Intracranial pressure responsiveness to positive end-expiratory pressure is influenced by chest wall elastance: a physiological study in patients with aneurysmal subarachnoid hemorrhage

Background Respiratory system elastance (ERS) is an important determinant of the responsiveness of intracranial pressure (ICP) to positive end-expiratory pressure (PEEP). However, lung elastance (EL) and chest wall elastance (ECW) were not differentiated in previous studies. We tested the hypothesis that patients with high ECW or a high ECW/ERS ratio have greater ICP responsiveness to PEEP. Methods An esophageal balloon catheter was placed to measure esophageal pressure. PEEP was increased from 5 to 15 cmH2O. Airway pressure and esophageal pressure were measured and EL, ECW and ERS were calculated at the two PEEP levels. Patients were classified into either an ICP responder group or a non-responder group based on whether the change of ICP after PEEP adjustment was greater than or less than the median of the overall study population. Results The magnitude of the increase in esophageal pressure (median [interquartile range]) at end-expiratory occlusion was significantly increased in the responder group compared with that in the non-responder group (4.1 [2.7–4.1] versus 2.7 [0.0–2.7] cmH2O, p = 0.033) after PEEP adjustment. ECW and the ECW/ERS ratio were significantly higher in ICP responders than in non-responders at both low PEEP (p = 0.021 and 0.017) and high PEEP (p = 0.011 and 0.025) levels. No significant differences in ERS and EL were noted between the two groups at both PEEP levels. Conclusions Patients with greater ICP responsiveness to increased PEEP exhibit higher ECW and a higher ECW/ERS ratio, suggesting the importance of ECW monitoring. Electronic supplementary material The online version of this article (10.1186/s12883-018-1132-2) contains supplementary material, which is available to authorized users.


Fujian, China
Additional File 1

Procedure of the placement of esophageal balloon catheter
We used the SmartCath-G adult nasogastric tube with an esophageal balloon (7003300, CareFusion Co., Yorba Linda, CA, USA) in this study. Patients were remained in a supine position with the head of the bed elevated to 30° during the study period. After anesthetizing the nose and oropharynx with 10% lidocaine spray, the esophageal balloon catheter was inserted through the nostril to a depth of 60 cm.
The intra-gastric position of the distal part of the catheter was confirmed by aspiration of gastric juice and auscultation of air insufflations into the stomach. After confirmation of the catheter position, the balloon was inflated with 1.5 mL of air, and the proximal part of the catheter was connected to a pressure transducer. Subsequently, the catheter was slowly withdrawn, and the dynamic occlusion test was performed. An end-expiratory occlusion was performed until three to five spontaneous inspiratory efforts were made against the end-expiratory occlusion. The ratio of the change in esophageal pressure (P ES ) to the change in airway pressure (∆P ES /∆P AW ) was calculated. The catheter was considered 2 correctly positioned when the ΔP ES /ΔP AW ratio during the occlusion test is in the range of 0.8 to 1.2 [1].
In the absence of spontaneous breathing, the positive pressure occlusion test was performed by applying manual compression on the rib cage during the end-expiratory occlusion [2,3].

Respiratory mechanics measurements
An end-inspiratory occlusion and an end-expiratory occlusion were performed for 3 s. P AW and P ES during the last second of occlusion were recorded. The mean P AW and P ES during a complete cardiac cycle was measured to avoid the influence of cardiac artifacts ( Figure E1).
3 Figure E1 Measurement of airway pressure (P AW ) and esophageal pressure (P ES ). An approximately 3-s end-inspiratory occlusion and an end-expiratory occlusion were performed. Move the cursors (the yellow and blue vertical lines) to the last and last second peaks, and the the average value between the two cursors were automatically calculated and displayed by the software.
Respiratory system elastance (E RS ) can be calculated as: Where P PLAT and PEEP TOTAL represent P AW at end-inspiratory and end-expiratory occlusion, respectively. The difference between P PLAT and PEEP TOTAL was called as airway driving pressure (∆P AW ).
E RS can be differentiated into lung elastance (E L ) and chest wall elastance (E CW ) [4]: Using the P ES as a surrogate for pleural pressure, E CW can be calculated as: Where P ES-EI and P ES-EE are respective P ES determined at end-inspiratory and end-expiratory occlusion, and the difference between P ES-EI and P ES-EE was called as chest wall driving pressure (∆P ES ).
E L can be calculated as: Where (P PLAT -P ES-EI ) and (PEEP TOTAL -P ES-EE ) are the respective transpulmonary pressure at endinspiratory and end-expiratory occlusion, and the difference of these two parameters represents the transpulmonary driving pressure (∆P L ).
E RS , E CW and E L were obtained at the two tested PEEP levels.