Peak pressure, plateau pressure and compliance

Question 17.3 from the first  paper of 2010 and the near-identical Question 12 from the second paper of 2007 ask the candidate to interpret a ventilator pressure-time graph, identify that there is auto-PEEP, and calculate static compliance on the basis of the plateau pressure and tidal volume. The college were also interested to see whther the trainees could look back far enough into their primaries to recall the determinants of peak airway pressure. Because this is one of those simple things that most of us have revised and re-forgotten seven times already, this chapter is a necessary aide-memoire.

Determinants of peak airway pressure

• resistance
• compliance
• tidal volume
• PEEP
• Inspiratory flow rate
• flow pattern

Formula for calculation of lung compliance

• Lung compliance, in ml per cmH2O, is the tidal volume divided by (Pplat minus PEEP)

Other notes of interest for thsi topic include  an introduction chapter for ventilator waveform analysis, and the chapter on flow, volume, pressure, resistance and compliance

Other frequently forgotten definitions:

• Peak pressure =  sum of airway pressure and alveolar pressure.
• Airway pressure = pressure in the circuit which exists while there is flow.
• Plateau pressure = the relationship between volume and compliance, in the absence of flow
• Compliance = volume divided by pressure
• Tidal volume = flow multipied by time