Guidant Defibrillators: Electrophysiology

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Electrophysiology is the study of electrical properties of cells and tissues.

If a microelectrode is placed on the surface of a resting myocardial cell and a second (indifferent) microelectrode is placed in a remote location such as the extracellular space then no electrode potential (zero potential) is recorded due to the high impedance of the cell membrane. If however, a glass microelectrode with a tip diameter of less than 0.5 µm penetrates the cell membrane then a potential gradient is detected with a resting membrane potential of -80 to -90 mV in most cardiac cells. However, this can range between 50 to -95 mV recorded during diastole depending on the cardiac cell type. The transmembrane resting potential is generated by the difference in ionic concentrations on either side of the cell membrane. The contributing ions include Sodium, Potassium, Chloride and Calcium [Guyton AC 1991].

Potassium is the major cation responsible for the production of a transmembrane potential due to an intracellular concentration of 150 mmol/l and extracellular of about 5 mmol/l with a 30:1 ratio [Carmeliet E 1992].

An opposite gradient exists for sodium ions (Na+) resulting in a higher extracellular concentration of Na+ ions compared with the intracellular concentration. The membrane however is much less permeable to Na+ than K+ ions and so the Na+ gradient does not change the resting gradient appreciably. The Na+/K+ pump actively removes Na+ ions from the cell against its electrochemical gradient at the same time as it pumps K+ ions into the cell against its chemical gradient. This pump is fueled by a Na+/K+-ATPase enzyme that hydrolyses ATP for energy and is bound to the membrane. It requires both Na+ and K+ to function and for every three Na+ ions pumped out from the cell two K+ ions are pumped inwards so an electrical gradient is established. There is a net pumping out of positive charge. The activity of the pump must maintain the same ionic balance even as heart rate increases since the cell loses a slight amount of K+ and gains a slight amount of Na+ with each depolarisation. Cardiac glycosides such as digoxin act directly to block this pump [Zipes P 1995].

If you or a loved one has had an implantable cardioverter-defibrillator such as the recalled Guidant:

1. CONTAK RENEWAL 2, Model H155

2. CONTAK RENEWAL, Model H135, or

3. PRIZM 2 DR, Model 1861

An experienced attorney can help protect you. Defibrillators are designed to save a life, not put it at a greater risk. Fill out the simple form below to contact an experienced defibrillator lawyer before your time limit to file suit runs out.

Weitz & Luxenberg is no longer accepting Guidant Defibrillator cases.

see also:

Membrane Channels Did Guidant destroy your membrane channels? Free legal help here!
Membrane channels & Guidant: free legal review from Weitz & Luxenberg

Excitation/Threshold Excitation and threshold potential: Guidant Lawsuit Information, FREE
FREE Guidant Defibrillator Lawsuit info

Sudden Cardiac Death Guidant CONTAK RENEWAL 2 H155: Sudden Cardiac Death
Sudden Cardiac Arrest and Guidant CONTAK RENEWAL 2 H155