A Mathematical Model of Mean Airway Pressure Based Upon Positive End-Expiratory Pressure, <Emphasis Type="Italic">I</Emphasis>:<Emphasis Type="Italic">E</Emphasis> Ratio, and Plateau Pressure

Pmean/PPL = [(I :E )+ R]/[(I :E )+ 1] where R is defined as: R = Ppeep/PPL. Based upon this model, it can be shown that (1) increasing I :E ratio will increase Pmean/PPL in a self-limiting logarithmic manner; (2) Pmean/PPL is a linear function with respect to R; (3) increases in R are associated with a diminished effect of I :E ratio on Pmean/PPL; (4) similarly, increases in I :E ratio are associated with a diminished effect of R on Pmean/PPL; (5) overall, changes in Pmean/PPL will consistently be effected more by changes in R than by changes in I :E ratio. This model illustrates the interrelationship between plateau pressure, PEEP, and I :E ratio as they affect mean airway pressure. Furthermore, it appears to be useful in explaining the clinically reported discrepancies regarding the efficacy of inverse ratio ventilation (IRV), especially when simultaneously applied with varying levels of PEEP. In addition, for a given plateau pressure, it is also possible to mathematically optimize PEEP and I :E ratio combinations so as to avoid excessive amounts of either.