Yet medicine advances because researchers keep asking the question: is there a better way? A growing body of evidence suggests that while simple spirometry remains valuable, it may not always provide the most meaningful picture of how COPD affects an individual patient. Therefore, new research is exploring whether another physiological measurement, one that is more closely linked to the underlying disease process, could be found using equipment as simple, inexpensive, and practical as that used for spirometry.

Among those pursuing this line of thought is researcher Andras Gedeon of Mincor AB and his colleagues, whose recent work introduces an innovative new method for measuring functional residual capacity using a remarkably straightforward approach. Their work does not claim to replace existing practice overnight. Instead, it presents an opportunity for clinicians and researchers to consider how COPD assessment could evolve to better reflect the disease itself.

The significance of their approach becomes clearer when we examine why simple spirometry became the standard in the first place, and why its limitations deserve renewed attention. COPD is characterized by progressive airflow limitation, but airflow obstruction is only one aspect of the disease. As lung tissue loses elasticity and the small airways narrow or collapse during expiration, patients gradually retain more air in their lungs. This phenomenon, known as hyperinflation, fundamentally alters breathing mechanics. The lungs remain partially inflated even after normal exhalation, making each subsequent breath more difficult.

Patients rarely describe this process in physiological terms. Instead, they simply say they cannot catch their breath. The sensation of breathlessness often becomes the dominant feature of daily life. Walking across a room, carrying on shopping, or even dressing can become exhausting. As hyperinflation worsens, respiratory muscles must work harder, exercise tolerance falls, and quality of life deteriorates.

This raises an important question for clinicians. If hyperinflation lies at the heart of many of the symptoms that patients experience, how can we measure it directly as often and as easily as we would like to?

Traditionally spirometry measures airflow and volume during forced expiratory maneuvers. It tells us how rapidly a patient can exhale a certain volume of air, and this information about airflow restriction has important diagnostic value.

However, forced breathing is not how people normally breathe. The required breathing maneuvers depend heavily on patient cooperation and effort. Elderly patients, frail individuals, and those experiencing acute illness may struggle to perform repeated maximal respiratory maneuvers consistently. Even under ideal circumstances, test quality can vary depending on technique, coaching, and patient understanding and effort.

Perhaps more importantly, airflow limitation does not always correspond closely with how patients actually feel or how their disease progresses. Two patients with similar spirometry results may experience very different levels of breathlessness and exercise limitation and may have different disease progression.

Over the past two decades, research has highlighted that measures reflecting lung volumes often correlate more closely with symptoms and prognosis than airflow obstruction. In particular, functional residual capacity, the volume of air remaining in the lungs after normal passive exhalation, has been shown to be a sensitive and valuable measure.

Functional residual capacity represents an important physiological characteristic of the respiratory system. In COPD, this resting lung volume frequently increases as damaged lungs lose their natural elastic recoil and air becomes trapped during expiration.

Importantly, this increase often occurs early in the disease process. As hyperinflation develops, functional residual capacity rises before many patients experience severe airflow limitation. This means that measuring resting lung volume provides valuable insight into disease progression that may not be fully reflected in flow resistance measurements.

Furthermore, enlarged resting lung volumes have been associated with clinically relevant symptoms. Breathlessness often correlates more closely with hyperinflation than with measurements of obstruction. Some studies have suggested that hyperinflation may also carry better prognostic information than airflow resistance.

These observations do not diminish the importance of spirometry. Rather, they suggest that COPD may benefit from a broader physiological assessment than is currently available in most clinical settings. The challenge has never been recognising the value of functional residual capacity. Pulmonary physiologists have long appreciated its importance and, whenever possible, measured it using advanced laboratory equipment.

The obstacle to regular use has been practical. Existing methods for measuring functional residual capacity generally require sophisticated pulmonary function laboratories, specialised equipment, complex procedures, and trained personnel. Body plethysmography, gas dilution techniques, and advanced imaging can provide adequate information, but these approaches are expensive, time consuming, and often unavailable outside specialist centres.

Primary care practices, community respiratory clinics, rural hospitals, and many outpatient services simply lack routine access to these technologies. As a result, clinicians continue to rely primarily on simple spirometry because it remains the only practical option. This is precisely why the work of Andras Gedeon and his collaborators becomes relevant.

They asked if functional residual capacity could be measured using an approach simple enough to be performed rapidly with inexpensive, easily available equipment. Their proposed solution draws upon fundamental gas laws of physics and a very specific, highly remarkable observation of the properties of lung tissue.

Normally, one would not expect when introducing carbon dioxide into the lung that the gas laws of elementary physics could be applied. This is because the lung tissue can readily and quickly both take up and release carbon dioxide. In contrast, the trace amount of inert gases, such as helium and methane, employed by sophisticated laboratory methods do not react with the lung tissue at all and so the lung can in this case be considered a container with passive walls and the gas-laws of physics can be applied.

However, in 1982 Robert Crapo and his colleagues made an extraordinary observation when studying the lungs of 90 healthy subjects. They found that the lung tissue always added 22% to the FRC volume irrespective of the age, gender or body size of the subject. This remarkable finding makes it feasible to use the fundamental gas laws also when introducing carbon dioxide into the lung.

The method by Andras Gedeon employs a brief period of normal breathing through an added external dead space of precisely known volume while monitoring end tidal carbon dioxide concentration (the last value at the end of an expiration). A standard commercially available IR-CO₂ analyzer routinely used in clinical monitoring can be used. By analyzing how the carbon dioxide concentration increases during the first few breaths of this short rebreathing maneuver, the investigators calculate an effective lung volume for carbon dioxide from which functional residual capacity can be determined using the relation found by Crapo.

At first glance, the approach seems incredible in its simplicity. Yet simplicity is exactly what gives it potential significance. The required equipment is readily available and far less complex than traditional laboratory systems. The measurement itself takes less than half a minute to perform. Because it relies on normal tidal breathing rather than repeated forceful respiratory manoeuvres, it has the potential to reduce the dependence on patient effort.

For healthcare professionals who regularly care for COPD patients, these features immediately raise interesting possibilities. Imagine being able to obtain physiologically meaningful information about lung hyperinflation without referring every patient to a specialist pulmonary laboratory. Imagine incorporating such measurements into routine outpatient assessment, community respiratory services, perioperative evaluation, or even home-based monitoring.

These possibilities remain aspirational rather than established. Nevertheless, they are sufficiently compelling to deserve serious consideration. The investigators carefully describe what their study demonstrates and, equally importantly, what it does not.

That distinction is particularly important because new ideas in medicine only become valuable when they are examined with both an open mind and scientific discipline.

The recently published study demonstrated that functional residual capacity can be determined in healthy volunteers, with good agreement compared with an established reference method. The procedure was completed quickly, required only simple equipment, and showed that repeated measurements improved precision so as to be comparable with that of the reference method. These findings provide encouraging evidence that the physiological principle underlying the technique is sound and that the method deserves further investigation.

However, healthcare professionals should also recognise the limits of the current evidence. The study was designed to establish whether the technique could accurately determine functional residual capacity in healthy individuals under carefully controlled conditions. It did not consider the effect of diseases of the lung.

Does this mean the method is unlikely to work in COPD? Quite the opposite.

Evidence from previous research both on animals and humans provides grounds for cautious optimism. Variations of carbon dioxide rebreathing techniques have already demonstrated their usefulness for measuring FRC in mechanically ventilated swine, with induced severe lung injury. More recent work has also shown that effective lung volume for carbon dioxide can be measured in mechanically ventilated critically ill patients with different and severe pulmonary diseases, suggesting that the physiological relationships reported by Crapo underpinning this approach remain largely intact even when lung function is seriously impaired. Taken together, these findings strengthen the rationale for extending the investigation to patients with COPD to obtain direct clinical evidence for this patient group.

Such investigations should address a series of important clinical questions. How accurately does the method perform across the full spectrum of COPD severity as compared to state-of-the-art methods for measuring FRC? Can it identify hyperinflation earlier than conventional spirometry? Can repeated measurements help clinicians to better monitor disease progression or evaluate the response to treatment? The answers to these questions could influence how COPD is assessed in the future.

For researchers, these studies could offer an attractive opportunity. The equipment described in this work is inexpensive and easily available, making it feasible for other investigators to evaluate, challenge, improve, and expand upon the technique. Although the basics are established the practical implementation, in particular the breathing protocol, can be designed in many ways. Although patient collaboration is expected to be less critical, nevertheless finding the optimal procedure should be of essence for obtaining data with good precision in the shortest time.

The work led by Andras Gedeon combines an innovative way to control the amount of carbon dioxide introduced into the lung and a less well-known but most important characteristic of lung tissue. Together they result in a remarkably simple method and equipment for FRC measurement. These encouraging initial results will motivate further work.

Spirometry was introduced by John Hutchinson 180 years ago and since that time it has been a most essential diagnostic tool with a firmly established place in respiratory medicine. If functional residual capacity can one day be measured simply, accurately, and inexpensively during normal breathing, it could become another regular and most valuable cornerstone in COPD assessment.

In the end, patients with COPD would ultimately benefit from such a development. Few opportunities in respiratory medicine therefore seem more deserving of a continued effort.