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In ALI patients, the prone position induces a decrease in chest wall compliance significantly correlated with oxygenation improvement. We studied the effects on respiratory mechanics and oxygenation of artificial change of chest wall compliance (obtained positioning a 10 kg `sand bag' (SB) upon the upper thorax) in 11 supine mechanically ventilated ALI patients (volume control mode, PEEP=14± 3 cmH2O, i.e., 2-5 cmH2O above the PV curve inflection point (`Pflex PEEP'), VT=556± 137 ml, RR=17± 4 br/min, VE=9.1± 1.3 L/min, FiO2=80± 22%).

We measured gas exchange, hemodynamics, total respiratory system compliance (CRS) partitioned into its lung (CL) and chest wall (CCW) components (end-inspiratory occlusion and esophageal balloon technique), end-expiratory lung volume (EELV; helium dilution), abdominal ventilation distribution (Vabd/Vtot; Respitrace®, SensorMedics) and intraabdominal pressure (IAP; intrabladder technique).

SB positioning led to an EELV decrease (from 1.22± 0.75 to 0.99± 0.66 l, P<0.05) and, interestingly, to an increase of CCW and CRS (from 43± 23 to 58± 35 and from 34± 15to 40± 18 ml/cmH2O, respectively, P<0.05). Mean PaO2,CL and Vabd/Vtot did not change, even if patients who improved oxygenation were the same who reduced their CCW (r=0.78, P<0.01) and increased CL (r=0.64, P<0.05) and Vabd/Vtot (r=0.68, P<0.05). Moreover,patients with lower baseline IAP had a greater CCW reduction (r=0.60, P<0.05).

We tested SB positioning also at an higher PEEP level (5 cmH2O above `Pflex PEEP'): increasing PEEP induced an increase of EELV (from 1.22± 0.75 to 1.37± 0.82 l,P<0.05), Vabd/Vtot (from 41± 19 to 50± 20%, P<0.05) and PaO2 (from 100± 24 to 121± 38 mmHg, P<0.05) but, at this higher PEEP level, after SB application, oxygenation decreased in all patients (from 121± 38 to 105± 30 mmHg, P<0.05).


Effects of artificial change of chest wall compliance depend on PEEP level and the patient's pathophysiological characteristics: in patients with low IAP values, SB positioning could decrease the regional compliance of upper thorax and favor a caudal displacement of the diaphragm, increasing the ventilation distribution to the abdominal compartment. This can result in an oxygenation improvement, probably due to a decrease of `true' shunt and/or a more homogeneous ventilation to perfusion ratio.