Normal Spirometry trace

In Pregnancy

Anatomical

• Diaphragm ascends ~4cm due to foetus
• AP and lateral diameters of thorax increase 2~3cm due to relaxin effect on chest
  wall
• Thoracic circumference increases 5~7cm (AP and lateral diameters increased 2~3cm)
• Airway dilatation → decreased resistance (35%) and increased deadspace (45%)
• Increased cardiac output, intravascular volume, and decreased oncotic pressure
  • Increased plasma volume → increased pulmonary blood volume → decreased compliance
  • Venous engorgement → Upper airway oedema

Mechanical

• Lung volume changes occur from 20 weeks
• TLV decreases 5% from baseline
• FRC decreased 20% due to both ERV and RV
• Significant postural change in FRC of 70%
• Progesterone → increased sensitivity to CO2 → Minute ventilation increases 50%
  • TV increases (25%)
  • RR increases
  • Results in hypocarbia and compensated respiratory alkalosis
    • pCO2 ~ 26-32mmHg
    • HCO3 ~ 18-20mmol/l
  • Further increase in MV during contractions
• Inspiratory capacity increased by 10%
• Closing capacity unchanged
• Increased O2 consumption during labour (60%)

After birth

• FRC and RV return to normal within 48 hours
• TV returns to normal within 5 days

Sakurai 2016

2012A 13: 5 (50%) of candidates passed.
This question was generally handled well. Significant variation exists in normal values between textbooks, and examiners took account of this variation. Some candidates failed to provide any normal values or gave values not in accord with the traces they provided. Some candidates confused spirometry (volume/time) with flow volume loops or even a forced expiratory volume trace. These scored no marks. The second part was not well answered by some candidates with failure to describe changes – or even contradicting correctly graphed changes (for example the reduction in residual volume with pregnancy described as increased). Memorising curves needs to be accompanied by understanding. No marks were awarded for detailed discussion of the Haldane effect, as this was not asked. Candidates are reminded to read and answer the question.

JC 2020

2017B 18: 87% of candidates passed this question.
This core respiratory physiology topic was well answered by most candidates. Candidates
generally were able to draw a spirogram. A common omission was inspiratory capacity.

Obstructive Lung Disease

e.g. Asthma, COPD

Physiological changes

• ↑ AWR caused by bronchial SM contraction, secretions, wall oedema, and reduced radial traction (from damaged lung parenchyma)
• Closing capacity rises towards or > FRC
• ↓ elastic potential energy is stored to generate driving pressure gradient (this is caused by ↑ lung compliance resulting from damaged lung parenchyma)

Thus

1. ↓ FRMAX and ↓ steep rise in expiratory FR during effort-dependent part of the curve
2. ↓ steep and “scooped” appearance of flow curve in effort-independent part of the curve due to dynamic airway compression occurring at higher lung volumes

Pulmonary function tests:

1. Peak Flow
   • Reduced PEF rates
2. Simple spirometry:
   • shows an increase in lung volumes
   • decreased inspiratory capacity
   • (increased FRC but not measured)
3. Forced expired spirometry is diagnostic
   • FEV1/FVC < 70%
   • FEV1 % predicted <50%
   • FEF 25 – 75% is prolonged
4. Flow volume loops
   • higher volumes
   • decreased flow
   • scooped effort dependent portion

Gladwin 2016

2011B 18: 0 (0 %) of candidates passed this question.
The majority of candidates were able to state that obstructive lung disease resulted in a reduction in the FEV1 and the FVC. Very few gave any indication of how much these parameters might be reduced in severe obstructive lung disease or that FVC is usually only slightly reduced, whereas the FEV1 is markedly reduced and therefore so is the FEV1/FVC ratio. Candidates were also expected to indicate that peak expiratory flow rate is also reduced (by 50% or more in severe disease), that carbon monoxide diffusing capacity is also reduced, and residual volume and total lung capacity are increased. A reasonable proportion of candidates commented that the forced expiratory flow (FEF) 25-50% being reduced and this results in a ‘scalloped’ flow volume loop. Accurately reproducing a normal flow volume loop and a flow volume loop of a patient with severe obstructive lung disease on the same graph could have depicted most of the above points and scored the majority of marks available.
The second part of this question was poorly answered. Whilst the majority correctly stated that an increase in airways resistance was present, few stated what changes gave rise to this. Dynamic airway compression, was mentioned by many, but why this occurred was explained poorly. The effect of closing capacity rising above the FRC was seldom mentioned and why the FRC increased again was seldom mentioned or explained. A detailed knowledge of what is a core subject in intensive care medicine was expected.
Syllabus: B1a 2a, b; B1j 2a
Recommended sources: Respiratory Physiology: The Essentials. West 6th Ed pgs 26, 96-99, 131-142.