Effects of PEEP on Respiratory Mechanics after Open Heart Surgery (6)

This plateau was reached in less than 5 s and hence the pressure at 5 s was taken to represent the end-inspiratory static elastic recoil pressure of the respiratory system (Pst,rs). Whenever the time required for passive expiration to proceed to completion is greater than the expiratory duration set by the ventilator, the end-expiratory lung volume during mechanical ventilation will exceed the relaxation volume (elastic equilibrium volume) of the respiratory system, and the respiratory system will exert a positive static elastic recoil pressure at end expiration. This pressure is termed intrinsic PEEP (PEEPi). PEEPi was measured with the end-expiratory occlusion technique. If PEEPi is present, Pao increases following airway occlusion until a plateau is reached that corresponds to PEEPi. The static compliance of the total respiratory system (Cst,rs) was calculated by dividing the tidal volume by the end-inspiratory Pst,rs after subtracting PEEP and PEEPi if present.
Change in end-expiratory lung volume due to PEEP (AEELV) was measured from the RIP sum signal. The drift of the RIP signal in vitro is less than 0.1 ml/min and in vivo under similar conditions it is approximately 1 ml/min. The effect of drift was avoided by using the breaths immediately preceding and those 0.5 to 1 min after the change of PEEP The cumulative sum of measured AEELVs at each PEEP level represents the total change in EELV.