An adapted air-filled esophageal open catheter method without balloon, using a disposable catheter and transducer allows for reproducible esophageal pressure measurements, without any specific material requirements.
Description of the air-filled esophageal catheter method
Here, we use a disposable low compliance polyvinyl esophageal suction catheter, originally intended for oral, nasopharyngeal or tracheobronchial suctioning, 49 cm long, 10 Fr and 3.3–2.0 mm outer–inner diameters. As shown in Fig. 1, the catheter is connected to an air-filled disposable blood pressure transducer bound to the monitor. A 1-l saline infusion bag is emptied, backfilled with air and pressurized at 100 mmHg by a pressure infusion bag with manometer (Additional file 1: Figure S1) and connected to the air-filled intravenous set, so that the transducer delivers a continuous air flow rate of ~ 2.5 ml/min. The pressurized system guarantees open-ended catheter patency, undamped values and signal stability. Air-labeled flags are disposed along the air-filled pressure line for safety. To facilitate the nasal or oral placement of the esophageal catheter and its visualization on chest X-rays, a siliconized guide wire of nasogastric enteral feeding tube is temporarily inserted in the catheter and bended to match the desired length. The catheter is positioned first in the stomach. Proper gastric position is assessed by auscultation of a 10-mL air flush and, after connection to the transducer by observation of positive deflections on waveform during inspiration or when gentle stomach compressions are imposed. The catheter is withdrawn to the lower-third of the esophagus until esophageal waveform is confirmed by small cardiac artifacts and spontaneous inspiratory negative deflections. Appropriate position of the catheter is confirmed in three ways: (i) by chest X-rays with the guide wire (Additional file 2: Figure S2); (ii) by visualization of cardiac artifacts on the esophageal waveform, and (iii) by equivalent changes in esophageal and airway pressures during the dynamic end-expiratory occlusion test maneuver. In passive breathing condition, gentle external chest compressions are performed during expiratory occlusion. In active breathing condition, spontaneous efforts occur against occlusion (Baydur’s maneuver) (Additional file 3: Figure S3). During the occlusion test, an esophageal-to-airway pressure change ratio (ΔPes/ΔPaw) close to unity (± 10–20%) validates the technique as an adequate estimate of pleural surface pressure and thereby the proper position of the catheter [3, 16].
Before any esophageal measurement, the open-ended catheter is flushed with 3 ml of air using a syringe in order to remove any distal secretion. The transducer is zeroed at atmospheric pressure. Values are recorded in mmHg and converted in cmH2O.
Esophageal catheter subocclusion by secretions is suspected when abrupt, vertical falls, staircase steps or increasing slopes disrupt the esophageal pressure wave (Additional file 4: Figure S4). Deobstruction of the catheter requires a flushing procedure to restore a proper signal.
For clinical purpose, we measure end-inspiratory and end-expiratory pressures in both airway and esophagus, during controlled and assisted modes. This allows computation of transpulmonary pressures in order to apply an optimal transpulmonary-guided lung-protective ventilation (Additional file 3: Figure S3). To facilitate bedside calculations we have designed an online transpulmonary pressures calculator [17]. For more details on the method, see the complete standard operating procedure in the Additional file 9.
Comparison with the reference balloon catheter
The esophageal balloon catheter used in our study was the 14 Fr multifunction nasogastric feeding catheter NutriVent™ (Sidam, Italy). We connected the balloon catheter via a rigid air-filled line to a disposable and locked pressure transducer bound with the monitor. In vivo calibration, including end-expiratory and end-inspiratory pressure–volume curves for different balloon volumes following Mojoli et al. [15] was performed and allowed the determination of the optimal filling volume of the esophageal balloon (Vbest), the esophageal wall elastance (Eew) and the pressure generated by the esophageal wall (Pew). Calibrated values of Pes (Pes minus Pew corresponding to Vbest) were not computed here. Additional ex vivo comparison between air-filled catheter and balloon catheter is presented in the online supplement (Additional file 5: Figure S5; Additional file 9).
Population
We studied 15 consecutive critically ill patients requiring prolonged mechanical ventilation (> 48 h), without contraindications such as coagulation, hemodynamic, esophageal or gastric disorder or recent cardiothoracic surgery considering the need for external chest compression during the occlusion test. Our methodological monocentric study was conducted in a tertiary 50-bed ICU university hospital in Liege, Belgium. All patients in passive breathing condition were sedated, paralyzed and ventilated in volume-controlled mode in semi-recumbent position. In active condition, all patients were ventilated in pressure support mode by a Servo-i Maquet respirator (Getinge, Sweden) that allows end-expiratory occlusion in this mode.
In vivo comparison of both esophageal pressure methods
All esophageal pressure measurements via the air-filled catheter and the balloon catheter were performed sequentially rather than simultaneously, to avoid catheter interference on the esophageal wall and potential bias. Air-filled catheter was withdrawn in the upper-third of esophagus during balloon catheter measurements. Inversely, balloon was deflated during air-filled catheter measurements, but without removal of the nasogastric feeding balloon catheter since the air-filled catheter is supposed to be used in ventilated patients usually requiring feeding catheter. Random order of reading was applied. Twelve measurements were performed repeatedly by three different observers, namely sequentially 3 series for each of them plus 3 supplemental series for the first observer. Each series of esophageal measurements consisted in six specific timepoints, i.e., during dynamic external chest compression, inspiratory occlusion and expiratory occlusion in the controlled mode (Additional file 3: Figure S3A), and during Baydur’s maneuver, inspiratory deflection and end-expiratory measure in the assisted mode (Additional file 3: Figure S3B).
Direct comparison of esophageal pressure with pleural pressure
Two patients (#7 and #8) presented right pleural drain that allowed pleural pressure measurements via fluid-filled regular pressure transducer. Recordings of simultaneous esophageal and pleural pressures traces together with ventilatory parameters could be performed using Philips IntelliVue MP70 with spirometry module and flow sensor. High-definition 500-Hz recordings required to analyze signal stability and frequency components of the pressure signals were performed using I-Care Pro Software (PLHealthcare & eSense, Belgium) connected to an MP70 interface. The frequency spectra were obtained by applying fast Fourier transforms (Python with NumPy package, Python Software Foundation) to each esophageal and pleural pressure signals, extracted from a 80-s multiparameter period recording, according to the Welch’s method [18].
Statistical analysis
Continuous variables are presented as mean ± standard deviation (range minimum–maximum) and qualitative variables as count (%). The agreement between the reference balloon catheter method (Balloon) and the air-filled catheter method (Catheter) was assessed using the Bland–Altman technique adapted for repeated measurements analysis [19,20,21]. Repeatability and reproducibility of esophageal pressure measurements were evaluated by intra-observer and inter-observer intraclass correlation coefficients. Paired t-tests were used to compare paired measurements of continuous variables in the patients' sample. P-value < 0.05 was considered significant. Statistics were performed using Stata (StataCorp 2021, College Station, TX) and RStudio (RStudio Team 2020, Boston, MA) software.