Scientific basis. An externally applied electrical field induces depolarization of phrenic nerve fibers, at mid-distance between the electrode and the cathode. If the stimulus is intense enough, all fibers are activated synchronously, giving predictable and reproducible results (see Stimulation Techniques in Section 3 of this Statement).
Methodology. For bilateral Es, the operator should stand behind the seated subject. The skin in the stimulated region is degreased and mildly abraded to decrease its electrical impedance, allowing lower current intensities. Monopolar or bipolar electrodes can be chosen (Figure 9). For monopolar electrodes, the anode is usually taped on the skin below the clavicle medially, and the cathode is held in the hand. Monopolar electrodes probably make it easier to find the nerve because a greater number of spots can be tested. However, because the electrical field is less focused than with bipolar electrodes, it may be more difficult to avoid costimulation of the adjacent sternocleidomastoid muscle or the brachial plexus. Bipolar electrodes are more specific but are also slightly more difficult to use. Various models of bipolar electrode are commercially available. They generally include felt tips 5 mm in diameter with an interelectrode distance of approximately 2 cm. When performing stimulation, the tips of the electrode should be soaked in saline and the cathode should be proximal.
The phrenic nerve is usually located beneath the posterior border of the sternocleidomastoid muscle, at the level of the cricoid cartilage (Figure 9). It is easier to locate and to isolate from the brachial plexus in subjects with long and slender necks. A simple technique to locate the nerve is to set the stimulator on repetitive stimulation mode, e.g., at a frequency of 1 Hz, with a relatively low intensity, and to try various sites. Identification of the correct site may be aided by careful observation of the abdomen; it will therefore be desirable to remove the shirt. once the nerve is identified, the operator marks the spot and the orientation of the electrode. current intensity is then increased, while monitoring the EMG to ob tain supramaximal stimulation (see subsequent paragraphs), which is generally achieved with 30- to 50-mA shocks. This procedure is performed on each side separately, before applying ES bilaterally. It is then advisable to reconfirm that stimulation is supramaximal. It is also possible to judge supramaxi-mality from the plateau of pressure response, but changes can be more difficult to interpret unless the plateau procedure is repeated to check that there is no loss of supramaximality.
Equipment. A constant-current stimulator capable of delivering square wave shocks of 0.1-millisecond duration and of modulable intensity is used, which should include two synchronized outputs to allow bilateral stimulation. Two triggered EMG amplifiers and a display should be available, so that the muscle action potential (M wave) can be checked online by the operator. Several manufacturers provide complete machines that offer a wide panel of sophisticated stimulation and EMG acquisition options. Stimulators and amplifiers can also be bought separately.
Advantages. If skillfully performed, ES generates a "pure" diaphragmatic contraction. The corresponding output is thus representative of diaphragm properties alone. ES can be reproducible in skilled hands.
Disadvantages. The stimulus intensities required to achieve supramaximal stimulation can be uncomfortable. From the technical point of view, there are several difficulties. First, maintaining optimal contact between the stimulating electrode and the nerve can be difficult. It may be necessary to impose a significant degree of pressure on the soft tissues of the neck, which can be painful for the subject and awkward for the operator, particularly in obese or old subjects, or those with hypertrophy of neck muscles. Skin-taped stimulating electrodes have been used in healthy volunteers (90), but they probably do not guarantee reproducible results in all settings. Neck-and electrode-stabilizing devices have been proposed (10, 85) that can be effective, but are cumbersome. Second, it is sometimes impossible to dissociate PNS from brachial plexus stimulation, particularly at high current intensities. This can be a source of discomfort for the subject, and can theoretically modify the characteristics of the rib cage, with which the contraction of the diaphragm interacts. Third, it can be impossible to locate the nerve, or to do so easily enough to obtain reliable supramaximal stimulation. Because of these difficulties, maintaining a constant symmetric maximal stimulus may need repetitive ES, which in itself can increase twitch pressure by potentiation or the staircase phenomenon (91, 92) (see also Twitch Potentiation).
The technical expertise required for effective ES may, thus, be a source of variability in research studies and limit its use in the clinical field, particularly in demanding settings such as the ICU or exercise.
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