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Non-invasive positive pressure ventilation is known to improve the outcome in patients with acute exacerbation of chronic obstructive pulmonary disease (COPD). By applying an external Continuous Positive Airway Pressure (CPAP) the intrinsic PEEP can be compensated and therefore respiratory muscle effort can be minimized. The exact measurement of intrinsic PEEP seems essential concerning an optimal ventilator setting in order to reduce work of breathing and avoid hyperinflation. The gold standard for measuring intrinsic PEEP is an esophageal catheter. However, esophageal pressure measurement is invasive and difficult to obtain in clinical routine for example in patients with acute COPD exacerbation. The purpose of this study was to evaluate two methods of non-invasively determining the intrinsic PEEP. Respiratory movements were recorded by inductive plethysmography simultaneously with respiratory flow to determine whether the time delay between both signals is in accordance with the intrinsic PEEP. Furthermore we generated the inspiration to expiration ratio to determine the intrinsic PEEP. In addition the study was to clarify if the detection of the intrinsic PEEP is possible in all sleep stages, whether sleep associated breathing disorders or other factors have an influence on the measurements and if the examination itself causes arousals.
Both methods were compared with the gold standard for measuring intrinsic PEEP. Therefore 12 patients were included with a history of COPD, all stable in GOLD stage II to IV and marked hyperinflation. Patients were measured in a sleep lab with standard polysomnography including thoracic and abdominal inductive plethysmography. In addition esophageal and gastric pressures were recorded via catheter. Several times during the night, nasal pressure was increased from 4 to 12 mbar in steps of one mbar, each step lasting 20 seconds or 60 seconds. These pressure ramps were passed both awake and in stable sleep. The software EDF-Trace (Sleep lab Marburg) automatically analyzed the data, which were also visually controlled. The results were illustrated in relation to the CPAP pressure and the differences in detected intrinsic PEEP by the different methods were verified on statistical significance using the correlation coefficient by Spearman-Rho.
Low statistical correlations between the different methods were found. Concerning thoracic inductive plethysmography the correlation coefficient was r=0,524, r=0,383 regarding abdominal inductive plethysmography and r=0,272 in comparison of the inspiration to expiration ratio to the esophageal catheter. In conclusion, both non-invasively methods are unable to replace the gold standard for measuring intrinsic PEEP. The thoracic inductive plethysmography has shown promising tendency, but it has to be examined furthermore in larger studies. The results also show that determining intrinsic PEEP affects sleep. In this small study it was discovered, that factors like the level of adipositas and the level of residual volume can affect the results potentially. Larger studies will be needed to verify their influence. Sleep associated breathing disorders were not a disturbing factor. In all patients applying an external pressure did not completely eliminate intrinsic PEEP and a further increase in external pressure did lead to a steady rise of the intrinsic PEEP. Therefore it is really important to find a safe non-invasively method which determinates the intrinsic PEEP exactly.