The primary findings of this study are: (1) the ∆Pes/∆Paw ratio was faintly different when computed with the Baydur and the positive pressure occlusion tests; (2) end-expiratory esophageal pressure was higher in sedated and paralyzed patients, with the esophageal balloon in the low position and with higher PEEP level; and (3) lung elastance was slightly lower when computed with the esophageal balloon in the low position while the PEEP reduced the chest wall elastance without affecting the lung and respiratory system elastance.
The estimation of respiratory mechanics, lung stress and muscle activity requires the measurement of transpulmonary pressure, calculated as the difference between airway and pleural pressure. Due to the impossibility of directly measuring pleural pressure in clinical practice, esophageal pressure has been proposed as a reliable surrogate [1–3]. Although the absolute values of esophageal pressure are not equal to the absolute values of pleural pressure, the difference between the changes in intrathoracic and esophageal pressure was found to be in the order of 1 %  and mainly depends on the elastance of the esophageal walls, on the surrounding structures and on the mechanical properties of esophageal balloon [5, 6, 11, 19]. Since the earliest animals and human studies, pleural pressure has usually been estimated indirectly from an esophageal catheter equipped by a balloon filled by air [19, 20]. Besides the appropriate air filling, a correct position of the balloon in the esophagus is fundamental to obtain an accurate estimation of the pleural pressure [4, 10, 12].
In spontaneously breathing subjects, the “optimal position” of the esophageal balloon catheter is usually found by applying the “Baydur occlusion test” . With this test in a small group of not intubated patients, it was found that the average ratio between changes in esophageal and airway pressure was 0.90 ± 0.40 with individual data ranging from 0.61 to 1.10 . In non-paralyzed anesthetized subjects, with the balloon optimally located, this ratio amounted to 0.98 ± 0.03 .
However, due to the impossibility to perform the “Baydur” test in deeply sedated or paralyzed patient, a positive pressure occlusion test has been proposed [4, 15].
Sedation and paralysis may affect the transmission of pleural pressure through the esophageal wall altering the validity of the positive pressure occlusion test compared with the “Baydur” test .
In the present study, we found that the ∆Pes/∆Paw ratio was significantly higher (+0.11) when calculated with positive occlusion test compared with Baydur’s test (paralysis vs no paralysis). During an occlusion test, either by compressing the thorax or by generating inspiratory efforts against a closed airway, there are actually slight changes in the lung volume and transpulmonary pressure due to the compressibility of intrathoracic gas, rather than constant. It could be also possible to have a slightly higher ∆Pes/∆Paw ratio obtained by compressing the thorax with gas compression, than the one obtained by Baydur’s maneuver with gas expansion. However, from the clinical viewpoint this difference is negligible. Contrary to dogs in which the ratio between esophageal and airway pressure was better during positive occlusion test compared with standard occlusion test, probably because the esophagus is composed of entirely striated muscle, in humans the middle and distal esophageal body is mainly constituted by smooth muscle with a much lower influence of sedative drugs and neuromuscular blockers [22–25]. This was also confirmed by Hedenstierna et al.  which showed no difference in the esophageal elastance (i.e., similar elasticity) between awake and anesthetized subjects. Furthermore, the higher level of sedation and the neuromuscular block which could decrease the lung volume did not affect the validity of the two tests.
In our study, ∆Pes/∆Paw ratio recorded by the Baydur and the positive pressure occlusion tests was not affected neither by the PEEP level nor by the esophageal balloon position.
Similar data were reported in healthy subjects in whom the changes in esophageal and airway pressure differed less than 10 % at different lung volumes, over the range of vital capacity  and with the esophageal balloon in different positions (middle and low) .
Based on the whole data, the Baydur and the positive pressure occlusion tests can be considered similar, suggesting that clinicians do not have to repeat the test or reposition the esophageal catheter when the subject is paralyzed or, on the contrary, is waked up.
Although in our study a considerable number of measurements of ∆Pes/∆Paw ratio showed a standard deviation ranged from 0.11 to 0.18, which can be due to intra-patient differences, differences between the four manual thoracic compressions or between the two inspiratory efforts intra- and inter-patients, all the ratios are close to unity and according to us the differences were not clinically relevant, including the effect of paralysis. Furthermore, all the explored interactions were not statistically significant. Therefore, our results showed that there is not a better condition to obtain a better match between ∆Pes and ∆Paw, and in fact, we consider ∆Pes/∆Paw ratio as adequate in every explored condition.
The low intra-patient repeatability could be due to differences in the manual thoracic compressions performed by the principal investigator and in the respiratory efforts performed by the patient; however, thank to these differences we explored a wide range of generated airway (from 1.5 to 23 cmH2O) and esophageal pressures (from 1.5 to 24 cmH2O). Within this wide range of pressures, the changes in esophageal and airway pressure were significantly related to a correlation coefficient of r = 0.984 and of r = 0.909 during the Baydur and the positive pressure occlusion tests, respectively.
Although questionable, the use of the end-expiratory esophageal pressure, considering the results of several recent studies based on the esophageal pressure to titrate PEEP [7, 28, 29], should at least require a correct measurement . In this study, end-expiratory esophageal pressure was significantly higher when the balloon was placed in the low position, in deeply sedated and paralyzed patients. It was also influenced by the PEEP level confirming the results of previous studies [7, 30].
When the esophageal balloon is in the middle part of the esophagus, it is mainly influenced by the weight of the heart, while in the low position by the weight of the heart plus the lung. Furthermore, the loss of muscle tone can increase the gravitational weight of these structures over the esophagus. Contrary to the ∆Pes/∆Paw ratio, the end-expiratory esophageal pressure is clearly influenced by the balloon position and paralysis. Although previous studies have shown the possible utility of setting PEEP according to the static end-expiratory esophageal pressure for minimizing the alveolar collapse [7, 30], the end-expiratory esophageal pressure was not found to be related to the lung weight or chest wall elastance . The end-expiratory esophageal pressure can be significantly higher in the presence of neuromuscular paralysis, in higher PEEP level and when esophageal balloon is placed in the low esophagus (Table 3). In addition, the static end-expiratory esophageal pressure has proved to be influenced by the elastic recoil of the balloon and of the esophagus and by the surrounding structures [10, 11, 31–33]. According to these artifacts, several correction factors have been introduced [28, 30]. However, the use of static end-expiratory esophageal pressure can be reasonable in the same clinical condition (PEEP, balloon position, paralysis) to evaluate the clinical changes over time.
The partitioned respiratory mechanics (lung and chest wall) and end-inspiratory transpulmonary pressure are often calculated in patients with acute respiratory failure to evaluate the effects of different tidal volumes or PEEP as a means of minimizing stress and strain and the ventilation-induced lung injury. To avoid any possible artifacts due to the presence of muscle activity, in the current study the respiratory mechanics were measured only in sedated and paralyzed patients. Elastance-derived end-inspiratory transpulmonary pressure and lung elastance were only minimally affected by the position of the esophageal balloon, and similarly to the results of the ∆Pes/∆Paw ratio, they can be considered independently from the position of the balloon. In addition, the PEEP only reduced the chest wall elastance without affecting the lung and total respiratory system elastance and, as expected, increased the elastance-derived end-inspiratory pressure.