Dissociation with interference and reciprocal beats have many features in common. They have to be differentiated from one another and from auricular parasystole. Herrmann and Ashman (1930) stressed the importance of the auricular rhythm and polarity of the P waves in the bipolar limb leads as differential criteria between the two rhythms, regular auricular rhythm and positive P waves indicating dissociation with interference. Either may occur when the auriculo-ventricular node is an independent pacemaker, or more rarely, when there is a heterogenetic centre of impulse formation in the ventricles. In both orthograde conduction remains intact although it may be impaired. Retrograde conduction is completely blocked in dissociation with interference whereas in reciprocal rhythm it is not altered, although for reciprocal beats to occur it is usually delayed to a certain critical level. The recognition of these disturbances of conduction is important in clinical medicine, for most reported cases have underlying organic heart disease, or are caused by intoxication such as that due to digitalis or to an increased content of pressor substances in the blood. Their presence can only be detected by graphic methods, and on ordinary clinical examination they are likely to be confused with sinus rhythm, or an idioventricular rhythm with extrasystoles in the ordinary sense. Thus either rhythm may be responsible for one variety of pulsus bigeminus.As early as 1897 Cushny confirmed the existence of two separate foci of stimulus formation in the heart acting simultaneously, the slower rhythm at times interfering with the faster centre of impulse formation: his calculations were made from canine myocardiographic tracings. Later Rothberger and Winterberg (1910), using the electrocardiograph, noted that transient dissociation with interference occurred in the dog's heart during the development of nodal rhythm. In 1923, Mobitz established dissociation with interference as a pararrhythmia sui generis. It is characterized by the co-existence of two separate foci of stimulus formation in the heart, usually a slow sinus rhythm and a faster nodal rhythm, the slower rhythm at times interfering with the faster rhythm by conducted beats. These conducted beats form the only link between the two centres, and they occur when a sinus impulse reaches the ventricles outside their refractory period. As in parasystole the slower pacemaker is protected against the faster nodal centre by an entrance block. With regard to reciprocal beats, Decherd and Ruskin (1943), have applied the experimental findings of Schmitt and Erlanger (1928) regarding uni-directional block to explain their return mechanism. They postulate an area of refractoriness in the vicinity of the auriculo-ventricular node, to the neighbourhood of which the impulse returns or sends back a subsidiary impulse to the ventricles. Reciprocal beats are only found in nodal rhythm with pre-ventricular activation.In clinical practice dissociation with interference is usually observed as a transie...

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The cardiac effects of serotonin were studied in 20 young mongrel dogs. Serotonin was slowly infused through a catheter inserted into the left carotid artery with the tip of the catheter located 1 or 2 cm. above the coronary orifices. The doses for different groups of dogs were 10, 5, 2, and 1 mg. in 500 cc. of normal saline, three to six times each week. The period of infusion lasted one to six weeks. A group of four dogs acted as controls. In dogs treated for more than two weeks with doses of 10 to 5 mg., and in those treated for more than four weeks with doses of 2 mg., the following pathological findings were observed: (a) fibrotic thickening of mitral, aortic, and tricuspid valves in this order of frequency; (b) myocardial hemorrhages partially organized ; (c) focal ischemic necrosis of myocardium with or without round cell infiltrations; (d) intimal thickening of small coronary arteries either in the form of cushions bulging into the lumen or extensive subendothelial thickening with asymmetrical reduction of the lumen. The intima of these arteries showed either an acellular homogeneous substance or a fibroblastic proliferation and fibroelastic hyperplasia. These findings were considered to be related to a direct action of serotonin which plays some role in the connective tissue function.

From the Belfast City Hospital When the A-V node is the dominant pacemaker of the heart, the effect of drugs on its fundamental properties can be studied, especially with reference to the duration of its refractory period and the rate of anterograde and retrograde conduction. Digitalis has been observed to precipitate reciprocal rhythm (White, 1915; Blumgart and Gargill, 1930, Case 1). Dechard and Ruskin (1943) related the concentration of digitalis in the body to the effect produced on the A-V node. They found that digitalis delays conduction in the A-V node, retrograde conduction proportionately more so than anterograde conduction, the effect increasing with the accumulation of the drug in the body, although in the initial stages of digitalization retrograde conduction was accelerated. Their recovery curves for anterograde conduction, obtained by graphically relating the R-P and P-R intervals, showed an apparent delay of recovery of the A-V node, being most conspicuous when retrograde conduction was much prolonged.