STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 359

TABLE 7.4 Typical Parameters Used in Iontophoresis Instruments

Clinical Method of Typical Typical Current

Application Current Delivery Waveform or Voltage

Delivery of water-soluble Current delivered between drug-delivery Dc 0 to 4 mA with limited

ionized medication and “dispersive” surface electrodes; 60-V compliance;

through the skin typical electrode impedance at electrode current density

beginning of treatment in the maintained under

range 20 to 100 k Ω ; impedance at the 500 mA/cm 2

end of treatment in the range 1 to 20 k Ω

Drawing glucose Current delivered between extraction Dc applied for 3 to 5 minutes, 200 µ A to 1 mA to yield

through skin by surface electrodes then glucose is analyzed for current density of

electroosmosis 7 to 10 minutes, followed by 0.3 mA/cm 2 at the

another extraction using dc cathode

in opposite direction for
3 to 5 minutes

also been investigated with some success as a way of delivering anti-in fl ammatory agents

(dexamethasone sodium phosphate lidocaine) to treat musculoskeletal disorders,

hyaluronidase to reduce edema, as well as medication for many localized skin disorders,
such as nail diseases, herpes lesions (e.g., delivery of acyclovir), psoriasis, eczema, and
cutaneous T-cell lymphoma.
An iontophoresis dose is expressed as

charge delivered (mA/min) current (mA) treatment time (minutes)

A typical iontophoretic drug delivery dose is 40 mA/min but can vary from 0 to 80 mA/min.
Most people feel little or no sensation at all during an iontophoretic treatment. Some peo-
ple feel a tingling or warm sensation under one or both of the electrodes. This is caused by
small blood vessels in the skin expanding due to the presence of direct current, or because
mast cells are responding to the current by releasing histamine.
It has also been demonstrated that electroosmosis can be taken advantage of in
reverse iontophoresis . Here, imposing an electric current across the skin extracts a sub-
stance of interest from within or beneath the skin to the surface. Sodium and chloride
ions from beneath the skin migrate toward the electrodes. Uncharged molecules, includ-
ing glucose, are also carried along with the ions by convective transport (electroosmo-
sis). This technique is now being used to monitor the subdermal concentration variation
of glucose, allowing diabetic patients to track their blood sugar without painful fi nger

pricks.

Modulation of Cardiac Contractility

A major determinant of contractile strength of cardiac muscle cells is the amount of
calcium reaching the contractile proteins during a beat. Reduced calcium transients are
believed to contribute to contractile dysfunction in heart failure. Shlomo Ben-Haim, the
founder of Impulse Dynamics (Haifa, Israel), discovered that extracellularly applied
electric

fields delivered during the absolute refractory period can modulate myocardial
contractility [Ben-Haim et al., 2002] (Table 7.5). Experimental evidence in situ as well
as in vivo (healthy dogs and pigs, heart failure dogs, and heart failure human patients)
indicates that electrical signals do modulate cardiac contractility [Burkhoff

fi

ff et al.,

2001].