What are transrespiratory pressure, transpulmonary pressure, transthoracic pressure, transairway pressure, and transmural pressure in the context of respiratory physiology?

Respiratory Pressure Definitions in Respiratory Physiology

These pressure measurements represent the fundamental building blocks for understanding respiratory mechanics, where each pressure gradient across specific anatomical structures determines the forces acting on the lungs, chest wall, and airways during breathing. 1

Core Pressure Definitions

Transpulmonary Pressure (PL)

• Transpulmonary pressure is the pressure difference between alveolar pressure and pleural pressure, representing the true distending pressure across the lung tissue alone. 2, 3
• This pressure reflects the elastic recoil pressure of the lung and determines actual lung stress independent of chest wall mechanics. 1
• Calculated as: PL = Airway pressure - Esophageal pressure (where esophageal pressure serves as a surrogate for pleural pressure). 3, 4
• Transpulmonary pressure is the most critical parameter for assessing risk of ventilator-induced lung injury, as it represents the actual distending force on alveolar tissue. 4, 5

Transthoracic Pressure (Trans-Chest Wall Pressure)

• Transthoracic pressure is the pressure difference between the pleural space and the body surface, representing the pressure across the chest wall structures. 1
• This pressure gradient determines chest wall distension and reflects the mechanical properties of the rib cage and abdominal wall. 1
• The American Thoracic Society/European Respiratory Society emphasizes that the pressure difference between pleural space and body surface represents both transthoracic and transpulmonary pressure simultaneously in a breathing person. 1

Transrespiratory Pressure

• Transrespiratory pressure is the total pressure difference across the entire respiratory system, calculated as airway opening pressure minus body surface pressure. 3
• This represents the sum of transpulmonary pressure and transchest wall pressure: Transrespiratory pressure = Transpulmonary pressure + Transchest wall pressure. 3
• In mechanically ventilated patients, plateau pressure represents transrespiratory pressure, not transpulmonary pressure—a critical distinction that affects ventilator management. 2, 3

Transairway Pressure

• Transairway pressure is the pressure difference between the airway opening and the alveolar space, representing the pressure gradient driving airflow through the conducting airways. 1
• This pressure overcomes airway resistance during breathing and is zero during conditions of no flow (end-inspiration, end-expiration). 1
• Dividing transairway pressure by flow yields airway resistance, a fundamental measurement in respiratory mechanics. 1

Transmural Pressure

• Transmural pressure refers to the pressure difference across any wall or membrane in the respiratory system, with the specific meaning depending on the structure being evaluated. 1
• For blood vessels: transmural pressure affects right ventricular afterload and venous return during mechanical ventilation. 2
• For airways: transmural pressure determines airway patency and collapse risk. 1
• The American Thoracic Society/European Respiratory Society states that pressure differences across structures are the relevant "pressures" for characterizing those structures, with positive pressure differences typically inflating the structure. 1

Critical Clinical Relationships

Pressure Partitioning

• Understanding how transrespiratory pressure partitions into lung and chest wall components is essential for safe mechanical ventilation, particularly in ARDS patients with altered chest wall mechanics. 3, 4
• In patients with high chest wall elastance (obesity, abdominal distension, chest wall edema), a substantial portion of airway pressure is spent distending the chest wall rather than the lung. 2, 3
• Esophageal pressure measurement allows clinicians to partition transrespiratory pressure and calculate actual transpulmonary pressure, enabling more precise assessment of lung stress. 3, 4, 6

Measurement Considerations

• Esophageal pressure serves as the most widely accepted surrogate for pleural pressure, measured via balloon catheters positioned 35-40 cm from the nostril in adults. 3, 4, 6
• The American Thoracic Society/European Respiratory Society emphasizes that pressures measured at a point represent the pressure in that space, though gravitational gradients exist (0.2 cm H₂O/cm height in thorax, 1 cm H₂O/cm height in abdomen). 1
• Proper balloon calibration is essential, as measurements can be affected by the volume of air in the balloon. 4, 6

Common Pitfalls to Avoid

• Never assume that airway plateau pressure alone reflects lung stress—this represents transrespiratory pressure and can substantially overestimate transpulmonary pressure in patients with stiff chest walls. 3
• Do not confuse the traditional definition of transpulmonary pressure (airway opening to pleural surface) with elastic recoil pressure (alveolar to pleural surface)—the latter is what matters clinically for assessing lung distension. 7
• Recognize that pleural pressure is not uniform throughout the thorax—there is a vertical gradient from non-dependent to dependent zones, so esophageal pressure represents mid-level pleural pressure. 5
• Avoid setting PEEP based solely on airway pressures without considering transpulmonary pressure—the goal is to achieve positive end-expiratory transpulmonary pressure to maintain alveolar recruitment. 2, 3