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Regional tidal lung strain in mechanically ventilated normal lungs.

Luis Felipe Paula^1,2, Tyler J Wellman¹, Tilo Winkler¹

¹Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts.

Journal of Applied Physiology (Bethesda, Md. : 1985)

|July 23, 2016

Summary

Mechanical ventilation can cause lung injury, especially in dependent regions. New methods for estimating lung strain reveal injury risk even at low global levels, highlighting the importance of regional analysis.

Area of Science:

Pulmonary Mechanics
Medical Imaging
Critical Care Medicine

Background:

Parenchymal strain is a critical factor in ventilator-induced lung injury (VILI).
Current imaging methods for estimating volumetric tidal strain (ε) have conceptual differences in reference volume calculation and accounting for lung recruitment.
Understanding regional strain variations is crucial for mitigating VILI.

Purpose of the Study:

To compare existing and novel methods for estimating regional tidal volumetric strains using computed tomography (CT).
To quantify the impact of tidal volume (V_T) and positive end-expiratory pressure (PEEP) on strain estimates.
To identify lung regions most susceptible to VILI under varying ventilation parameters.

Main Methods:

Eight pigs were mechanically ventilated with varying V_T (6 and 12 ml/kg) and PEEP (0, 6, 12 cmH₂O).
CT scans were acquired at end-expiration and end-inspiration.
Regional reference volumes were calculated using different methods, including corrections for intratidal and PEEP-induced recruitment, across eight ventral-dorsal regions.

Main Results:

All strain estimates showed significant vertical heterogeneity, with maximal strains in middependent lung regions.
Maximal strain values and their locations varied depending on the estimation method and V_T-PEEP conditions.
Regional strain values indicative of lung injury were observed even when global measurements remained below critical thresholds.
Strain increased with V_T and was consistently higher in middependent lung regions.
PEEP reduced strain estimates referenced to end-expiratory volumes but not those referenced to resting lung volumes.

Conclusions:

Middependent lung regions are at higher risk for VILI, even in healthy lungs under mechanical ventilation.
Different strain estimation methods highlight distinct lung regions and ventilation conditions associated with maximal strain, aiding in VILI research.
Regional strain analysis is essential for understanding and preventing VILI, as global measures may underestimate injury risk.

Keywords:

anesthesia computed tomography mechanical ventilation regional ventilation strain measurements

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