The laws of electrical stimulation of cardiac tissue

Classic description of the criteria for cardiac stimulation is by the strength-duration curve which may be presented for current, energy, or charge. Minima for each of these occur at different waveform durations. The fundamental mechanism by which the stimulus is effective is altering the polarization potential of the cell membranes in the heart. Partial depolarization of these cells initiates a cascade leading to complete depolarization which in turn triggers cardiac cell contraction. Active propagation of the depolarization from cell to cell is the mechanism for activation of the entire heart when stimulation is used for pacemaking. For depolarization with much stronger shocks, this depolarization mechanism in some cells is combined with hyperpolarization in other cells. The hyperpolarization causes anodal block of propagation. Two methods are used to study cardiac stimulation, the threshold concept and the probability concept, and each method has advantages and limitations. Many variables affect the capability of an electrical shock to stimulate the heart to a desired response. These include the current waveform the characteristics of the electrodes used to inject the current into the tissue, the disorder being treated, the mass of cells required to be effected, metabolic variables (such as temperature or pH of the the tissue), drugs, and the geometric arrangements of electrodes and body organs. Of particular engineering interest are different effectiveness of different electrical waveforms, and the electrode characteristics of half-cell potentials, polarization potentials and materials. Equivalent circuits are presented for the complex and nonlinear, interactive behavior of the electrical shock, electrodes, and tissues.