IMPACT OF POSITIVE END-EXPIRATORY PRESSURE ON CHEST-WALL AND LUNG PRESSURE-VOLUME CURVE IN ACUTE RESPIRATORY-FAILURE

To investigate whether chest-wall mechanics could affect the total res piratory system pressure-volume (P-V) curve in patients with acute res piratory failure (ARF), and particularly the lower inflection point (L IP) of the curve, we drew the total respiratory system, lung, and ches t-wall P-V curves (P-Vrs, P-VL, and P-VW, respectively) for 13 patient s with ARF, using the supersyringe method together with the esophageal balloon technique. Measurements were randomly repeated at four differ ent levels of positive end-expiratory pressure (PEEP) (0, 5, 10, 15 cm H2O) and from each P-V curve we derived starting compliance (C-start) , inflation compliance (C-inf), and end compliance (C-end). With PEEP of 0 cm H2O (ZEEP), an LIP on the P-Vrs curve was observed in all pati ents (7.5 +/- 3.9 cm H2O); in two patients an LIP was detected only on the P-VL curve (8.6 and 8.7 cm H2O, respectively); whereas in seven p atients an LIP was observed only on the P-Vw curve (3.4 +/- 1.1 cm H2O ). In four patients, an LIP was detected on both the P-VL and P-VW cur ves (8.5 +/- 3.4 and 2.2 +/- 1.0 cm H2O, respectively). The LIP was ab olished by PEEP, suggesting that a volume-related mechanism was respon sible for the observed LIP on both the P-VL and P-VW curves. At high l evels of PEEP, an upper inflection point (UIP) appeared on the P-Vrs a nd P-VL curves (11.7 +/- 4.9 cm H2O and 8.9 +/- 4.2 cm H2O above PEEP, respectively) suggesting alveolar overdistension. In general, Pa-O2 i ncreased with PEEP (from 81.7 +/- 35.5 mm Hg on ZEEP to 120 +/- 43.8 m m Hg on PEEP 15 cm H2O, p < 0.002); however, the increase in Pa-O2 wit h PEEP was significant only in patients with an LIP on the P-VL curve (from 70.5 +/- 16.2 mm Hg to 117.5 +/- 50.7 mm Hg, p < 0.002), the cha nges in Pa-O2 in patients without an LIP on the P-VL curve not being s ignificant (from 91.3 +/- 45.4 mm Hg to 122.2 +/- 41.1 mm Hg). We conc lude that in ventilator-dependent patients with ARF: (1) the chest-wal l mechanics can contribute to the LIP observed on the P-Vrs curve; (2) the improvement in Pa-O2 with PEEP is significant only in patients in whom LIP is on the lung P-V curve and not on the chest wall curve; (3 ) high levels of PEEP may over-distend the lung, as reflected by the a ppearance of a UIP; (4) measurement of P-Vrs alone may be misleading a s a guide for setting the level of PEEP in some mechanically ventilate d patients, at least in the supine position, although it helps to prev ent excessive alveolar overdistension by indicating the inflection vol ume above which UIP may appear.