Respiratory muscle dysfunction can contribute to the onset of respiratory failure and may worsen with interventions to unload the respiratory muscles and treat underlying conditions [1]. The respiratory system consists of two main components: lungs, the gas-exchanging organ, and the pump for lung ventilation. The pump includes the chest wall, respiratory muscles, central nervous system (CNS) controllers and pathways connecting these controllers to the respiratory muscles, such as spinal and peripheral nerves. Acute respiratory failure (ARF) is characterised by a sudden and significant impairment in the respiratory system's ability to maintain adequate gas exchange, developing within minutes or hours. In contrast, chronic respiratory failure (CRF) develops gradually over weeks, months or years.

Lung failure, caused by conditions such as pneumonia, emphysema or interstitial lung disease, leads to hypoxaemia with normocapnia or hypocapnia (hypoxaemic or type I respiratory failure) (figure 1) [1]. Acute lung failure can occur during asthma or COPD exacerbations, pneumonia, acute respiratory distress syndrome (ARDS) or pulmonary oedema. Chronic lung failure can develop in chronic obstructive and restrictive lung diseases (i.e. COPD and interstitial lung disease).

FIGURE 1

Types of respiratory failure. The respiratory system can be considered as consisting of two parts: 1) the lung and 2) the pump, including the chest wall, respiratory muscles, and central/peripheral nervous system. Adapted from Roussos and Koutsoukou [1].

Pump failure mainly results in hypercapnia (hypercapnic or type II respiratory failure) (figure 1) due to alveolar hypoventilation, although hypoxaemia can coexist [1]. Acute pump failure can occur in neuromuscular disorders (e.g., myasthenia gravis or Guillain–Barré syndrome), chest wall abnormalities (e.g., flail chest), increased abdominal pressure (e.g., pancreatitis) or CNS depression (e.g., drug overdose and brainstem injury). Chronic pump failure develops from progressive respiratory muscle dysfunction in conditions, such as amyotrophic lateral sclerosis (ALS) or muscular dystrophies, and in chronic chest wall disorders, such as kyphoscoliosis or severe obesity (obesity hypoventilation syndrome) that restrict lung expansion. The balance between restrictive forces on lung expansion and respiratory muscle capacity determines the likelihood of a respiratory muscle contribution to pump failure [2–4].

Both types of respiratory failure can coexist in the same patient. For example, severe COPD patients and chronic type I respiratory failure may develop additional ventilatory failure due to worsening of respiratory muscle dysfunction, causing muscle fatigue and carbon dioxide retention [3, 5]. Similarly, severe pulmonary oedema or asthmatic crises may initially cause hypoxaemia, followed by hypercapnia as the condition persists [1]. Prolonged mechanical ventilation for severe lung failure can also gradually lead to or exacerbate pre-existing respiratory muscle dysfunction, prolonging mechanical ventilation and complicating weaning [6].

Assessing respiratory muscle function is therefore essential for diagnosing and phenotyping patients, evaluating treatment efficacy and follow-up [7]. Recent advances in respiratory muscle neurophysiology and imaging, such as surface electromyography and respiratory muscle ultrasound, are increasingly used in clinical settings [8–10]. This review will discuss the potential of these novel methods alongside established techniques for assessing respiratory muscle function, including voluntary and evoked manoeuvres to measure airway opening, oesophageal and gastric pressures [7].

We will review established and emerging techniques for assessing respiratory muscle function in patients with ARF and CRF. Additionally, we will present current practices and evidence on treatment strategies aimed at supporting overworked respiratory muscles or enhancing their capacity through training and evaluate their impact on clinical outcomes. The presented information is not based on a systematic search of the literature including search terms and inclusion criteria. Instead, we performed a subjective selection of the literature that we deemed most relevant. To do this, we based our approach on recent systematic reviews [10–15] and task force statements [7, 16], and performed cited reference searches on these papers using Web of Science to find more recent original articles.

In the previous paragraphs we have provided a comprehensive overview of the clinical tools available for evaluating and treating respiratory muscle dysfunction, summarising both established techniques and recent methodological advancements. Notably, we highlighted the complementary roles of noninvasive assessment methods such as EMG and MUS, alongside established invasive and noninvasive pressure assessment techniques. While MUS is already frequently applied in clinical practice, EMG remains currently underused. The applications of these assessments are diverse, ranging from ARF to patients to those receiving chronic home mechanical ventilation. Currently available tools are useful for evaluating muscle function, titrating ventilatory support and guiding treatment decisions. MUS, in particular, is effective for bedside diagnosis and monitoring of diaphragm dysfunction, and it can predict clinical outcomes in critically ill, mechanically ventilated patients. EMG of the diaphragm and extradiaphragmatic muscles can optimise the dosing of unloading interventions and can help to optimise muscle stimulation during training interventions. Future advancements in information technology, hardware and software are expected to facilitate the broader adoption of these technologies in clinical settings in the years to come. If larger datasets, including results from multiple noninvasive and invasive respiratory muscle function assessments alongside clinical case data, become available, artificial intelligence based data analysis may emerge as a powerful tool in respiratory muscle function test interpretation. This could potentially help clinicians to refine and standardise diagnoses of respiratory muscle dysfunction in analogy with advances that have been made recently in pulmonary function test interpretation [160]. It could also potentially help to improve predictions of weaning and clinical outcomes in both acute and chronic settings by integrating results from multiple (non)invasive respiratory muscle assessments.

Evaluating respiratory muscle function is valuable for diagnosing, phenotyping and assessing treatment efficacy in patients with ARF and CRF.

EMG and MUS play an increasingly important role alongside established invasive and noninvasive pressure assessment techniques.

While MUS is already frequently applied in clinical practice, EMG remains currently underused.

Both are useful tools for evaluating muscle function, titrating ventilatory support, optimising muscle stimulation during training interventions and guiding treatment decisions.