Question 26.1

Created on Sun, 05/24/2015 - 22:09
Last updated on Sat, 09/26/2015 - 23:38
Pass rate: 75%
Highest mark: ?

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A 64 year old man is admitted to ICU with a 5 day history of increasing shortness of breath, non-productive cough and acute respiratory failure.

Clinical examination reveals reduced breath sounds and inspiratory and expiratory wheeze  bilaterally. Chest  X-Ray  reveals  hyperinflated lung  fields.  The  following data are from pulmonary function  testing performed 3 months ago.

Variable                   Predicted        Pre- bronchodilator Post- bronchodilator

FEV1 (L/min)        3.15                   0.77                         0.85

FVC (L/min)          4.05                   3.00                         3.38

FEV1/FVC  %        70                      20                            25%

The  patient is intubated and  volume-controlled  ventilation instituted. The  settings are SIMV,  rate of  8,  TV  500  ml,  FIO2  1.0,  PEEP  0.  Three sets  of  ventilatory parameters are provided below.  Based on the information above, select from A, B or C, which pattern will be most  likely to fit with  his respiratory dysfunction and explain why.

A

B

C

Peak      pressure
(cm water)

65

65

35

Plateau 
pressure         (cm water)

20

63

33

b) The patient is intubated and volume-controlled ventilation instituted.  This is the patient’s flow-volume loop. What abnormality is illustrated by the flow pattern?

c)  This  is  the  patient’s  flow-time  respiratory  waveform.  What   abnormality  is illustrated by this trace?

d) List 3 changes  to the ventilator settings  you could do to correct the abnormality noted in c)?

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College Answer

a) Answer:  A  (A  high  peak-plateau gradient  with  high  peaks  are consistent   with obstructive lung disease)

b) The patient is intubated and volume-controlled ventilation instituted.  This is the patient’s flow-volume loop. What abnormality is illustrated by the flow pattern?

Expiratory flow scooped out/Increased expiratory resistance

c)Incomplete emptying/potential for gas trapping

d) List 3 changes  to the ventilator settings  you could do to correct the abnormality noted in c)?

Decrease respiratory rate
Increase peak inspiratory flow
Decrease the I:E ratio (increase expiratory time//decrease inspiratory time)

Discussion

a) is straightforward. The patient with such an obstructive pattern of spirometry will generate a high peak airway pressure, but a reasonably normal plateau pressure (because beyond the obstructed air passages lies a relatively normal well-compliant lung parenchyma)

b) the flow-volume loop demonstrates a "scooped out" pattern. The return limb of the loop (helpfully labelled "expiration") demonstrates poor low flow, suggesting that in expiration the patient has trouble exhaling the gas. This is a feature of increased airway resistance.

c) The flow waveform (which are discussed elsewhere) demonstrates a failure of the flow to reach zero at the end of expiration, which suggests gas trapping.

d) In order to decrease gas trapping, one would decrease respiratory rate, decrease the I:E ratio, and decrease the peak inspiratory flow.