Pleural Pressure, Alveolar Pressure, and Transpulmonary Pressure

The pressure displayed by a ventilator acts on both the lung and the structures surrounding it. To understand lung inflation, identify the pressure inside the airspaces, the pressure outside the lung, and the difference between them. These distinctions also explain how a patient can generate substantial lung-distending pressure while the ventilator displays only modest pressure.

23.1 Every Pressure Needs a Reference

A pressure value is meaningful only when its reference is known. Absolute pressure in physics is measured relative to a vacuum. Respiratory monitors usually display gauge pressure, measured relative to the surrounding atmosphere. A displayed airway pressure of zero therefore means that airway pressure equals atmospheric pressure. It does not mean that the airway contains no gas or has zero physical pressure.

Negative gauge pressure is simply pressure below atmosphere. During spontaneous inspiration, respiratory muscles can lower pleural pressure below atmospheric pressure. The negative sign does not identify a vacuum. It identifies the direction of the difference from the chosen zero.

Respiratory mechanics commonly uses centimeters of water, written cm H₂O. The unit describes pressure, regardless of whether the device actually contains a water column. Pressures being subtracted must use the same units and reference. Combining a ventilator pressure in cm H₂O with an unconverted vascular pressure in mm Hg produces an invalid result.

There is a second use of the word absolute in clinical esophageal-pressure papers. An absolute esophageal value often means the baseline pressure at a particular point in the breath, as opposed to its change during breathing. That baseline value is generally still referenced to atmosphere. It should not be confused with pressure referenced to a vacuum.

A pressure difference across a wall asks a separate question. It compares pressure on one side with pressure on the other. If both measurements use the same atmospheric reference, that reference cancels during subtraction. The resulting difference describes a force tending to distend or compress the structure.

Zeroing is therefore part of the physiology measurement. An offset in one pressure channel changes a calculated baseline difference even if the waveform appears plausible. Time alignment matters as well: pressures from different moments in the breath should not be subtracted as though they were simultaneous.

23.2 Transpulmonary Pressure Compares the Inside and Outside of the Lung

Alveolar pressure, abbreviated P_A, is pressure within an alveolar airspace. Pleural pressure, P_pl, is the pressure acting at the lung's outer surface through the pleural interface. The difference helps determine the distension of the lung between those boundaries.

A positive difference means alveolar pressure exceeds pleural pressure. In an open, inflated lung at static equilibrium, this difference balances the lung's inward elastic recoil. A larger difference generally produces greater inflation, although the volume response depends on lung mechanics and which units are open.

Consider an illustrative relaxed breath with an open airway and no airflow at end-expiration. Alveolar gauge pressure is 0 cm H₂O. Pleural gauge pressure is −5 cm H₂O. The first number represents pressure inside the airspaces, and the second represents pressure outside the lung. Subtraction gives 0 − (−5) = +5 cm H₂O. Thus, zero alveolar gauge pressure can coexist with a positive pressure distending the lung.

Now consider alveolar pressure of +10 cm H₂O and pleural pressure of +5 cm H₂O. The difference is 10 − 5 = +5 cm H₂O again. Both pressures are higher relative to atmosphere, but their difference is unchanged. The numbers illustrate why alveolar or airway pressure alone cannot define lung distension.

These examples use a simple lung compartment. A real lung has regions with different pleural pressures, recoil properties, and airway communication. Its airspaces also share tissue attachments that transmit forces between neighboring regions. One calculated value does not describe the local mechanical conditions at every alveolus.

The distinction becomes especially important when a reported end-expiratory value is negative. It may indicate unfavorable distending conditions near the sampled region. It does not establish that every alveolus has collapsed, because regional pressure, airway closure, and measurement error must also be considered.

23.3 The Lung and Chest Wall Share the Applied Pressure

The lung and chest wall move together because they are mechanically coupled through the pleural interface. At the resting volume of a relaxed healthy respiratory system, the lung tends to recoil inward while the chest wall tends to expand outward. Their opposing tendencies help produce subatmospheric pleural pressure.

The chest-wall compartment in respiratory mechanics includes the rib cage, diaphragm, and abdominal influences on their movement. Its behavior therefore changes with posture, abdominal distension, and muscle activity, as well as with properties of the ribs and surrounding tissues.

Let P_bs represent pressure at the body surface. The pressure across the chest wall is pleural pressure minus body-surface pressure. The pressure across the whole respiratory system is alveolar pressure minus body-surface pressure. These differences add:

Pleural pressure cancels in the sum. With the body surface exposed to atmosphere, P_bs is conventionally zero gauge pressure. The identity separates the pressure across the lung from that across the chest wall. Interpreting either as passive elastic recoil additionally requires relaxed muscles and appropriate static measurements.

During passive inflation, some of the applied pressure expands the lung and some moves the chest wall. If the chest wall resists expansion more strongly, a larger rise in pleural pressure may accompany a given tidal volume. If lung elastance is high, more of the applied pressure appears across the lung. Elastance means the change in pressure required for a change in volume.

Katz and colleagues separated pulmonary and chest-wall elastances in ventilated patients with acute respiratory failure. Their physiological measurements demonstrated a substantial chest-wall contribution to total respiratory-system mechanics. A total compliance measurement therefore cannot always identify which compartment has become harder to inflate. 1

An elevated baseline pleural pressure and increased chest-wall elastance are also distinct. Baseline pressure describes the pressure at one operating volume. Elastance describes how much that pressure changes as volume changes. A high starting pressure does not establish an equally large tidal rise.

23.4 Airway Pressure Approaches Alveolar Pressure Only Under Suitable Conditions

Airway-opening pressure, P_ao, is measured at the proximal airway rather than inside each alveolus. During inspiration, gas must move through the endotracheal tube and conducting airways. A pressure difference is required to overcome their resistance. Proximal airway pressure can therefore exceed alveolar pressure while inspiratory flow continues.

When flow stops, the flow-related pressure drop decreases. During an adequate end-inspiratory hold in a passive patient, airway pressure can approach the pressure in communicating alveolar units. The plateau pressure is used for this purpose. A valid interpretation requires attention to effort, leaks, the duration of the hold, and the pressure trace rather than the displayed value alone.

The initial pressure fall after an occlusion includes the loss of flow-related pressure. Further changes may reflect tissue stress relaxation and redistribution of gas between regions with different mechanical time constants. A plateau is therefore a standardized estimate under defined conditions, rather than proof that the entire lung has one unchanging pressure.

At end-expiration, a passive expiratory occlusion can reveal total PEEP in communicating units. Total PEEP includes externally applied PEEP and pressure remaining because exhalation is incomplete. Simply using the ventilator's set PEEP can miss this intrinsic component.

Airway closure creates a different problem. A closed airway disconnects distal airspaces from the proximal pressure measurement. Even an occlusion with no measured flow cannot guarantee equilibration across that obstruction. Chen and colleagues described this phenomenon in ARDS and showed why static airway pressure can misrepresent the pressure in disconnected airspaces. 2

During flow, subtracting pleural or esophageal pressure from airway-opening pressure gives an airway-to-pleural estimate that includes an airway resistive contribution. It is often called dynamic transpulmonary pressure. It should not be treated as identical to the static alveolar-minus-pleural pressure that reflects elastic lung distension.

23.5 The Same Airway Pressure Can Produce Different Lung Distension

Consider two illustrative passive patients measured at end-inspiration. In the first, plateau pressure is 30 cm H₂O and the relevant pleural pressure is 20 cm H₂O. Plateau pressure represents estimated alveolar pressure, while pleural pressure represents the pressure surrounding the lung. Their difference is 30 − 20 = 10 cm H₂O.

In the second patient, plateau pressure is 26 cm H₂O and pleural pressure is 8 cm H₂O. The difference is 26 − 8 = 18 cm H₂O. This patient has the lower airway pressure but the larger lung-distending difference. The comparison explains why partitioning pressure can clarify a difficult mechanics assessment.

It does not establish that the first patient is safe or that the second is injured. Regional stress, tidal volume, recruitability, the amount of aerated lung, and repeated exposure all matter. The illustrative pleural pressures are assumed to represent the regions being compared; an actual esophageal measurement cannot establish that assumption for the entire lung.

Chiumello and colleagues measured respiratory mechanics and lung stress and strain across different PEEP and tidal-volume conditions. A given airway pressure produced variable lung stress because the partition between lung and respiratory-system elastance varied. Their results demonstrate the limitation of an airway-only description, without establishing one universally safe transpulmonary threshold. 3

The abdomen can raise pleural pressure by displacing the diaphragm and changing the thoracic operating volume. Obesity, positioning, and abdominal distension may therefore affect airway-pressure interpretation. Their presence does not determine an individual patient's pleural pressure or justify assuming a fixed correction.

Talmor and colleagues measured esophageal and transpulmonary pressures in patients with acute respiratory failure and found marked variation in the pressure surrounding the lung. Their physiological study supports measuring rather than inferring that contribution from airway pressure alone. It was not a randomized trial proving a clinical treatment strategy. 4

23.6 Esophageal Pressure Is an Estimate That Requires Validation

A balloon in the thoracic esophagus senses pressure transmitted through the esophageal wall. Esophageal pressure, P_es, provides an accessible estimate of pressure in the nearby thoracic environment. It does not directly sample the pleural space, and it does not map pressure across both lungs.

Placement usually involves advancing the catheter sufficiently to establish a gastric position and then withdrawing it into the thoracic esophagus according to the device and clinical protocol. The pressure pattern, anatomical position, and response to appropriate maneuvers help confirm location. Cardiac oscillations can support thoracic placement, but their presence alone does not validate the measurement.

An occlusion test examines pressure transmission. With the airway occluded, an inspiratory effort lowers intrathoracic pressure while producing little change in lung volume. In a suitably positioned and filled system, airway and esophageal pressure changes should be similar. The test compares changes, not merely their starting values.

Baydur and colleagues tested this principle in subjects in several positions. Repositioning the balloon could restore agreement between esophageal and mouth-pressure changes when the initial position gave poor transmission. Their work established the occlusion test as a way to evaluate the measurement, including the importance of catheter location. 5

In passive patients, suitably performed chest compression during an airway occlusion can provide a related transmission check. Active effort, airway obstruction, leaks, inappropriate balloon filling, and poorly synchronized signals can compromise interpretation. The technique and acceptance criteria should follow the validated protocol being used.

A satisfactory test supports the fidelity of pressure changes under those conditions. It does not prove that the baseline esophageal value equals pressure at every pleural location. Record the position and measurement conditions, and reconsider validation after important changes in posture, ventilatory conditions, or catheter behavior.

23.7 Balloon Filling and Calibration Affect Baseline Values and Swings

An esophageal balloon must contain enough gas to transmit the surrounding pressure faithfully. An underfilled system can compress or incompletely unfold, reducing the measured pressure and its respiratory swing. A trace may remain visible despite substantial measurement error.

Excess filling creates another error. Distending the balloon and esophagus generates pressure within the measuring system. The displayed value then includes pressure caused by the measurement itself. A larger number does not necessarily indicate greater surrounding pleural pressure.

Mojoli and colleagues tested several balloon catheters in a pressure chamber. The volume range giving acceptable accuracy depended on the catheter and surrounding pressure. Underfilling could underestimate both baseline pressure and pressure changes. These bench findings explain why one fixed filling volume cannot be assumed to work across all devices and patients. 6

In a subsequent study of mechanically ventilated patients, Mojoli and colleagues progressively filled the balloon and assessed pressure transmission and esophageal-wall effects. Their calibration procedure selected a suitable filling volume and estimated the baseline pressure added by esophageal distension. The optimal volume varied between measurements. 7

Baseline correction and accurate swing measurement answer related but different questions. If a stable offset adds the same pressure at inspiration and expiration, subtraction between those moments can cancel that offset. In contrast, calculating end-expiratory airway pressure minus an uncorrected baseline esophageal pressure retains the error. An offset that changes during breathing cannot be assumed to cancel.

Other artifacts require recognition. Swallowing and esophageal contraction can create transient pressure rises. Cardiac oscillations may obscure a respiratory endpoint. Catheter migration, secretions, tubing problems, or transducer drift can alter the signal. Inspect the raw trace and repeat relevant checks when values change unexpectedly.

Calibration improves the instrument's relationship to its local environment. It cannot remove the genuine regional differences between esophageal surroundings and distant pleural surfaces. Device accuracy and anatomical representativeness remain separate sources of uncertainty.

23.8 Regional Pleural Pressure Limits Any Single Transpulmonary Estimate

Pleural pressure varies with location. In a supine patient, dependent regions are influenced by the weight and deformation of the overlying lung and mediastinal structures. Their pleural pressures may be higher than those in nondependent regions. The gradient changes with lung injury, posture, volume, and the distribution of surrounding forces.

If alveolar pressures were equal, a region with higher pleural pressure would have a lower transpulmonary pressure. It would consequently operate at different distending conditions from a region with lower pleural pressure. Raising PEEP may help maintain aeration in one region while increasing distension elsewhere.

Yoshida and colleagues compared esophageal estimates with regional measurements in injured pigs and human cadavers. Direct airway-minus-esophageal estimates more closely reflected dependent-to-middle regions near the esophagus. Elastance-based approaches provided different information about inspiratory pressure in nondependent regions. Their findings demonstrate why the estimation method and sampled region matter. 8

The direct method subtracts a baseline esophageal value from the corresponding static airway value. Tidal partitioning instead asks how much the pressure across the lung changes during inflation. It subtracts the tidal esophageal rise from the airway rise:

The delta symbol means change. The equation requires matched passive, no-flow endpoints and a trustworthy esophageal signal. It describes a tidal difference and does not, by itself, establish the lung's baseline transpulmonary pressure.

Elastance-based estimates use the measured pressure–volume changes to partition respiratory-system mechanics. Some approaches then extrapolate an inspiratory lung pressure using additional assumptions about elastic behavior and the recoil reference. They should not be substituted silently for a direct baseline subtraction. Identify the method, its assumptions, and whether the reported value is a baseline pressure or a tidal change.

23.9 Inspiratory Effort Can Increase Distending Pressure Without Raising Airway Pressure

During passive ventilation, the ventilator supplies the pressure that inflates the respiratory system. During assisted breathing, respiratory muscles also contribute. Diaphragm contraction lowers pleural pressure, increasing the difference between the airway or alveolar pressure and the pressure outside the lung.

Consider an illustrative airway pressure of 20 cm H₂O. If esophageal pressure is +5 cm H₂O, the airway-to-esophageal difference is 20 − 5 = 15 cm H₂O. If stronger effort lowers esophageal pressure to −5 cm H₂O while airway pressure remains 20, the difference becomes 20 − (−5) = 25 cm H₂O. The additional 10 cm H₂O comes from a fall in surrounding pressure rather than an increase in the displayed airway pressure.

During flow, these are dynamic airway-to-esophageal differences. The actual alveolar-minus-pleural pressure also depends on the airway pressure drop and regional conditions. The example establishes how muscle effort can add to distending forces; it does not measure regional stress in a patient.

Effort can also be distributed unevenly. In an injured lung, diaphragm contraction may generate a greater local pressure fall in dependent regions. Gas can move from other lung regions into those regions before or alongside gas entering through the airway. This internal redistribution is called pendelluft.

Yoshida and colleagues demonstrated effort-related pendelluft and increased regional inflation in an experimental lung-injury model, following a clinical observation. Substantial redistribution could occur despite a limited global tidal volume. This provides mechanistic evidence that ventilator volume and pressure can miss local inflation, while not proving that every spontaneous breath causes injury. 9

Esophageal pressure can help identify inspiratory effort and patient–ventilator interaction. Its swing should be assessed with flow, volume, airway pressure, and timing. It is not identical to respiratory-muscle pressure or work, because chest-wall recoil and the chosen reference also contribute to those calculations.

23.10 Physiological Rationale Does Not Establish a Universal PEEP Target

A low end-expiratory transpulmonary estimate provides a plausible explanation for inadequate distending pressure near the sampled region. Raising PEEP could increase that pressure and maintain aeration. The response depends on how much added airway pressure raises pleural pressure, whether lung units can recruit, and how other regions and the circulation respond.

The original EPVent trial randomized patients to esophageal-pressure-guided ventilation or a conventional ARDS Network strategy. It stopped after 61 participants at a prespecified criterion and showed improved oxygenation and respiratory-system compliance with the guided strategy. Its small size and physiological primary endpoint did not establish a reliable survival benefit. 10

EPVent-2 tested an esophageal-guided PEEP strategy against an empirical high-PEEP–oxygen strategy in 200 patients with moderate-to-severe ARDS. The primary ranked outcome combining death and ventilator-free days was not significantly different. Twenty-eight-day mortality was also similar. The comparator and protocols differed from those of the original study, so the trials were not repetitions of an identical comparison. 11

A later risk-based and mechanistic reanalysis of EPVent-2 found associations involving illness severity and achieved end-expiratory transpulmonary pressure. Those analyses generated hypotheses about which patients and pressure conditions might matter. Associations with an achieved pressure are not equivalent to randomized assignment to that pressure, and post-hoc findings require prospective confirmation. 12

Esophageal manometry can therefore clarify a physiological problem without dictating one PEEP target for every patient. A pressure-guided adjustment must still be followed through its effects on aeration, compliance, oxygenation, overdistension, cardiac function, and perfusion. Better oxygenation alone does not establish a better overall outcome.

23.11 Work Through the Measurements Before Making the Clinical Inference

Consider an illustrative passive ventilated patient with validated esophageal measurements. Total PEEP is 12 cm H₂O, and end-expiratory esophageal pressure is 14 cm H₂O. Plateau pressure is 28 cm H₂O, and end-inspiratory esophageal pressure is 18 cm H₂O. Tidal volume is 400 mL.

At expiration, 12 represents the estimated alveolar pressure in communicating units, while 14 estimates the surrounding pressure near the esophagus. Their difference is 12 − 14 = −2 cm H₂O. At inspiration, subtract the corresponding surrounding estimate from plateau pressure: 28 − 18 = 10 cm H₂O.

The airway pressure rises by 28 − 12 = 16 cm H₂O during the tidal inflation. Esophageal pressure rises by 18 − 14 = 4 cm H₂O. Subtracting these changes gives 16 − 4 = 12 cm H₂O as the estimated tidal transpulmonary change. The same result comes from subtracting the end-expiratory estimate from the end-inspiratory estimate: 10 − (−2) = 12.

Convert the tidal volume to 0.4 L before calculating elastance. Respiratory-system elastance is 16/0.4 = 40 cm H₂O/L. The estimated chest-wall tidal elastance is 4/0.4 = 10 cm H₂O/L. The estimated lung tidal elastance is 12/0.4 = 30 cm H₂O/L. The two components add to the respiratory-system value: 30 + 10 = 40.

These are simplified tidal estimates. They assume passive conditions, reliable static endpoints, an appropriate volume measurement, and sufficiently stable mechanics over the interval. Recruitment, nonlinear recoil, regional heterogeneity, and esophageal artifacts can complicate their interpretation.

The negative end-expiratory estimate raises a question about local distending conditions. It does not establish the PEEP change required, show that the entire lung is closed, or guarantee that additional PEEP will help. Review the waveforms, measurement validity, airway communication, regional aeration, and hemodynamic response before translating the calculation into a treatment decision.

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