Respiratory Mechanics and Transpulmonary Pressure

Basic Respiratory Mechanics

The relationship between transpulmonary pressure and resulting volume and flow changes in the lungs is fundamental to understanding respiratory mechanics, as stated by the American Thoracic Society, with lung resistance and reactance components playing a crucial role 1, 2
The respiratory system's response to pressure is described by impedance, which includes both resistance and reactance components, with resistance representing the in-phase relationship between pressure and flow, as noted by the European Respiratory Society 3, 1, 4

Reactance represents the out-of-phase relationship, determined by elastic properties at low frequencies and inertive properties at higher frequencies, with the American College of Chest Physicians recommending consideration of these factors in respiratory care 3
Lung mechanics is most clearly understood when transpulmonary pressure is related to tidal volume and flow, with the American Thoracic Society emphasizing the importance of this relationship in understanding lung function 1, 2, 4

During normal breathing, transpulmonary pressure represents the elastic recoil pressure of the lung at very slow breathing frequencies, with the American College of Chest Physicians noting that this is essential for maintaining proper lung function 4
Dividing transpulmonary pressure by flow yields lung resistance, while dividing by volume change yields lung elastance, with the European Respiratory Society providing guidelines for these calculations 1

Transpulmonary pressure affects right ventricular afterload, while changes in pleural pressure affect venous return, with the Society of Critical Care Medicine recommending consideration of these factors in mechanical ventilation 5, 6, 7
In ARDS, monitoring transpulmonary pressure helps prevent lung overdistension and right ventricular failure, with the American Thoracic Society emphasizing the importance of this monitoring in improving patient outcomes 8, 6

The normal intrapleural pressures are -8 cmH₂O during inspiration and -3.4 cmH₂O during expiration in spontaneous breathing, as reported by Thorax 9
During vigorous inspiratory efforts, the transmicrovascular pressures are especially high because the inflation force originates from the pleura, according to Intensive Care Medicine 10

In ARDS, monitoring transpulmonary pressure helps prevent pulmonary overdistension and right ventricular failure, with compliance significantly reduced due to the loss of functional alveolar units, as noted in Critical Care 11
For the same tidal volume, there is no advantage in terms of stress and strain generated in the lung between pressure-controlled and volume-controlled ventilation, as reported by Critical Care 11 and Critical Care 12
A stiffer chest wall resists distortion better and allows the diaphragm to produce more pressure for a given level of force, according to the American Journal of Respiratory and Critical Care Medicine 13

The Society of Critical Care Medicine recommends considering transpulmonary pressure measurement as part of comprehensive monitoring in mechanically ventilated patients, although this requires appropriate training 14, 15
Plateau pressure ≤30 cmH₂O is a standard target, but this represents transrespiratory pressure, and in patients with increased chest wall elastance, higher plateau pressures (≤29-32 cmH₂O) may be acceptable because transpulmonary pressure remains lower 14, 15

The European Society of Intensive Care Medicine suggests that monitoring transpulmonary pressure specifically helps prevent lung overdistension by ensuring end-inspiratory transpulmonary pressure remains safe, and guides PEEP titration to maintain positive end-expiratory transpulmonary pressure and prevent alveolar collapse 14, 15