Question 26.1
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 |
65 |
65 |
35 |
|
Plateau |
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)?
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 and decrease the I:E ratio. This would have the effect of increasing the peak inspiratory flow, as the same volume would have to be pushed into the patient over a shorter period of time, and in the state of bronchospasm this would likely produce higher peak airway pressures, but the college did not ask for this.