What is lung mechanics?

What is Lung Mechanics

Lung mechanics refers to the physical and mechanical properties governing how the lungs move air in and out, encompassing the pressure-volume-flow relationships, elastic recoil forces, airway resistance, and work required for breathing.

Core Components of Lung Mechanics

Lung mechanics fundamentally describes how transpulmonary pressure (the pressure difference between the alveoli and pleural space) relates to resulting volume and flow changes in the lungs 1. This relationship is characterized by several key properties:

Compliance and Elastance

• Compliance represents the change in lung volume for a given change in transpulmonary pressure, with normal values of 1.2-2.0 ml/cmH₂O/kg body weight in adults 2
• Elastance is the inverse of compliance, reflecting the lung's tendency to return to its resting state after distension 3
• In disease states like ARDS, compliance can drop to less than 25% of normal (approximately 20 ml/cmH₂O), indicating severe lung injury 4, 2

Resistance and Impedance

• Airway resistance is calculated by dividing transpulmonary pressure by flow, representing the opposition to airflow through the conducting airways 1
• Impedance encompasses both resistance (the in-phase relationship between pressure and flow) and reactance (the out-of-phase relationship determined by elastic and inertive properties) 1

Functional Aspects of Lung Mechanics

Work of Breathing

The respiratory muscles must generate sufficient pressure to overcome the viscoelastic mechanical load of the respiratory system 5. In healthy lungs, this work is minimal, but in disease states:

• Small airway and alveolar collapse cause decrements in lung compliance that are nearly universal in patients with acute lung injury 4
• Static inflation pressures for typical tidal volumes may exceed 25 cmH₂O when lung compliance approaches 20 ml/cmH₂O or less 4

Gas Exchange Mechanics

The mechanical properties directly impact gas exchange efficiency 4:

• Intrapulmonary shunting (normally <5% of cardiac output) can exceed 25% in ARDS due to persistent perfusion of atelectatic and fluid-filled alveoli 4
• Hypoxic pulmonary vasoconstriction, which normally limits shunt by reducing perfusion to poorly ventilated units, may be ineffective in lung injury 4

Mechanical Forces in Breathing

Cough Mechanics as an Example

Cough demonstrates the complex interplay of lung mechanical forces 4:

• Inspiratory phase: Inhalation to high lung volumes optimizes the expiratory muscle length-tension relationship 4
• Compressive phase: Glottic closure allows generation of high intrathoracic pressures (the driving force for expiratory flow) 4
• Expiratory phase: High intrathoracic pressures dynamically compress central airways, enhancing airstream velocity to 12 L/s 4

Tissue Mechanical Properties

Lung mechanics extends beyond air movement to include tissue-level forces 4:

• Cells experience dynamic forces and constant cyclic stretching, which is exaggerated during mechanical ventilation 4
• The macroscopic stiffness of tissue differs from local microscopic stiffness sensed by cells through focal adhesions 4
• Altered mechanical properties in disease (such as increased stiffness in fibrosis) affect cellular behavior and disease progression 4

Clinical Measurement and Monitoring

Pressure Measurements

• Normal intrapleural pressures: -8 cmH₂O during inspiration and -3.4 cmH₂O during expiration in spontaneous breathing 1
• Transpulmonary pressure monitoring helps prevent lung overdistension and guides PEEP titration in mechanically ventilated patients 1
• Plateau pressure (≤30 cmH₂O) represents transrespiratory pressure, though higher values may be acceptable with increased chest wall elastance 1

Dynamic Assessment

Respiratory mechanics can be assessed through 4:

• Regional compliance measurements using electrical impedance tomography
• Pressure-volume curves during mechanical ventilation
• Flow-volume relationships during spontaneous breathing

Pathophysiological Alterations

In Sepsis and ARDS

Lung mechanics become severely compromised 4:

• Extravascular lung water accumulation creates interstitial edema when fluid deposition exceeds lymphatic clearance capacity
• Pulmonary hypertension develops from vasoconstriction, intravascular obstruction, and later fibrosis
• The volume of aeratable lung is reduced by alveolar edema and surfactant dysfunction

Mechanical Ventilation Considerations

Understanding lung mechanics is essential for protective ventilation strategies 6:

• End-inspiratory transpulmonary pressure (stress) may better indicate injury potential than plateau pressure alone
• The stress-strain relationship guides ventilator settings to minimize ventilator-induced lung injury
• Static and dynamic compliance measurements inform PEEP titration strategies 2