Effects of PEEP and tidal volume on elastances and distribution of volume changes of the different chest wall compartments

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We used optoelectronic plethysmography [1] to analyze six sedated and paralyzed ALI/ARDS patients receiving continuous positive pressure ventilation (CPPV) with different settings of PEEP (5, 10, 15 cmH₂O) and tidal volume (TV, 300, 600, 900 ml). The aim of the present work is to study the effects of PEEP and TV on volume distribution in three different chest wall compartments (upper thorax, V_UT, lower thorax, V_LT and abdomen, V_AB) and on their elastances (E_UT, E_LT and E_AB, respectively) assessed by the end-inspiratory occlusion technique. Results are expressed in the table as mean ± SD.

We found that both PEEP and TV, but not their interaction, have an effect on volume distribution in the UT and AB (P<0.05). At increasing PEEP and TV, the increased abdominal and decreased upper thoracic contribution to inspired volume were associated to a decreased compliance of the UT. Finally, we found significant (P<0.001) exponential relationships between UT and AB contributions to TV and E_UT.

We conclude that at high PEEP and TV, the elastance of the upper thorax decreases and it causes a more `abdominal' distribution of TV.

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