Chapter 5
Tests of respiratory muscle strength
Respiratory muscle strength can be measured via a variety of techniques of varying technical difficulty and degrees of invasiveness (1). This chapter discusses the most commonly performed tests of respiratory muscle strength in standard clinical respiratory laboratories, the measurement of maximal respiratory pressures.
The maximal inspiratory pressure (PImax or MIP) and maximal expiratory pressure (PEmax or MEP) are measurements of static pressure and reflect the maximum pressure generated by the respiratory muscles against an occluded airway and the elastic recoil pressure of the lung and chest wall (1, 2). In clinical practice, PImax is usually measured following maximal exhalation, at or close to residual volume (RV), and PEmax following maximal inhalation, at or close to total lung capacity (TLC). Subjects perform a maximal inspiratory or expiratory effort against an occluded mouthpiece (with a small leak to reduce buccal muscle use and glottic closure) (1). Occasionally PImax and PEmax may be measured from functional residual capacity (FRC) rather than TLC or RV. One study has shown that in healthy individuals, the differences in measured respiratory pressures at these different lung volumes are not clinically significant (3).
The sniff nasal inspiratory pressure (sNIP) is a more dynamic measure of inspiratory muscle strength. sNIP is a measure of the pressure generated via a maximal short, sharp inspiratory effort through an unobstructed nostril, generally from FRC. Pressure is measured via a catheter passed through a plug occluding the other nostril (1, 4).
Measures of maximal respiratory pressure assess global respiratory muscle function rather than specific muscles. The primary muscles used during active inspiration include the diaphragm, inspiratory intercostals, scalene and sternomastoid muscles. The abdominal wall muscles and expiratory intercostals are the primary muscles used during active expiration (2).
Test quality
Maximal respiratory pressures and sNIP are entirely effort-dependent tests. Poor volition, resulting in reduced values, may be due to submaximal effort or respiratory muscle dysfunction. Accurate technical comments regarding effort are crucial in this instance.
PImax and PEmax (1)
- Acceptability criteria
- Maximal effort.
- PImax—measured at or near the predetermined lung volume (RV or FRC).
- PEmax—measured at or near the predetermined lung volume (TLC or FRC).
- Must have tight lip seal.
- Support cheeks with hands during PEmax.
- Inspiratory and expiratory pressures must be maintained for at least 1.5 s, so that the mean pressure sustained over 1 s can be recorded.
- The peak pressure may be higher than mean pressure over the 1 s period; however, the peak pressure is believed to be less repeatable.
- Maximal effort.
- Repeatability criteria
- Minimum of three acceptable efforts varying less than 20%.
- Maximal respiratory pressures will be affected by effort (reduced), leak around the mouthpiece (reduced) and orofacial muscle use (potentially increased).
- The type of mouthpiece used (flanged versus straight) will impact on results, with flanged mouthpiece producing slightly lower results compared to the straight mouthpiece. This should be taken into account when choosing reference values.
sNIP (1, 5)
- Acceptability criteria
- Measured at the end of relaxed expiration (FRC).
- Maximal effort.
- Smooth rise and sharp peak on pressure tracing.
- Measured at the end of relaxed expiration (FRC).
- Repeatability criteria
- Minimum of 8–10 acceptable efforts, 3 highest within 10%.
- sNIP will be affected by effort, nasal obstruction and mouth leak.
Factors to consider during interpretation
- Respiratory muscles are used for many non-respiratory-related purposes (maintenance of posture, for example) and may have more strength than is required for respiration. Hence, despite there being deficits in maximal respiratory pressures, other aspects of lung function, such as vital capacity, may not be affected (1).
- Although a result in the normal range assists with excluding significant respiratory muscle dysfunction, an abnormal result may reflect poor test performance rather than reflecting true respiratory muscle weakness (1). Using multiple assessment methods for assessing respiratory muscle strength may be helpful in reducing the false positive rate for respiratory muscle weakness (6).
- PImax may be reduced in isolation where obstruction with hyperinflation (of TLC) is present (see Chapter 4). In this case, the flattened diaphragm is at a mechanical disadvantage to generate maximal pressures (1).
- sNIP may be affected by obstructive lung disease because of the lack of equilibration of pressures across the lung (1, 2).
- Although a result in the normal range assists with excluding significant respiratory muscle dysfunction, an abnormal result may reflect poor test performance rather than reflecting true respiratory muscle weakness (1). Using multiple assessment methods for assessing respiratory muscle strength may be helpful in reducing the false positive rate for respiratory muscle weakness (6).
Interpretation
Parameters used in interpretation are as follows:
- PImax—maximal inspiratory pressure (units: cmH2O or kPa)
- PEmax—maximal expiratory pressure (units: cmH2O or kPa)
- sNIP—sniff nasal inspiratory pressure (units: cmH2O or kPa)
Additional parameters from other tests that may assist in assessing respiratory muscle strength include the following:
- VC: measured in both upright and supine postures
- Static lung volumes: TLC, RV and RV/TLC
Limits of the normal range are as follows:
- For PImax, PEmax and sNIP, only abnormally low results are of interest. Therefore, the lower limit of normal (LLN) is set at a z-score of −1.64.
- Note: Reference values for maximal respiratory pressures (PImax, PEmax) are varied and normal ranges are wide (1).
- Clinically significant inspiratory or expiratory muscle dysfunction can probably be excluded for absolute values of PImax > 80 cmH2O or PEmax > 100 cmH2O, respectively (1, 2).
- Choose a reference set that used similar methodology to that used by the laboratory. The type of mouthpiece used makes a difference to values obtained (see Test Quality earlier).
Note: The normal ranges for sNIP are wide, possibly reflecting the wide range of normal muscle strength in individuals (1).
- Clinically significant inspiratory muscle weakness is unlikely to be present for absolute values of sNIP > 70 cmH2O (males) and >60cm H2O (females) (1). Keep in mind that sNIP represents the integrated pressure from all inspiratory muscles and there may be muscle weakness in individual muscles, which is unable to be detected using this test.
- Clinically significant inspiratory or expiratory muscle dysfunction can probably be excluded for absolute values of PImax > 80 cmH2O or PEmax > 100 cmH2O, respectively (1, 2).
- See Chapters 2 and 3 for limits for spirometry and static lung volumes.
Table 5.1 outlines an interpretation strategy for maximal respiratory pressures using the LLN. An alternative interpretation strategy is that clinically significant inspiratory or expiratory muscle dysfunction can probably be excluded for absolute values of PImax > 80 cmH2O or PEmax > 100 cmH2O, respectively (1, 2).
Table 5.1 Interpretative statements for measurement of respiratory muscle strength.
| PImax | PEmax | sNIP | Interpretationa |
| Within normal limits | Within normal limits | Maximal respiratory pressures are within normal limits, excluding clinically significant respiratory muscle dysfunction | |
| <LLN | <LLN | Maximal respiratory pressures are reduced. This may reflect global respiratory muscle dysfunctiona | |
| <LLN | Within normal limits | The maximal inspiratory pressure is reduced in the presence of normal maximal expiratory pressure. This finding suggests inspiratory muscle weaknessb ,c | |
| Within normal limits | <LLN | The maximal inspiratory pressure is within normal limits in the presence of a reduced maximal expiratory pressure. This finding suggests expiratory muscle weakness | |
| Within normal limits | Sniff nasal inspiratory pressure is within normal limits, excluding clinically significant inspiratory muscle weakness | ||
| <LLN | Maximal inspiratory pressure is reduced. This may reflect inspiratory muscle weaknessc |
a Interpretative statements for measurement of respiratory muscle strength are provided in Table 5.1. The interpretation assumes that the test is of good quality and maximal effort has been achieved. When a value is less than the LLN, then consider the quality of the test. The low result may be due to poor effort rather than a true low result.
b PImax may also be reduced in isolation in conditions where hyperinflation (of TLC) is present (see Chapter 3). In this case, the flattened diaphragm is at a mechanical disadvantage to generate maximal pressures, rather than the inspiratory muscles being weak.
c When both PImax and sNIP are reduced, the likelihood of inspiratory muscle weakness increases.
Other tests of respiratory function may also provide evidence of respiratory muscle dysfunction. However, the patterns seen are not specific for respiratory muscle dysfunction, and the results need to be interpreted in context with other findings and the clinical history of the subject. Examples include the following:
- A reduced VC is a common finding in significant respiratory muscle weakness.
- A reduced TLC and elevated RV/TLC (hence, reduced VC), particularly when there is no evidence of obstruction on spirometry (and often FRC is within the normal range), may indicate respiratory muscle dysfunction. The reduced TLC reflects the inability to fully inflate the lungs, and the elevated RV/TLC reflects the inability to fully exhale due to respiratory muscle weakness (1).
- A >30% fall in VC between upright and supine postures may suggest clinically significant diaphragm weakness (1).
- A reduced TLCO and VA may also occur in the presence of respiratory muscle weakness often with an elevated KCO (suggesting incomplete alveolar expansion), though this is not always the case (7).
Comparisons to previous results
The literature suggests that changes in PImax and PEmax > 21–29 cmH2O over weeks are considered clinically significant (in this book, changes >30 cmH2O are considered significant) (8, 9). Changes >23 cmH2O over a month are considered to be clinically significant for sNIP (8).
There is some evidence that change in PImax between visits may, in part, be due to training effect rather than a physiological change, and this should also be taken into consideration. This difference is not observed using sNIP (10).
Examples of interpretation of respiratory muscle strength
Respiratory muscle strength may be measured alone, but often is measured in conjunction with other tests. Interpretation is performed using the following steps as applicable:
- Check for requirements of cautionary statements is related to the following parameters:
- Reference values (are values appropriate for this subject? See Chapter 1 for details)
- Quality of test (read technical comments, check raw data if required).
- Reference values (are values appropriate for this subject? See Chapter 1 for details)
- Read clinical notes.
- Interpret parameters of lung function measured (e.g. spirometry, static lung volumes, gas transfer) with respect to standard patterns of abnormality and with respect to changes seen with respiratory muscle impairment.
- Interpret measures of respiratory muscle strength.
- Write technical interpretation (Table 5.1).
- Compare current results to previous results.
- Put results into clinical context.
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