Evaluating ventilator settings as related to mechanical power: tidal volume vs. respiratory rate dynamics

Abstract

We read with great interest the article by Buiteman-Kruizinga et al. [1] which investigated the effect of individual components of mechanical ventilation on mechanical power and provided evidence that the respiratory rate may be the most attractive ventilator setting to adjust when targeting a lower mechanical power. The authors utilised data from three randomised clinical trials that involved patients receiving invasive mechanical ventilation in the form of either volume- or pressure-controlled ventilation for reasons other than acute respiratory distress syndrome (ARDS). In addition, the authors grouped the patients by an upper quartile cut-point of 17 J.min^-1 for mechanical power but did not segregate patients based on the mode of mechanical ventilation utilised.

This exploration aligns well with our study in which we modelled the effects of individual ventilator settings such as tidal volume, respiration rate and positive end-expiratory pressure on mechanical power across various ventilation modes and ARDS severity using a mathematical simulator [2]. Some of our findings align well with those of Buiteman-Kruizinga et al. However, our data showed that tidal volume reduction may be the most effective and consistent strategy for reducing mechanical power to ≤ 17 J.min^-1, especially under volume-controlled ventilation with constant flow, which yielded the lowest mechanical power compared with pressure-controlled ventilation and descending ramp volume-controlled ventilation.

Notably, Buiteman-Kruizinga et al. emphasised respiratory rate as a prime target to lower mechanical power in patients without ARDS, whereas our modelling data suggest tidal volume reduction is more efficient in simulated ARDS scenarios. An obvious reason for this divergence is the fact that the patients in the study by Buiteman-Kruizinga et al. did not have ARDS while our study included simulated patients with mild, moderate and severe ARDS. However, we believe another important reason may be the fact that Buiteman-Kruizinga et al. did not segregate patients based on the mode of mechanical ventilation but mixed mechanical power values obtained under volume- and pressure-controlled ventilation despite mechanical power being expressed differently under these two widely used modes of mechanical ventilation [3]. While respiration rate is reflected as directly proportional to mechanical power in both expressions under volume- and pressure-controlled ventilation, tidal volume is not only reflected as directly proportional to mechanical power but also may be reflected through its indirect effect on driving pressure and peak airway pressure that are included in mechanical power expressions. For the same simulated lung mechanics, our data showed that volume-controlled ventilation results in lower generated mechanical power compared with pressure control given the same tidal volume and breathing frequency. Also, our simulations showed that whenever mechanical power exceeded some safe threshold (e.g. > 17 J.min^-1), decreases in tidal volume were more efficient than decreases in respiratory rate in bringing mechanical power back to the safe target during either volume- or pressure-controlled ventilation. Another reason that can explain the divergence in some of the results may be the controlled nature of simulated environments [2] vs. real-world heterogeneity [1].

Nonetheless, both studies converge on the notion that excessive mechanical power – irrespective of the setting it originates from – is associated with adverse outcomes and should be mitigated proactively. The complementary nature of our findings reinforces the need to tailor ventilation strategies to individual patient profiles and pathophysiology. Future prospective clinical studies directly comparing tidal volume vs. rate manipulation across ARDS phenotypes and ventilator modes would be invaluable to guide bedside decision-making.