Respiratory Mechanics and Mortality During Ventilation

• In ICU
• Thu, 14 Aug 2025

Lung-protective ventilation strategies, particularly the use of low tidal volumes, are widely recommended in patients with acute respiratory distress syndrome (ARDS). However, evidence from meta-analyses does not support a single, universally optimal tidal volume per predicted body weight. This is likely because predicted body weight estimates total lung size rather than the aerated portion of the lung that is vulnerable to ventilator-induced lung injury (VILI). The concept of the “baby lung” has helped clarify this discrepancy, emphasising that in ARDS, the functional, aerated lung is significantly reduced and heterogeneous in distribution.

A more physiologically relevant approach is to scale tidal volume to the size of the aerated lung, which is difficult to measure directly at the bedside. Instead, respiratory system compliance, which remains relatively stable across ventilated alveolar units, can serve as a useful surrogate for aerated lung size. This leads to the concept of driving pressure as a proxy for lung strain. A 2015 study by Amato et al. identified driving pressure as the key mediator of survival benefits associated with lung-protective ventilation, a finding subsequently validated by others. Importantly, reducing tidal volume in patients who already have low driving pressure may actually be harmful if it leads to excessive respiratory rates or increased effort.

Most ARDS patients spend the majority of their time on mechanical ventilation in assisted rather than controlled modes, making monitoring and controlling tidal volume more complex. In these settings, the total driving pressure includes both ventilator support and patient-generated inspiratory effort. Bedside tools such as inspiratory and expiratory holds and occlusion manoeuvres make it feasible to assess both static and dynamic driving pressures even during assisted ventilation.

Grassi et al. conducted a large multicentre observational study examining how respiratory mechanics during the early phase of assisted ventilation relate to clinical outcomes in ARDS. They found that higher driving pressures and lower respiratory system compliance were associated with increased mortality, whereas tidal volume, kept within a relatively safe range (around 8 mL/kg), was not. This finding supports the view that tidal volume alone is a limited marker of ventilation safety. While experimental data have highlighted patient effort as a key contributor to self-inflicted lung injury, the study found no significant association between inspiratory effort and mortality. This may be due to clinicians actively adjusting pressure support to minimise effort and respiratory distress, especially in patients with reduced compliance.

Non-survivors in the study required higher levels of pressure support, likely reflecting attempts to compensate for poor compliance rather than a harmful effect of support per se. This underscores compliance as a key outcome determinant and suggests that both lung and diaphragm protection become more challenging when compliance is low. Additionally, the findings raise the question of whether stricter targets for driving pressure should be pursued or whether such metrics simply reflect disease severity.

Finally, interventions like sedation or PEEP optimisation may help manage driving pressure, but their effects are highly individualised and lack robust outcome data. Since this study evaluated patients early in their disease course, it remains uncertain whether its findings apply in later fibrotic phases of ARDS, when compliance may no longer correlate with the size of the aerated lung.

In summary, the study highlights the importance of comprehensive bedside monitoring, particularly of driving pressure, compliance, and patient effort, in optimising ventilator settings during assisted ventilation in ARDS and tailoring treatment to individual physiology.

Source: AJRCCM

Image Credit: iStock

References:

Rosà T, Bongiovanni F, Grieco DL (2025) Respiratory Mechanics and Mortality During Assisted Ventilation. Am J Respir Crit Care Med.