Their argument arrives at a pivotal moment. Volume guarantee, a feedback mode that automatically adjusts oscillatory pressure to keep delivered breath volumes near a clinician-chosen target, has been gaining traction in neonatal and pediatric intensive care. A systematic review of high-frequency oscillatory ventilation with volume guarantee in infants, published in the same journal in 2025, helped fuel enthusiasm for the approach, and editorial commentary has asked whether the neonatal community is ready for volume targeting during oscillation at all. What the new correspondence adds is a caution grounded in bench physics: the number a clinician types into the ventilator does not, by itself, define the mechanical dose the lung actually receives.

The technical heart of the argument rests on how oscillatory ventilators work. In high-frequency oscillatory ventilation, gas movement is driven by a rapidly oscillating diaphragm or piston superimposed on a constant mean airway pressure that holds the lung open. The oscillatory pressure amplitude determines how far gas swings back and forth with each cycle, while the frequency, typically measured in hertz, determines how many of those swings occur each second. Delivered tidal volume emerges from the interaction of these settings with the impedance of the respiratory system, a combination of resistance and compliance that varies dramatically between patients, between endotracheal tube sizes, and over the course of an illness. Two infants with identical target tidal volumes may therefore experience very different pressure swings, very different frequencies, and very different patterns of lung stress and strain.

Bench evidence underpins the correspondence’s central claim. In a study published in Archives of Pediatric Critical Care Medicine in 2026, Lilitwat and colleague Yodchai Vorakunthada systematically varied ventilator settings in a pediatric mechanical test lung and measured the tidal volumes actually delivered during oscillation. Their experiments demonstrated that frequency, pressure amplitude, mean airway pressure, inspiratory-expiratory ratio, and airway resistance all influence the volume of gas that reaches the test lung. In other words, delivered tidal volume is not a fixed consequence of a target setting; it is an emergent result of the entire configuration of the ventilator and the mechanical properties of the circuit and airway. A target achieved at a high frequency with a low amplitude is not mechanically equivalent to the same target achieved at a low frequency with a high amplitude, even though the volume readout looks identical.

This distinction matters because the mechanical dose delivered to the lung is what drives both benefit and injury. In conventional ventilation, decades of research have established that excessive tidal volumes and pressures stretch and stress alveoli, triggering ventilator-induced lung injury. During oscillation, the relationship between volume and injury is subtler, since the breaths are small and the protective effect of the strategy depends on maintaining lung volume with mean airway pressure while avoiding both overdistension and collapse. If clinicians judge the intensity of support, or the risk of injury, by the target tidal volume alone, they may overlook the contributions of amplitude and frequency to the oscillatory pressure swings, or the role of mean airway pressure in shaping end-expiratory lung volume. The correspondence suggests that this incomplete accounting could complicate the interpretation of clinical trials comparing ventilation strategies.

The authors point to prior work showing that oscillators themselves differ substantially in how they deliver volume and pressure. A 2015 in vitro comparison of eight high-frequency oscillatory ventilators, led by David Tingay and colleagues and published in Neonatology, found marked variability in performance characteristics between devices, raising the question of whether all oscillators are, in fact, created equal. A 2017 study in Pediatric Critical Care Medicine by Raymond Wong and colleagues examined volume and pressure delivery during pediatric high-frequency oscillatory ventilation and documented discrepancies between set and delivered values. More recently, a 2021 study in Pediatric Pulmonology by Gonzalo Solís-García and colleagues reported that in preterm infants with respiratory distress syndrome, low target volumes combined with high frequencies could achieve adequate ventilation, reinforcing the idea that frequency and volume interact in determining the adequacy of gas exchange.

Against this backdrop, the correspondence frames a practical recommendation for the research community. Future studies of high-frequency oscillatory ventilation with volume guarantee, the authors argue, should report not only the target tidal volume and the achieved tidal volume, but also the oscillatory frequency, the pressure amplitude, proxies of airway resistance, the size of the endotracheal tube, and the carbon dioxide response of the strategy. This expanded reporting standard would allow readers to reconstruct the actual mechanical conditions of each study and to compare results across devices, patient populations, and institutions. Without such detail, the authors warn, the literature risks accumulating studies whose headline volume targets conceal fundamentally different mechanical exposures, making meta-analyses and clinical guidelines less reliable than they appear.

The recommendation also has implications for bedside practice. Clinicians adjusting a volume guarantee setting on an oscillator typically watch the achieved tidal volume and the resulting carbon dioxide levels, titrating the target to keep blood gases in range. The correspondence does not dispute the value of this workflow, but it emphasizes that the same achieved volume can be reached through different combinations of frequency and amplitude, and that these combinations carry different mechanical implications for the lung. A patient with high airway resistance, for example, whether from a small endotracheal tube, airway secretions, or disease itself, may require different pressure amplitudes to achieve the same volume as a patient with low resistance. Tracking resistance proxies and tube size alongside volume data gives clinicians a fuller sense of why a ventilator is behaving as it is, and when a change in the underlying mechanics, rather than the target, is driving a change in ventilation.

It is worth noting the nature of the publication itself. The piece is a correspondence, a format used for scholarly commentary and interpretation rather than original data collection, and the authors state explicitly that no datasets were generated or analyzed for it. Its contribution is a synthesis of existing bench and clinical evidence, assembled to make a methodological point at a time when volume guarantee during oscillation is moving from bench curiosity toward routine clinical use. The authors acknowledge using a language model solely to assist with organization and language refinement, while retaining full responsibility for the scientific content. Both authors declare no competing interests, and the journal’s records show the manuscript was received in late August 2026, accepted in mid-September, and published on October 1, 2026.

For a field that has spent decades refining how to protect the smallest lungs, the message is deceptively simple: a number is not a dose. As volume guarantee becomes more widespread on oscillatory ventilators, the correspondence urges researchers and clinicians alike to resist the convenience of a single target value and to report and reason about the full set of mechanical variables that shape what the lung actually experiences. The next generation of trials, armed with richer descriptions of frequency, amplitude, resistance, and tube size, will be better positioned to answer the question that matters most to patients and families, namely which combinations of settings deliver the benefits of oscillatory ventilation while minimizing its hidden mechanical costs.

Subject of Research: Mechanical determinants of delivered tidal volume during high-frequency oscillatory ventilation with volume guarantee

Article Title: Target tidal volume incompletely describes mechanical dose during high-frequency oscillatory ventilation

Article References: Ostermaier, E. M., & Lilitwat, W. (2026). Target tidal volume incompletely describes mechanical dose during high-frequency oscillatory ventilation. Pediatric Research. https://doi.org/10.1038/s41390-026-05571-7

Image Credits: AI Generated

DOI: 10.1038/s41390-026-05571-7

Keywords: high-frequency oscillatory ventilation, tidal volume, volume guarantee, mechanical ventilation, neonatal intensive care, pediatric research, ventilator-induced lung injury, pressure amplitude, airway resistance, mean airway pressure, endotracheal tube, bench testing

The core of the debate lies in a trade-off that every intensivist performs at the bedside, often several times a day. When lung-protective limits cap the tidal volume, the ventilator must increase the respiratory rate to sustain adequate minute ventilation, the total volume of air moved in and out of the lungs each minute. Minute ventilation is simply the product of tidal volume and respiratory rate, so mathematically the two settings can be traded against one another while the total remains constant. In theory, halving the tidal volume and doubling the rate should deliver the same gas exchange. In practice, the lungs and the body do not obey such neat arithmetic, and the consequences of shifting the balance in one direction or the other have remained surprisingly difficult to pin down.

The reason the trade-off is not neutral lies in the architecture of the respiratory system. Not all inhaled air reaches the alveoli, the tiny sacs where oxygen and carbon dioxide are actually exchanged. A substantial fraction of each breath fills the conducting airways, the trachea, bronchi and bronchioles, where no gas exchange takes place. This volume is known as anatomical dead space. When tidal volume is reduced, each breath carries a proportionally larger burden of dead space gas, air that is inhaled and exhaled without ever contributing to gas exchange. To compensate, the respiratory rate must rise even further than simple minute ventilation calculations would suggest, because the effective, alveolar ventilation falls faster than the total ventilation. Higher rates, in turn, shorten the time available for exhalation, raising the risk of air trapping and the build-up of intrinsic positive end-expiratory pressure, a phenomenon that can quietly strain the lungs and the right side of the heart.

Jung and her colleagues frame this problem through the lens of ventilatory efficiency, a concept that captures how much of the work of breathing actually translates into useful gas exchange. Ventilatory efficiency is commonly assessed by relating ventilation to carbon dioxide elimination, and it deteriorates when dead space fraction rises or when the matching between ventilation and perfusion within the lung worsens. The Hannover group contends that when clinicians and researchers trade tidal volume against respiratory rate, they typically track pressures, volumes and driving pressure, but rarely ask whether the resulting pattern of breathing is actually efficient. Two ventilator configurations that look equivalent on a monitor, delivering identical minute ventilation and similar airway pressures, can differ substantially in how much carbon dioxide they clear per liter of air moved, and that difference carries metabolic and mechanical consequences for the patient.

The correspondence appears alongside and responds to a study by Grieco and colleagues, also published in Intensive Care Medicine, which examined how respiratory mechanics modify the impact of ventilator settings on clinical outcomes in patients with acute brain injury. That work adds an important dimension to the debate, because patients with injured brains are exquisitely sensitive to changes in carbon dioxide levels, intrathoracic pressure and the hemodynamic effects of ventilation. In this population, the choice between a low tidal volume with a high rate and a slightly larger tidal volume with a lower rate is not merely a question of lung protection. It can influence cerebral blood flow, intracranial pressure and brain tissue oxygenation, making the trade-off a matter of neurological as well as pulmonary consequence.

To probe the physiological side of the question directly, the Hannover authors point to their own prospective physiological pilot study, published in Annals of Intensive Care, in which they systematically increased respiratory rate during low tidal volume ventilation and measured what happened to ventilatory efficiency and to the mechanical costs of breathing. The study, led by Jung with colleagues including Markou and Storch, was designed to isolate the effect of rate under controlled conditions, allowing the researchers to observe how the efficiency of carbon dioxide elimination and the energetic burden on the respiratory system changed as breaths became faster and smaller. Although pilot studies of this kind involve limited numbers of patients and are intended to generate hypotheses rather than definitive answers, they provide the kind of granular physiological data that large outcome trials often cannot capture.

The significance of this line of research extends beyond the physiology laboratory. Mechanical ventilation is one of the most common interventions in critical care, and the energy cost of breathing, whether performed by the patient’s own muscles or by a machine, is a real metabolic burden for patients whose reserves are already depleted. Inefficient ventilation means that the body, or the ventilator, must move more air to achieve the same gas exchange, and every liter of air moved imposes stress on the lung tissue, on the airways and on the circulation. If a high rate strategy systematically degrades ventilatory efficiency, then the apparent safety of very low tidal volumes may come with a hidden price that current protective ventilation protocols do not account for. Conversely, if efficiency is preserved across a wide range of rate settings, clinicians would gain welcome flexibility in tailoring ventilation to individual patients.

The Hannover authors also emphasize that respiratory mechanics, the elastic and resistive properties of the individual patient’s respiratory system, modify how these trade-offs play out. A stiff, poorly compliant lung in acute respiratory distress syndrome behaves very differently from a more compliant one, and the same ventilator settings can impose very different stresses depending on the underlying disease, the body size and the geometry of the airways. This is precisely the message reinforced by the Grieco study in brain-injured patients, where the impact of ventilator settings on outcomes depended on respiratory mechanics. The implication is uncomfortable but important: there may be no universal answer to the question of how to distribute ventilation between volume and rate, and the optimal balance may need to be determined for each patient rather than dictated by a single protocol.

What the correspondence ultimately calls for is a change in perspective rather than a change in practice. The authors do not dispute the value of low tidal volume ventilation, which remains one of the most robustly supported interventions in critical care. Instead, they argue that ventilatory efficiency deserves a place at the table when ventilator settings are chosen and when clinical trials are designed. Modern ventilators already measure many of the variables needed to estimate dead space and efficiency at the bedside, and incorporating these measurements into routine monitoring could allow clinicians to see not just how much air they are moving, but how effectively that air is working. As the field moves toward increasingly personalized ventilation strategies, the question posed by Jung, Gillmann and Stueber is likely to shape both research agendas and bedside decision-making in the years ahead.

For now, the message for the intensive care community is one of measured caution and renewed curiosity. The trade between tidal volume and respiratory rate is not a zero-sum game played out on the ventilator display; it is a physiological negotiation with consequences for gas exchange, energy expenditure, hemodynamics and, in vulnerable populations such as patients with brain injury, the injured organ itself. By insisting that ventilatory efficiency be considered in that negotiation, the Hannover group has added a compelling voice to a growing conversation about what protective ventilation truly means. The answer to their title question, on the evidence they marshal, appears to be a qualified yes, and the next generation of studies will need to determine exactly how that consideration should change the numbers entered on the ventilator each morning.

Subject of Research: Ventilatory efficiency in the trade-off between tidal volume and respiratory rate during protective mechanical ventilation

Article Title: Should ventilatory efficiency be considered when trading tidal volume against respiratory rate?

Article References: Jung, C., Gillmann, H.-J., & Stueber, T. (2026). Should ventilatory efficiency be considered when trading tidal volume against respiratory rate?. Intensive Care Medicine. https://doi.org/10.1007/s00134-026-08627-8

Image Credits: AI Generated

DOI: 10.1007/s00134-026-08627-8

Keywords: mechanical ventilation, tidal volume, respiratory rate, ventilatory efficiency, dead space, acute respiratory distress syndrome, intensive care medicine, respiratory mechanics, carbon dioxide elimination, acute brain injury, lung protection, minute ventilation

The study, published in Global Surgical Education, the journal of the Association for Surgical Education, was conceived as an example of translational simulation. Rather than using simulators purely to teach skills, the researchers treated the simulation laboratory as a controlled experimental platform for evaluating a crisis-care innovation that could not ethically or logistically be studied in living patients. The approach matters beyond this single device configuration. Professional societies cautioned against shared ventilation based on theoretical reasoning and bench-level data, but the magnitude of the risk had never been measured in a physiologically representative environment. The Ohio State team set out to fill that gap with reproducible numbers.

The experimental setup was deliberately built from components that any hospital could obtain. Two Laerdal SimMan 3G Plus patient simulators were connected in parallel to a single ventilator running in volume-control assist-control mode. The inspiratory and expiratory ports of the ventilator were split using T-connectors, with HEPA filters placed on each circuit to limit cross-infection and one-way valves positioned to prevent retrograde airflow from the stiffer lung unit to the more compliant one. The only custom components were four 3-D-printed adapters, designed at the university’s Center for Design and Manufacturing Excellence, to connect commercial flowmeters to standard 22-millimeter ventilator tubing. Similar off-the-shelf adapters, the authors note, are available in many markets.

The physiological core of the experiment was compliance mismatch. One simulator was programmed with normal lung compliance, while the other was set to markedly reduced compliance—33 percent of normal—reproducing the impedance imbalance that, in a parallel circuit, drives airflow preferentially toward the easier-to-inflate lung. This is the exact mechanism that the Society of Critical Care Medicine, the American Association for Respiratory Care, the American Society of Anesthesiologists, the Anesthesia Patient Safety Foundation, the American Association of Critical-Care Nurses, and the American College of Chest Physicians cited in their 2020 joint statement against multiple patients per ventilator. In a parallel circuit, air follows the path of least resistance, so the more compliant lung receives a disproportionate share of each breath while the stiffer lung may be severely under-ventilated.

The numbers the simulation produced are striking. At a set tidal volume of 900 milliliters, the normal-compliance simulator received approximately 436 milliliters per breath at baseline. When compliance was reduced to 33 percent, the delivered volume in that limb collapsed to just 141 milliliters—a drop of 295 milliliters, or 68 percent, and a volume the authors describe as incompatible with meaningful ventilation. At the higher setting of 1100 milliliters, the low-compliance limb fell from 487 to 414 milliliters, a 15 percent reduction. These measurements were highly stable: across seven consecutive breaths per condition, coefficients of variation stayed below 9 percent in all six experimental conditions, ranging from 1.4 to 8.9 percent.

The second hypothesis tested whether a pragmatic intervention could mitigate the maldistribution. The researchers fitted inline flowmeters with adjustable valves to the inspiratory tubing of each limb and gradually closed the valve on the higher-flow circuit until the floats in both flowmeters indicated comparable inspiratory flow rates. The effect was dramatic at the 900-milliliter setting: delivered volume in the low-compliance limb recovered from 141 to approximately 431 milliliters, restoring 99 percent of the normal-compliance baseline. At 1100 milliliters, the volume rose from 414 to 501 milliliters, slightly exceeding baseline—consistent, the authors explain, with manual titration to a matched float position rather than to a fixed volume target.

These results empirically confirm what the six societies had argued on theoretical grounds, and they do something the original joint statement could not: they quantify the risk. A patient whose lungs are three times stiffer than a neighbor’s could silently receive less than a third of the intended breath volume while the ventilator displays apparently reassuring numbers. The more compliant lung, meanwhile, faces elevated risks of volutrauma and barotrauma from overdistension. Pressure-controlled ventilation modes may limit barotrauma but do not solve the fundamental problem of unequal volume distribution when compliances diverge. The simulation also demonstrated that flow regulation, while a substantial improvement, does not achieve fully independent control of each patient’s ventilation.

That partial success carries real implications for crisis preparedness. The authors are careful not to suggest that ventilator sharing is safe, but their data show that if sharing were ever deemed necessary as an absolute last resort, it need not be entirely uncontrolled. Inline flowmeters with adjustable valves—a commercially available, inexpensive addition—can redistribute tidal volume toward individualized targets. Just as importantly, the equipment list, circuit configuration, and testing protocol described in the paper can be replicated at any institution with high-fidelity simulation capability, allowing hospitals to test configurations, identify failure modes, and pre-position equipment before a crisis arrives rather than improvising during one.

The study also doubles as an educational platform, which is central to the journal’s mission. The same circuit that generated the data converts directly into a mastery-learning scenario for surgical residents, critical-care fellows, and interprofessional ICU teams. Learners are tasked with recognizing tidal volume maldistribution between two mismatched simulated patients, titrating the flowmeter valves, and identifying the failure modes—unilateral hypoventilation, overdistension injury to the compliant lung, and the loss of independent control of PEEP, oxygen concentration, respiratory rate, and inspiratory time—that underlie the societies’ warning. A facilitator guide, predefined learning objectives, and a structured debriefing framework are provided in the supplementary appendix so other centers can adopt the scenario. The point, the authors emphasize, is not to teach trainees how to routinely split a ventilator but to let them observe, in a controlled environment, why guideline bodies advise against it.

Perhaps the study’s most important contribution is methodological. A 2021 survey of acute care hospitals found that while 96 percent used simulation for education, only 15 percent used it for research, 37 percent for testing, and 30 percent for error analysis. The Ohio State work illustrates how simulation can function as a clinical laboratory for crisis-driven innovations that occupy the gap between theoretical feasibility and clinical implementation—a role the authors argue is underutilized but critically important for disaster and pandemic preparedness. The team is candid about the limits of their pilot: flowmeters measure flow rather than tidal volume directly, the simulators cannot reproduce the spontaneous breathing efforts, secretions, and rapidly evolving lung mechanics of critically ill patients, and the circuit tested only a single assembly under static conditions. Future work, they suggest, should incorporate dynamic compliance changes, more than two simulators, and in situ testing of the complete workflow including team communication and troubleshooting. But as a proof of concept, the study shows that the questions raised in a pandemic’s darkest hours do not have to wait for the next one to be answered.

Subject of Research: Simulation-based evaluation of tidal volume maldistribution during shared mechanical ventilation

Article Title: High-fidelity simulation as a translational and educational platform for crisis ventilation: quantifying and mitigating tidal volume maldistribution during shared mechanical ventilation

Article References: Husain, S., Syed, A., Bockbrader, J., Dunlea, T., Winfield, S., & Vazquez, D. (2026). High-fidelity simulation as a translational and educational platform for crisis ventilation: quantifying and mitigating tidal volume maldistribution during shared mechanical ventilation. Global Surgical Education – Journal of the Association for Surgical Education, 5(1), Article 185. https://doi.org/10.1007/s44186-026-00589-1

Image Credits: AI Generated

DOI: 10.1007/s44186-026-00589-1

Keywords: shared mechanical ventilation, ventilator splitting, tidal volume, lung compliance, translational simulation, high-fidelity simulation, crisis preparedness, flowmeters, patient simulators, COVID-19, ventilator shortage, medical education

The study drew on the COmPLETE cohort, a cross-sectional investigation conducted at the University of Basel in Switzerland in collaboration with the Royal Brompton Hospital in London. All participants were free from cardiopulmonary disease, screened for active respiratory infection, and assessed with spirometry interpreted according to European Respiratory Society and American Thoracic Society guidelines using race-neutral Global Lung Initiative reference values. Each volunteer then completed a maximal cardiopulmonary exercise test on a cycle ergometer, with ventilation measured on a breath-by-breath basis and averaged over 10-second intervals. The cohort was remarkably balanced: women made up 52 percent of participants, body mass index averaged 23.7 kilograms per square meter, and lung function sat comfortably within normal limits, with predicted forced expiratory volume in one second at 107 percent and predicted forced vital capacity at 110 percent. In other words, these were genuinely healthy adults, not patients, which makes the age-related changes all the more telling.

The researchers framed their investigation around a growing appreciation in physiology that the irregularity, or complexity, of a biological signal can itself be a marker of health. In cardiology, reduced heart rate variability is a well-established signal of autonomic dysfunction and disease risk. But the application of entropy analysis to breathing during exercise has been far more limited. Sample entropy works by asking how predictable a time series is: a highly repetitive, regular pattern scores low, while a complex, irregular pattern scores high. The team calculated sample entropy for minute ventilation, breathing frequency, and tidal volume in every participant, using an embedding dimension of two and a tolerance coefficient of 0.2. Their initial hypothesis, modeled on the heart rate variability literature, was that aging would reduce ventilatory irregularity, constraining the breathing pattern in older adults. The data told a different story.

First, the conventional measures. As expected, exercise capacity declined steadily with age. Peak power output, peak oxygen uptake, peak minute ventilation, peak tidal volume, and peak breathing frequency all fell progressively across the decades, with correlations ranging from modest to strong. Peak oxygen uptake dropped from roughly 45 milliliters per kilogram per minute in the twenties to under 23 in the oldest group. But the striking finding was the shape of the change in breathing strategy. At the first ventilatory threshold, breathing frequency remained essentially flat across the entire age range, hovering around 20 to 22 breaths per minute regardless of whether a participant was 25 or 85. Yet the ratio of breathing frequency to minute ventilation traced a non-linear curve: stable from age 20 to about 55, then inflecting sharply upward. At peak exercise the same pattern emerged, with the frequency-to-ventilation ratio rising markedly in the 70-to-79 and 80-plus decades, indicating a shift toward a tachypneic, or rapid-and-shallow, breathing strategy.

This shift makes mechanical sense. Healthy aging is associated with loss of lung elastic recoil, stiffening of the chest wall, reduced respiratory muscle strength, and declining tidal volumes relative to lung capacity. As the lungs lose their spring, breathing deep becomes expensive. Older adults compensate by breathing more often rather than more deeply, achieving the required ventilation through frequency rather than depth. The study found that the tidal volume to forced vital capacity ratio rose with age at both threshold and peak exercise, confirming that older breathers were working closer to their volumetric limits. Age and peak breathing frequency emerged as independent predictors of ventilatory reserve utilization, the percentage of estimated maximal voluntary ventilation consumed during exercise. Meanwhile ventilatory efficiency deteriorated, with the ventilatory equivalent for carbon dioxide nadir climbing from 24 in the youngest group to 32 in those over 80, meaning older adults had to move substantially more air to eliminate the same amount of carbon dioxide.

Then came the entropy results, which overturned the researchers’ expectations. After adjusting for sex, body mass index, peak oxygen uptake, forced expiratory volume in one second, and exercise duration, generalized additive models revealed a non-linear relationship between age and ventilatory sample entropy across all three signals. Entropy values held steady through early and middle adulthood, then climbed progressively from approximately age 60 onward. The pattern appeared in minute ventilation entropy, tidal volume entropy, and breathing frequency entropy, with the models for minute ventilation explaining just over a quarter of the deviance. Far from becoming more rigid and predictable, the breathing of older adults became demonstrably more chaotic during exercise. The team had hypothesized a decline in irregularity with age, mirroring the well-documented decline in heart rate variability, and the opposite occurred.

The authors offer several plausible mechanisms, while cautioning that the observational design cannot settle the question definitively. One possibility is altered central respiratory drive or impaired integration of afferent feedback within the brainstem, a form of neural dedifferentiation consistent with the broader concept of loss of physiological complexity in aging. Elevated entropy has previously been documented in dysfunctional breathing and in post-COVID unexplained breathlessness, conditions where patients report exertional breathlessness without any underlying cardiorespiratory pathology. Intriguingly, the age-related rise in entropy coincides with the period when around a quarter of people over 65 report breathlessness in the absence of diagnosed disease. The stability of breathing frequency at the first ventilatory threshold suggests that at submaximal intensities, age-related mechanical constraints have not yet compromised ventilation; the irregularity and tachypneic shift become evident only as exercise intensity climbs into higher domains. Sex differences added another layer: males showed higher minute ventilation entropy than females, possibly because larger lung volumes and airway dimensions in males produce a broader, less constrained ventilatory response that registers as a more irregular time series.

The study also connected ventilatory irregularity to fitness. In secondary analyses, higher peak oxygen uptake was associated with lower sample entropy across all three ventilatory signals, and peak oxygen uptake was the primary determinant of minute ventilation and frequency entropy in models adjusted for age and body mass index. The authors are careful to note that this may reflect shared age-related influences rather than a direct mechanistic relationship, since cardiorespiratory fitness declines steadily after the fifth decade, roughly the same point at which entropy begins its ascent. Still, the possibility that fitness protects the fine-grained organization of breathing control is a tantalizing lead. The findings also fit into a developmental arc suggested by prior work in children and adolescents, where entropy values decline during maturation, indicating increasing refinement of respiratory control. Taken together, these studies sketch a lifespan trajectory in which breathing control is refined through childhood and adolescence, stabilizes through adulthood, and then de-differentiates in later life, echoing patterns observed in frailty research across multiple physiological systems.

Limitations are candidly acknowledged. The analysis was retrospective and cross-sectional, so longitudinal data would be needed to track how individual breathing patterns evolve with age. All testing occurred at a single Swiss center in a predominantly Caucasian cohort, which limits generalizability, and no perceptual breathlessness ratings such as Borg scale scores were collected, preventing a direct link between entropy and symptom intensity in this cohort. The use of estimated rather than directly measured maximal voluntary ventilation is a further caveat, though the two are strongly correlated in healthy people. Crucially, the proportion of the exercise test spent above the first ventilatory threshold was comparable across all age decades, between 52 and 54 percent, suggesting that differences in relative exercise intensity did not confound the entropy findings. The authors also provide normative entropy values across the age span, a reference standard intended to anchor future studies of breathlessness in older adults.

The bigger picture is compelling: the study demonstrates that the aged respiratory system is forced into a mechanically inefficient, frequency-dominant pattern accompanied by rising signal irregularity, a kind of neuromechanical dissociation that may underpin the heightened perception of breathlessness so commonly reported with healthy aging. Because breathing frequency tracks perceived exertion closely, the tachypneic shift documented here provides a physiological bridge between the aging lung and the everyday experience of getting winded. Future prospective studies correlating entropy with breathlessness scores, and longitudinal designs tracking entropy within individuals over time, will be needed to convert these cross-sectional patterns into clinical insight. But the message already resonates: aging does not simply dim the respiratory system, it rewires its control, and the mathematical fingerprint of that rewiring becomes visible somewhere around our sixtieth birthday.

Subject of Research: Age-related changes in ventilatory pattern and irregularity during exercise in healthy adults

Article Title: The ventilatory response pattern to exercise across the age‐span in healthy adults

Article References: Williams, Z. J., Cenerini, G., Schwendinger, F., Knaier, R., Wagner, J., Hull, J. H., & Schmidt‐Trucksäss, A. (2026). The ventilatory response pattern to exercise across the age‐span in healthy adults. Physiological Reports, 14(17), Article e71065. https://doi.org/10.14814/phy2.71065

Image Credits: AI Generated

DOI: 10.14814/phy2.71065

Keywords: healthy aging, ventilatory response, sample entropy, cardiopulmonary exercise testing, breathing frequency, tidal volume, exertional breathlessness, respiratory mechanics, physiological complexity, cardiorespiratory fitness, ventilatory threshold, COmPLETE study