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Lung function tests

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Lung function tests

Lung volumes and capacities

Tidal volume

- volume that flows into the lungs with each inspiration during quiet breathing. (7ml/kg)

Inspiratory reserve volume

- air inspired with a maximal inspiratory effort in excess of the tidal volume. (Normal approx. 3.3 l in men and 1.9 l in women)

Expiratory reserve volume

- volume expired by active expiration after passive expiration (Normal approx 1.0 l (m) and 0.7 l (f))

Residual volume

- volume left in the lungs after maximal expiratory effort. (Normal 1.2 l (m) and 1.1 l (f)).
- determined by the force of the expiratory muscles opposed by the tendency of the thorax to recoil outwards at low volumes
- beyond the third decade of life RV increases due to dynamic compression or closure of airways.
- as a fraction of TLC RV increases from 25% at 20 years to 40% at 70 yrs
- any airway narrowing or loss of elastic recoil accelerates the increase in RV
- increased elastic recoil ( eg pulmonary fibrosis) associated with decreased RV

Inspiratory capacity

IRV + TV

Functional residual capacity

ERV + RV ie volume remaining in the lungs at the end of a normal expiration
- of interest in relation to closing volume (lung volume above residual volume at which airway collapse occurs during expiration)
- determined by the balance between the inward elastic recoil of lung and outward pull of thoracic cage.
- increased in conditions associated with decreased elastic recoil, in particular emphysema and in conditions where localized damage causes cysts or bullae.
- reduced by upward movement of the diaphragm, obseity, an in presence of painful thoracic or abdominal wounds
- affected by the tone of the diaphragm, intercostal and other respiratory muscles
- reduction of FRC, eg by induction of anaesthesia, or an increase in closing volume by loss of the normal support of the bronchial tree may make CC > FRC. (CC=CV+RV) Airway collapse then occurs during normal tidal breathing with resultant hypoxia. Seen both during anaesthesia and in the postoperative period. Exacerbated by the absence of sighs which are inhibited by surgical pain and opioids. (Normally occur at approximately 1 min intervals - help to re-expand collapsed alveoli.) In normal adults CC rises above FRC (supine) at about 45 yrs and above FRC (erect) at about 65 yrs.

Vital capacity

IRV + TV + ERV ie maximum breath volume
- important as a measure of respiratory sufficiency, especially in patients with restrictive lung disorder or a neuromuscular disease
- determined by power of respiratory muscles, elastic properties of the chest wall and lung parenchyma, size and patency of airways at low lung volumes, sex and body size
- VC < 15 ml/kg (and VT < 5ml/kg) indicates likely need for mechanical ventilation, VC <10 ml/kg associated with impending respiratory failure and ventilatory support is invariably required when VC < 3 ml/kg even in the presence of normal lungs.

Total lung capacity

- IRV + TV + ERV + RV. ie volume of gas in the lung at the end of maximal inspiration
- averages 3-5 l in a 70 kg adult
- in absence of gross respiratory muscle weakness lung elastic recoil is major determinant
- increased when elastic recoil is decreased (eg emphysema) and decreased when recoil increased (eg pulmonary fibrosis)

Respiratory minute volume

- (at rest): approx 6 l/min

Alveolar ventilation

- (at rest): approx 4.2 l/min

Timed vital capacity

- 83% of total in 1 sec; 97% in 3 secs

Ventilatory capacity

- maximal ability to move gas in and out of lungs
- usually quantified by measuring maximum expiratory flow rates
- maximum expiratory flow is determined by elastic recoil of lung and resistance of intrathoracic airways
- greatest at high lung volumes where elastic recoil is greatest and airways resistance is least
- except at onset of expiration maximal expiratory flow largely independent of muscular effort
- beyond modest effort an increase in driving pressure is matched by an increase in resistance due to compression of intrathoracic airways
- gross muscle weakness will decrease maximum expiratory flow

Method of measurement

Maximum expiratory flow volume curve.

Gives flow rate over whole range of VC

Forced expiratory volume in 1 sec. (FEV1).

Usual bedside measure of maximum expiratory flow
- useful as a test of lung function despite the fact that expiration usually passive because main factor limiting ventilation is airway closure in expiration and this is the main factor affecting FEV1
- other factors are abdominal muscle function, effort and inspiratory capacity (determines how deep a breath subject can take before starting forced expiration)
- usually 50-60 ml/kg

Peak expiratory flow rate

- can be measured using Wright peak flow meter. This measures volume expired in first 0.1 sec of forced expiration. Gives reading in L/min
- very effort dependent

Respiratory muscle power

Maximum inspiratory pressure

- useful measure of power of inspiratory muscles
- measured during maximum inspiratory effort against an occluded airway at RV or FRC
- normal value varies with age and sex: exceeds -12 kPa in young females and -17.3 kPa in young males
- value < -2.5 kPa suggests that spontaneous ventilation will probably be inadequate

Maximum expiratory pressure

- measure of power of expiratory muscles

Transdiaphragmatic pressure

- measure of diaphragmatic function
- measured during spontaneous ventilation, maximum inspiratory effort and phrenic nerve stimulation
- estimated from difference between pleural and abdominal pressures. Measured using balloon catheters in mid-oesophagus and stomach respectively

Distribution of ventilation

- normally airflow ceases almost simultaneously throughout lung at end of expiration
- in diffuse airways disease airways close irregularly and progressively as expiration proceeds
- evenness can be assessed using single breath nitrogen test or pulmonary nitrogen washout

Single breath nitrogen test

- gives information on evenness of distribution of ventilation and on closing volume
- normal change in nitrogen concentration per 500 ml of expired air <1.5% during phase III ("alveolar plateau")
- lung volume at which phase III changes to phase IV is closing volume

Pulmonary nitrogen washout

- subject given 100% O2 to breath for 7 min after breathing room air
- maldistribution indicated by nitrogen concentration >2.5% in expired gas

Gas transfer

- measure of lungs' overall capacity to transfer gas rather than just diffusing capacity of alveolar-capillary membrane
- gas transfer factor = volume of gas taken up/(PAGas - PcGas)
- gas is usually carbon monoxide: rate of diffusion resembles oxygen and so completely taken up by Hb that mean capillary pressure is effectively zero
- value given as ml/min/mmHg of alveolar pressure of CO
- factors which affect TLCO:

  • epithelial-endothelial surface area (increases with size of subject)
  • pulmonary capillary blood volume and Hb concentration (increases in polycythaemia and pulmonary capillary distension; decreases with PE)
  • rate of reaction of CO with Hb
  • thickness of alveolar-capillary membrane
  • distribution of ventilation and ventilation-perfusion relationships

- effect of lung volume or unevenly distributed ventilation can be corrected by dividing TL by effective alveolar volume (VA). Latter is volume into which CO distributes during measurement of TL. Determined by helium dilution. 10% helium is administered during test along with CO and air
- in parenchymal disease characterised by destruction or diffuse infiltration both TL and TL/VA decreased while in diseases characterized by loss of lung tissue (eg fibrotic replacement) TL decreases but TL/VA may be normal.


© Charles Gomersall December 1999

 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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