Esophageal Pressure
The most common technique for PES measurement is to use a catheter with a thin- walled latex balloon sealed at its distal port and filled with air [47, 58]. After anesthetizing the nose and oropharynx, the catheter is inserted through the nostril. The empty balloon catheter is advanced into the stomach, at which time the balloon is inflated. If no diaphragmatic paralysis is present, the presence of an increase in pressure during a spontaneous inspiration generally indicates that the balloon is in the stomach. Afterward, the catheter is slowly withdrawn until a negative pressure deflection replaces the positive deflection, indicating that the balloon is in the lower third of esophagus [59]. The distal part of the catheter is connected to a pressure transducer, which, in turn, can be connected to a dedicated acquisition system, a patient-monitoring system, or an auxiliary pressure port of the ventilator.
There are different types of esophageal catheters available on the market, each one characterized by different length, diameter, and compliance and filling volume of the balloon. These characteristics influence the measurement and must always be taken into account to ensure an accurate estimate of PPL. The filling volume represents a critical issue. Volumes too small determine an underestimation of PES, while overfilling will stretch the balloon leading to overestimation [60]. New commercially available catheters [61] allow to combine the balloon with a regular nasogastric tube that can be used for enteral feeding, and it is known to not impair the measurement [62].
When inspiratory efforts are made against an occluded airway, the deflections in PES should match PAO. Thus, a regression of PES vs. PAO should yield a slope of unity. In practice, slopes that differ from 1.0 by up to 10% are common [63]. In sedated and paralyzed patients, the occlusion test is performed by applying manual compression on the chest during airway occlusion [64]. Factors that should be checked periodically influencing measurement accuracy include the position of the balloon, the amount of air injected into the balloon, the patient’s position, and the lung volume. Cardiac artifacts and esophageal contraction due to peristalsis can distort the PES signal [65].
