The assessment of transpulmonary pressure in mechanically ventilated ARDS patients.

PURPOSE

The optimal method for estimating transpulmonary pressure (i.e. the fraction of the airway pressure transmitted to the lung) has not yet been established.

METHODS

In this study on 44 patients with acute respiratory distress syndrome (ARDS), we computed the end-inspiratory transpulmonary pressure as the change in airway and esophageal pressure from end-inspiration to atmospheric pressure (i.e. release derived) and as the product of the end-inspiratory airway pressure and the ratio of lung to respiratory system elastance (i.e. elastance derived). The end-expiratory transpulmonary pressure was estimated as the product of positive end-expiratory pressure (PEEP) minus the direct measurement of esophageal pressure and by the release method.

RESULTS

The mean elastance- and release-derived transpulmonary pressure were 14.4 ± 3.7 and 14.4 ± 3.8 cmH₂O at 5 cmH₂O of PEEP and 21.8 ± 5.1 and 21.8 ± 4.9 cmH₂O at 15 cmH₂O of PEEP, respectively (P = 0.32, P = 0.98, respectively), indicating that these parameters were significantly related (r(2) = 0.98, P < 0.001 at 5 cmH₂O of PEEP; r(2) = 0.93, P < 0.001 at 15 cmH₂O of PEEP). The percentage error was 5.6 and 12.0 %, respectively. The mean directly measured and release-derived transpulmonary pressure were -8.0 ± 3.8 and 3.9 ± 0.9 cmH₂O at 5 cmH₂O of PEEP and -1.2 ± 3.2 and 10.6 ± 2.2 cmH₂O at 15 cmH₂O of PEEP, respectively, indicating that these parameters were not related (r(2) = 0.07, P = 0.08 at 5 cmH₂O of PEEP; r (2) = 0.10, P = 0.53 at 15 cmH₂O of PEEP).

CONCLUSIONS

Based on our observations, elastance-derived transpulmonary pressure can be considered to be an adequate surrogate of the release-derived transpulmonary pressure, while the release-derived and directly measured end-expiratory transpulmonary pressure are not related.