Question 17.3

Pass rate: 93%

Highest mark: ?

The figure below illustrates an airway pressure waveform of a single breath during volume controlled ventilation, incorporating an end inspiratory pause and an auto- PEEP manoeuvre.

X Axis – Time in seconds
Y axis – Airway pressure in cm water

(a)        What are the factors which determine variable B?

(b)        If the delivered tidal volume was 500 ml, what is the calculated compliance?

(c)        List the change(s) you would make to the ventilator settings to treat an increase in the value of variable D.

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College Answer

(a)        What are the factors which determine variable B?

Resistance, compliance, tidal volume, PEEP, insp flow rate and flow pattern

(b)        If the delivered tidal volume was 500 ml, what is the calculated compliance?

25 ml/cm water   [TV/(Plateau-PEEP)]

(c)        List the change(s) you would make to the ventilator settings to treat an increase in the value of variable D.

Increase expiratory time
Decrease I:E ratio, decrease RR, reducing MV

Discussion

This is a question interrogating the candidate's familiarity with the pressure-time curve of the ventilator.

Local resources of questionable quality exist:

a) Variable B is the peak airway pressure. This variable is determined by the lung compliance, tidal volume, airway resistance and PEEP.

To some extent the flow pattern also matters (the higher the inspiratory flow, the greater the contribution from airway resistance)

b) The calculation of compliance is not something we do every day, but people should probably be at least dimly aware of the equation which is involved. Static compliance is the volume change achieved per unit of pressure change; thus one can calculate it by dividing the tidal volume by the difference between the plateau pressure and PEEP. It would be important to also mention the reciprocal of compliance, which is elastance - pressure change required to elicit a unit of volume change.

c) Variable D is the Auto-PEEP. This patient is gas-trapping.

In order to address this one, one can

  • minimise PEEP
  • decrease the I:E ratio to 1:4 or so
  • decrease the inspiratory rise time (to decrease total inspiratory time)
  • decrease the respiratory rate
  • decrease the tidal volume

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