Is Esophageal Pressure Always Positive?
No, esophageal pressure (Pes) is not always positive—it is usually negative during normal breathing, reflecting the subatmospheric pleural pressure, but becomes positive during forced expiration, coughing, or Valsalva maneuvers. 1
Normal Respiratory Physiology
During quiet breathing at functional residual capacity (FRC), esophageal pressure is typically negative (subatmospheric), serving as a surrogate for pleural pressure (Ppl). 1
The ATS/ERS guidelines explicitly state that Pes is "usually, but not always, negative" when defining transdiaphragmatic pressure calculations. 1
During inspiration, esophageal pressure becomes more negative as the diaphragm contracts and intrathoracic pressure decreases. 1
During expiration, esophageal pressure becomes less negative or may become positive, particularly during active expiration. 1
When Esophageal Pressure Becomes Positive
During forced expiratory maneuvers, esophageal pressure can reach highly positive values exceeding 300 mmHg, as documented during maximum voluntary cough efforts. 1
The ATS/ERS statement notes that when measuring high positive pressures (such as during maximal expiratory maneuvers), balloon volumes may need adjustment from the standard 0.5 mL to prevent complete emptying of the balloon. 1
During coughing, esophageal pressure spikes dramatically during both the compression and expulsion phases, reflecting the massive increase in intrathoracic pressure required to generate effective cough flow. 1
Clinical Measurement Considerations
Esophageal pressure measurement requires proper balloon catheter positioning approximately 35-40 cm from the nostril in adults, with validation through the dynamic occlusion test matching Pes to airway opening pressure during Mueller maneuvers. 1
The reference point matters: for respiratory muscle measurements, gastric pressure (Pga) is conventionally taken as zero at resting end expiration, while Pga is usually positive with respect to atmosphere due to hydrostatic pressure in the abdomen. 1
Resting esophageal pressure values depend on body position and respiratory phase, with supine positioning affecting measurements differently than upright positioning due to gravitational effects. 1
Key Clinical Pitfall
A common misconception is that esophageal pressure remains constant or always negative. Clinicians must recognize that Pes varies continuously throughout the respiratory cycle and can swing from highly negative (during strong inspiratory efforts) to highly positive (during expiratory efforts or coughing). 1 This dynamic range is essential for proper interpretation of respiratory mechanics, transpulmonary pressure calculations, and assessment of patient-ventilator interactions in mechanically ventilated patients. 2