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Online ISSN 1827-1596
Chiumello D. 1, Carlesso E. 1, Aliverti A. 3, Dellacà R. L. 3, Pedotti A. 3, Pelosi P. P. 4, Gattinoni L. 1,2
1 Department of Anesthesia and Intensive Care, IRCCS Foundation, Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena, Milan, Italy;
2 Department of Anesthesia and Intensive Care, University of Milan, Milan, Italy;
3 Department of Bioengineering, Polytechnic of Milan, Milano, Italy;
4 Department of Clinical and Biological Sciences, Insubria University, Varese, Italy
Aim. The pressure-volume (PV) curve in acute lung injury and acute respiratory distress syndrome (ALI/ARDS) patients has been proposed for estimating the underlying pathology, lung recruitment and setting mechanical ventilation. The supersyringe method may lead to artifacts due to thermodynamics and gas exchange. Another possible confounding factor is the volume shift, primarily blood, out of the chest wall when the intrathoracic pressures rise. We set out to quantify the volume shift and investigate its mechanisms.
Methods. Ten ALI/ARDS patients (5 males/5 females, PaO2/FiO2 222±67) were studied in the Intensive Care Unit, University Hospital. PV curve was performed by a supersyringe (0.100 L, 14 steps ∆Vgas) while recording the chest wall volume difference (∆Vcw) by the optoelectronic plethysmography. Differences in airway (∆Paw) and esophageal (∆Pes) pressures were measured during the maneuver. Volume shift was defined as ∆Vcw-∆Vgas, corrected for thermodynamic and gas exchange.
Results. Starting compliance (P<0.05), inflation/deflation compliance (P<0.01), hysteresis (P<0.01) and unrecovered volume (P<0.01) were significantly affected by volume shift. The volume shift was directly correlated to the product ∆Paw*inflation time (R2=0.87, P<0.001), to the ratio of ∆Pes to ∆Paw (R2=0.80, P<0.01) and to central venous pressure (R2=0.42, P<0.05) and inversely correlated with the deflation time (R2=0.58, P<0.05). At 20 cmH2O of airway pressure the volume shift between the inflation and deflation limbs of the PV curve amounted to 0.099±0.058 L.
Conclusion. The volume shift, constituted mainly of blood, significantly affects both inspiratory and expiratory PV curve. Caution is needed when interpreting the PV parameters (Minerva Anestesiol 2007;73:1-10).