In vivo calibration of esophageal pressure in the mechanically ventilated patient makes measurements reliable

Abstract

Background: Esophageal pressure (Pes) can provide information to guide mechanical ventilation in acute respiratory failure. However, both relative changes and absolute values of Pes can be affected by inappropriate filling of the esophageal balloon and by the elastance of the esophagus wall. We evaluated the feasibility and effectiveness of a calibration procedure consisting in optimization of balloon filling and subtraction of the pressure generated by the esophagus wall (Pew).

Methods: An esophageal balloon was progressively filled in 36 patients under controlled mechanical ventilation. VBEST was the filling volume associated with the largest tidal increase of Pes. Esophageal wall elastance was quantified and Pew was computed at each filling volume. Different filling strategies were compared by performing a validation occlusion test.

Results: Fifty series of measurements were performed. VBEST was 3.5 ± 1.9 ml (range 0.5-6.0). Esophagus elastance was 1.1 ± 0.5 cmH2O/ml (0.3-3.1). Both Pew and the result of the occlusion test differed among filling strategies. At filling volumes of 0.5, VBEST and 4.0 ml respectively, Pew was 0.0 ± 0.1, 2.0 ± 1.9, and 3.0 ± 1.7 cmH2O (p < 0.0001), whereas the occlusion test was satisfactory in 22%, 98%, and 88% of cases (p < 0.0001).

Conclusions: Under mechanical ventilation, an increase of balloon filling above the conventionally recommended low volumes warrants complete transmission of Pes swings, but is associated with significant elevation of baseline. A simple calibration procedure allows finding the filling volume associated with the best transmission of tidal Pes change and subtracting the associated baseline artifact, thus making measurement of absolute values of Pes reliable.

Keywords: Calibration; Esophageal artifact; Esophageal balloon catheter; Esophageal elastance; Esophageal pressure; Mechanical ventilation; Pleural pressure; Protective ventilation; Transpulmonary pressure; Ventilator-induced lung injury.

Figures

Fig. 1

Static esophageal balloon pressure-volume curves. Relationship between balloon filling volume and static values of Pes, both at end-expiration (PesEE, circles) and at end-inspiration (PesEI, squares). On the end-expiratory pressure-volume (PV) curve, the intermediate linear section was graphically detected and analyzed for its lower and upper limits (VMIN and VMAX, respectively). The range between VMIN and VMAX was considered to correspond to appropriate balloon filling, with volumes below VMIN denoting underfilling and volumes above VMAX denoting overdistention. The elastance of the esophagus (cmH2O/ml) was considered equivalent to the slope of this section of the end-expiratory PV curve. Within the appropriate filling range, we identified VBEST, i.e., the filling volume associated with the maximum difference between PesEI and PesEE. Pes esophageal pressure

Fig. 2

Validation test at different esophageal catheter-filling volumes. Panels (a) and (c): Paw and Pes over time, during a single mechanical respiratory breath and an end-expiratory occlusion maneuver with chest compressions. Arrows refer to the start of the occlusion maneuver. Panels (b) and (d): Pes/Paw relative changes during the end-expiratory occlusion maneuver. Panels (a) and (b) correspond to an esophageal balloon catheter filling of 0.5 ml, while panels (c) and (d): correspond to a filling of 2.5 ml. The slope of the Pes/Paw relationship was 0.48 with an injected volume of 0.5 ml (panel b) and 1.02 with an injected volume of 2.5 ml (panel d). Paw airway pressure, Pes esophageal pressure

Fig. 3

Examples of inspiratory and expiratory static esophageal balloon pressure-volumes curves. Circles refer to end-expiratory esophageal pressure (PesEE); closed circles refer to VMIN and VMAX as graphically detected (respectively lower and upper limits of the intermediate linear section of the curve). Squares refer to end-inspiratory esophageal pressure (PesEI); closed squares refer to VBEST (the filling volume associated with the largest PesEI – PesEE difference). Panel (a) 32-year-old male patient, body mass index (BMI) 27 kg/m2; pulmonary alveolar proteinosis and kyphoscoliosis; positive end-expiratory pressure (PEEP) 0 cmH20; tidal volume (TV) 550 ml, plateau pressure (Pplat) 20 cmH2O. VMIN and VBEST 0.5 ml. Esophageal elastance 1.3 cmH2O/ml, pressure generated by the esophageal wall (Pew) at VBEST 0.0 cmH2O. Panel (b) 82-year-old female patient, BMI 22 kg/m2; respiratory failure after pulmonary endoarterectomy; PEEP 7 cmH20, TV 500 ml, Pplat 25 cmH2O. VMIN 0.5 ml and VBEST 3 ml. Esophageal elastance 1.3 cmH2O/ml, Pew at VBEST 3.3 cmH2O. Panel (c) 31-year-old male patient, BMI 63 kg/m2; legionella pneumonia and morbid obesity; PEEP 12 cmH20, TV 450 ml, Pplat 30 cmH2O. VMIN 2.5 ml and VBEST 4.0 ml. Esophageal elastance 1.2 cmH2O/ml, Pew at VBEST 1.8 cmH2O. Panel (d) 70-year-old male patient, BMI 23 kg/m2; intra-abdominal hypertension due to large retroperitoneal hematoma; PEEP 10 cmH20, TV 370 ml, Pplat 29 cmH2O. VMIN 1.5 ml and VBEST 6 ml. Esophageal elastance 0.8 cmH2O/ml, Pew at VBEST 3.6 cmH2O. Pes esophageal pressure

Fig. 4

Effects of different esophageal balloon filling volumes on the validation test and the esophagus artifact. Panel (a) The validation occlusion test performed at VBEST was associated with the ∆Pes/∆Paw ratio closest to 1 (0.96 ± 0.06; p < 0.0001 compared to all the other filling strategies) and the highest success rate (98 %; p < 0.001 compared to all the other filling strategies). Panel (b) Pressure generated by the esophageal wall (Pew) as a reaction to optimal filling volume (VBEST) was 2.0 ± 1.9 cmH2O and ranged from 0.0 to 6.0 cmH2O. Pew measured at lower filling volumes (V0.5 and VMIN) was lower (p < 0.0001) and Pew measured at near-full balloon inflation (V8.0) was higher (p < 0.0001). Open symbols refer to individual data; bars refer to mean values. ∆Pes/∆Paw ratio between esophageal pressure (Pes) and airway pressure (Paw) changes during the validation occlusion test

Fig. 5

Subtraction of the esophageal artifact: effect on absolute values of Pes. Compared to PesCAL, bias (mean difference, continuous line) and precision (±1.96 SD of the difference, dotted lines) of PesVBEST were 2.1 ± 3.6 cmH2O. In individual patients, overestimation of Pes [and underestimation of transpulmonary pressure (PL)] due to esophageal elastance may be clinically significant, eventually leading to inappropriate high positive end-expiratory pressure (PEEP) levels and/or end-inspiratory lung overdistention. Pes esophageal pressure