Two-dimensional generator function for the vector cardiogram for use in volume conduction models of the thorax

An electrocardiogram (ECG) measures the electrical activity of the heart on the surface of the skin. Volume conduction models of the thorax can be designed to simulate such measurements. However, to drive such simulations a generator function is required to describe the electrical activity of the heart. Although such simulations, varying in complexity, are discussed in literature, there is a need for a simplified, though comprehensive approach that can be used as a concise introduction to this topic, or for cases where one is primarily interested in first-order approximations of this problem. In this article an overview of the vector interpretation of the ECG, also known as a vector cardiogram (VCG), is presented in the two-dimensional frontal plane of a human. The derivation of the equivalent electric dipole (i.e. the cardiac vector) from the VCG, which can be used as a current-source generator function for volume conduction model simulating the ECG, is discussed. A procedure for implementing such a volume conduction model with the finite element technique, using this simplified two-dimensional generator function, is discussed and the results are presented. The general features observed in recorded ECG leads agree with those predicted by this simple model.

Influence of motoneuron firing synchronization on SEMG characteristics in dependence of electrode position

The frequency content of the surface electromyography (SEMG) signal, expressed as median frequency (MF), is often assumed to reflect the decline of muscle fiber conduction velocity in fatigue. MF also decreases when motor unit firings synchronize, and we hypothesized that this effect can explain the electrode-dependent pattern in our previous recordings from the trapezius muscle. An existing motoneuron (MN) model describes the afterhyperpolarization following a spike as an exponential function on which membrane noise is superimposed. Splitting the noise into a common and an individual component extended the model to a MN pool with a tunable level of firing synchrony. An analytical volume conduction model was used to generate motor unit action potentials to simulate SEMG. A realistic level of synchrony decreased the MF of the simulated bipolar SEMG by ∼30% midway between endplate position and tendon but not above the endplate. This is in accordance with experimental data from the biceps brachii muscle. It was concluded that the pattern of decrease of MF during sustained contractions indeed reflects MN synchronization.