What is transrespiratory pressure in respiratory medicine, particularly for patients with chronic obstructive pulmonary disease (COPD), asthma, or acute respiratory distress syndrome (ARDS)?

What is Transrespiratory Pressure?

Transrespiratory pressure is the total pressure difference across the entire respiratory system, calculated as the difference between airway opening pressure and body surface pressure (atmospheric pressure), representing the sum of transpulmonary pressure (pressure across the lung) and transchest wall pressure (pressure across the chest wall). 1

Fundamental Definition and Components

Transrespiratory pressure encompasses the complete pressure gradient needed to inflate both the lungs and chest wall together. 1 This differs critically from two related but distinct concepts:

• Transpulmonary pressure (PL): The pressure difference between alveolar pressure and pleural pressure, representing the distending pressure across the lung tissue alone 1, 2, 3
• Transchest wall pressure: The pressure difference between pleural pressure and atmospheric pressure 1

The mathematical relationship is: Transrespiratory pressure = Transpulmonary pressure + Transchest wall pressure 1

Clinical Significance in Mechanical Ventilation

Normal Physiology vs. Mechanical Ventilation

In spontaneously breathing subjects, approximately 50% of alveolar pressure changes transmit to pleural pressure, though this transmission decreases substantially in diseased, stiffer lungs. 1 During positive pressure ventilation, the transrespiratory pressure determines the total work required to inflate the respiratory system against both lung and chest wall elastance. 1

ARDS-Specific Considerations

In ARDS patients, understanding transrespiratory pressure becomes critical because high chest wall elastance can substantially alter the relationship between airway pressures and actual lung distension. 1 The acutely injured lung has reduced compliance, and when combined with elevated chest wall elastance (common in obesity, abdominal distension, or edema), the same airway pressure produces different transpulmonary pressures. 1, 4

Esophageal pressure measurement allows clinicians to partition transrespiratory pressure into its lung and chest wall components, enabling more precise assessment of actual lung stress. 1, 2 This becomes particularly important when setting PEEP, as the goal is to achieve positive end-expiratory transpulmonary pressure (not just positive airway pressure) to maintain alveolar recruitment. 1, 2

Measurement and Practical Application

Esophageal Manometry

Esophageal pressure serves as a surrogate for pleural pressure, measured via balloon catheters positioned 35-40 cm from the nostril in adults. 1 The validity is confirmed by matching esophageal pressure to airway opening pressure during static Mueller maneuvers. 1 This measurement enables calculation of transpulmonary pressure as: PL = Airway pressure - Esophageal pressure 1, 2

Elastance-Derived Method

Transpulmonary pressure can also be estimated from the ratio of lung elastance to respiratory system elastance, without requiring esophageal manometry. 2, 5 This elastance-derived method may better estimate transpulmonary pressure in non-dependent lung zones at risk for overdistension. 2 Studies demonstrate good correlation between airway pressure stress index and transpulmonary pressure stress index, particularly when chest wall elastance is <30% of total respiratory elastance. 5

Critical Pitfalls in ARDS Management

The most dangerous misconception is assuming that airway plateau pressure alone reflects lung stress—in patients with high chest wall elastance, substantial airway pressure may be dissipated across the chest wall, resulting in lower-than-expected transpulmonary pressure and inadequate lung recruitment. 1, 4 Conversely, in patients with normal chest wall mechanics but severe lung injury, the same plateau pressure produces higher transpulmonary pressure and greater risk of overdistension. 4

During spontaneous breathing efforts in ARDS, patients can generate excessively negative pleural pressure swings, which when added to ventilator-delivered pressure, create dangerously high transpulmonary pressures exceeding 20-25 cmH₂O—the proposed safety threshold for limiting ventilator-induced lung injury. 1, 2 This explains why deep sedation or neuromuscular blockade may be necessary in severe ARDS to prevent patient self-inflicted lung injury. 1

Obesity and Elevated Baseline Pleural Pressure

Elevated basal pleural pressures correlate strongly with BMI, meaning obese ARDS patients require higher PEEP levels to achieve the same transpulmonary pressure as non-obese patients. 4 Failure to account for this leads to systematic under-recruitment in obese patients when using standard PEEP tables. 4

Relationship to Driving Pressure

While transpulmonary pressure represents static distending pressure, lung driving pressure (ΔPL = end-inspiratory PL - end-expiratory PL) reflects the tidal distending pressure and may be the most important predictor of ventilator-induced lung injury. 1, 2 Limiting end-inspiratory transpulmonary pressure to 20-25 cmH₂O appears physiologically sound for mitigating injury. 2