EFFECTS OF PEEP ON RESPIRATORY MECHANICS ARE TIDAL VOLUME AND FREQUENCY-DEPENDENT

How the effects of frequency, tidal volume (VT) and PEEP interact to d etermine the mechanical properties of the respiratory system is unclea r. Airway flow and airway and esophageal pressures were measured in te n intubated, anesthetized/paralyzed patients during mechanical ventila tion at 10-30 breaths/min and Vr of 250-800 mi. From these measurement s, Fourier transformation was used to calculate elastance (E) and resi stance (R) of the total respiratory system (subscript rs), lungs (subs cript L) and chest wall (subscript cw) at 5, 10 and 0 cm PEEP. As PEEP increased from 0-5 cmH(2)O, all elastances and resistances decreased (P < 0.05). Increasing PEEP to 10 cmH(2)O decreased E-L, R-rs, and R-L further (P < 0.05). The changes in E-rs, E-L, R-rs and R-L caused by PEEP were less (P ( 0.05) as VT increased, while changes in R-rs, R-L and E-rs were less (P < 0.05) as frequency increased. VT dependences i n E-rs and R-rs were enhanced (P < 0.05) at O cmH(2)O PEEP. The ratio of E-L to chest wall elastance was not affected by PEEP (P > 0.05), bu t increased (P < 0.05) with increasing VT at 5 and 10 cmH(2)O PEEP. We conclude that it is critical to standardize ventilatory parameters wh en comparing groups of patients or testing clinical intervention effic acy and that the differential effects on the lungs and chest wall must be considered in optimizing the application of PEEP. (C) 1997 Elsevie r Science B.V.