Lung stress and strain during mechanical ventilation: any difference between statics and dynamics?

Area of Science:

• Critical care medicine
• Pulmonary physiology
• Respiratory mechanics

Background:

• Mechanical ventilation involves tidal volume (VT) and positive end-expiratory pressure (PEEP), generating dynamic and static lung strains.
• Understanding the distinct contributions of these strains to lung injury is crucial for optimizing ventilator strategies.

Purpose of the Study:

• To investigate whether different combinations of dynamic and static lung strains, when resulting in the same overall global strain, consistently cause lung edema.
• To determine the differential impact of dynamic versus static lung strains on lung injury and mortality.

Main Methods:

• A laboratory investigation using 28 healthy pigs subjected to 54 hours of mechanical ventilation.
• Animals were exposed to varying combinations of dynamic (VT) and static (VPEEP) strains, maintaining a constant global strain of 2.5.
• Lung edema was assessed by comparing initial and final lung weights, alongside evaluations of mortality, lung mechanics, gas exchange, histology, and inflammation.

Main Results:

• Ventilation with entirely dynamic strain (high VT, zero PEEP) led to pulmonary edema in all animals.
• Conversely, ventilation with mainly static strain (low VT, high PEEP) did not result in edema.
• Intermediate combinations showed variable lung weights, while smaller dynamic and larger static strains reduced mortality, improved lung mechanics and oxygenation, and decreased histological injury and inflammation.

Conclusions:

• Lung edema formation is influenced not only by the magnitude of global lung strain but also by the proportion of dynamic and static components.
• Larger static strains appear less detrimental than large dynamic strains in the context of lung injury.
• These findings suggest that the interplay between dynamic and static lung strains is critical in the development of ventilator-induced lung injury.