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1 Servizio di Anestesia e Rianimazione, Azienda Ospedaliera di Parma, 43100 Parma; and 2 Unità Operativa di Pneumologia, Ospedali Riuniti di Bergamo, Azienda Ospedaliera, I-24128 Bergamo, Italy
The lower inflection point (LIP)
on the total respiratory system pressure-volume (P-V) curve is widely
used to set positive end-expiratory pressure (PEEP) in patients with
acute respiratory failure (ARF) on the assumption that LIP represents
alveolar recruitment. The aims of this work were to study the
relationship between LIP and recruited volume (RV) and to propose a
simple method to quantify the RV. In 23 patients with ARF, respiratory
system P-V curves were obtained by means of both constant-flow and
rapid occlusion technique at four different levels of PEEP and were
superimposed on the same P-V plot. The RV was measured as the volume
difference at a pressure of 20 cmH2O. A third measurement
of the RV was done by comparing the exhaled volumes after the same
distending pressure of 20 cmH2O was applied (equal pressure
method). RV increased with PEEP (P < 0.0001); the
equal pressure method compares favorably with the other methods
(P = 0.0001 by correlation), although individual data
cannot be superimposed. No significant difference was found when RV was
compared with PEEP in the group of patients with a LIP
5
cmH2O and the group with a LIP >5 cmH2O
(76.9 ± 94.3 vs. 61.2 ± 51.3, 267.7 ± 109.9 vs.
209.6 ± 73.9, and 428.2 ± 216.3 vs. 375.8 ± 145.3 ml
with PEEP of 5, 10, and 15 cmH2O, respectively). A RV was
found even when a LIP was not present. We conclude that the recruitment
phenomenon is not closely related to the presence of a LIP and that a
simple method can be used to measure RV.
acute respiratory failure; mechanical ventilation; pressure-volume curve; positive end-expiratory pressure
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