[Study of excitation circulation around an unexcitable myocardial obstacle]

An experimental model of tachy-arrhythmias due to myocardial excitation wave circulation round an unexcitable obstruction was explored. Artificial openings of various sizes and shapes in an isolated rabbit left atrium posed as obstacles. Acetylcholine, having a potent effect on the refractory period of atrial cells, was used to alter refractory characteristics. It has been demonstrated that differences in the sensitivity to the action of pharmacologic agents on both re-entry types can actually be absent. Acetylcholine is shown to shorten the period of circulation round the hole, i.e. act on such circulation in the same fashion as it does on the leading cycle, in a wide range of hole diameters, approaching the ones comparable with the size of the atrium. Sensitivity to acetylcholine only disappears when a labyrinth is set up with artificially prolonged circulation pathway. The action of acetylcholine can be attributed to the absence of tissue with fully recovered excitability between the anterior and posterior fronts of the wave circulating round the obstacle.