Diagnostic methods to assess inspiratory and expiratory muscle strength

Abstract

Impairment of (inspiratory and expiratory) respiratory muscles is a common clinical finding, not only in patients with neuromuscular disease but also in patients with primary disease of the lung parenchyma or airways. Although such impairment is common, its recognition is usually delayed because its signs and symptoms are nonspecific and late. This delayed recognition, or even the lack thereof, occurs because the diagnostic tests used in the assessment of respiratory muscle strength are not widely known and available. There are various methods of assessing respiratory muscle strength during the inspiratory and expiratory phases. These methods are divided into two categories: volitional tests (which require patient understanding and cooperation); and non-volitional tests. Volitional tests, such as those that measure maximal inspiratory and expiratory pressures, are the most commonly used because they are readily available. Non-volitional tests depend on magnetic stimulation of the phrenic nerve accompanied by the measurement of inspiratory mouth pressure, inspiratory esophageal pressure, or inspiratory transdiaphragmatic pressure. Another method that has come to be widely used is ultrasound imaging of the diaphragm. We believe that pulmonologists involved in the care of patients with respiratory diseases should be familiar with the tests used in order to assess respiratory muscle function.Therefore, the aim of the present article is to describe the advantages, disadvantages, procedures, and clinical applicability of the main tests used in the assessment of respiratory muscle strength.

Keywords: Diagnostic tests, routine; Diaphragm; Muscle weakness; Respiratory function tests; Respiratory muscles.

Figure 1 -. Measurement of MEP and MIP with a digital pressure manometer (model MVD 300; Globalmed, Porto Alegre, Brazil). In A, positive MEP values. In B, negative MIP values.

Figure 2 -. Variation in inspiratory pressure during measurement of MIP with a one-way valve. The highest value usually occurs within 15 to 20 seconds.

Figure 3 -. Comparison between inspiratory mouth pressure (Pm) and inspiratory esophageal pressure (Pes) during mouth occlusion (Baydur maneuver), for ascertaining the correct location of the esophageal catheter. Note the good correlation between the two measurements.

Figure 4 -. Transdiaphragmatic pressure. The top curve represents esophageal pressure, the middle curve represents gastric pressure, and the bottom curve represents transdiaphragmatic pressure (Pdi). In this example, there are differences in the esophageal and gastric pressure measurement range.

Figure 5 -. Example of simultaneous recording of esophageal and gastric pressures during forced inspiration. Note that as esophageal pressure becomes more negative, gastric pressure becomes positive, creating a mirror image of the two curves.

Figure 6 -. Phrenic nerve magnetic stimulation coil placed on the anterior cervical area of a volunteer.

Figure 7 -. Ultrasound imaging of the diaphragm. In A, an ultrasound scan for assessment of diaphragm motion. The top image is a B-mode image, and the gray arrow indicates the diaphragm, which is seen as a more echogenic line. The bottom image is the top image in M-mode and serves to measure diaphragm excursion (distances between A-A and B-B points) during breathing at rest. In A, diaphragm motion was 19.5 and 18.1 mm and was, therefore, normal. In B, an ultrasound scan for assessment of diaphragm thickening. The top image is a B-mode image, and the white arrow indicated the diaphragm, which is seen as a more echogenic line. The bottom image is the top image in M-mode and serves to measure diaphragm thickening during inspiration (A-A points) and the next expiration (B-B points). In B, diaphragm thickening was 1.3 mm and was, therefore,