The rhythm of the heart beat. II—Disturbance of rhythm produced by late premature beats

The disturbance of cardiac rhythm by a premature beat was first studied by Marey (1875), who found that the lengthening of the cycle following the premature beat usually just compensated for the shortening of the cycle preceding it, the two cycles together being equal to two normal cycles. The phase of the rhythm therefore suffered only a transient disturbance by the premature beat. Engelmann (1895) obtained similar results when premature beats were set up by stimulation of the frog's ventricles, but later (1897) be stated that there was no lengthening of the cycle following the premature beat set up by stimulation of the vena cava. On the basis of these observations be put forward the suggestion that the ventricle beats in response to discrete impulses discharged rhythmically from the sinus. A premature beat set up by stimulation of the ventricle would be prevented from reaching tbs sinus by the next normal impulse from there, and so the essential rhythm of the heart (the sinus rhythm) would remain undisturbed, i. e., there would be a completely compensatory pause following the ventricular premature beat. Cushny and Matthews (1897) working for the first time with premature beats of mammalian hearts found that an auricular premature beat was only followed by a compensatory pause when set up relatively late in a cardiac cycle. The pause following earlier premature beats was too short to be compensatory. Hering (1900) confirmed both Engelmann and Marey by finding complete compensation after ventricular premature beats, but this was rare with auricular premature beats and be stated that premature beats set up by stimulation of the vena cava were followed by a pause equal to a normal cycle (cf. Engelmann), though his figures show a slight lengthening.