Question 17.3

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:

• Intrinsic PEEP and the expiratory hold manoeuvre
• Alveolar pressure and the inspiratory hold manoeuvre
• Interpreting the shape of the pressure waveform

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 patern 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. Compliance is pressure required per volume of lung distension; thus one can calculate it by dividing the tidal volume by the diference between the pleateau pressure and PEEP.

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
• increase the inspiratory rise time
• decrease the respiratory rate
• decrease the tidal volume