The results of our study indicate that EIT is able to assess the dynamic changes of regional lung aeration in response to step increase and decrease in airway pressure at the bedside. Regional lung inflation and deflation could be described using a two-compartment model. EIT-derived measures of regional respiratory mechanics τ1 and τ2 as well as the fractions of the fast and slow compartments clearly distinguished the ARDS patients from the patients with normal lungs. Regional differences in these measures were detected between the ventral and dorsal lung areas reflecting the different filling and emptying behavior of the lung tissue in the dependent and non-dependent regions. PEEP exerted a significant effect on regional τ1 and τ2 during both inflation and deflation. The fractional contribution of the fast and slow compartments depended on PEEP during step decrease but not during the step increase in airway pressure.
ARDS vs. Normal lungs
EIT-based analysis of the temporal behavior of regional lung filling and emptying discriminated the ARDS patients from the patients with healthy lungs under all studied conditions. τ1 and τ2 were always higher in the patients with normal lungs than in the patients with ARDS most likely reflecting the higher lung compliance of the healthy subjects[19–21]. The only exception was τ2 in the ventral lung regions at the highest PEEP. In this case, regional τ2 fell markedly in the patients with normal lungs due to postulated overdistension and became similar to the value found in this region in ARDS patients.
Consistent with alveolar recruitment taking place in the injured lungs during step inflation and derecruitment during deflation is the finding of lower fractions of the fast compartment in the ARDS patients during both step increase and decrease in airway pressure. However, we observed that the fractional contribution of the two compartments to lung emptying was similar in ARDS patients and in patients with healthy lungs during expiration to the highest PEEP. The fractions of the slow compartment were higher in ARDS patients than in the patients with normal lungs at the lower PEEP values implying more rapid derecruitment in the injured lungs.
Ventral vs. Dorsal regions
Regional differences between the EIT-derived measures of regional lung filling and emptying behavior τ1 and τ2 as well as between the relative sizes of the fast and slow compartments existed between the ventral and dorsal regions. These can be attributed to the effect of gravity. Derecruitment preferably occurred in the dependent lung regions during expiration and these regions became recruited again during inspiration, whereas overdistension was more likely present in the non-dependent lung areas.
Since regional alveolar distension is lower in the dependent than in the non-dependent lung regions at lung volumes below full inflation, a larger volume gain will occur in the dependent than in the non-dependent regions during sustained step inflation. This was found in our study as revealed by higher τ1 in the dorsal regions during inflation in both patient groups. The differences between the regions tended to be the largest when zero end-expiratory pressure was used. Regional τ2 values in the ventral and dorsal regions were alike during inflation at the lowest PEEP but fell more rapidly in the ventral regions. This phenomenon is attributable to regional lung overdistension which was observed at the PEEP of 15 cm H2O in the patients with healthy lungs and already at 8 cm H2O in the ARDS patients.
The effects of step decrease in airway pressure on regional dynamics of lung aeration can not be regarded as a simple reversal of the effects elicited by the rapid inflation of the lungs using a sustained step increase in airway pressure. Regional τ1 was lower in the dorsal than in the ventral regions during step deflation. τ2 was also lower in these regions when the deflation ended at the two lower PEEP values in the healthy lungs and at the lowest value in the injured lungs. τ2 was generally higher during deflation than inflation suggesting that once the lung was recruited during sustained inflation the regional loss in volume was slower than its gain during the step inflation.
Regional fraction of the fast compartment was higher in the ventral than in the dorsal regions. This can be attributed to more pronounced lung recruitment in the regions near the spine and has already been postulated in an experimental study using an animal model of ARDS. The slow compartments were also larger in the dorsal regions during deflation to low PEEP values. This phenomenon may reflect regional derecruitment which was more probable the lower the PEEP value at which the deflation ended.
A PEEP-dependent fall in τ1 was found in both patient groups during step inflation as a result of decreasing respiratory system compliance and airway resistance at higher lung volumes[20, 24, 25]. The dependency of τ1 on PEEP also existed during deflation but it was only observed in the ventral regions. These results are in harmony with two clinical studies in patients with ARDS and chronic obstructive lung disease where a reduction of overall expiratory τ was found with increasing PEEP[10, 26].
τ2 also exhibited a PEEP-dependent behavior with the lowest values found at the highest PEEP. This was expected because the lowest degree of recruitment or derecruitment occurred when the step inflation started and the step deflation ended at the highest PEEP of 15 cm H2O. High PEEP was shown to decrease expiratory resistance by preventing airway closure.
We expected that the slow compartment size would become smaller at higher PEEP during the inflation maneuver but this was not confirmed by the current results. The relative size of the slow compartment was previously shown to fall when lung tissue was recruited in an animal model of ARDS. However, these results are not directly comparable with our findings obtained in patients using another imaging modality. In the present findings, the effect of recruitment on the slow compartment size may have been masked by other opposing effects like overdistension with increasing lung tissue resistance slowing down the filling of the regions with air. During the deflation maneuver, a marked dependency on PEEP was observed. The size of the fast compartment was the highest during lung emptying to 0 cm H2O of pressure, the corresponding average values exceeded 94% in the lung healthy subjects. This corresponds to previous findings showing an almost mono-exponential emptying pattern under these conditions.
The results of our study require cautious interpretation. The used set-up, the protocol, the imaging method and the assessment of regional and not global lung behavior had an impact on the obtained results and differed from previous studies on respiratory mechanics in ARDS.
The patient examinations were accomplished in a clinical setting in intubated, mechanically ventilated patients. Therefore, the findings reflect not only the dynamic behavior of the respiratory system alone but were also influenced by the endotracheal tube and the respirator circuit. Therefore, the τ1 and τ2 values were higher in our patients than in other studies[10, 26–28].
The large pressure differences applied during the inflation and deflation maneuvers resulted in high flow rates. Smaller PEEP differences and, thus, lower flow rates were applied in a few previous studies[29, 30]. Since the endotracheal tube is a flow-dependent resistive element it contributes to the determined τ along with the tubing and ventilator resistance affecting the analysis of the respiratory system mechanics. High flows in the endotracheal tube and respirator circuitry increase the resistance and lead to higher respiratory τ.
During large airway pressure steps, additional phenomena may have influenced the EIT-derived findings: Firstly, pulmonary fluid and blood content may have been affected by the sustained inflation and deflation. This may have contributed to the small sloping plateau of the EIT waveforms in the patients with normal lungs during inflation and also to the steeper slope observed in the ARDS patients, where an additional effect of lung recruitment was probable. The relatively high τ2 values in a relatively small slow compartment found in the lung healthy patients are consistent with this effect. Secondly, continuing gas exchange during the later phases of the deflation maneuvers may have induced a prolonged decrease in ΔZ and contributed to the relatively high τ2 values. Thirdly, age-dependent differences in lung mechanics may have impacted the results since the ARDS patients were older than the patients with healthy lungs. Fourthly, slightly different sections of the lung may have been examined by EIT at different PEEP steps because of a shift in the cranio-caudal axis. This phenomenon has been described in animal and clinical EIT studies but it is less dramatic compared when other imaging modalities are used since EIT examines a broader than a strictly two-dimensional section of the chest.