and units. As a general rule, diagrams should also be explained and help to illustrate or relate to a written point.
For factors affecting FRC, to score full marks, it should be clearly stated if the factor causes an increase or decrease in FRC. This topic is well covered in the recommended respiratory texts.

Functional residual capacity

• Volume of air in lungs at the end of normal tidal expiration
• Occurs at equilibrium point between the recoil of chest/diaphragm and lungs to collapse inwards
• Sum of residual volume and expiratory reserve volume
• Normal FRC 30ml/kg (approx 2.2l in 70kg male)
• Functions:
  • Oxygen reservoir
  • PaO2 buffer
  • Prevents atelectasis
  • Minimizes work of breathing
  • Lung kept at most compliant segment of compliance curve
  • Decreases pulmonary vascular resistance

Consequence of 1L loss of FRC

• ↑Work of breathing
  • FRC is the volume at the minimum work at rest
  • Compliance curve shifted to a flatter portion (greater change in pressure required for change in volume)
  • FRC < closing capacity → ↑dynamic air closures/↑air trapping → ↑WOB
  • ↑atelectasis (↑with supine position) → ↓ lung compliance → ↑WOB
  • ↑airways resistance 2° ↓caliber of airway → ↑WOB
• ↑V/Q mismatch
  • 2° to dynamic airways closure, gas trapping, atelectasis
  • ↑shunt
  • ↓PaO2 due to inability of zone 1 to compensate for the larger zone 3
• ↑Pulmonary Vascular Resistance (PVR)
  • PVR minimal at FRC
  • ↓FRC → ↓radial traction on extraalveolar vessels → ↓radius of vessels → ↑PVR
    • ↑PVR → ↑ RV Afterload
• ↓FRC → ↑West Zone 4 → ↑pressure on extraalveolar vessels by poorly inflated lung → ↓radius → ↓flow
• ↓O Store and Buffer capacity
  • Greater variability in saturations thoughout the respiratory cycle
  • More rapid desaturation with hypoxia

Sakurai / Gladwin / JC 2020

2017A 08: 70% of candidates passed this question.
High scoring answers began with a definition and normal values, followed by a detailed list of the consequences of decreasing the FRC.
Some candidates included descriptions of the normal function of FRC, conditions that decrease FRC and ways of improving reduced FRC. These were not required and did not attract marks. Diagrams require correctly labelled axes, values & units.

2010B 15: 5 (33%) of candidates passed this question.
This is core knowledge and it was expected candidates would describe physiological consequences accurately. Good answers included a definition of FRC and correct value. A number of candidates omitted this. It was also expected that candidates mention that as FRC falls, alveolar closure occurs, lung compliance decreases and airway resistance increases work of breathing increases, pulmonary vascular resistance, and thus right ventricular afterload increases. Many candidates described alveolar closure as causing increased dead space ventilation rather than altered V/Q.
Syllabus: B1e References: Nunn’s respiratory physiology, pages 51-56

Functional residual capacity

• Volume of air in lungs at the end of normal tidal expiration
• Occurs at equilibrium point between the recoil of chest/diaphragm and lungs to collapse inwards
• Sum of residual volume and expiratory reserve volume
• Normal FRC 30ml/kg (approx 2.2l in 70kg male)
• Functions:
  • Oxygen reservoir
  • PaO2 buffer
  • Prevents atelectasis
  • Minimizes work of breathing
  • Lung kept at most compliant segment of compliance curve
  • Decreases pulmonary vascular resistance

Measurement of FRC

• Cannot be directly measured by spirometry
• Body plethysmography
  • Subject contained within glass tight box
  • Breath against occluded airway
  • Changes in alveolar pressure measured at mouth
  • Boyle’s law used to calculate FRC
• Nitrogen wash-out technique
  C₁V₁=C₂V₂
  • Initial alveolar concentration of nitrogen = 79% (if breathing room air)
  • Subject breathes 100% O2
  • Volume of total nitrogen exhaled measured (e.g 4L)
  • FRC = 4L/79% = approx 5L
• Helium wash-in technique
  C₁V₁=C₂V₂
  • Known volume and concentration of helium inhaled and allowed to
    equlibrate but not diffuse into blood
  • Concentration and volume of exhaled helium measured and used to
    calculate FRC, where

Sakurai / Gladwin / JC 2020

2017B 24: 59% of candidates passed this question.
Most candidates could state 2 methods of measuring FRC. Some candidates (especially for nitrogen wash out) failed to provide enough information e.g. statements such as “if the amount of nitrogen is measured then FRC can be derived” were insufficient to score many marks.

2015B 04: 25% of candidates passed this question.
This question requested a definition AND a description of measurement (one method if correctly discussed could and did generate a pass mark) although additional marks were awarded if multiple measurement methods were mentioned or described. Detailed descriptions of the factors effecting FRC and its functions were NOT requested and scored no marks. “Fowlers method” uses 100% oxygen and nitrogen analysis to calculate anatomical dead space – NOT FRC – so scored no marks. Both Helium dilution and nitrogen washout (with 100%oxygen) enable calculation of FRC using C1V1=C2V2 where V2 = V1+FRC. Body plethysmography requires more complex calculations of P1V1= P2V2 (Boyles Law) applied twice = for the box and then the lung. Few candidates had a clear understanding of this method. Most answers did not demonstrate the depth of understanding of the measurement techniques that was required to score highly

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

2019B 07: 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.