From Kilograms to Compliance: Rethinking Lung-Protective Ventilation as an Equity Issue

Two patients in an intensive care unit can each receive a tidal volume of 6 milliliters per kilogram of predicted body weight, fully in line with current guideline recommendations from the European Society of Intensive Care Medicine, and yet face substantially different risks of ventilator-induced lung injury. On paper, both appear to receive equally protective ventilation. In reality, the same prescribed volume can impose very different levels of stress on the lung, and the difference is not random: it falls systematically on groups whose lungs are smaller than the standard equation assumes. That is the argument advanced in a new perspective article published in Intensive Care Medicine by Dario von Wedel, Marcelo B. P. Amato and Maximilian S. Schaefer, who contend that physiology-guided ventilation should be understood not merely as a refinement of intensive care practice but as an equity intervention.

The roots of the problem reach back to the ARMA trial, the landmark study that established low tidal volumes as one of the most fundamental advances in intensive care and substantially improved survival in mechanically ventilated patients. ARMA scaled tidal volumes to predicted body weight, a calculated value derived from a patient’s height and sex that was originally designed for pharmacokinetic purposes rather than for estimating lung size. The approach shaped a generation of clinical practice, and for good reason: targeting 6 milliliters per kilogram of predicted body weight reduced mortality compared with traditional, larger volumes. But the equation embeds a fixed, sex-corrected relationship between height and lung size, and it assumes that this relationship holds for every patient in every disease state and with every body composition.

Accumulating evidence now shows that patients with the same predicted body weight, and therefore the same estimated lung size, can have markedly different functional lung sizes, meaning the volume of aerated lung that actually accommodates the tidal breath. One patient may have consolidations, edema, or the shrunken functional lung that clinicians describe as the baby lung, all of which reduce the volume of lung available for ventilation. More strikingly, recent work demonstrates that a patient may simply be female and still receive a volume that is too large for her lungs. The predicted body weight equation systematically overestimates female lung sizes, both in critically ill and in healthy populations. When clinicians target tidal volumes scaled to predicted lung size, this overestimation translates directly into excessive lung stress, ventilator-induced lung injury and, ultimately, higher mortality in the populations disadvantaged by the equation.

The consequences are visible in the data on outcomes. Critically ill women frequently experience worse outcomes than men, and several contributing mechanisms have been identified. Women receive higher tidal volumes when these are standardized to predicted body weight, and those higher volumes produce higher driving pressure, the difference between plateau pressure and positive end-expiratory pressure. Driving pressure is widely regarded as a measure that scales tidal volume to functional lung size and serves as an indicator of ventilator-induced lung injury risk. Because the static equation overestimates the size of female lungs, equal treatment delivers unequal lung stress. Similar biases have been demonstrated across race, ethnicity and age, suggesting that the problem is not confined to a single demographic variable but reflects the inability of any static, population-average equation to absorb heterogeneity in dynamic physiological measures.

The authors emphasize that the bias does not stop at tidal volume. Novel summary estimates of ventilation intensity inherit the same distortion. Mechanical power, an integrative measure of the energy delivered to the lung per unit time, incorporates absolute rather than scaled tidal volumes, and therefore also carries the imprint of equations that misjudge lung size in certain groups. In other words, even the tools developed to quantify the aggressiveness of ventilation more comprehensively can distribute harm unevenly across the patients they were designed to protect. Standardizing ventilation to functional lung size, rather than to a calculated body weight, would avoid this bias among patients with smaller lungs, including women.

The conceptual key to the alternative is the baby lung concept, which sharpened functional lung size as the physiological target of lung-protective ventilation. Functional lung size is strongly correlated with static respiratory system compliance, which is calculated as the tidal volume divided by the driving pressure. Because compliance can be measured at the bedside during brief end-inspiratory and end-expiratory occlusions in a passive patient, it offers a direct, physiology-based window into how much lung is actually receiving the breath. Tidal volume scaled to respiratory system compliance, tracked through driving pressure, has therefore emerged as a physiology-guided target of lung-protective ventilation, one that adapts to the individual patient rather than to a population average.

What has been largely ignored, the authors argue, is the consequence of this choice for healthcare equity, defined as the absence of systematic disparities between different patient groups. Prescribing the same 6 milliliters per kilogram of predicted body weight to every patient is equal treatment in a formal sense. But because the equation systematically overestimates functional lung size in specific groups, equal treatment delivers unequal lung stress, and equity therefore requires unequal tidal volumes: smaller volumes for patients whose lungs are genuinely smaller. The individual mechanisms behind these disparities may differ, spanning biological, social or measurement pathways, yet the remedy is the same in every case, ventilation tailored to the individual functional lung size. Compliance-based measures capture differences both across subgroups and along the trajectory of an individual patient over time, and because they are grounded in directly measured physiology, they may be more agnostic to demographic factors that risk introducing algorithmic bias.

Historically, personalization has been treated as an add-on to the standard of care rather than as part of it. Driving pressure has been an established target for more than a decade, yet it remains an expert refinement rather than a routine element of ventilator management. The authors argue that for historically disadvantaged groups in whom algorithmic bias is most prevalent and severe, including women, older patients and minoritized racial and ethnic groups, personalization may be the necessary step to equalize the quality of care rather than an optional enhancement. Complementary bedside tools reinforce this direction: esophageal manometry and electrical impedance tomography offer appealing approaches for identifying the best ventilator settings for each individual patient, and integrative measures that scale the energy delivered to functional lung size extend the same logic to the total mechanical workload of ventilation.

The practical pathway they describe is concrete. Clinicians may target driving pressures well below established thresholds of harm, for example 10 centimeters of water. A reasonable starting point is a tidal volume of 6 milliliters per kilogram of predicted body weight, followed by measurement of the plateau pressure and then a stepwise reduction of tidal volume, within the guideline-supported range of 4 to 8 milliliters per kilogram, until the driving pressure target is achieved. Individualizing positive end-expiratory pressure further helps reduce driving pressures while tailoring care to the individual patient’s needs. Achieving this shift also requires an educational component: trainees must learn to interpret the patient’s physiology on the ventilator instead of reading a height chart, and the physiological metrics of respiratory system compliance and driving pressure must be elevated from optional refinements to primary titration targets.

The authors close with a call for clinicians and researchers to work together on implementation, investigating how physiology-guided ventilation is best delivered at the bedside and teaching clinicians to deliver it. The strategies involved are available and well researched, they note, and the remaining task is to embed them in routine practice. Framed in their words, the next revolution in mechanical ventilation will not be another static equation. It will be the decision to stop using one, to stop targeting population averages, and to start considering each patient’s physiology by moving from kilograms to compliance. For the two patients who opened the argument, receiving identical volumes by the book, equity in lung protection may ultimately be achieved with unequal tidal volumes, each matched to the lung that actually has to accommodate the breath.

Subject of Research: Equity implications of predicted body weight-based lung-protective ventilation and physiology-guided tidal volume titration

Article Title: Physiology-guided ventilation as an equity intervention: from kilograms to compliance

Article References: von Wedel, D., Amato, M. B. P., & Schaefer, M. S. (2026). Physiology-guided ventilation as an equity intervention: from kilograms to compliance. Intensive Care Medicine. https://doi.org/10.1007/s00134-026-08633-w

Image Credits: AI Generated

DOI: 10.1007/s00134-026-08633-w

Keywords: mechanical ventilation, ventilator-induced lung injury, predicted body weight, driving pressure, respiratory system compliance, healthcare equity, algorithmic bias, baby lung, PEEP, intensive care, ARDS, personalized medicine