What is lung mechanics?

What is Lung Mechanics

Lung mechanics refers to the physical and mechanical properties that govern how the lungs generate pressure, flow, and volume changes during breathing, encompassing measurements of compliance, resistance, elastance, and the pressure-volume-flow relationships that determine respiratory function.

Core Components of Lung Mechanics

Lung mechanics is fundamentally understood through the relationship between transpulmonary pressure (the pressure difference across the lung) and the resulting volume and flow changes 1. The respiratory system's mechanical behavior is described by several key properties:

Primary Mechanical Properties

• Compliance: The measure of lung distensibility, expressed as volume change per unit pressure change (ml/cmH₂O), with normal adult values ranging from 1.2-2.0 ml/cmH₂O per kilogram body weight 2
• Elastance: The inverse of compliance, representing the lung's tendency to recoil to its resting position 1
• Resistance: The in-phase relationship between pressure and flow, representing opposition to airflow through the airways 1
• Impedance: The overall respiratory system response to pressure, including both resistance and reactance components 1

Pressure Relationships

The mechanical function of the lungs depends critically on pressure gradients 3:

• Transpulmonary pressure represents the elastic recoil pressure of the lung during slow breathing and is calculated by dividing this pressure by flow to yield lung resistance, or by volume change to yield lung elastance 1
• Normal intrapleural pressures are -8 cmH₂O during inspiration and -3.4 cmH₂O during expiration in spontaneous breathing 1
• During vigorous inspiratory efforts, transmicrovascular pressures become especially high because the inflation force originates from the pleura 1

Physiological Mechanisms

Respiratory Muscle Function and Pressure Generation

The respiratory muscles provide the pressure required to overcome the viscoelastic mechanical load of the respiratory system 4. The mechanical pump function involves:

• Inhaling to high lung volumes and glottic closure prior to expiration (as seen in cough mechanics) facilitate generation of high intrathoracic pressures 5
• These high intrathoracic pressures provide the driving force for airstream flow and dynamically compress central airways 5
• The expiratory muscles are lengthened and "strengthened" during inspiration, optimizing the length-tension relationship for subsequent expiratory efforts 5

Dynamic Airway Behavior

Airflow and lung volume changes are determined by the relationships between pressures at different sites (airway opening, alveolar, pleural, and muscular) and the mechanical properties of airways resistance, lung compliance, and chest wall compliance 4.

Clinical Measurement and Application

Assessment Methods

Respiratory mechanics can be measured through various approaches 6, 3:

• Static compliance is measured during conditions of no airflow, such as during an inspiratory hold maneuver on mechanical ventilation, to eliminate the resistance component 2
• Plateau pressure measures end-inspiratory distending pressure and has become a key parameter in mechanical ventilation 3
• Esophageal manometry allows determination of transpulmonary pressure, which may be a better indicator of potential for lung injury than plateau pressure alone 3

Disease States

Lung mechanics change characteristically in disease 6, 7:

• In ARDS, static compliance may be reduced to less than one-fourth of normal values (approximately 20 ml/cmH₂O), and monitoring transpulmonary pressure helps prevent pulmonary overdistension and right ventricular failure 1, 2
• Decreased static compliance indicates stiffened lungs requiring more pressure for a given volume change 2
• Alveolar edema, surfactant dysfunction, and pulmonary fibrosis all reduce compliance by different mechanisms 2

Mechanical Forces and Tissue Behavior

Tissue-Level Mechanics

Lung diseases are often characterized by alterations in physiological mechanical properties of the extracellular matrix and lung tissue as a whole 5. These changes affect cellular behavior through:

• Macroscopic tissue stiffness differs from local microscopic stiffness sensed by cells through focal adhesions 5
• Cells in the lungs experience dynamic forces and constant cyclic stretching, which is exaggerated during mechanical ventilation 5
• Biomechanical cues from the local microenvironment influence cell signaling and function 5

Advanced Concepts

Beyond basic models, more complex mechanical concepts describe lung function more accurately 4:

• Nonlinearity: Mechanical properties change with lung volume and flow rates
• Inhomogeneous ventilation: Different lung regions exhibit different mechanical properties
• Viscoelasticity: Time-dependent mechanical behavior of lung tissue
• Stress and strain: These concepts assess the potential for lung injury during mechanical ventilation, with stress representing transpulmonary pressure and strain representing volume change relative to resting lung volume 3

Clinical Pitfalls

When assessing lung mechanics, several important considerations apply:

• Measurements should be made during zero-flow conditions to eliminate the resistance component when determining static compliance 2
• In spontaneously breathing patients, esophageal pressure measurements may be needed to accurately determine transpulmonary pressure 2
• For the same tidal volume, there is no advantage in terms of stress and strain generated between pressure-controlled and volume-controlled ventilation 1
• A stiffer chest wall resists distortion better and allows the diaphragm to produce more pressure for a given level of force 1