The purpose of this Section is to describe the tests used to assess respiratory muscle strength. To test strength, pressures can be measured either during voluntary maneuvers or during involuntary contractions, particularly in response to phrenic nerve stimulation.
A. Volitional Tests of Respiratory Muscle Strength: Volitional tests are often simple for patients to perform, but it can be difficult to be certain that a maximum effort has been made. This can lead to difficulty in the interpretation of low results.
1. Maximum Static inspiratory and Expiratory Pressure i. Pimax and PEmax are commonly used, clinically useful measurements. Some individuals have difficulty with the technique and the interpretation of low results can be problematic.
ii. Maximum static transdiaphragmatic pressure (Pi,di,max) provides specific information on diaphragm strength, but can be a difficult maneuver in naive subjects and patients. Pi,di,max has a wide normal range and has limited usefulness in clinical practice.
2. Maximum Sniff Pressures: Maximum sniff efforts can be achieved by patients with little practice; sniff pressures are reproducible and have a narrower normal range than static mouth pressures or Pi,di,max. Sniff esophageal pressure assesses global inspiratory muscle strength and sniff Pdi is a clinically useful measure of diaphragm strength. Sniff nasal pressure provides a useful noninva-sive measure of inspiratory muscle strength and has been validated in patients with neuromuscular disease.
3. Maximum Cough Pressure: Cough gastric pressure is measured as an index of abdominal muscle strength. Pga,co is a useful test to supplement PEmax, particularly in patients unable to perform the PEmax maneuver reliably. To date, few data are available for normal values of Pga,co.
B. Nonvolitional Tests of Respiratory Muscle Strength
1. Phrenic Nerve Stimulation: Phrenic nerve stimulation is specific for the diaphragm and is not influenced by the central nervous system.
i. Electrical phrenic nerve stimulation (ES) can achieve selective supramaximal stimulation of the diaphragm, but requires considerable skill, is sometimes uncomfortable for patients, and is difficult to achieve in some clinical settings (e.g., the ICU).
ii. Magnetic phrenic nerve stimulation (MS) is technically easier for the operator and less uncomfortable for the patients. Cervical magnetic stimulation (CMS) elicits a bilateral diaphragm contraction. CMS is less specific than ES, and coactivates muscles innervated by the brachial plexus. Achieving and confirming supramaximal nerve stimulation can be difficult, and recording the diaphragm EMG can pose problems. Unilateral anterior-lateral MS is more specific than CMS, and results are similar to ES. Unilateral MS allows the investigation of hemidiaphragm function, and bilateral anterior-lateral MS reliably achieves supramaximal stimulation.
iii. Twitch transdiaphragmatic pressure (Pdi,tw) provides an index of diaphragm, or hemidiaphragm, strength. Normal values are available and Pdi,tw is a useful clinical measurement. Twitch mouth pressure (Pmo,tw) can provide a noninvasive measure of diaphragm strength, but inadequate transmission of alveolar pressure to the mouth, in patients with airway obstruction or when there is glottic closure, is a substantial practical problem limiting clinical application.
iv. Pdi,tw is a research technique useful to assess the degree of activation of the diaphragm during voluntary efforts (the technique of twitch occlusion) and Pdi,max can be estimated from submaximal efforts.
2. Abdominal Muscle Stimulation: Magnetic stimulation over the eighth to tenth thoracic vertebrae posteriorally, and the recording of twitch gastric pressure, provide a clinically applicable nonvolitional test of abdominal muscle strength. Data on normal values for twitch gastric pressure are limited.
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