Respiratory muscle strength can be assessed by measuring the maximal inspiratory pressure (MIP, PImax, or negative inspiratory force [NIF]) and the maximal expiratory pressure (MEP or PEmax). The MIP reflects the strength of the diaphragm and other inspiratory muscles, while the MEP reflects the strength of the abdominal muscles and other expiratory muscles. An alternative or additional test of inspiratory muscles strength is maximal sniff nasal inspiratory pressure (SNIP) (discussed below). Common indications for measurement of the MIP, SNIP, and MEP include:
- Respiratory muscle weakness is suspected, such as a patient with dyspnea, weak cough, and known neuromuscular disease
- Spirometry detected a pattern of restriction: reduced forced expiratory volume in one second (FEV1), reduced forced vital capacity (FVC), normal FEV1/FVC ratio, and reduced total lung capacity (TLC) and respiratory muscle weakness is in the differential diagnosis
- Spirometry detected a low vital capacity (VC) of unknown etiology and respiratory muscle weakness is in the differential diagnosis
- Spirometry detected a low maximal voluntary ventilation (MVV) maneuver of unknown etiology and respiratory muscle weakness is in the differential diagnosis
- Evaluation of whether known respiratory muscle weakness has improved, remained stable, or worsened
- To determine whether there is an increased risk of incident mobility disability in older individuals, hospitalizations and death in patients with COPD, mortality in patients with heart failure [1-4]
Measurement, interpretation, quality assurance, and clinical applications of the MIP and MEP are discussed in this topic review. Alternative tests of respiratory muscle strength, particularly SNIP, are also mentioned. Assessments of other aspects of respiratory function (eg, airflow, lung volumes, gas exchange) are described separately. (See "Overview of pulmonary function testing in adults" and "Diffusing capacity for carbon monoxide" and "Reference values for pulmonary function testing" and "Flow-volume loops" and "Office spirometry".)
Measurement of the MIP and MEP can be made with a mechanical pressure gauge that is connected to a mouthpiece (picture 1). However, electronic devices are widely available, which may be hand-held or connected to a computer (figure 1) [5-7]. The device should contain a small hole (1 mm diameter and 20 to 30 mm in length), which allows an air leak. This prevents the patient from generating pressure by using their cheek muscles .
Maximal inspiratory pressure (MIP) — Place a rubber mouthpiece with flanges on the device. Instruct the patient to seal his or her lips firmly around the mouthpiece, exhale slowly and completely, and then "pull in hard, like you are trying to suck up a thick milkshake." Demonstrate the maneuver and have the patient repeat it. The patient should maintain inspiratory pressure for at least 1.5 seconds and the largest negative pressure sustained for at least 1 second (not a transient spike) should be recorded . These durations are estimated by the individual supervising the test. Allow the patient to rest for about one minute and then repeat the maneuver five times. Provide verbal or visual feedback after each maneuver. The goal is for the variability among measurements to be less than 10 cm H2O . Measurements should be rounded to the nearest 5 cm H2O.