MEASUREMENT OF STATIC COMPLIANCE OF THE TOTAL RESPIRATORY SYSTEM IN PATIENTS WITH ACUTE RESPIRATORY-FAILURE DURING MECHANICAL VENTILATION - THE EFFECT OF INTRINSIC POSITIVE END-EXPIRATORY PRESSURE

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Abstract
In mechanically ventilated patients with acute respiratory failure, the static compliance of the total respiratory system was conventionally obtained by dividing the tidal volume by the difference between the plateau pressure measured at the airway opening (Pao) during an occlusion at end-inspiratory and positive end-expiratory pressure (PEEP) set by the ventilator. This analysis was valid only if the elastic recoil pressure of the respiratory system was 0 at the end of expiration, indicating that the system reached its elastic equilibrium point. To test if this was always the case, in 14 mechanically ventilated patients with acute respiratory failure, measurements were made of Pao and of flow and volume changes. In only 4 of the patients did expiratory flow become nil before end-expiration and inspiratory flow started synchronously with the onset of the positive-pressure swing delivered by the ventilator, indicating that in these 4 patients the end-expiratory elastic recoil pressure was 0. By contrast, in the remaining 10 subjects, expiratory flow was still present when the ventilator began to increase Pao, indicating that the end-expiratory elastic recoil pressure was not 0: In all these 10 patients, a positive .DELTA.Pao (as much as 7.5 cm H2O) had to be applied by the ventilator before the actual onset of inspiratory flow. This .DELTA.Pao represented the pressure required to counterbalance the end-expiratory elastic recoil before inspiratory flow began and was termed intrinsic PEEP. To obtain a valid measurement of respiratory compliance, the tidal volume was divided by the difference between end-inspiratory plateau Pao and total PEEP, which included externally applied and intrinsic PEEP. In the 10 patients with intrinsic PEEP, the value of static respiratory compliance computed without taking intrinsic PEEP into account underestimated the correct value by as much as 48%. Apart from its implications on measurements of respiratory system compliance, intrinsic PEEP may have adverse effects in terms of hemodynamics and weaning.

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