Version May 2024
INTRODUCTION — Many chronic respiratory diseases and neuromuscular disorders are associated with respiratory muscle weakness (table 1). Respiratory muscle strength can be assessed using several methods, most of which are noninvasive, although a small proportion of patients require additional invasive testing.
Measurement and interpretation of the three major tests of respiratory muscle strength, maximal inspiratory pressure (PImax), maximal expiratory pressure (PEmax), and sniff nasal inspiratory pressure (SNIP) are discussed in this topic review. Assessments of other aspects of pulmonary function (eg, airflow, lung volumes, gas exchange) and evaluation of patients with suspected respiratory muscle weakness are described separately.
●(See "Selecting reference values for pulmonary function tests" and "Overview of pulmonary function testing in adults".)
●(See "Selecting reference values for pulmonary function tests" and "Overview of pulmonary function testing in adults".)
●(See "Flow-volume loops".)
●(See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation".)
When performing tests of respiratory muscle strength, we adhere to the standards set out by the American Thoracic Society and the European Respiratory Society [1,2].
INDICATIONS — Common indications for measurement of respiratory muscle strength include:
●Patients with suspected respiratory muscle weakness (eg, patient with known neuromuscular disease with a weak cough, unexplained dyspnea or dyspnea on exertion, and/or orthopnea). (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Respiratory muscle strength testing' and "Diagnostic evaluation of adults with bilateral diaphragm paralysis", section on 'Clinical manifestations' and "Diagnosis and management of nontraumatic unilateral diaphragmatic paralysis (complete or partial) in adults", section on 'Clinical manifestations'.)
●Patients with lung function tests that show reduced vital capacity (VC). (See "Approach to the patient with dyspnea", section on 'Pulmonary function tests' and "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Pulmonary function testing' and "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Differential diagnosis'.)
●Patients with known respiratory muscle weakness to monitor improvement, stability, or deterioration. (See "Respiratory muscle weakness due to neuromuscular disease: Management", section on 'Treatment of the underlying disorder' and "Respiratory muscle weakness due to neuromuscular disease: Management", section on 'Physiologic evidence of respiratory muscle weakness'.)
MAXIMAL INSPIRATORY AND EXPIRATORY PRESSURE
Devices and attachments — Measurement of the maximal inspiratory pressure (PImax) and maximal expiratory pressure (PEmax) can be performed using devices that are widely available, some of which are self-contained handheld devices (figure 1) and others that are connected to a computer for data acquisition [3-5]. Devices can be simple manually operated devices with a separate pressure transducer, or automated electronic devices integrating all components.
●Side port ─ The device should contain a side port that allows a small air leak to prevent the patient from generating pressure by using their cheek muscles (eg, a hole in the device that is 2 mm wide and 20 to 30 mm long) [6].
●Patient-device interface ─ The patient-device interface can be any of the following:
•Flanged mouthpiece ─ A sterile rubber flanged mouthpiece, which can be sterilized between patients, is commonly used due to its ease of use by patients and the ability to form a leak-free seal (figure 1).
•A facemask seal ─ A facemask seal, similar to that used for noninvasive ventilation, can be used for individuals who cannot obtain a tight seal with the lips (picture 1).
•A tube interface ─ Large-diameter stiff rubber tubing can be used for patients capable of generating large pressures who rupture the seal with the lips at the mouthpiece during testing, especially during the PEmax maneuver (picture 2)
●Transducer ─ The pressure generated against an occlusion during inspiration or expiration (ie, the PImax and PEmax, respectively) is measured using a suitable pressure transducer (ideally electronic transducer) that is part of the device or connected to the device via a narrow-gauge catheter. The output from the transducer is displayed on the device itself or on a computer. Units of pressure are measured in centimeters of water (cm H2O).
Technique — The technique required for testing PImax and PEmax measures the pressure generated against an occluded airway during maximal inspiration or expiration, respectively. During all maneuvers, patient cooperation and effort should be noted and recorded. Obtaining a tight seal with the lips is important. (See 'Poor patient effort, technique, and repeatability' below and 'Poor patient-device interface' below.)
Maximal inspiratory pressure (PImax) — The maximal inspiratory pressure (PImax, sometimes also known as MIP) reflects the strength of the diaphragm and other inspiratory muscles (eg, rib cage, sternocleidomastoid). (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Muscles of respiration and cough'.)
To achieve a maximum value, it is important to explain and demonstrate the maneuver to the patient before and during the procedure. The patient should be seated and wearing a nose clip (unless an oronasal mask is used).
●We instruct the patient to take a few tidal breaths through the mouthpiece or mask and then exhale slowly and completely (ie, to residual volume). We next instruct the patient to breathe in as hard as possible. The patient should maintain the inspiratory effort for at least 1.5 seconds. The largest negative pressure sustained for at least one second (not a transient spike) should be recorded (figure 2) [1,2].
●We allow the patient to rest for approximately one minute and repeat the maneuver five times providing verbal or visual feedback after each maneuver. Sometimes additional maneuvers are required (eg, if the last measurement was the highest for the test session or if the second highest measurement is not at least 90 percent of the highest measurement). The goal is for the variability among maximum value measurements to be less than 10 cm H2O [7]. (See 'Poor patient effort, technique, and repeatability' below.)
●We report the maximum value achieved across all efforts (not a mean value) [1]. (See 'Interpretation (PImax, PEmax, SNIP)' below.)
Maximal expiratory pressure (PEmax) — The maximal expiratory pressure (PEmax, also known as MEP) reflects the strength of the abdominal muscles and other expiratory muscles. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Muscles of respiration and cough'.)
We explain and demonstrate the maneuver to the patient before and during the procedure. The patient is seated and wearing a nose clip (unless a facemask is used).
●We instruct the patient to take a few tidal breaths through the mouthpiece or mask. We next instruct the patient to take a maximal inspiration through the mouthpiece or mask (ie, to total lung capacity) and then blow out as hard as possible, similar to inflating a very stiff balloon. The patient should maintain expiratory effort for at least 1.5 seconds and the largest positive pressure sustained for at least one second (not a transient spike) should be recorded (figure 2) [1,2].
●We allow the patient to rest for about one minute and then repeat the maneuver five times, providing verbal or visual feedback after each maneuver. Sometimes additional maneuvers are required. The goal is for the variability among maximum value measurements to be less than 10 cm H2O [7]. (See 'Poor patient effort, technique, and repeatability' below.)
●We report the maximum value achieved across all efforts (not a mean value). (See 'Interpretation (PImax, PEmax, SNIP)' below.)
SNIFF NASAL INSPIRATORY PRESSURE (SNIP) — The maximal sniff nasal inspiratory pressure (also called the sniff nasal force [SNIF]) is an indicator of inspiratory muscle strength. Because a mouthpiece is not used, SNIP is particularly helpful for patients with facial weakness who cannot obtain a tight seal around the mouthpiece for the maximal inspiratory pressure (PImax) test [1]. For similar reasons, SNIP is also useful for children (over 5 years of age) [8,9].
However, SNIP is less reliable in individuals (adults and children) with occluded nostrils or upper airway obstruction (eg, nasal polyps or edema) since an adequate sniff maneuver can only be achieved if the contralateral nostril is not obstructed and allows the free passage of air. SNIP is also not suitable for patients with hyperinflation due to obstructive airways disease due to reduced transmission of pressure from the chest to the nose [1,10-12].
Device — Measuring SNIP involves a nasal bung, which is inserted into one nostril. The bung is connected by a catheter to a suitable pressure transducer (figure 1) that is connected to a readout system.
Different sized sniff bungs are available commercially, although suitable devices can be constructed using fine bore tubing and rubber bungs or dental impression putty.
Technique — SNIP measures the pressure generated by maximal inhalation through one patent nostril. The sniff maneuver involves a coordinated contraction of the diaphragm and other inspiratory muscles which generally produces pressures in excess of those obtained during a maximal inspiratory pressure (PImax) maneuver [13]. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Muscles of respiration and cough'.)
The sniff nasal inspiratory pressure test is performed using the following technique:
●We insert a bung attached to the pressure transducer into one nostril (completely obstructing flow through that nostril).
●We instruct the patient to sniff as strongly as possible through the contralateral unobstructed nostril.
●We record the peak negative pressure (in the obstructed nostril).
●We allow the patient to rest for one minute and repeat the sniff maneuver until a maximal peak negative pressure is obtained (less than 10 cm H2O variation); this may require up to 10 efforts [10]. Since the test involves a simple natural maneuver it is relatively easy for patients to achieve maximal efforts with a little practice. Thus, less coaching is generally needed in between repeated attempts when compared with the PImax maneuver.
●We report the maximum value across all efforts (not a mean value). Patient cooperation and effort should be noted. (See 'Interpretation (PImax, PEmax, SNIP)' below.)
ADDRESSING SOURCES OF ERROR — The validity of maximal inspiratory pressure (PImax), sniff nasal inspiratory pressure (SNIP), and maximal expiratory pressure (PEmax) are directly related to the quality of the test from which they were measured. In patients with low-normal or low values, potential sources of error that should be considered include patient effort and technique, the patient-device interface, and equipment issues.
Poor patient effort, technique, and repeatability — Patient effort and technique are probably the most common causes of error for low PImax and PEmax. As both tests require volitional effort, we report the degree of patient cooperation and effort alongside the test results.
We suggest the following to minimize the impact of poor effort and technique:
●Experienced technician - Since measurements of respiratory muscle strength are often unfamiliar maneuvers, only an experienced technologist should supervise testing. Such an individual should provide careful instructions as to the test procedure and provide encouragement and motivation along with feedback on test performance.
●Grading effort and repeatability among multiple efforts - We grade the quality of each test according to patient effort and repeatability (table 2). In this system, excellent effort and repeatability (<5 cm H2O difference among measurements) receive the highest quality grade, while poor effort or poor repeatability (>20 cm H2O difference among measurements) receives the lowest grade. In general, 80 to 90 percent of healthy adults achieve less than 10 cm H2O difference among measurements [14,15].
Many clinical laboratories perform only five maneuvers for the PImax and PEmax each. However, in our experience, the best values during a test session are sometimes obtained after more than five efforts (eg, up to 15) [7,16]. In addition, it is prudent that up to three additional maneuvers be performed if the last measurement was the highest for the test session, or if the second highest measurement is not at least 90 percent of the highest measurement.
Poor patient-device interface — Error can be due to an inadequate patient-device interface.
For PImax and PEmax maneuvers, the patient needs to form a tight seal with their lips around the mouthpiece of the device (the SNIP maneuver is less affected by the patient-device interface since a seal with the lips is not needed). However, some patients with orofacial muscle weakness are not able to obtain a good seal with the lips. For such patients, we suggest the following:
●The patient may be able to use their hands to press their lips around the mouthpiece during each maneuver [17].
●Alternatively, the technologist or physician can press the patient's lips against the mouthpiece to obtain a good seal.
●A facemask interface (picture 1) or a tube interface (ie, a wide diameter rubber tube pressed firmly against the face around the lips (picture 2)) can be substituted for the mouthpiece. (See 'Devices and attachments' above.)
●The sniff nasal inspiratory pressure can be used instead of the PImax. (See 'Devices and attachments' above and 'Sniff nasal inspiratory pressure (SNIP)' above.)
For patients who break the seal of a standard mouthpiece or facemask due to high pressures during the maneuver (more commonly seen in patients without respiratory muscle weakness), a tube interface can be used. The latter is based on several small observational studies which demonstrated that patients without respiratory muscle weakness frequently broke their seal (particularly during expiratory efforts) with a standard mouthpiece or facemask, but not a tube interface [18,19].
Equipment error — We adhere to guidelines describing the appropriate characteristics for equipment required to measure respiratory muscle function [1,2]. Potential sources of error include the following:
●Type of pressure gauge and display ─ Electronic devices with either a self-contained digital display or computer connection are preferable. A display of the pressure waveform produced during the maneuver is also helpful as visual inspection can help with quality control and ensure correct performance of the maneuver (picture 3). This can be of particular relevance for SNIP when a short, sharp effort is required.
Our rationale for electronic devices is that the dial on mechanical devices can be difficult to read accurately, particularly if there are marked fluctuations in generated pressure.
●Transducer performance ─ A sensitive transducer with a resolution of at least 0.5 cm H2O, a range 300 cm H2O, and a flat frequency response up to 15 Hz is required. Validation of equipment performance characteristics should be performed with the appropriate tubing and any additional fittings connected as these can significantly affect transducer performance. This is particularly important for SNIP measurement since rapid pressure changes occur during the maneuver.
●Sampling frequency ─ For noncommercial systems that are assembled “in house” by the pulmonary function laboratory, the pressure should be recorded using a sample frequency of at least 100 Hz. Most commercially available devices have this feature built into them, eliminating operating characteristics as a source of error.
●Calibration ─ Regular verification of system accuracy should be performed by comparison with a manometer [20]. Systems should be calibrated before use and at regular intervals depending on how frequently tests are performed (eg, monthly if tests are performed more than once a month).
INTERPRETATION (PIMAX, PEMAX, SNIP) — Interpreting tests of respiratory muscle strength requires knowledge of the following:
●Normal reference ranges. (See 'Data interpretation (comparison with the reference range)' below.)
●The clinical context (eg, diagnostic, prognostic). (See 'Clinical interpretation' below.)
Data interpretation (comparison with the reference range) — Maximal inspiratory pressure (PImax), sniff nasal inspiratory pressure (SNIP), and maximal expiratory pressure (PEmax) are interpreted by comparing values to a reference range derived from patients with normal respiratory muscle strength. However, reference ranges are poorly defined and are wide-ranging, leading to inaccuracy, especially when the result falls within the lower limit of normal or just below normal.
Reference ranges for PImax, PEmax and SNIP have been published according to sex and age, since values decline with age and are lower in women than in men (table 3 and table 4) [6,21]. Reference ranges for SNIP are also available for children [12].
When interpreting data from tests of respiratory muscle strength we suggest the following approach:
●Mid- to high-normal range or above the upper limit of normal ─ Values that lie in the mid- to high-normal range or above the upper limit of normal reliably exclude clinically significant respiratory muscle weakness (ie, good negative predictive value).
●Lower quarter of the normal range or below the lower limit of normal ─ Values that lie within the lower quarter of the normal range or below the lower limit of normal (LLN) may reflect respiratory muscle weakness but may also reflect other conditions (ie, poor positive predictive value) [22,23]. This includes the following:
•Poor effort ─ Poor effort is one of the most common reasons for low values and is generally commented upon by the technician. (See 'Poor patient effort, technique, and repeatability' above.)
•Technical factors ─ Technical factors can also contribute to low values. These include suboptimal technician coaching, poor patient-device interface, and equipment errors, which are discussed above. (See 'Addressing sources of error' above.)
•Lung hyperinflation ─ Patients with lung hyperinflation due to severe airway obstruction may generate a low PImax despite normal inspiratory muscle strength [24-27]. This is due to shortening of diaphragmatic muscle fibers and the mechanical disadvantage conferred by lung hyperinflation [28]. In such patients expectations of PImax should be adjusted downward as lung hyperinflation increases (figure 3) [24].
•General factors ─ General factors such as older age, weak handgrip strength, malnutrition, poor health, low physical activity, and short stature have all been associated with low values [5,14,15]. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Differential diagnosis'.)
Thus, low values do not reliably confirm respiratory muscle weakness and the diagnosis should be supplemented with other findings, repeat testing, and/or invasive testing. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Diagnosis' and 'Additional tests (invasive)' below.)
Clinical interpretation — Respiratory muscle strength testing plays a key role in the diagnosis, prognosis, and follow-up of patients with respiratory muscle weakness.
Diagnosis — In patients with suspected respiratory muscle weakness, the PImax, PEmax, and SNIP are considered in conjunction with other measures, most commonly supine and upright vital capacity (VC) or forced vital capacity (FVC) for the diagnosis (or exclusion) of respiratory muscle weakness. Inconclusive studies may prompt additional investigations [29]. Further details regarding criteria for the diagnosis of respiratory muscle weakness are provided separately. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Diagnostic evaluation' and 'Additional tests (invasive)' below.)
The sensitivity and specificity of respiratory muscle strength tests have been compared with the VC for the diagnosis of respiratory muscle weakness:
●The PImax is more specific for respiratory muscle weakness than a fall in VC on supination [30,31], although the latter may be more specific for the detection of diaphragmatic weakness than the PImax [32,33].
●The VC falls late in progressive neuromuscular disease (eg, amyotrophic lateral sclerosis [ALS]) while the PImax falls earlier and correlates better with disease progression [34]. (See 'Clinical course and criteria for detecting change' below.)
●Both the PImax and VC have similar sensitivity (55 versus 53 percent) and specificity (83 versus 89 percent) for detecting hypercapnic respiratory failure [11].
Diagnostic accuracy for inspiratory muscle weakness is improved when PImax and SNIP are both measured. Although SNIP and PImax are both used to assess inspiratory muscle strength, the agreement between the results of these testing modalities is variable and hence these two tests should be regarded as complementary rather than interchangeable. In a study of 182 patients referred to a specialist laboratory for assessment of possible respiratory muscle weakness, 40 percent were diagnosed as weak by PImax, 42 percent by SNIP, and 32 percent by PImax plus SNIP but the combination of PImax and SNIP reduced the diagnosis of weakness by 19 percent compared to either test alone [29].
PImax, SNIP, and PEmax may help classify respiratory muscle weakness as primarily affecting the inspiratory muscles or expiratory muscles. As examples:
●A low PImax and SNIP but a normal PEmax suggests isolated inspiratory muscle weakness (usually diaphragmatic).
●A low PImax, SNIP, and PEmax suggests generalized skeletal muscle weakness (eg, ALS).
●A normal PImax and SNIP and low PEmax suggests isolated expiratory muscle weakness, although this is rare.
However, from a practical standpoint, this differentiation may not be helpful in determining the underlying cause of respiratory muscle weakness (eg, ALS versus myotonic dystrophy) since many neuromuscular disorders may have mixed involvement of respiratory and skeletal muscle weakness.
Severity and prognosis — The PImax, SNIP, and PEmax can be used to quantify the severity of respiratory muscle weakness and predict its clinical consequences. This is illustrated by the following observations:
●A PImax below one-third of normal predicts hypercapnic respiratory failure (PaCO2 >45 mmHg) [11].
●A SNIP that is 35 percent of normal predicts likely ventilatory failure in patients with ALS (figure 4) [11,35].
●A PEmax less than 60 cm H2O predicts a weak cough with difficulty clearing secretions [36,37].
●A reduced PImax is associated with increased mortality [38].
The PImax, SNIP, or PEmax usually decline before the symptoms or signs of respiratory muscle weakness emerge. This is due to the large reserve of respiratory muscle strength that normally exists, such that considerable muscle strength must be lost before symptoms or signs develop [39]. This is particularly true for sedentary patients without heart or lung disease. Such patients place minimal demand on their respiratory muscles and may not become symptomatic until they have severe respiratory muscle weakness [22].
The most powerful biomarker for mortality stratification for ALS is diaphragmatic strength as measured by phrenic nerve stimulation, but the predictive power of SNIP is also excellent, whereas VC shows little or no decline until 12 months before death or ventilation [40]. (See "Respiratory muscle weakness due to neuromuscular disease: Management", section on 'Chronic ventilatory support' and "Symptom-based management of amyotrophic lateral sclerosis", section on 'Respiratory function management'.)
Clinical course and criteria for detecting change — Serial measurements of the PImax, SNIP, and PEmax can be used to evaluate whether the respiratory muscle weakness has improved, remained stable, or worsened. This is particularly helpful for evaluating the response to therapy (eg, immunosuppressive therapy for myasthenia gravis or Guillain-Barré syndrome) and for planning an intervention in a progressive neuromuscular disease (eg, noninvasive or mechanical ventilation). (See "Respiratory muscle weakness due to neuromuscular disease: Management", section on 'Chronic ventilatory support' and "Symptom-based management of amyotrophic lateral sclerosis", section on 'Respiratory function management'.)
Detecting a "significant" physiologic change must be distinguished from normal test variation. We use a change of more than 25 cm H2O as the threshold to identify change of respiratory muscle strength that does not lie within a normal variance. The rationale for this criterion is based upon studies in healthy individuals, which demonstrate that 95 percent of normal test-to-test variation is less than 25 cm H2O [41,42]. However, it is ideal that each laboratory perform their own internal study using patients with neuromuscular disease to confirm this cutoff as appropriate in the population served. In addition, it is important to bear in mind that a “significant” physiologic change may or may not correlate with a clinically meaningful change in the patient’s respiratory symptoms [43-45].
Threshold values that may prompt mechanical ventilation are discussed separately. (See "Respiratory muscle weakness due to neuromuscular disease: Management", section on 'Chronic ventilatory support' and "Symptom-based management of amyotrophic lateral sclerosis", section on 'Respiratory function management'.)
ADDITIONAL TESTS (INVASIVE) — In most patients, the maximal inspiratory pressure (PImax), maximal expiratory pressure (PEmax) and sniff nasal inspiratory pressure (SNIP) are sufficient to make a diagnosis of respiratory muscle weakness when taken together with other clinical features. However, when the diagnosis of respiratory muscle weakness is in doubt, more invasive testing can be performed, depending on availability and need. These complex measurements are best made in specialized centers by a pulmonologist and/or neurologist.
Invasive testing includes the following:
●Sniff esophageal pressure ─ Sniff esophageal pressure is measured using a pressure catheter in the lower esophagus. Similar to SNIP, it uses the sniff maneuver to measure inspiratory muscle strength and is therefore suitable for patients with poor seal around the mouthpiece. However, unlike SNIP, sniff esophageal pressure is less likely to be inaccurate in patients with hyperinflation since there are no issues with the transmission of pressure from the thorax to the esophagus (sniff esophageal pressure) as opposed to transmission of pressure from the thorax to the nose (SNIP) [1,10-12,46]. (See 'Sniff nasal inspiratory pressure (SNIP)' above and 'Addressing sources of error' above.)
●Gastric pressure ─ Gastric pressure, measured with a pressure catheter in the stomach following maximal cough efforts, reflects volitional expiratory (particularly abdominal) muscle strength [36].
Nonvolitional abdominal (expiratory) muscle strength can also be measured by recording gastric pressure following surface magnetic stimulation posteriorly in the mid-line at the level of T10 [47].
Assessing cough strength is discussed separately. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Assessing cough strength'.)
●Tests of diaphragm strength ─ Several tests can be used to test diaphragm strength. These tests are discussed separately. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Diaphragmatic function' and "Diagnostic evaluation of adults with bilateral diaphragm paralysis", section on 'Specific diaphragmatic tests' and "Diagnosis and management of nontraumatic unilateral diaphragmatic paralysis (complete or partial) in adults", section on 'Sniff test'.)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Pulmonary function testing".)
SUMMARY AND RECOMMENDATIONS
●Indications ─ The three most common noninvasive tests that measure respiratory muscle strength include the following (see 'Introduction' above and 'Indications' above):
•Maximal inspiratory pressure (PImax)
•Maximal expiratory pressure (PEmax)
•Sniff nasal inspiratory pressure (SNIP)
The PImax and SNIP reflect the strength of the diaphragm and other inspiratory muscles, while the PEmax reflects the strength of the abdominal and other expiratory muscles. (See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Muscles of respiration and cough'.)
Common indications for their measurement include clinically suspected respiratory muscle weakness, lung function tests consistent with respiratory muscle weakness (eg, restrictive pattern, low vital capacity), and evaluation of whether respiratory muscle weakness has improved, remained stable, or worsened.
●Testing technique ─ The technique required for testing PImax and PEmax involves measuring the pressure generated against an occluded airway during maximal inspiration or expiration, respectively (figure 1). These tests are effort-dependent and obtaining a tight seal at the lips on the flanged mouthpiece is important for success. Several efforts are made with coaching and feedback provided in between each maneuver. (See 'Maximal inspiratory and expiratory pressure' above.)
The technique for SNIP involves measuring the pressure generated by maximal inhalation through one patent nostril. The sniff maneuver is a little easier than PImax or PEmax maneuver and is more suitable for patients who cannot obtain a tight seal with their lips on the mouthpiece. It is not suitable for patients who have upper airway obstruction (eg, nasal polyps) or hyperinflation from obstructive lung disease. (See 'Maximal inspiratory and expiratory pressure' above and 'Sniff nasal inspiratory pressure (SNIP)' above.)
●Interpretation ─ Test results are compared with normal reference ranges (table 3 and table 4) and should take into consideration potential errors associated with testing and the clinical context (eg, diagnostic, prognostic). (See 'Data interpretation (comparison with the reference range)' above.)
•Mid- to high-normal ─ Patients with values for PImax, SNIP, and/or PEmax that lie in the mid- to high-normal range or above the upper limit of normal generally do not have respiratory muscle weakness (ie, good negative predictive value).
•Low or low-normal ─ Test results that lie within the lower quarter of the normal range or below the lower limit of normal (LLN) may reflect respiratory muscle weakness but may also reflect poor effort or difficulty with performing the maneuvers (ie, poor positive predictive value). Other factors that can contribute to low values include lung hyperinflation, older age, weak handgrip strength, malnutrition, poor health, low physical activity, short stature, suboptimal technician coaching, poor patient-device interface, and technical errors. (See 'Addressing sources of error' above.)
Respiratory muscle strength testing plays a key role in the diagnosis of respiratory muscle weakness, determination of its severity, prediction of its clinical sequelae, evaluation of its clinical course, and need for intervention. Further details regarding criteria for the diagnosis and management of respiratory muscle weakness are provided separately. (See 'Clinical interpretation' above and "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Diagnostic evaluation' and "Respiratory muscle weakness due to neuromuscular disease: Management", section on 'Chronic ventilatory support'.)
●Additional tests ─ In most patients PImax, PEmax and SNIP are sufficient to make a diagnosis of respiratory muscle weakness when taken together with other clinical features and pulmonary function testing. When the diagnosis of respiratory muscle weakness is in doubt, additional (typically invasive) testing can be performed, depending on availability and need.
These include the sniff esophageal pressure (measures inspiratory pressure), gastric pressure (measures expiratory pressure), and tests of diaphragmatic strength. These tests generally require the insertion of a pressure-measuring catheter. Test of diaphragmatic strength are described separately. (See 'Additional tests (invasive)' above and "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation", section on 'Diaphragmatic function' and "Diagnostic evaluation of adults with bilateral diaphragm paralysis", section on 'Measurement of transdiaphragmatic pressure' and "Diagnostic evaluation of adults with bilateral diaphragm paralysis", section on 'Imaging studies' and "Diagnostic evaluation of adults with bilateral diaphragm paralysis", section on 'Diaphragmatic electromyography'.)
ACKNOWLEDGMENTS — The UpToDate editorial staff acknowledges Paul L Enright, MD and John Moxham, MD, who contributed to earlier versions of this topic review.
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