Anti-inflammatory and anesthetic medications (dexamethasone and lidocaine) are the most common medications delivered using iontophoresis in physical therapy.
What is iontophoresis?
Iontophoresis is a non-invasive method to deliver medication through the skin to a specific area using a continuous direct current. Anti-inflammatory and anesthetic medications (dexamethasone and lidocaine) are the most common medications delivered using iontophoresis in physical therapy1.
Iontophoresis requires two electrodes, one positively and one negatively charged. Placement of the electrodes is dependent on the polarity of the medication being delivered. For example, dexamethasone has a negative polarity. Therefore, the electrode containing dexamethasone will be placed over the affected body part and attached to the negative charge in the electrical circuit. The negative current repels the negative dexamethasone ions, driving them through the patient's skin.2,3 Conversely, lidocaine has a positive polarity so the electrode would be attached to the positive charge in the electrical circuit.
Iontophoresis or ion transfer is introduction of substances into the body for therapeutic purposes using a direct current. Each substance is separated into its ionic components by the action of the current and deposited subcutaneously according to the imposed polarity on the electrode. Therapeutic results depend on the ion introduced, the pathology present and the desired effects.
The current required for iontophoresis is continuous direct galvanic current, which can be obtained from standard low-voltage generators. The treatment is not the current itself, but rather the ions introduced through that current. The completely non-invasive concept of iontophoresis became very attractive to physical therapists because of the minimal ionic concentrations required for effective administration.
Formula for iontophoresis:
The basic formula for using iontophoresis is:
I x T x ECE = grams of substance introduced, Where:
I : (Intensity) measured in amperes.
T : (Time) measured in hours.
ECE : (Electro-Chemical Equivalent) represents standardized figures for ionic transfer with known currents and time factors.
As the determination of the ECE for many complex substances is very difficult, fewer milligrams of these complex substances will penetrate the skin.
Principles of Ion transfer:
* Ionic polarity:
The basis of successful ion transfer lies in physics principle “like poles repel and unlike poles attract’. So, the ions are repelled into the skin
by an identical charge on the electrode surface placed over it. Sub-dermally, the ions introduced re-combine with the existing ions floating in the blood stream, forming the necessary new compounds for therapeutic interactions.
* Low-level amplitude:
Most researches have indicated that low-level amplitude is more effective than high-level intensities, as higher intensities adversely affect ionic penetration. It should be put into consideration that a few ions, ready to combine, may be better than a sum of ions, repelling each other because of their similar charges. The physics involved with ion transfer necessitates low intensity (less than 5 ma) and low percentage or ion sources (1 to 5%).
It was found that the negative electrode (cathode) is more irritating than the positive one (anode), due to the formed sodium hydroxide under its position. So, the negative electrode should be made larger than the positive electrode (usually twice), even if the negative electrode is the active one. According to the laws of physics, electrons flow from negative to positive, regardless of electrode size. So, enlarging the negative electrode size lowers the current density on the negative pad, leading to reduction of irritation.
1. Ion penetration: Actually, penetration does not exceed 1 mm, with subsequent deeper absorption through the capillary circulation. The bulk of deposited ions is found at the site of the active electrode, where they are stored as either soluble or insoluble compound, to be depleted by the sweep of circulating blood.
2. Acid / alkaline reactions: As the anode (+) produces an acid reaction (a weak HCL acid), it is considered sclerotic, which tends to harden tissues, serving as an analgesic agent due to local release of oxygen. On the other hand, as the cathode (-) produces an alkaline reaction (a strong sodium hydroxide), it is then considered sclerolytic, which is a softening agent due to the hydrogen release, serving in the management of scars and burns.
3. Hyperemia: Both the positive and negative electrodes produce hyperemia and heat due to the resulting vasodilatation. The cathodal hyperemia is generally more pronounced and takes more time to disappear than that of the anode. Generally, hyperemia under both electrodes lasts within one hour, causing no more discomfort to the patient.
4. Dissociation: Under normal circumstances, ionizable substances
dissociate in solution releasing ions, which with the passage of direct current into the solution migrate toward the other pole. This is the concept of ion transfer. Due to the variability in resistance of various tissues to current flow, electrode placement becomes of an utmost importance.