Session S43.2

Propagation of Electrical Excitation in Isolated Rabbit Hearts: Influence of Stimulation Protocol, Spatial Coupling and Ion Channel Dynamics

S Bauer, S Fruhner*, I Romero, H Engel,
H Koch, M Bär

Technische Universität
Berlin, Germany

Propagation of electrical excitation in the rabbit heart is computed using a simple realistic ionic model. A rabbit specific anatomy of the ventricles ("San Diego rabbit heart") provides the geometry for the finite-element simulations. Excitation was initiated by either a local stimulation near the apex (sine protocol) or an almost simultaneous Purkinje-like stimulation of the inner wall (Purkinje protocol). The simulations are compared to data obtained from an autonomously beating rabbit heart in Langendorff perfusion as well as from rabbit hearts exposed to drugs that either block sodium channels (1µMol ajmaline) or gap junctions (10µMol palmitoleic acid). The latter corresponds to an effective decrease of the spatial coupling due to lower intracellular conductivity in the tissue. Time-resolved surface maps of extracellular bioelectric potentials were analysed and compared to maps generated from the simulations. Experimental electrograms were obtained by a matrix of 8 x 8 electrodes placed on the surface of both ventricle's epicardium. After filtering the data, activation maps were computed and subsequently surface propagation speed maps were obtained.
Comparisons between data and model results were made in particular regarding the average propagation speed in both ventricles and the QRS durations for normal and drug-exposed hearts. The effects of the drugs on the average propagation velocity in the left ventricle were successfully modeled by decreasing the sodium conductivity to 53% (for ajmaline) and the intracellular conductivities to 70% (for palmitoleic acid) of their normal values. Both drugs cause a decrease of the propagation velocities in the range of 15–30%. The ratio r of the average propagation speeds in the left and right ventricle differs between 0.9 and 1.7 in the experiments. This ratio differs strongly between different rabbit hearts, but is only weakly affected by both drugs. The simulations show a variation of r between 0.9 and 1.57. Experiments show also a weak increase (+10%) of the QRS time upon addition of palmitoleic acid and a strong increase (+66%) upon addition of ajmaline. Corresponding simulations show an increase of the QRS time of 20–25% for both drugs and are almost independent of the stimulation protocol. The qualitative shape of the QRS is better reproduced with the sine protocol, while the experimentally found maximum propagation speed of 1.4 m/s is reached only using the Purkinje protocol (1.5 m/s).
Altogether, all experimentally found features can be reproduced qualitatively. Quantitative agreement is not always achieved. Large differences of experimental results for different rabbit hearts as well as the strong dependence of the excitation and propagation properties on the particular stimulation protocol suggest that accurate information of the individual anatomy of a heart (including geometry and structure of the Purkinje system) is needed for quantitative prediction by simulations.

(Abstract Control Number: 150)