Applications and Perspectives

The use of PNS-derived pressures to study the mechanical action of the diaphragm assumes that stimulation is bilateral and supramaximal, and that care is taken to control for lung volume and twitch potentiation.

Research applications. Bilateral ES is an important tool in human diaphragm research, used to study the properties of the diaphragm and the mechanisms of its fatigue independently of volitional influences. The twitch occlusion technique, as the only means to separate the peripheral and central components of diaphragm dysfunction, is also an important, if complex, research tool. The use of paired stimuli could facilitate research on the frequency characteristics of diaphragm fatigue (174).

Cervical magnetic stimulation provides slightly different information from ES, mainly because of rib cage muscle coacti-vation. However, its simplicity for the subject and the operator makes it a valuable tool for clinical research, especially in difficult settings, such as the ICU, or when repeated studies are needed, such as the evaluation of therapeutic interventions.

Clinical applications. Contraindications. There are virtually no contraindications to ES and MS, but their use requires some precautions. MS should not be performed in patients with a cardiac pacemaker. Patients with orthopedic implants, even metallic ones, can be studied with MS (e.g., candidates for phrenic pacing after high cervical cord injury) (198) after sufficient time for consolidation.

Clinical use. Pressure responses to PNS play an important part in the clinical evaluation of inspiratory muscle function. Diaphragmatic weakness can be established and quantified with PNS, particularly when voluntary maneuvers are equivocal. The use of MS with Pmo may be an even simpler nonvolitional measure.

The preoperative assessment of patients for possible phrenic pacing after high cervical cord lesions constitutes a particular application. PNS provides information on phrenic nerve con-

TABLE 4. TWITCH TRANSDIAPHRAGMATIC PRESSURES MEASURED IN A VARIETY OF STUDIES

First Author (Ref.)

Note

Bellemare (56) Aubier (118) Hubmayr (83) Mier (179)

Laghi (108) Similowski (109)

Similowski (131) Mancini (182)

Bilateral Supramaximal ES: Healthy Young Volunteers

8-33

Needle stimulation Implanted wire stimulation

15 control subjects referred for suspected diaphragm weakness; significant overlap between groups; Pdi,tw was discriminant only for diagnosis of severe diaphragm weakness Supine position

3 subjects; comparison between transcutaneous electrical stimulation and needle stimulation

Comparison CMS-ES

3 subjects; validation of a cervical apparatus aiming at standardizing PNS for repeated studies: data reproducible among 25 repeated studies Comparison CMS-ES Comparison CMS-ES

Bilateral Supramaximal ES: Older Healthy Volunteers

19.8-31.7 6 subjects aged 50-72 yr, serving as control subjects in a study of diaphragm properties in hyperinflated patients with COPD 18.8 6 subjects aged 50 ± 8 yr, serving as control subjects in a study of diaphragm function in chronic heart failure

Cervical Magnetic Stimulation: Healthy Young Volunteers

19.8-31.7 6 subjects aged 50-72 yr, serving as control subjects in a study of diaphragm properties in hyperinflated patients with COPD 18.8 6 subjects aged 50 ± 8 yr, serving as control subjects in a study of diaphragm function in chronic heart failure

Cervical Magnetic Stimulation: Healthy Young Volunteers

Similowski (100)

33.4

First-generation stimulator; supramaximality of PNS not

always certain

Wragg (107)

36.5

Comparison CM-ES

Wragg (l4é)

36.5

Study of diaphragm twitch potentiation; values in the table are

the unpotentiated ones

Laghi (184)

38.9

12 subjects

Hamnegard (129, 187)

17-34

Similowski (109)

27.5

Comparison CM-ES

Laghi (108)

37.7

Comparison CM-ES

Cervical Magnetic Stimulation: Older Healthy Volunteers

Polkey (190)

25.4

Patients with COPD

Similowski (131)

10-26

Bilateral ES

Wanke (183)

15.2

Bilateral ES

Polkey (134, 190)

18.2

CMS

Definition of abbreviations: CMS = cervical magnetic stimulation; COPD = chronic obstructive pulmonary disease; ES = electrical stimulation; Pdi,tw = twitch transdiaphragmatic pressure; PNS = phrenic nerve stimulation. Values represent means or ranges.

Definition of abbreviations: CMS = cervical magnetic stimulation; COPD = chronic obstructive pulmonary disease; ES = electrical stimulation; Pdi,tw = twitch transdiaphragmatic pressure; PNS = phrenic nerve stimulation. Values represent means or ranges.

duction time, which is important in making the decision to undertake pacing (198). it also provides an estimate of the degree of diaphragm disuse atrophy, which is an important determinant of the reconditioning strategy (199).

Given that methodological precautions are rigorously respected, PNS is one of the more reproducible respiratory muscle tests, and this makes it suitable for follow-up studies (see Phrenic Nerve Stimulation in Section 3 of this Statement).

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