Concealed conduction.

THE CONCEPT of concealed conduction was introduced into electrocardiography to explain a lengthening of, or change in, the refractory period of the atrioventricular conduction system caused either by atrial activity that fails to propagate to the ventricles or by ventricular activity that fails to reach the atria. Standard electrocardiograms do not provide a direct demonstration of the partial invasion of the atrioventricular eondueting system under such conditions and usually fail to indicate the mechanism or exact site of block. Studies of the electrical activity of single cardiac fibers have shown that a local conduction delay or complete failure of propagation can occur at the junction of two fibers whose action potentials differ in duration. Such disturbances of conduction are observed whenever the propagation of excitation in one fiber precedes complete repolarization of another adjacent fiber. Records of the transmembrane action potentials of single cardiac fibers have shown that rather marked differences in action-potential duration are present at the peripheral junction of Purkinje fibers with papillary muscle and at the junction of fibers of the atrioventricular node and bundle of His. Less abrupt differences in action-potential duration are encountered within the atrioventricular node and within the peripheral Purkinje system. These observations suggest that failure of atrioventricular transmission might take place at any point between the atria and the ventricles because of local differences in the duration of electrical activity. This proposition has been studied by recording the trans-