The utility of spirometry and single breath gas transfer measurements to identify low total lung capacity

Total Lung Capacity (TLC) is widely recognized as the gold standard for measuring lung volume, but its assessment typically requires complex and expensive equipment, such as body plethysmography or gas dilution techniques. These methods are not always readily available outside specialized healthcare settings. As a result, it’s crucial to explore whether other lung function tests, like spirometry, could serve as reliable substitutes for TLC measurement, especially in routine diagnostic settings.

Spirometry is the most common lung function test due to its simplicity, cost-effectiveness, and widespread availability. It provides valuable data like Forced Vital Capacity (FVC), which reflects the maximum volume of air a patient can exhale forcefully. A reduced FVC is often used as an indicator of low TLC, especially in the presence of normal FEV1/FVC ratios. However, studies have shown that FVC is not a perfect predictor of TLC, particularly because conditions like obstructive lung disease or respiratory muscle weakness can distort FVC results.

Another lung function test, the single breath gas transfer test, measures the diffusing capacity of the lungs and includes an assessment of alveolar volume (VA), which is measured using an inert tracer gas. VA has been suggested as a potential surrogate for TLC, but its accuracy can be compromised by obstructive diseases, which may cause ventilation inhomogeneity and lead to an underestimation of VA compared to TLC.

A key point of interest is whether VA, when assessed alongside the lower limit of normal (LLN), could effectively identify low TLC, particularly in the context of restrictive lung disease. Such an approach would offer a less invasive, quicker, and cost-efficient alternative to full TLC plethysmography, making it especially beneficial in community diagnostic centers where the goal is to detect lung impairments before referring patients to specialized care.

The study aimed to evaluate how well spirometric restriction (reduced FVC) and low VA could identify patients with low TLC, both overall and across different severities of airflow obstruction. They also explored whether the relationship between these measures differed depending on the patient's underlying diagnosis.

The results show that spirometry alone is not an accurate tool for identifying low TLC, but it can effectively rule out restriction when the FVC is normal. On the other hand, low VA proved to be a reliable marker for identifying low TLC, offering a practical, less burdensome alternative to traditional TLC measurements. This could significantly reduce both costs and patient burden while improving the efficiency of restrictive lung disease diagnosis in general practice.

By integrating these findings into everyday clinical workflows, healthcare providers could streamline lung function testing and provide more timely, accurate diagnoses without the need for expensive or complex equipment.