Practical assessment of risk of VILI from ventilating power: a conceptual model

Marini, J.J., Thornton, L.T., Rocco, P.R.M. et al. Practical assessment of risk of VILI from ventilating power: a conceptual model. Crit Care 27, 157 (2023). https://doi.org/10.1186/s13054-023-04406-9

Abstract

This article introduces a conceptual model aimed at assessing the risk of Ventilator-Induced Lung Injury (VILI) using parameters readily available at the bedside. It emphasizes the importance of understanding the energy dynamics involved in mechanical ventilation, particularly focusing on the concept of 'mechanical power' and 'damaging energy per cycle'. The model proposes using static circuit pressures and an estimate of the maximally tolerated non-dissipated airway pressure to gauge the potential hazard of delivered energy. This approach seeks to improve the comprehension of key factors involved in lung protective ventilation, making it a valuable tool for clinicians.

Introduction

The management of Acute Respiratory Distress Syndrome (ARDS) through mechanical ventilation requires careful consideration to avoid VILI. Current strategies focus on limiting tidal volumes and pressures; however, these parameters alone may not fully capture the risk of lung injury. The concept of 'mechanical power', which accounts for the energy dissipated and conserved during ventilation, offers a more comprehensive view but still lacks precision in guiding ventilatory practices. This paper builds on the idea of 'damaging energy per cycle', proposing a simplified mathematical model to help clinicians assess the risk of VILI more effectively.

Model Overview

The model suggests that the risk of VILI can be assessed by considering the energy dynamics of each ventilation cycle, specifically focusing on the 'elastic' energy components related to volume changes. By estimating a threshold for maximally tolerated non-dissipated airway pressure, clinicians can partition the total intracycle energy into 'safe' and 'hazardous' components. This approach allows for a more nuanced understanding of how different ventilatory settings might contribute to lung injury.

Key Concepts

1. Mechanical Power: Incorporates both dissipated (resistive) and conserved (elastic) energy components of ventilation, highlighting the role of ventilating frequency and energy cost per tidal cycle.
2. Damaging Energy per Cycle: Focuses on the energy associated with non-resistive pressures, correlating with potentially damaging strain under dynamic conditions.
3. Threshold Pressure for Damage: An estimate of the pressure at which elastic energy becomes potentially hazardous, varying between patients and within different lung regions.

Clinical Application

The model provides a framework for estimating the 'safe' and 'hazardous' portions of elastic energy delivered during mechanical ventilation. By adjusting tidal volume, driving pressure, and PEEP based on this assessment, clinicians can tailor ventilation strategies to minimize the risk of VILI. The model emphasizes the importance of considering both the static pressures measured at the airway opening and the underlying energy dynamics to guide lung protective ventilation.

Limitations and Future Directions

While the model offers a conceptual tool for understanding the energetics of VILI, it acknowledges the limitations of using airway pressures as proxies for tissue stresses and strains. Future refinements of the model could incorporate more precise measurements and account for the complex interplay of factors that influence VILI risk, including alveolar geometry, flow patterns, and vascular pressures.

Conclusion

This conceptual model represents a step forward in the effort to integrate key factors involved in lung protective ventilation. By focusing on the energy dynamics of mechanical ventilation, it provides clinicians with a potentially valuable tool for assessing and mitigating the risk of VILI. Further research and refinement of the model are needed to enhance its clinical utility and precision.

Watch this video on "How to normalize the mechanical power" by Luciano Gattinoni ISICEM 2023

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Discussion Questions

1. Understanding Mechanical Power in Ventilation: How does the concept of mechanical power integrate into current practices of lung protective ventilation, and why is it considered a comprehensive variable for assessing VILI risk?
2. The Role of Damaging Energy per Cycle: Discuss the significance of identifying 'damaging energy per cycle' in the context of mechanical ventilation. How does this concept help in refining ventilatory strategies to minimize VILI?
3. Threshold Pressure for Damage: The model introduces a hypothetical threshold for maximally tolerated non-dissipated airway pressure. How might this threshold vary between patients, and what factors could influence its determination in clinical practice?