Changes In Refractory Period Research Articles

Relations between ventricular refractoriness and regional myocardial blood flow after acute coronary occlusion

Myocardial ischemia has been associated with dispersion of ventricular refractory periods and this dispersion has been related to the ventricular arrhythmias seen with coronary occlusion. This study relates the degree of change in measured ventricular refractory period with the degree of regional myocardial blood flow abnormality after coronary occlusion. When regional myocardial blood flow is less than 70 percent of that of nonischemic areas, refractory periods are significantly ( P < 0.001) shortened. The greatest change in refractory periods occurs in areas with a regional myocardial blood flow that is 21 to 40 percent of that of non-ischemic areas. Marked (less than 20 percent) and minimal (61 to 80 percent) reductions in regional myocardial blood flow are associated with less, but still significant, shortening of ventricular refractory periods. Thus dispersion of refractoriness can be related to the inhomogeneity of regional myocardial blood flow after acute coronary occlusion. Interventions designed to salvage ischemic myocardium by increasing regional myocardial blood flow may affect dispersion of ventricular refractory periods in complex and divergent ways.

Electrophysiological actions of disopyramide phosphate on canine ventricular muscle and purkinje fibers.

Disopyramide phosphate is a new antiarrhythmic drug that has been shown to possess significant antiarrhythmic effects in animals and man. In the present investigation, the effects of 2, 5, and 10 mug/ml of disopyramide phosphate were studied on the electrophysiological properties of canine Purkinje fibers and ventricular muscle superfused in vitro. Transmembrane action potentials were recorded from Purkinje fibers in the region of maximum action potential duration (gate), from Purkinje fibers proximal and distal to the gate, and from ventricular muscle. Disopyramide phosphate produced a concentration-dependent decrease in the slope of phase 4 diastolic depolarization of spontaneously beating Purkinje fibers. In all electrically stimulated fibers, the drug decreased the amplitude and the maximum upstroke velocity of the action potential. This depression of phase 0 characteristics was accompanied by a decrease in conduction velocity. In Purkinje fibers located at the gate, a concentration-dependent parallel shift to the right and a depression of the maximum of the membrane responsiveness curve occurred. Effects on action potential duration were variable. Repolarization was altered so that action potentials with dissimilar durations recorded from sites proximal to, at, and distal to the gate became equal. The total action potential duration and the effective refractory period of gate Purkinje fibers were prolonged, but the change in action potential duration was always greater than the change in effective refractory period so that the ratio of the change in duration to the change in refractory period was always greater than one.

Local ventricular repolarization changes due to sympathetic nerve-branch stimulation

Functional distributions of individual cardiac nerves distal to the stellate ganglia were determined in 30 open-chest, anesthetized dogs by mapping sites of refractory-period shortening during stimulation of the nerves. On the right, recurrent cardiac nerve stimulation produced marked shortening of refractory periods in the interventricular septum, and lesser changes on the anterior heart surface. On the left, ventromedial cardiac nerve stimulation shortened refractory periods in a similar distribution, but changes were not marked as with the recurrent cardiac nerve. The other left-side nerve that produced repolarization changes, the ventrolateral cardiac nerve, produced marked refractory-period changes on the posterior heart surface. Its distribution showed little overlap with that of the ventromedial or recurrent cardiac nerves. T waves inverted in an electrocardiographic Y lead during recurrent cardiac and ventromedial cardiac nerve stimulation, while ventrolateral cardiac nerve stimulation increased the positivity of T waves in that lead. The cardiac distributions of individual nerves documented in this study provide an anatomical basis for localized alterations in ventricular electrophysiologic properties.

Confinement of premature impulses in functional compartments of regions of the A-V conducting system.

The local refractory periods of the multiple branches of the distal A-V conducting system progressively increase from proximal to distal, reaching a maximum a few millimetres proximal to the termination of a given tract of conducting tissue in muscle, and progressively decreasing beyond the point of maximum duration. The multiple areas of maximum local refractory periods function collectively to provide a limiting segment, or gate, which determines the functional refractory period of preparations of large segments of A-V conducting tissue. Artificial or natural changes in the refractory period of a single tract of conducting tissue alter the functional properties of such preparations. Reversal of the changes in, or ablations of, the single abnormal tracts of tissue reverses the changes in functional refractory period of the entire preparation. These observations may relate to the way in which local cardiac disease may cause re-entrant arrhythmias.

Action of aconitine on the cold-blooded heart.

SummaryThe effect of aconitine on the primary properties of heart muscle has been studied in the cold-blooded heart. Its constant action was to increase the rate of spontaneous beating or to initiate a spontaneous rhythm in driven ventricular strips. This action appeared to be independent of change in resting excitability and to result from enhancement of intrinsic rhythmicity. Vagal action on the auricle depressed the pacemaker, decreased contraction amplitude and shortened refractory period. Contractility and refractory period changes were independent of rate change. In the atropinized auricle high concentrations shortened refractory period by direct muscle action. During the stage of shortened refractory period there was a critical interval when a single extra shock might elicit multiple responses. In the turtle ventricle, which has no vagal endings, high concentrations shortened refractory period, decreased conduction velocity and raised threshold for electrical stimulation.