PRESSURE-VOLUME CURVES IN ACUTE RESPIRATORY-FAILURE - AUTOMATED LOW-FLOW INFLATION VERSUS OCCLUSION

Pressure-volume (P-V) curves of the respiratory system allow determina tion of compliance and lower and upper inflection points (LIP and UIP, respectively). To minimize lung trauma in mechanical ventilation the tidal volume should be limited to the P-V range between LIP and UIP. A n automated low flow inflation (ALFI) technique, using a computer-cont rolled Servo Ventilator 900C, was compared with a more conventional te chnique using a series of about 20 different inflated volumes (Pst-V c urve). The pressure in the distal lung (Pdist) was calculated by subtr action of resistive pressure drop in connecting tubes and airways. Com pliance (Cdist), Pdist(LIP) and Pdist(UIP) were derived from the Pdist -V curve and compared with Cst, PsSt(LIP), and Pst(UIP) derived from t he Pst-V curve. Nineteen sedated, paralyzed patients (10 with ARDS and 9 with ARF) were studied. We found: Cdist = 2.3 + 0.98 . Cst ml/cm H2 O (r = 0.98); Pdist(LIP) = 0.013 + 1.09 . Pst(LIP) cm H2O (r = 0.96). In patients with ARDS: Pdist(LIP) = 4.71 + 0.84 Pst(UIP) cm H2O (r = 0 .94). In ARF, we found differences in UIP between the methods, but dis crepancies occurred above tidal volumes and had little practical impor tance. They may reflect that Pdist comprises dynamic phenomena contrib uting to pressure in the distal lung at large volumes. Compliance, but not LIP and UIP, could be accurately determined without subtraction o f resistive pressure from the pressure measured in the ventilator. We conclude that ALFI, which is fully automated and needing no ventilator disconnection, gives useful clinical information.