Time-Controlled Adaptive Ventilation (TCAV): a personalized strategy for lung protection

Al-Khalisy, H., Nieman, G.F., Kollisch-Singule, M. et al. Time-Controlled Adaptive Ventilation (TCAV): a personalized strategy for lung protection. Respir Res 25, 37 (2024). https://doi.org/10.1186/s12931-023-02615-y

Abstract

Acute Respiratory Distress Syndrome (ARDS) changes lung inflation dynamics during mechanical ventilation, leading to repetitive alveolar collapse and expansion (RACE) which predisposes the lung to ventilator-induced lung injury (VILI). Two main strategies currently used to minimize VILI include low tidal volume (LVT) with low-moderate positive end-expiratory pressure (PEEP) and the open lung approach (OLA). The LVT approach protects already open lung tissue from overdistension while resting collapsed tissue by excluding it from mechanical ventilation. The OLA aims to reinflate potentially recruitable lung over time, using higher PEEP to prevent progressive loss of end-expiratory lung volume (EELV) and RACE. Despite these strategies, ARDS-related mortality remains high due to the clinical and pathologic heterogeneity of ARDS. A new ventilation strategy, Time-Controlled Adaptive Ventilation (TCAV), applied via Airway Pressure Release Ventilation (APRV), leverages the heterogeneous time- and pressure-dependent collapse and reopening of lung units to personalize VT and EELV. TCAV adjusts expiratory duration based on respiratory system compliance (CRS), measured by the slope of the expiratory flow curve during passive exhalation, to promote alveolar stabilization and minimize RACE. This strategy aims to reduce VILI and improve ARDS outcomes.

Introduction

ARDS remains a significant clinical challenge, primarily managed through mechanical ventilation. However, mechanical ventilation can cause VILI, significantly increasing ARDS-related mortality. Current protective ventilation strategies, such as LVT and OLA, have not substantially reduced ARDS mortality, possibly due to their one-size-fits-all approach, which does not account for the clinical and pathophysiologic heterogeneity of ARDS. Understanding ARDS pathophysiology is crucial for developing personalized ventilation strategies.

Conventional Protective Ventilation Strategies

Low Tidal Volume Approach The ARDSNet LVT approach (6 mL/kg of ideal body weight) protects normal lung from overdistension while allowing collapsed lung tissue to rest. This strategy aims to keep plateau airway pressure below 30 cmH2O using PEEP to prevent EELV loss and minimize RACE-induced atelectrauma. Despite being the standard of care, LVT has not significantly improved ARDS-related mortality.

Open Lung Approach The OLA applies sufficient PEEP, with or without recruitment maneuvers, to prevent progressive derecruitment and EELV loss. However, OLA has not reduced ARDS mortality and often requires deep sedation and neuromuscular blocking agents, which have inconsistent benefits. High-frequency oscillatory ventilation (HFOV), an OLA strategy, also failed to improve ARDS outcomes, likely due to heterogenous flow distribution and parenchymal strain.

A Personalized Approach to Mechanical Ventilation

The high mortality associated with ARDS and the limitations of LVT and OLA highlight the need for a ventilation strategy that stabilizes and gradually recruits collapsed alveoli while avoiding RACE. The TCAV method, applied via APRV, achieves this by controlling the duration of passive exhalation flow to retain lung volume and avoid airway closure during expiration. This method leverages the time dependencies of alveolar opening and closing, gradually reopening collapsed lung tissue over hours or days.

Stabilize and Gradually Recruit Approach

Time-Controlled Adaptive Ventilation (TCAV) The TCAV method sets and adjusts APRV parameters to maintain an open lung using a brief expiration duration that prevents alveolar collapse. The PHigh is applied for an extended duration (Thigh) to gradually recruit and maintain open lung units, while a brief TLow during the Release Phase halts expiratory airway closure. The Slope of the Expiratory Flow curve (SLOPEEF), a breath-by-breath reflection of CRS, is used to titrate TLow, personalizing the approach based on lung injury severity.

Using Time to Prevent Alveolar Collapse The TLow is the most critical component of TCAV. If set too long, it causes progressive atelectrauma. The optimal TLow varies with factors like airway size and lung injury severity. As lung injury progresses, recoil forces increase, necessitating a shorter TLow to maintain EELV. The SLOPEEF guides TLow adjustments, terminating expiration when expiratory flow decelerates to 75% of PEF. This strategy personalizes and adapts ventilation based on real-time changes in CRS.

Using Time to Gradually Reopen Alveoli During TCAV, the lung spends most of its time at PHigh, facilitating gradual recruitment without causing volutrauma. The brief TLow prevents lung units from closing before the next application of PHigh, maintaining alveolar stability. The extended CPAP Phase gradually opens lung tissue, while the brief Release Phase functions as an inflate-and-brake mechanism to avoid re-collapse.

Summary and Conclusions

ARDS-related mortality remains high despite lung protective ventilation strategies like LVT, HFOV, and OLA. These strategies fail to eliminate VILI mechanisms due to their one-size-fits-all approach. TCAV offers a personalized and adaptive ventilation strategy by gradually reopening collapsed lung tissue and preventing closure during expiration. This method adapts to the changing pathophysiology of individual patients using the SLOPEEF as a dynamic feedback tool. While further research and clinical trials are needed, TCAV presents a promising approach to improving ARDS outcomes by addressing the underlying mechanisms of VILI.

Watch the following video on Clinical Application and Case Studies Applying the TCAV Method of APRV by Dr?ger

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Discussion Questions

1. How does the TCAV method compare to traditional ventilation strategies in terms of reducing VILI and improving ARDS outcomes?
2. What are the key factors in personalizing the expiratory duration (TLow) using the slope of the expiratory flow curve (SLOPEEF)?
3. How can future research address the limitations and validate the long-term benefits of TCAV in ARDS patients?

Javier Amador-Casta?eda, BHS, RRT, FCCM

Interprofessional Critical Care Network (ICCN)

[email protected]

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