Effect of Inspiratory Flowrate and Circuit Compressible Volume on Auto-PEEP during Mechanical Ventilation

The term "auto-PEEP" has been applied to PEEP that occurs during mechanical ventilation when there has been incomplete exhalation of a delivered breath at the onset of the next inspiration. We hypothesized that shortening inspiratory time by increasing flowrate and/or using a circuit with low compressible volume (LCV) would decrease auto-PEEP. Methods: We first constructed a laboratory model to test our hypothesis under controlled conditions. Using a Manley test lung connected to an MA-1 ventilator, we created auto-PEEP by applying a screw clamp to the endotracheal tube and quantitated auto-PEEP by use of calibrated transducers on either side of the obstruction. To confirm the bench studies clinically, we randomly varied inspiratory flow and circuitry-keeping corrected tidal volume (V T ) and frequency (f) constant- in five adult patients with auto-PEEP. Results: In the bench studies, when auto-PEEP was 20 cm H 2 O with standard ventilator circuitry (corrected VT = 550 ml and f = 24/min), increasing inspiratory flow from 60 to 100 L/min reduced auto-PEEP to 7.5 cm H 2 O. Switching to LCV circuitry at the same V T and f produced auto-PEEP of 10 cm H 2 O at 60 L/min flow and 5 cm H 2 O at 100 L/min. In the clinical studies of adult patients, at flowrates of 60 and 100 L/min, mean auto-PEEP values were 15.8 and 9.0 cm H 2 O, respectively, with standard tubing; with LCV tubing the respective auto-PEEP values were 12.6 and 5.8 cm H 2 O. (P < 0.05 in both clinical studies.) We conclude that higher inspiratory flowrates and a low compressible volume circuit are effective in reducing auto-PEEP in mechanically ventilated adults. (Respir Care 1986;31:1075-1079.)