Background: Respiratory muscle weakness is an important clinical problem. Tests of varying complexity and invasiveness are available to assess respiratory muscle strength. The relative precision of different tests in the detection of weakness is less clear, as is the value of multiple tests.
Methods & Patients: We analysed the respiratory muscle function tests of clinical referrals who had multiple tests assessed in our laboratories over a 6 year period. Thresholds for weakness for each test were determined from published and in-house laboratory data. The patients were divided into three groups; those who had all relevant measurements of global inspiratory muscle strength (group A, n=182), those with full assessment of diaphragm strength (group B, n=264), and those for whom expiratory muscle strength was fully evaluated (group C, n=60). We studied the diagnostic outcome of each inspiratory, diaphragm and expiratory muscle test, both singly and in combination, and calculated the impact of using more than one test to detect weakness.
Results: The clinical referrals were primarily for the evaluation of neuromuscular diseases and dyspnoea of unknown cause. A low maximal inspiratory mouth pressure (PImax) was recorded in 40.1% of referrals in group A, while a low sniff nasal pressure (Sniff Pnasal) was recorded in 41.8% and a low sniff oesophageal pressure (Sniff Poes) in 37.9%. When assessing inspiratory strength with the combination of all three tests 29.6% of patients had weakness. Using the two non-invasive tests, PImax and Sniff Pnasal, in combination we obtained a similar result (low in 32.4%). Combining Sniff Pdi (low in 68.2%) and Twitch Pdi (low in 67.4%) reduced the diagnoses of patients with diaphragm weakness to 55.3% in group B. 38.3% of the patients in group C had expiratory muscle weakness as measured by PEmax, compared to 36.7% when weakness was diagnosed by cough Pgas, and 28.3% when assessed by Twitch T10. Combining all three expiratory muscle tests reduced the number of patients diagnosed as having expiratory muscle weakness to 16.7%.
Conclusion: The use of single tests, such as PImax, PEmax and other available individual tests of inspiratory, diaphragm and expiratory muscle strength, tend to overdiagnose weakness. Combinations of tests increase diagnostic precision, and in the population studied they reduced the diagnosis of inspiratory, specific diaphragm, and expiratory muscle weakness by 19 – 56%. Measuring both PImax and Sniff Pnasal resulted in a relative reduction of 19.2% of patients falsely diagnosed with inspiratory muscle weakness. The addition of Twitch Pdi to Sniff Pdi increased diagnostic precision by a smaller amount, 18.9%. Having multiple tests of respiratory muscle function available both increases diagnostic precision, and makes assessment possible in a range of clinical circumstances.
- Respiratory Function
- Respiratory Muscle Tests
- Respiratory Muscle Weakness