Question 26.1

Last updated 04/01/2019 - 18:27
 Topic: Respiratory Medicine and Mechanical Ventilation
Pass rate
76.1%
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Other SAQs in this paper:

The following question is based on the shown pulmonary function tests (PFTs). Assume in each case that the test result is adequate and reproducible.

Key:

FVC

L

Forced Vital Capacity

FEV1

L

Forced Expiratory volume in 1 second

FEV1/FVC

%

Ratio of the above

RV

L

Residual volume at end expiration

TLC

L

Total Lung Capacity

DLCO corr

ml/min/mmHg

Diffusing capacity for carbon monoxide, corrected for Hb

You are asked to evaluate a previously well, 36-year-old male who has presented to Emergency Department (ED) with shortness of breath and increased work of breathing. This has been progressive over the past week. He has had PFTs performed recently as an outpatient: 

Predicted

Actual

% Predicted

Post Bronchodilator

% Change

FVC (L)

4.20

3.15

75

3.62

+15

FEV1 (L)

3.40

2.14

63

2.56

+20

FEV1/FVC (%)

80

68

71

+4

RV (L)

2.31

3.03

131

TLC (L)

6.41

6.53

102
  1. What pattern of abnormality is shown?
  1. Should it become necessary, what implications will this have for your ventilation strategy?  (30% marks)

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College answer

Obstructive, reversible, evidence of gas trapping but not hyperinflation (1 mark)   
At risk of dynamic hyperinflation, may need high inspiratory pressures, low PEEP, long expiratory time (3 Marks) 

Discussion

Formal pulmonary function tests have also been asked about in Question 21.2 from the first  paper of 2014  and the identical Question 9.1 from the first  paper of 2011. 

To approach this systematically:

  • FVC is the forced vital capacity, and here it is decreased, which ordinarily simply means that the spirometry effort was poor, but the college assure us that each test was "adequate and reproduceable" so the patient must truly have reduced FVC.
  • FEV1 is the forced expired volume over 1 second, and is a measure of maximal air flow. A decreased FEV1 which improves with bronchodilators demonstrates a reversible obstructive pattern.
  • FEV1/FVC ratio is a measure of airway resistance which incorporates both the abovementioned metrics.
  • RV is the residual volume. A high RV suggests end-expiratory gas trapping or bullous dead space. This patient's RV is clearly increased; on the basis of this the college say there is "evidence of gas trapping" .
  • TLC is the total lung capacity. In this case, the TLC is near normal. On the basis of this, the college say that there is no hyperinflation.

The ventilation strategy for somebody with such an obstructive pattern of respiratory function and such propensity to hyperinflation resembles that which might be used for status asthmaticus, and would consist of the following strategies:

  • Use the largest tube possible.
  • Use lowest FiO2 to achieve SpO2 of 90-92%
  • Use a small tidal volume, 5-7ml/kg
  • Use a slow respiratory rate, 10-12 breaths per minute (or even less!)
  • Use a long expiratory time, with I:E ratio 1:3 or 1:4
  • Increase inspiratory flow rate to maximum. .
  • Reset the pressure limits (i.e. ignore high peak airway pressures).  .
  • Use a volume-control mode of ventilation.
  • Use minimal PEEP.
  • Keep the Pplat below 25cmH2o to prevent dynamic hyperinflation. 
  • Titrate PEEP to work of triggering once the patient is breathing spontaneously.

References

Pellegrino, Riccardo, et al. "Interpretative strategies for lung function tests."European Respiratory Journal 26.5 (2005): 948-968.

The American Thoracic Society has a page which features an excellent bibliography of the articles which support their interpretation standards.

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