Applications

See Table 2.

Single fiber and motor unit analysis. Single fiber and motor unit signal analyses are useful for diagnosis of nerve or muscle pathology. For the diagnosis of neuromuscular disease, limb muscles are more commonly studied than respiratory muscles, because they are more readily accessible. A number of investigators have demonstrated the usefulness of needle electromyo-graphy of the diaphragm for the diagnosis of neuromuscular disease, particularly neuropathic processes such as Guillain-Barre syndrome, lower motor neuron involvement with spinal cord injury, and polyneuropathy of critical illness (31, 32, 34).

Bolton (34) has pointed out that the relatively high-frequency, low-amplitude potentials of the normal diaphragm are often difficult to differentiate from myopathic potentials. Nevertheless, several neuromuscular diseases present primarily with respiratory muscle weakness; as experience is gained with single fiber and motor unit analysis of respiratory muscles, these techniques applied to respiratory muscles may provide the earliest evidence of the neuro- or myopathic process.

TABLE 2. APPLICATIONS FOR

RESPIRATORY MUSCLE ELECTROMYOGRAMS

Type of Test

Condition

Finding

Needle EMG

Denervation

iMUAPs, fibrillation potentials, and

positive sharp waves

Demyelination

iMUAPs, without potentials

Chronic denervation

iNo., tsize of MUAPs

Myotonia

Myotonic discharges

Myopathy

Short, polyphasic potentials

Interference pattern EMG signal

Paralysis or dyscoordination

Respiratory muscle activation pattern

Quantification of neural drive

Changes in FRA or RMS

Efficiency of contraction

APdi/AEdi

Fatigue

Spectral analysis

Definition of abbreviations: APdi/AEdi = ratio of tidal respiratory change in transdiaphragmatic pressure to tidal respiratory change in integrated diaphragm EMG; EMG = electromyography; FRA = full wave rectified and averaged signal; MUAP = motor unit action potential; RMS = root mean square of the signal.

Definition of abbreviations: APdi/AEdi = ratio of tidal respiratory change in transdiaphragmatic pressure to tidal respiratory change in integrated diaphragm EMG; EMG = electromyography; FRA = full wave rectified and averaged signal; MUAP = motor unit action potential; RMS = root mean square of the signal.

Interference pattern signal. The interference pattern EMG (raw EMG from surface electrodes) of respiratory muscles is useful for the determination of the timing and level of muscle activation during respiratory activities. Thus, EMGs can help to determine which of the many respiratory muscles are active in various phases of respiration, in various body positions, in various states of consciousness, and in various clinical conditions (see Electromyography in Section 8 of this Statement). Specifically, the absence of voluntary or involuntary EMG activity can be used as evidence of paralysis of specific respiratory muscles. EMGs can also help to quantify the respiratory muscle activation responses to loaded breathing and to co2-stimulated breathing or to monitor and control mechanical ventilation (58, 59). Furthermore, when related to pressure or force developed by respiratory muscles, EMGs can help to assess the electromechanical "efficiency" of respiratory muscle function (see Electromechanical Effectiveness in Section 6 of this Statement).

Interindividual comparisons of absolute FRA or RMS values do not meaningfully reflect respiratory drive, because of varying filtering influences of electrode placement relative to the contracting muscles and/or anatomic differences between subjects, for example, the amount and type of interlaying tissue. However, changes in these indices in response to interventions such as changes in inspired co2 concentration, loaded breathing, exercise, states of consciousness, drugs, or other influences, reflect changing motor output of the central nervous system (CNS) to respiratory muscles. Because some respiratory muscles are silent during quiet breathing, it is not practical to normalize respiratory muscle EMG activity to that observed during resting tidal breathing. It is often more practical to normalize EMG activity to that observed during sniff inhalations or maximal inspiratory efforts (22).

The interference pattern EMG of respiratory muscles may also be useful for the assessment of respiratory muscle fatigue (see Electromyography in Section 5 of this Statement). Localized muscle fatigue is accompanied by a reduction in muscle fiber action potential conduction velocity (1), which is reflected in the diaphragm EMG power spectrum as a shift toward lower frequencies (41, 52). This "spectral shift" is most commonly quantified as a reduction in fc. Spectral shifts have been detected in healthy subjects during inspiratory resistive breathing (38, 60), in patients with weak inspiratory muscles constrained to breathe with prolonged inspiratory duty cycles (61) or with exertion-induced inspiratory muscle overload (62), and in patients with chronic obstructive pulmonary disease during exertion (43), and they are associated with changes in respiratory effort sensation (63). The EMG frequency spectrum is often influenced by other factors, such as the signal-to-noise ratio, electrode position, and recruitment of muscles that potentially contribute to cross-talk. Technical factors affecting the EMG power spectrum must always be taken into account before any physiologic interpretations of a spectral shift can be made (1, 15-19, 41-48).

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