Nonspatial determinants of electrograms in guinea pig ventricle

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Abstract
Theory states that the extracellularly recorded potential (epsilon) is determined by spatial and nonspatial factors. Spatial factors include the boundary between areas with different transmembrane voltages (Vm) and the relationship of the boundary to the extracellular electrode. Nonspatial factors include the Vm across the boundary [transmembrane potential gradient (TPG)] and a conductivity term (C sigma). Few studies have investigated the nonspatial factors experimentally due to the difficulty in separating the nonspatial from the spatial determinants. Our model rendered the spatial factors constant, permitted the simultaneous recording of epsilon and Vm, and allowed the manipulation of Vm and C sigma across the boundary. Epsilon and the TPG were related predictably with changes in [K+]o, [Na+]o, temperature, conduction, stimulus rate or prematurity, and hypoxia. In the presence of a constant TPG, epsilon could be affected by a change in C sigma caused by hypoxia and a metabolic poison. The effects of the nonspatial determinants on epsilon could be modeled using electrical circuit analogues. Nonspatial determinants must be considered in studies using electrocardiographic measures as an index of ischemia.