An adapted air-filled esophageal catheter method without balloon, using disposable catheter and transducer allows reproducible esophageal pressure measurements, without specific ventilator requirements.
Here we use a disposable low compliance polyvinyl esophageal suction catheter, originally intended for oral, nasopharyngeal or tracheobronchial suctioning, of 49 cm long, 10Fr and 3.3-2.0 mm outer-inner diameters. As shown in Figure 1a, the catheter is connected via a Luer-lock transfer connector to an air-filled disposable blood pressure transducer bound to the monitor. The transducer is identical to the one intended for blood pressure monitoring and contains a flush device delivering a continuous flow of 3 mL/h with a differential pressure of 200 mmHg between infusion bag pressure and physiological pressure monitored. A one-liter saline infusion bag is emptied, backfilled with air and pressurized over 250 mmHg by a pressure bag and connected to the air-filled IV set with drip chamber and roll clamp. Air-labeled flags are disposed along the air-filled pressure line for safety.
- Placement of the esophageal catheter:
To facilitate the nasal or oral placement of the esophageal catheter and its visualization on chest X-ray, a siliconized guide wire of nasogastric enteral feeding tube is temporarily inserted in the catheter and bended to match desired length. The catheter follows the naso- or orogastric feeding tube if present, until its extremity is positioned first in the stomach. The pressure transducer line is connected to the catheter with the appropriate connector, and the roll clamp on the infusion set is opened to allow continuous flush and signal. A stable pressure waveform is then visualized. Proper gastric position is confirmed by auscultation of a 10 mL of air flush and 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 The catheter is withdrawn to the lower third of the esophagus, taking a nasal-tragus-xyphoid distance – 5 cm for nasal placement, and mobilized until esophageal waveform is confirmed by small cardiac artifacts and spontaneous inspiratory negative deflections.
- Esophageal pressure measurements:
The open-ended catheter is flushed with air before starting to remove any distal secretion. The remaining air is then aspirated to prevent artifacts. The line is left open to air to equilibrate for a few seconds. One ml of air is injected to allow optimal pressure transduction. We record the zero with the transducer open to atmospheric pressure. Appropriate position of the catheter is confirmed in three ways, by chest X-ray with the guide wire, by visualization of cardiac artefacts on esophageal waveform, and by equivalent changes in esophageal and airway pressures during dynamic end-expiratory occlusion maneuver. In passive breathing condition, gentle external chest compressions are performed during expiratory occlusion. In active breathing condition, spontaneous efforts occur against occlusion. Esophageal to airway pressure change ratio (DPes/(DPaw) should be close to one (± 10-20%). Values are recorded in mmHg and converted in cmH2O. Cost of our novel method is 17.6€ for one complete disposable circuit.
- Diaphragm and lung-protective ventilation:
In passive breathing condition, supine patients are sedated and ventilated in volume control mode. End-inspiratory and end-expiratory occlusions are performed to obtain plateau airway pressure (Pplat), plateau esophageal pressure (Pes,plat), total positive end-expiratory pressure (PEEPtot), end-expiratory esophageal pressure (Pes,ee), DP (DP = (Pplat – PEEPtot)) and DPes (DPes = (Pes,plat – Pes,ee)). Ecw (Ecw = DPes/Vt) is calculated as the ratio of DPes to Vt, El (El = Ers – Ecw) is obtained by subtracting Ecw from respiratory system elastance (Ers; Ers = DP/Vt). Elastance ratio (ER; ER = El/Ers) is the lung to the respiratory system elastance ratio. Three transpulmonary pressures are calculated, with potential therapeutic targets [1, 14]: i) elastance-derived end-inspiratory PL (PLei,ER; PLei,ER = Pplat x El/Ers) < 20 cmH2O; ii) end-expiratory PL (PLee; PLee = (PEEPtot – Pes,ee)) > 0 cmH2O; iii) driving PL (DPL; DPL = (DP - DPes) = (PLplat – PLee)) < 10-12 cmH2O.
In active condition, semi-recumbent patients are ventilated in pressure support mode with spontaneous inspiratory efforts. Peak airway pressure (Ppeak), inspiratory esophageal pressure (Pes,i), PEEP, end-expiratory esophageal pressure (Pes,ee), DPes (DPes = (Pes,i – Pes,ee)) and dynamic airway driving pressure (DPdyn; DPdyn = (Ppeak – PEEP)) values are measured. Three transpulmonary pressures are also calculated, with potential therapeutic targets [1, 14]: peak PL (PLpeak; PLpeak = (Ppeak – Pes,i)) < 20 cmH2O, DPes < -10 to -15 cmH2O and dynamic PL swing (DPLdyn; DPLdyn = (DPdyn – DPes)) < 15 cmH2O.