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[Respiratory pattern and respiratory strain in automatic tube compensation and inspiratory pressure support].

STUDY OBJECTIVE
To investigate whether automatic tube compensation (ATC) or conventional pressure support (PS) is suitable to compensate for the work of breathing imposed by the breathing circuit without altering the breathing pattern.


METHODS
Breathing pattern and work of breathing were measured in healthy volunteers. After a 20 min period of quiet breathing through a mouth piece (control) the volunteers were breathing through a 8.0 mm ID endotracheal tube (ETT) with four different settings: CPAP at 0 mbar, ATC, PS 5 mbar, PS 10 mbar. Each mode was applied for a 20 min period. At the end of each period data from 10 consecutive breaths were analyzed and averaged. Tidal volume (VT), breathing frequency (f), and minute ventilation (Ve) were determined from the stored gas flow tracings. Work of breathing was assessed as the pressure time product (PTP) calculated from the transdiaphragmatic pressure (Pdi) using a combined esophageal and gastric balloon catheter.


RESULTS
During the control period the breathing pattern was as follows: VT = 882 +/- 277 ml, f = 13.7 +/- 5/min, Ve = 11.5 +/- 4.2 L/min. Maximal Pdi was 9.2 +/- 5.4 mbar and PTP was 11.3 +/- 7.1 mbar x s. Breathing CPAP through the ETT resulted in a slight increase in Pdi (10.8 +/- 5.4 mbar) and PTP (14.8 +/- 10.4 mbar x s) with an unchanged breathing pattern. However, for the same amount of unloading from respiratory workload ATC did not alter the breathing pattern, whereas PS 5 mbar and PS 10 mbar resulted in a clear increase in VT (1014 +/- 202 ml, 1336 +/- 305 ml, respectively).


CONCLUSION
From the presented data in healthy volunteers it might be concluded that ATC and PS 5 mbar and 10 mbar are suitable modes for unloading the respiratory system from work imposed by the breathing circuit. ATC does not alter the breathing pattern in contrast to PS which results in an increased tidal volume. Therefore, the exact compensation of the work imposed by the ETT during ATC seems to be advantageous over ATC to assess the actual breathing pattern.