Pleural Pressure and Mechanics

Pleural (intrapleural) pressure is the pressure in the thin liquid-filled space between the lung and the chest wall. It is normally subatmospheric and is the link that mechanically couples the chest wall to the lung: changes in pleural pressure produced by the respiratory muscles are transmitted across the pleural space to distend or relax the lung. The difference between alveolar and pleural pressure — the transpulmonary pressure — is the true distending pressure of the lung.

Definition

Pleural pressure is the pressure within the pleural space surrounding the lung, normally negative relative to atmosphere; transpulmonary pressure is the difference between alveolar pressure and pleural pressure and represents the net pressure distending the lung.

Scope

This topic covers the origin and sign of pleural pressure, the concept of transpulmonary pressure as the lung's distending pressure, the vertical gradient of pleural pressure within the thorax and its consequences for regional ventilation, and the use of esophageal pressure as a clinical surrogate. It is a reference account and does not provide clinical management advice.

Core questions

• Why is pleural pressure normally subatmospheric, and how do the lung and chest wall recoil forces produce it?
• How does transpulmonary pressure relate alveolar and pleural pressure to lung distension?
• Why does pleural pressure vary down the height of the lung, and how does that gradient affect regional ventilation?
• How can esophageal pressure be used to estimate pleural pressure?

Key concepts

• Intrapleural pressure
• Transpulmonary pressure
• Pleural pressure gradient
• Regional ventilation
• Esophageal pressure surrogate
• Chest wall-lung coupling

Key theories

Transpulmonary pressure as distending pressure

The lung is distended not by absolute pressures but by the difference between the pressure inside the alveoli and the pleural pressure surrounding it; this transpulmonary pressure determines lung volume and is the quantity that elastic recoil must balance.

Vertical pleural pressure gradient and regional ventilation

Because of gravity and the weight of the lung, pleural pressure is less negative at the bottom of the upright lung than at the top, so dependent regions sit on a steeper part of the pressure-volume curve and receive proportionally more of each breath; the same gravitational gradient also shapes regional blood flow.

Mechanisms

At rest the lung's inward elastic recoil pulls away from the chest wall, which recoils outward; these opposing forces stretch the small volume of pleural fluid and keep pleural pressure below atmospheric, holding the lung expanded. During inspiration the muscles enlarge the thorax, pleural pressure falls further, transpulmonary pressure rises, and the lung expands. Pleural pressure is not uniform: gravity makes it more negative at the apex than the base of the upright lung, so apical alveoli are more expanded but less compliant, while basal alveoli, starting smaller, expand more for a given pressure change and so are better ventilated. Because pleural pressure cannot easily be measured directly in living subjects, the pressure in a balloon in the esophagus is used as a practical surrogate.

Clinical relevance

Transpulmonary pressure is the physiological basis for assessing the true mechanical load on the lung, and esophageal-pressure estimation of pleural pressure is used to interpret respiratory mechanics in patients on mechanical ventilation. The vertical pleural pressure gradient explains why ventilation is normally distributed unevenly through the lung. This entry describes mechanisms and measurement and is not a source of individualized treatment advice.

Evidence & guidelines

The physiological basis of pleural and transpulmonary pressure is established in classic mechanics studies and textbooks; the practical use and limitations of esophageal-pressure measurement as a pleural-pressure surrogate have been reviewed in the critical-care literature.

History

The recognition that the lung is distended by transpulmonary pressure, and that pleural pressure varies down the thorax, emerged from mid-twentieth-century work relating regional pressures to ventilation and blood flow, including West's studies of gravitational gradients in the lung. Esophageal manometry was developed in parallel as a way to estimate pleural pressure, and its clinical application has been revisited in modern critical-care reviews.

Debates

How faithfully does esophageal pressure represent pleural pressure?

Esophageal pressure is the standard surrogate for pleural pressure, but mediastinal weight, posture, and regional gradients mean a single esophageal value cannot capture pleural pressure everywhere in the lung, so its absolute interpretation remains debated.

Key figures

• John B. West
• Jere Mead
• Stephen Loring
• Laurent Brochard

Seminal works

• west-1964
• akoumianaki-2014

Frequently asked questions

Why is pleural pressure normally negative?

The lung recoils inward and the chest wall recoils outward; these opposing elastic forces pull on the small volume of pleural fluid between them, holding the pleural pressure below atmospheric and keeping the lung expanded against the chest wall.

What is transpulmonary pressure?

It is the difference between alveolar pressure and pleural pressure — the net pressure acting to distend the lung. Lung volume is set by this difference rather than by either pressure alone, and elastic recoil balances it at rest.