Effects of volume shift on the pressure-volume curve of the respiratory system in ALI/ARDS patients

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 ΔVgas) while recording the chest wall volume difference (ΔVcw) by the optoelectronic plethysmography. Differences in airway (ΔPaw) and esophageal (ΔPes) pressures were measured during the maneuver. Volume shift was defined as ΔVcw-ΔVgas, 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 ΔPaw* inflation time (R2=0.87, P<0.001), to the ratio of ΔPes to ΔPaw (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 cm H2O 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.