Effects of artificial changes in chest wall compliance on respiratory mechanics and gas exchange in patients with acute lung injury (ALI)

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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 cmH<sub>2</sub>O, i.e., 2-5 cmH<sub>2</sub>O above the PV curve inflection point (`P_flex PEEP'), V_T=556± 137 ml, RR=17± 4 br/min, V_E=9.1± 1.3 L/min, F_iO₂=80± 22%).

We measured gas exchange, hemodynamics, total respiratory system compliance (C_RS) partitioned into its lung (C_L) and chest wall (C_CW) components (end-inspiratory occlusion and esophageal balloon technique), end-expiratory lung volume (EELV; helium dilution), abdominal ventilation distribution (V_abd/V_tot; 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 C_CW and C_RS (from 43± 23 to 58± 35 and from 34± 15to 40± 18 ml/cmH₂O, respectively, P<0.05). Mean PaO₂,C_L and V_abd/V_tot did not change, even if patients who improved oxygenation were the same who reduced their C_CW (r=0.78, P<0.01) and increased C_L (r=0.64, P<0.05) and V_abd/V_tot (r=0.68, P<0.05). Moreover,patients with lower baseline IAP had a greater C_CW reduction (r=0.60, P<0.05).

We tested SB positioning also at an higher PEEP level (5 cmH₂O above `P_flex PEEP'): increasing PEEP induced an increase of EELV (from 1.22± 0.75 to 1.37± 0.82 l,P<0.05), V_abd/V_tot (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).

Conclusion

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.