Comprehensive Guide to Low Frequency Currents in Physiotherapy

Characterization and Waveform of Faradic Current

Faradic current is defined as an interrupted direct current, specifically a unidirectional current characterized by variable intensity and interrupted pulses. These pulses possess a short duration of less than $10\,ms$ and operate at a low frequency, though the repetition rate is notably high, typically ranging from $50\,pulses/s$ to $100\,pulses/s$. Primarily, this current is utilized for the stimulation of normally innervated muscles. The traditional waveform consisted of two distinct and unequal phases: a low-intensity phase with a long duration and a high-intensity phase with a short duration. In modern physiotherapy equipment, transistors and filters are employed to eliminate the first phase entirely, leaving only the high-intensity, short-duration pulses for therapeutic use.

General Effects and Safety Profile of Faradic Current

Depending on the specific frequency and intensity settings, faradic current is capable of producing muscle stimulation, trophic effects, and analgesic effects. Because it remains a form of direct current, it carries inherent risks such as skin irritation or chemical corrosion; therefore, clinicians must carefully control the intensity and the overall treatment duration. Despite these potential risks, the chemical effects of the current are considered minimal, and the procedure is generally comfortable for the patient.

Physiological Effects of Faradic Current

The application of faradic current produces several distinct physiological responses. Regarding sensory nerves, the current causes a mild pricking sensation due to the stimulation of those nerves. This stimulation triggers reflex vasodilation of superficial blood vessels, which results in the reddening of the skin. On the muscles, the current produces contractions that are similar in nature to voluntary contractions, leading to increased metabolism, higher demand for oxygen ($O_2$) and nutrients, and increased production of waste products. The repeated cycle of contraction and relaxation acts as a pumping mechanism, which significantly improves both venous and lymphatic drainage. Furthermore, faradic current stimulates motor nerves. When the intensity is sufficient, at a frequency of approximately $50\,Hz$, the muscle undergoes tetanic contraction. Because prolonged stimulation leads to muscle fatigue, the current is often surged or interrupted to permit periods of relaxation.

Clinical Indications for Faradic Current

Faradic current is used primarily to induce contraction in muscles with normal innervation. Specifically, it is used for the facilitation of muscle contraction in instances where a patient cannot voluntarily contract a muscle. In cases of muscle re-education, it is applied to restore a sense of movement in muscles that have lost voluntary control due to prolonged disuse. It is also indicated for neuropraxia of the motor nerve, where nerve conduction is temporarily blocked without degeneration. In such cases, voluntary impulses cannot reach the muscle, but stimulation below the lesion site can still trigger contraction. In cases of neurotmesis (severed nerves), where axons degenerate, faradic current is preferred in the later stages of recovery to prevent muscle atrophy and fibrosis, although it does not directly regenerate the nerve. Additionally, it is used to enhance circulation and improve vascular supply through alternate contraction and relaxation.

Application Techniques for Faradic Systems

There are three primary methods for applying faradic current: motor point stimulation, nerve stimulation, and the bath method. In motor point stimulation, an indifferent electrode is placed near the muscle's origin, and an active electrode is placed directly over the motor point to produce an effective contraction of an individual muscle. The nerve conduction method involves placing the indifferent electrode on a convenient area near the target muscle and the active electrode over a site where the nerve trunk is superficial; stimulating the nerve thus causes contraction in all muscles supplied by that nerve. The bath method involves passing the electrical current through water in a tray or tub, which is particularly useful for small muscles in the hands or feet where individual stimulation is difficult. This can be bipolar, where both electrodes are in the tray, or unipolar, where one electrode is in the tray and the other is elsewhere on the body.

Fundamentals of Galvanic Current (Direct Current)

Galvanic current is a direct current (DC) characterized by constant intensity, low frequency impulses, and long-duration pulses exceeding $10\,ms$. It is specifically used for the stimulation of denervated muscles. Unlike faradic current, it has a low repetition rate of approximately $30\,impulses/min$. While effective, it carries disadvantages, primarily the risk of chemical tissue damage under the electrodes. Under the anode ($+$), hydrochloric acid ($HCl$) forms, while sodium hydroxide ($NaOH$), or soda lye, forms under the cathode ($-$). A modified form, known as galvanic interrupted current, is commonly used because it provides approximately $95\%$ of the galvanic effect while being better tolerated by patients because of the interrupted flow.

Physiological Responses to Galvanic Current

Galvanic current has specific effects on denervated muscles; with adequate intensity and an impulse duration of at least $100\,ms$, it can produce muscle contractions. These contractions are notably sluggish, with both the contraction and relaxation phases occurring slowly. Regarding sensory nerve stimulation, galvanic current produces a stabbing or burning sensation, leading to reflex vasodilation and erythema (skin redness). When stimulating motor nerves, each stimulus produces a brief muscle twitch followed by immediate relaxation, though this effect is generally considered weak and is not highly beneficial for muscle strengthening.

Indications and Methodology for Galvanic Treatments

The clinical indications for galvanic current include drug delivery (iontophoresis), denervated muscle stimulation, pain relief, improvement of circulation, and diagnostic testing. Application techniques include the labile technique, where the active electrode moves continuously over the treatment area while the indifferent electrode remains fixed (useful for large areas), and the stable technique, where both electrodes are stationary (used for localized treatment). Methods also include the bipolar method, using two equal-sized electrodes over the area, and the monopolar method, which uses one small active electrode and one large indifferent electrode, concentrating the effect under the active electrode.

Iontophoresis Principles and Practice

Iontophoresis is a technique utilizing galvanic (direct) current to drive medically useful ions through the skin into tissues, also referred to as ion transfer. This process is based on the principle that like charges repel one another. Consequently, negatively charged medications are placed under the cathode ($-$), and positively charged medications are placed under the anode ($+$). For this to be effective, medications must be in ionic form. Interrupted DC helps the ions pass through the skin, where they become chemically active and produce local therapeutic effects. Treatments typically last between $10\,min$ and $20\,min$. Indications for iontophoresis include localized inflammation, tendon and joint pain, calcifications, and hyperhidrosis (excessive sweating).

Examples of Ions and Medications in Iontophoresis

Various drugs are used depending on the desired therapeutic outcome. Dexamethasone is negatively charged and used as an anti-inflammatory. Lidocaine is positively charged and used for pain relief. Calcium, which is positively charged, is used to treat muscle spasms. Acetic acid is negatively charged and is used for treating calcifications. All applications place the active electrode directly over the area requiring treatment and utilize a calculated dose of direct current.