Positive end-expiratory pressure and lung compliance: effect on delivered tidal volume

The effects of positive end-expiratory pressure (PEEP) and lung compliance (CL) on delivered tidal volume (VTdel) and ventilator output were evaluated in the following anaesthesia machine/ventilator systems: Narkomed III with a Model AV-E ventilator (III/AV-E system) and an Ohmeda Modulus II with either a 7810 anaesthesia ventilator (II/7810 system) or a Model 7000 anaesthesia ventilator (II/7000 system). With a standard circle anaesthesia breathing circuit connected to a test lung simulating CL, gas flow was measured and integrated over time at each combination of VT settings (VTset), 500 ml or 1000 ml; CL settings, 0.15 to 0.01 L.cm H2O-1 decreased incrementally; and PEEP settings, 0 to 30 cm H2O increased in 5-cm H2O increments. The integral of gas flow at the Y-piece of the breathing circuit was recorded as VTdel and at the output of the ventilator bellows as ventilator output. As CL decreased to 0.01 L.cm H2O-1 and PEEP increased to 30 cm H2O, at VTset of 500 ml and 1000 ml, respective VTdel decreased linearly to 251 +/- 6 ml and 542 +/- 7 with the III/AV-E, 201 +/- 5 and 439 +/- 5, with the II/7810, and 181 +/- 4 and 433 +/- 7 ml with the II/7000 (P < 0.05 among the three systems). Loss in VTdel due to PEEP alone, which increased only slightly when VTset was increased, accounted for an increasingly greater percentage of VTset as it was decreased, which was less pronounced with low CL.(ABSTRACT TRUNCATED AT 250 WORDS)