PREMATURE CONTRACTION, STAIRCASE, SUPERPOSITION OF CONTRACTIONS AND OPTIMAL RHYTHM IN THE MAMMALIAN HEART, WITH SPECIAL REFERENCE TO THE VENTRICLES
Authors: Macwilliam J. A. 
Published: December 2014

1. Under experimental conditions, with the use of various anaesthetics, the ventricular muscle of the cat and rabbit exhibits different kinds of behaviour in regard to the effects on contraction force of variations in the length of the intervals between the beats—these may be provisionally termed types I., II., and III.

2. Type I. is the normal, showing smaller premature curves and enlarged delayed beats, the optimal response being obtained after very long intervals. In type II. beats delayed to a sufficient extent are markedly reduced, there being an early decline in contractility beyond a certain interval, which may be very short. When the existing rhythm is too slow a certain degree of prematurity is often associated with enlarged so‐called “supernormal” excursions; these are not really supernormal but represent a return towards the normal from a depressed level of strength. The influence of the optimal length of the interval is very pronounced. Bigeminus with a regular succession of enlarged premature contractions is often seen. Type III. is essentially a modification of type I., although showing, under the influence of chloroform and the like, some features suggestive of those of type II., such as superposition of contractions.

3. A staircase of beats is seen under certain conditions, whether the beats result from artificial stimulation or occur spontaneously.

4. Superposition of contractions, more or less complete, may often be obtained in types II. and III. The superposed contraction may equal or exceed the primary beat in height.

5. The influence of certain (optimal) rates of rhythm may be strikingly demonstrated in type II., the beats showing much larger excursions than at rates that are either much slower or quicker. The arterial pressure may be either raised or lowered during these phases of optimal rhythm. With certain ventricular rates during auricular fibrillation the ventricle excursions may be greatly altered without parallel changes in arterial pressure.

6. The specific characters of the different types are not necessarily evidenced by the arterial pulse record, although enlarged premature contractions of the ventricular record are sometimes associated with (higher) premature curves in the aortic records.

7. One type can often be converted into another by appropriate treatment with drugs, experimental procedures, etc., without there being any constant relation to altered physical conditions.

8. Type I. has been constantly found in the auricle of both cat and rabbit. The ventricle of the cat frequently shows type II., while type I. is also common; type I. is often changed to type II. after an asphyxial phase (from interruption of artificial respiration), injection of adrenaline, etc. In decerebrated cats type II. is commonly strongly marked. In the ventricles of the rabbit the normal type I. may be converted into type II. by treatment, e.g. injection of adrenaline following depression by a potassium salt given intravenously, asphyxia, and so on.

9. The essential features of the different types may be recognised in the heart temporarily emptied by closure of the venae cavae.

10. In the different types the influence of variations in the length of the diastolic intervals upon the size of the beats comes into consideration when the rate of the heart is increased or diminished through the vagi or the cardiac augmentor nerves, as results of section or stimulation of these nerves. The same applies to the effects of alteration in the heart's rhythm from any cause, e.g. from change in the rate of the pacemaker or from artificial excitation of the auricle or ventricle at regular rates.

11. Irregular spacing of the ventricular beats, whether due to irregular impulses from the auricles along the A‐V bundle (as in auricular fibrillation) or to direct stimulation of the ventricles, or the occurrence of extra‐systoles, is attended by much greater and more complex disturbances in the ventricular record in type II. than in type I.

12. Evidence of various kinds is available to show that the specific characters of the types described are due to differences in the dynamic conditions of the muscle and are not explicable on mechanical grounds, such as variations in the fullness of the ventricles at the beginning of systole, changes in the arterial resistance, and the like.

13. The special phenomena of type II. are dependent on altered dynamic conditions associated with rapid “recuperation” the early establishment of conditions favouring optimal mechanical response being succeeded after the optimal phase by a speedy decline to conditions involving a less effective response to excitation. The brief duration and early occurrence of the optimal period in type II. are in contrast to what is present in the normal type I.