Assessing respiratory muscle function is crucial for clinicians, physiologists and researchers. Several methodological developments over the past twenty years have increased our understanding of respiratory muscle function and responses to interventions in health and disease. This recently published ERS task force statement assessed the field of respiratory muscle testing in health and disease, in adults and in children and critically ill patients in the intensive care unit. This statement critically evaluated the most recent scientific and methodological developments regarding respiratory mechanics and muscle assessment with an original and novel approach, i.e. by addressing the validity (i.e. the extent to which a test or variable is related to the function of a physiological system or to patient-meaningful variables, such as symptoms or exercise), precision, reproducibility, prognostic information (i.e. relationship with the natural history of the disease), discrimination (i.e. whether a variable can differentiate the severity of the disease as conventionally measured), clinical meaningful difference (i.e. the minimal difference in a tested variable that is considered to be functionally worthwhile or clinically important) and responsiveness to interventions of various methods. A particular emphasis was given to evaluation during exercise, which is a useful condition to stress the respiratory system.
In this ERS statement remarkable advances in respiratory muscle and lung mechanics assessment in the past few decades have come up, and three of them merit to be highlighted here.
First, the noninvasive and readily available measurements of upright and supine vital capacity (VC) in the evaluation of respiratory muscle function, especially the diaphragm, and the novelty is that a 15% decrease in the supine position (15% which represents twice the CV of the measure could be considered the LLN) may orient towards a unilateral diaphragm weakness.
Second, indices of respiratory muscle effort during exercise such as the oesophageal pressure tidal swings (Poes,tid) can serve as an index of global respiratory muscle effort during exercise, can identify differences in disease severity in patients with COPD (i.e. by Global Initiative for Chronic Obstructive Lung Disease stages), are sensitive to changes over time and to interventions and are related to the perception of dyspnoea during exercise. Poes,tid has been successfully applied as a bedside monitoring tool in sleep studies, and during weaning trials. Although it is difficult to establish a minimal clinically important difference of these indices of respiratory muscle effort, given the paucity and heterogeneity of the studies, a clinically meaningful difference of 14-16% from baseline condition has been shown to correlate with a clinically meaningful reduction of exertional dyspnoea after pharmacological intervention such as bronchodilators.
Third, the increasing availability of new and novel respiratory muscle imaging techniques such as the ultrasound to assess diaphragm dimensions and activity in terms of static measurement of end-expiratory diaphragm thickness, dynamic evaluation of the ratio of inspiratory to expiratory diaphragm thicknesses, reported as thickening ratio, and diaphragmatic excursion. This technique is readily available at the bedside and allows a simple, rapid and direct evaluation of the diaphragm that is more sensitive than fluoroscopy for the identification of muscle activity. Ultrasound-related indices of diaphragmatic dimensions and activity have recently been associated with diaphragm dysfunction in critically ill patients receiving mechanical ventilation and have also been reported as possible predictors of weaning outcome and duration of mechanical ventilation. Other imaging tools such as optoelectronic plethysmography (OEP) and structured light plethysmography (SLP) can be considered as emerging, non-contact, noninvasive method to assess breathing pattern and diaphragm (dys)function either in healthy or in patients with respiratory diseases.
This ERS statement is meant to launch new attitudes for clinicians, physiologists and researchers and encourages them to apply and fully translate it to the clinical care of individual patients. This requires a huge effort especially in this era in which less and less time is dedicated to training in the practical realization and interpretation of the more advanced tests of respiratory muscle function worldwide. This contributes to a vicious circle in which only a bunch of pulmonologists know and perfectly handle these tests that are available only in specialised centres. How to fight this disappointing and unfortunate situation? It seems critical that new generations of pulmonologists should be intensively exposed to clinical physiology concepts and practices.