Question 22.3

Created on Tue, 06/06/2017 - 04:50
Last updated on Sat, 07/15/2017 - 18:04
Pass rate: 7.8
Highest mark: 72.5%

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The following ventilator waveform shown below (Figure 3) is from a patient who is sedated and ventilated    in the pressure regulated volume control mode (a pressure control mode with a target tidal volume). The patient has an arterial oxygen saturation of 94%.

That's obviously not the original college image.
The ventilator settings from the college image were as follows:

  • PRVC mode
  • 30% FiO2
  • 7 PEEP
  • 24 resp rate (actual patient rate was 25, but there were no spontaneous breaths)
  • VT 450ml
  • MVe is 10.7 L/min
  • I:E ratio of 1:2
  • Ppeak of 24
  • Pmean of 7

Give the likely lung pathology.   (10% marks)

Give two features shown in the flow waveform that support your answer. (20% marks)

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College answer

a) Less than 24 cm H2O

Flow has not reached zero by the end of inspiration indicating there is still a pressure gradient between the ventilator and the alveoli. As result the alveolar pressure must be less than the inspiratory pressure delivered by the ventilator.  [Note: 24 was not an acceptable answer]         

 b) Obstructive airways disease

High inspiratory flow at the end of inspiration despite adequate inspiratory time
Failure of expiratory flow to return to zero by the end of expiration

Discussion

The examiners had chosen to answer the question "what is the likely lung pathology" with the cryptic phrase "less than 24 cm H2O". To analyse this, one needs to take into context the fact that the college then went on to remark that to merely write "24" would be somehow unacceptable. This leads one to believe that this whole answer is not the product of chance keystrokes accidentally self-assembling into a nonsequitur. We must somehow work "less than 24 cm H2O" into a valid answer to this SAQ. If the author were not well supplied with contraband exam papers, trying to reverse-engineer this and find an appropriate ventilator waveform would have been a painful and ridiculous exercise.

Fortunately, they let you keep your paper at the end. The Siemens Maquet Servo-I model ventilator screen used for this SAQ did in fact have the number "24" on it. That number represented the peak inspiratory pressure, usually seen in the status screen at the top right hand corner of the monitor. The monitor was also alarming "Paw High", and it had been silenced. The SAQ text also gives us the patient's oxygen saturation, a neither-here-nor-there 94%. From these data, we must somehow generate a lung pathology. Not only that, but it must be a pathology which might somehow give rise to "24" as a plausible description.

"Obstructive airways disease" would not be that answer. Nothing about the way this patient is being ventilated fits with that description, and the waveforms did not agree (the expiratory waveform returned to zero, contrary to the examiner's comment). The high inspiratory flow rate at the end of inspiration is correctly identified; in the college image the ventilator cycled to expire at a flow rate which would be about 50% of peak. However, "high inspiratory flow at the end of inspiration despite adequate inspiratory time" is difficult to interpret as a sign of obstructive airways disease, as such disease typically results in low flow rates through the obstructed airways.  

 "Flow has not reached zero by the end of inspiration" is a valid thing to say. However, it is not essential for it to reach zero.  When the mandatory volume is achieved by the decelerating flow ramp and the ramp ends up at zero at the end of inspiration, that probably just means that the lung compliance is poor. 

"As result the alveolar pressure must be less than the inspiratory pressure delivered by the ventilator". This is correct. In fact this statement cannot be challeged because underpins the entire practice of ventilation, as in order for any inspiratory flow to occur there needs to be a pressure gradient where the alveolar pressure is lower than the pressure in the ventilator tubing.  

The ventilator settings are in fact quite bizarre, as they call for a mandatory minute volume of over 10L. The peak pressure is a bit high, and the lung compliance is a bit reduced (~ 26 ml/cmH2O), but the patient is being ventilated with a level of PEEP which does not make sense for an ARDS patient, for example. 

Thus far, Dr Hyde has offered the most reasonable explanation for the bizarre college answer. In his opinion, the answer "less than 24" is the vestigial remnant of an deleted first question to this SAQ,  which has become trapped in the official paper through poor proofreading. 

"I wonder if they initially had a 3rd question in there that got vetted out just before the paper was published. I can imagine there being 3x 10% questions there, and question a) being along the lines of "what's the alveolar pressure?" - which would then make sense of their answer of <24 as we know that at a Pplat of 24 there is still ongoing flow before the breath cycles."

I am grateful for this explanation, as is the only one which makes sense. 

References