Clinical Uses Of Electrical Stimulation

Clinical electrical stimulation is simply the application of electrical currents to a body, be it for function or therapy. As we just discussed, the current of electrons passing through the wires is converted into a current of ions moved within the tissue, which are in turn capable of transporting electrical charge across the membranes of excitable tissues. The purpose of these applied currents is to cause the targeted depolarization of nerve and/or muscle to threshold.

The most common clinical applications of electrical stimulation are:

1. Cardiac pacing. Electrical stimulation of the heart's chambers relieves or eliminates the symptoms of bradycardia (a heart rate that is too slow). Rhythmic stimulation (pacing) increases the heart rate to meet the oxygen needs of the body. Cardiac pacing is discussed in detail in Chapter 8.

2. Cardiac defibrillation. High-energy stimulation of the heart (in the form of an electrical shock) interrupts a rapid heart rhythm (tachycardia) so that a more normal rhythm can be restored. Cardiac defibrillation is discussed in detail in Chapter 8.

3. Cardiomyoplasty. A skeletal muscle (e.g., the latissimus dorsi, which attaches at one end to the upper part of the upper arm bone and spreads out like a fan to attach to the spine and ribs) is dissected free from its normal attachments and then wrapped around the heart. The muscle is then stimulated to contract in synchronism with the heart. Since skeletal muscle is prone to fatigue, it must first be trained by converting its fibers to fatigue-resistant type 1 muscle fibers. Training is done with a low stimulation rate with only one pulse in a burst and over a period of six weeks increasing the repetition rate and the number of pulses in the burst.

4. Electroventilation. Electrical stimulation of the phrenic nerve or the diaphragmatic muscles is used to support ventilation. Candidates for breathing pacing include patients who require chronic ventilatory support because of spinal cord injury, decreased day or night ventilatory drive (e.g., sleep apnea), intractable hiccups (chronic hiccups often lead to severe weight loss and fatigue and can have fatal consequences), and damaged phrenic nerve(s). The physiological respiratory function provided by these devices is far superior to that provided by mechanical ventilators since the air inhaled is drawn into the lungs by the musculature rather than being forced into the chest under mechanical pressure.

5. Diagnostic stimulation of nerves and muscles. Nerve conduction studies are performed routinely to assess peripheral nerve function. Electrical stimulation is applied to a nerve and the nearby EMG signal is measured. This is done to determine the speed of transmission along the nerve. It also helps to determine if there is a blockage in the nerve or where the nerve connects to the muscle. In a similar way, electrical stimuli delivered at the wrist or behind the knee are used to evoke brain responses to sensory inputs. The somatosensory-evoked potentials are detected by coherent averaging of the EEG. From this information, the evaluator may determine whether there is a delay in conduction to the brain, a blockage at any point, or abnormally low or high activity in the brain. Another common diagnostic use of nerve stimulation is monitoring the depth of neurological blocks present in a patient following the administration of muscle relaxant drugs (e.g., prior to surgery, and after surgery following the administration of antagonist drugs).

6. Diagnostic stimulation of the brain. Very brief high-voltage pulses or pulse bursts to stimulate percutaneously human motor cortex, visual cortex, or spinal cord are used for intraoperative monitoring as well as for diagnosis of neurological diseases.

7. Pain relief. The technique of applying electric currents to the spinal cord or a peripheral nerve to relieve pain is known as electroanalgesia. Its use with both permanently implanted and nonsurgically applied devices is common practice in the treatment of patients suffering from chronic pain.

8. Control of epileptic seizures. Electrical stimulation of the vagus nerve [also known as vagus nerve stimulation (VNS)] involves periodic mild electrical stimulation of the vagus nerve in the neck by a surgically implanted device. VNS has been found effective in controlling some epilepsies when antiepileptic drugs have been inadequate, their side effects intolerable, or neurosurgery has not been an option. In some cases VNS has also been effective in stopping seizures. It carries minimal side effects (e.g., mild tingling sensations and voice hoarseness during stimulation), but unlike many medications, there seem to be no significant intellectual, cognitive, behavioral, or emotional side effects to VNS therapy. VNS is now the second most common treatment for epilepsy in the United States, and the improvement in seizure control is comparable to that of new antiepileptic drugs.

9. Control of Parkinsonian tremor. Electrical stimulation of neuron clusters deep inside the brain [also known as deep brain stimulation (DBS)] is now used to inactivate the subthalamic nucleus, which is overactive in Parkinson's disease. A multielectrode lead is implanted into the ventrointermediate nucleus of the thalamus. The lead is connected to a pulse generator that is surgically implanted under the skin in the upper chest. When the patient passes a magnet over the pulse generator, the device delivers high-frequency pulse trains to the subthalamic nucleus to block the tremor.

10. Gastric "pacing." Electrical stimulation of the stomach is currently being used to reduce symptoms of nausea and vomiting for patients suffering from gastroparesis (a stomach disorder in which food moves through the stomach more slowly than normal).

11. Restoration of lost sight. Electrical stimulation of the retina, the optical nerve, and the visual cortex is now developed to the point at which implants for functionally restoring sights to blind patients will soon be available commercially. Functional sight may be given to patients blinded by retinitis pigmentosa by using integrated circuits embedded in contact with the retina. The ICs contain an array of photovoltaic cells that directly power an array of microstimulators and electrodes to convert the image into a directly mapped electrical image, bypassing degenerated photoreceptors and directly stimulating the remaining nerve cells in the retina. For patients with blindness caused farther down the optical nerve, the possibility exists of stimulating the visual cortex directly using micro-electrode arrays to generate coherent images from phosphenes (sensation of a spot of light) elicited by the electrical stimulation.

12. Restoration of lost hearing. Cochlear implants stimulate spinal ganglion cells of the auditory nerves, bypassing nonfunctional hair cells to restore limited hearing in some types of deafness. The cochlear implant system really consists of an implanted stimulator connected to an electrode array inserted in the cochlea and an external speech processor that codes the speech into stimulation patterns that can be translated back into sounds by the brain. The external speech processor also powers the implant via an inductive energy transfer link. Cochlear implants are now common and provide substantial benefits to many profoundly deafened children and adults. Benefits vary by person and range from increased perception of environmental sounds to the ability to use a telephone.

13. Restoration of lost or impaired neuromuscular function. Functional electrical stimulation (FES), also known as functional neuromuscular stimulation (FNS), is a rehabilitation strategy that applies electrical currents to the nerves that control paralyzed muscles in order to stimulate functional movements such as standing or stepping. FNS systems include either skin-surface or implanted electrodes, a control unit which often also receives motion information back from sensors, and a stimulus generator. A number of FNS units are now either available commercially or under clinical investigation. Typical applications of FNS include controlling foot drop, enabling lower-limb paraplegics to stand or sit, and restoring hand function to the paralyzed upper limb.

14. Maintenance or increase in range of movement. Electrical muscle stimulation (EMS) is used to strengthen muscle and facilitate voluntary motor function. Although EMS devices are often advertised for muscle toning and weight reduction, they are authorized by the FDA only as prescription devices for maintaining or increasing range of motion, relaxation of muscle spasm, prevention or retardation of disuse atrophy, muscle reeducation, increasing local blood circulation, and postsurgical stimulation of calf muscles to prevent the formation of blood clots.

15. Electroconvulsive therapy (ECT). This is a relatively painless procedure that is effective in treating major depression. A short, controlled set of electrical pulses is given for about a minute through scalp electrodes to produce generalized seizures. Biological changes that result from the seizure are believed to result in a change in brain chemistry which is believed to be the key to restoring normal function. Because patients are under anesthesia and have taken muscle relaxants, they neither convulse nor feel the current.

TABLE 7.1 Typical Parameters Used in Various Clinical Applications Involving the Stimulation of Tissues with Electrical Currents

Clinical Application

Typical Method of Current Delivery

Typical Waveform

Typical Current or Voltage

Cardiac pacing

Cardiac defibrillation

Cardiomyoplasty

Electroventilation

Diagnostic stimulation of peripheral nerves

Diagnostic stimulation of brain (cortex) and spinal cord

Pain relief

Vagus nerve stimulation (VNS)

Implanted electrodes in contact with heart; electrode impedance 250 Q to 1kQ Gelled skin-surface electrodes placed on chest; electrode impedance ~50 Q Implanted electrodes in contact with heart; electrode impedance 30 to 60 Q Gelled skin surface electrodes placed on chest; electrode impedance 50 to 100 Q Platinum-iridium wire electrodes sewn across skeletal muscle a few centimeters apart, looped under the nerve branches that run along the surface of the muscle; electrode impedance 50 to 100 Q

Temporary electroventilation with gelled anterior axillary skin-surface electrodes; impedance 250 Q to 1kQ Implanted electrodes in contact with phrenic nerve or innervation point of diaphragmatic muscles

Bipolar pair of 2- to 5-mm-diameter spherical dry electrodes with interelectrode distance of 2 to 5 mm applied to skin over target nerve Bipolar pair gelled electrodes (or corkscrew electrodes for intraoperative monitoring) with interelectrode distance of ~7 cm applied to skin Implanted electrodes in contact with spinal cord or targeted peripheral nerve to block the sensation of pain Gelled skin-surface electrodes

(impedance 200 Q to 1 kQ) placed on painful region; often known as transcutaneous electrical nerve stimulation (TENS) Implanted electrodes in contact with vagus nerve; electrode impedance 1 to 7 kQ

0.1- to 2-ms capacitor-discharge pulse with charge-balancing phase

Balanced biphasic current pulse

20 to 40 ms in duration Biphasic capacitor discharge

5 to 10 ms in duration Monophasic or biphasic capacitor-discharge pulse 5 to 10 ms in duration Burst of capacitive discharge pulses with charge-balancing phase, 0.06 to 1.0 ms in duration; 1 to 16 pulses per burst, at a pulse repetition rate of ~10 to 60 Hz; burst delivered in synchrony with cardiac activity at a burst-to-beat ratio of 1:1 to 1:16 0.8-s bursts of balanced monophasic pulses 10 |ls in duration at 35 Hz

0.8-s bursts of balanced biphasic current pulses 1 to 10 ms in duration or (phrenic nerve) 25 to 100 ms in duration (muscles); repetition rate 30 Hz Monophasic current pulses 50 |J,s to 2 ms in duration

50-^s-wide transformer-isolated square wave

Monophasic or biphasic pulses ~210|ls in duration delivered at 30 to 80 Hz

Monophasic or biphasic pulses 50 to 150 |ls in duration delivered at 10 to 150 Hz

Monophasic current pulses (with charge-balancing phase) 130 to 1000 |ls in duration delivered at ~30 Hz for 30 s every 5 minutes

0.1 to 8 V peak delivered from 5- to 10-^F capacitor 50 to 200 mA with 30-V

compliance 2 to 10 A with capacitor bank charged to <1 kV 30 to 40 A with capacitor bank charged to <3 kV

0.1 to 8 V peak delivered from 5- to 10-^F capacitor

200 mA to 1.5 A with up to 1500-V compliance

1 to 10 mA with 12-V compliance

0 to 100mA with up to 400-V compliance

100 to 1000 V with a maximum current of 1.5 A (at a rate of current rise of 0.1 A/^s)

10 to 150 mA with <150-V compliance

0.25 to 35 mA with 12-V compliance

TABLE 7.1 (Continued)

Clinical Application

Typical Method of Current Delivery

Typical Waveform

Typical Current or Voltage

Deep brain stimulation (DBS)

Gastric pacing

Restoration of lost sight

Cochlear stimulation

Functional neuromuscular stimulation (FNS)

Electrical muscle stimulation (EMS)

Thin electrode implanted deep into parts of the brain that are involved in control of movement; electrode impedance 600 Q to 2kQ

Implanted electrodes stitched to the stomach muscle wall of the antrum 10 cm proximal to the pylorus; electrode impedance 200 Q to 1 kQ

Implanted electrode array in contact with retina; typical electrode impedance 1 to 10 kQ

Implanted electrode array in contact with brain's visual cortex; typical electrode impedance 10 to 100 kQ

Implanted electrode array in contact with cochlea; typical electrode impedance 1 to 10 kQ

Implanted electrodes in contact with muscle; electrode impedance 200Q to 2kQ

Gelled skin-surface electrodes placed over target muscle

Gelled skin-surface electrodes placed over target muscles

Electroconvulsive therapy (ECT)

Interferential mode: at least two pairs of skin surface electrodes delivering high-frequency signals that interfere at the target muscles; gelled skin-surface electrodes with impedance 100 Q to 1.5 kQ at 4 kHz

Gelled skin-surface electrodes applied to the forehead; impedance 250 Q to 1.5 kQ

60- to 450-^s charge-balanced capacitor-discharge pulses delivered at 2 to 185 Hz; burst on/off times depend on patient needs

Monophasic or biphasic pulses ~210 |ls in duration delivered at 30 to 80 Hz

Balanced biphasic current pulse 100 |ls to 5 ms in duration; repetition rate 60 to 500 Hz Balanced biphasic current pulse 100 |ls to 2 ms in duration; repetition rate 10 to 250 Hz

Balanced biphasic current pulse 20 |ls to 1.2 ms in duration; repetition rate up to 2 kHz Balanced biphasic current pulse 25 to 500 ms in duration; repetition rate up to 100 Hz <1-ms pulse (typically around 300 |ls) with a frequency

< 100 Hz (due to the absolute refractory period of normal muscle)

Biphasic 10- to 15-ms waveforms ~ 10 Hz for denervated muscle <1-ms pulse (typically around 300 |ls) with a frequency of

< 100 Hz (due to the absolute refractory period of normal muscle)

Biphasic 10- to 15-ms waveforms ~ 10 Hz for denervated muscle Sinusoidal current; one channel at a frequency of 4 kHz, second channel at 4 kHz ± selectable beat frequency

10-s burst of 0.25-ms pulses delivered at 10 to 100 Hz

10 to 600 |JA with 6-V compliance

1 to 60 |JA with 6-V compliance

30 |JA to 2 mA with 12-V compliance

1 to 10 mA with 20-V compliance

10 to 150 mA with <150-V compliance

10 to 150 mA with <150-V compliance

0 to 100mA RMS with 150-V compliance

Up to 1 A with 2.5-kV voltage compliance

The primary factors determining whether sufficient current flows to yield a desired clinical effect are impedance of the body tissues in the path of the current, electrode size and position, stimulation parameters, and the electrical characteristics of the tissue to be excited. These parameters are usually interrelated, as shown in Table 7.1 for the various clinical areas in which electrical stimulation is used.

The most commonly used stimulation signal waveshapes are those shown in Figure 7.6. The charge-balanced pulses of Figure 7.6a ensure that no net charge is introduced to the

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