Question 27

a)             Draw and label the pressure time curve for a patient  with normal lungs being ventilated  with constant flow volume controlled ventilation  with a respiratory rate of 20 an inspiratory to expiratory ratio of 1:2, PEEP 5 cm water.

b)         Using the same scale, and assuming the same ventilator settings, draw a representative pressure time curve for a patient  with acute severe asthma.

c)         Briefly, explain the basis for any changes that you have represented.

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College Answer

Normal Pressure Time Curve should include the following:
Baseline pressure above zero equals PEEP
Peak Inspiratory pressure (PIP)
Plateau pressure should be included with the long inspiratory time (1second) from rate 20 and I:E 1:2 described in the question
Return of pressure to baseline PEEP
Time axis should have seconds, 1 second in inspiration, 2 in expiration 

Plateau Pressure of 15-20 cm H2O and PIP less than 30 cm H20 as patient described as having normal lungs.

Acute Severe Asthma :

Changes: Raised PIP with unchanged Plateau pressure, but accepting that PEEP and plateau pressure may be increased with successive breaths if illustrated (due to gas trapping / auto peep)

Basis: increase in inspiratory airflow resistance but not lung or chest wall compliance, unless significant gas trapping with ensuing AutoPEEP


This college answer, together with the diagrams and explanations, is relatively thorough. Except for the fact that the college diagram needs some correction. The college mention nothing of an inspiratory pause, which is clearly present on their ventilator graphics. Without an inspiratory pause, they would not be able to illustrate the difference between plateau pressure and peak inspiratory pressure.

A more indepth discussion of ventilator pressure-time waveforms takes place elsewhere.