What is the recommended approach for inspiratory muscle testing in patients with respiratory symptoms or conditions, such as Chronic Obstructive Pulmonary Disease (COPD) or asthma, according to the American Thoracic Society (ATS)/European Respiratory Society (ERS) guideline?

Inspiratory Muscle Testing Based on ATS/ERS Guidelines

The ATS/ERS guideline recommends measuring maximal inspiratory pressure (PImax) at or near residual volume using a flanged mouthpiece with a small leak, performing at least three maneuvers sustained for 1-1.5 seconds that vary by less than 20%, and recording the maximum value. 1

Primary Testing Methods

Maximal Static Inspiratory Pressure (PImax)

• Patient should be seated with a flanged mouthpiece and nose clip, with measurements performed at functional residual capacity (FRC) or residual volume (RV). 1
• A small leak (1-2mm diameter) must be incorporated into the system to prevent glottic closure and ensure measurement of true respiratory muscle pressure rather than buccal pressure. 1
• Each effort should be maintained for at least 1-1.5 seconds, and the maximum value of three maneuvers that vary by less than 20% should be recorded. 2
• Normal lower limits are PImax ≥75 cm H2O for men and ≥50 cm H2O for women; values numerically less than 70 cm H2O in males or 60 cm H2O in females suggest significant inspiratory muscle weakness. 2, 1

Sniff Nasal Inspiratory Pressure (SNIP)

• The sniff maneuver is a technically simple, natural maneuver that is easier for most patients to perform than static maximal efforts and requires little practice. 1
• Sniff nasal pressure (Pnas) values numerically greater than 70 cm H2O (males) or 60 cm H2O (females) are unlikely to be associated with significant inspiratory muscle weakness. 1
• The sniff should be performed with the contralateral nostril closed (SNIPCL) rather than open, as this technique yields systematically higher and more reliable values that correlate better with PImax. 3
• This test is particularly useful when patients have difficulty maintaining a proper seal around the mouthpiece or in clinical evaluation of respiratory muscle strength. 1

Advanced Diaphragm-Specific Testing

Transdiaphragmatic Pressure During Sniff (Pdi,sn)

• Pdi,sn values greater than 100 cm H2O in males and 80 cm H2O in females are unlikely to be associated with clinically significant diaphragm weakness. 1
• Normal mean values are 148 ± 24 cm H2O for men and 121 ± 25 cm H2O for women. 1, 2
• This requires esophageal and gastric balloon catheters and is recommended only as a research tool or in specialized respiratory muscle function laboratories. 1

Critical Methodological Considerations

Equipment and Technique

• A flanged mouthpiece is preferred over a tube mouthpiece, as it provides more consistent measurements and reduces the influence of cheek muscles. 1
• The measurement system must include a valve allowing normal breathing followed by maximal maneuver, with the small leak preventing pressure generation by facial muscles. 1
• Measurements are critically dependent on lung volume and muscle length; PImax should be measured at RV where inspiratory muscles are at optimal length. 2

Reproducibility and Interpretation

• PImax and PEmax are highly dependent on patient effort and have significant learning curves, with average coefficients of variation around 25%. 4
• Multiple measurements (at least 5 in children, minimum 3 in adults) are recommended due to learning effects and variability. 2
• Using single tests tends to overdiagnose weakness; combining PImax and SNIP reduces false-positive diagnoses of inspiratory muscle weakness by approximately 19%. 5

Clinical Applications and Pitfalls

When to Test

• Respiratory muscle testing should be performed in patients with unexplained dyspnea, neuromuscular disorders, suspected diaphragm weakness, or those at risk for respiratory failure. 1, 6
• Serial measurements are valuable for monitoring disease progression and response to therapy in neuromuscular diseases. 1, 6

Common Pitfalls to Avoid

• Do not rely solely on vital capacity, as it is a non-specific and relatively insensitive measure that may miss moderate inspiratory muscle weakness. 4, 6
• Avoid testing during acute illness or when patients are breathless, as submaximal efforts are most likely in these circumstances. 4
• Be aware that PImax may underestimate weakness in patients with bulbar involvement; SNIP may be more reliable in this population. 6
• Do not interpret isolated low values without considering the wide normal range reflecting natural variation in muscle strength between individuals. 1, 2

Complementary Assessments

• Measure supine vital capacity to improve detection of diaphragmatic involvement, as the supine position stresses the diaphragm. 6
• Consider cough peak expiratory flow (cough PEF) to assess airway clearance capability; values below 160-270 L/min suggest compromised cough efficiency. 6
• Arterial blood gases typically remain normal until severe weakness develops (respiratory muscle strength <40% predicted, vital capacity <50% predicted). 1