A.Garrard Macleod

The determination and the significance of the areas of the ventricular deflections of the electrocardiogram

Abstract By measuring the areas of the ventricular deflections of the electrocardiogram it is possible to determine the mean electrical axis of QRS, which gives the direction in which the excitatory process spreads over the average element of ventricular muscle, and the mean electrical axis of T, which gives the inverse of the direction in which the recovery process spreads over the average element of ventricular muscle. If all the ventricular muscle passed through the period of excitation in the same time and in the same way, the area of QRS and the area of T would be equal in absolute magnitude, but opposite in sign, and the area of QRST would be zero. The area of QRST is a measure of the electrical effects produced by local variations in the excitatory process. The mean electrical axis of QRST gives the direction of the line along which these local variations are greatest. The local variations in the excitatory process which determine the mean electrical axis of QRST are dependent upon factors that act upon different parts of the ventricular muscle with different intensities. They are not materially influenced by the course of the excitatory process over the ventricular muscle.

Electrocardiograms that represent the potential variations of a single electrode

Abstract In order to simplify the analysis of the curves obtained by leading from the precordium and for certain other purposes, we have devised leads that record the potential variations of a single electrode. Electrodes are placed on the right arm, left arm, and left leg in the usual way and connected through like resistances to a central terminal. The resistances used for this purpose should be large in comparison with the resistance of the body in standard leads. Theoretical considerations and experiments on a model indicate that under these circumstances the potential variations of the central terminal are negligible. The curves obtained by leading from an exploring electrode in contact with any part of the body to the central terminal represent the variations in potential produced by the heartbeat in the region in contact with the former. The potential variations of the right arm, left arm, and left leg are recorded by leading from the electrodes placed on these extremities to the central terminal. They may be compared with the potential variations that occur in various parts of the precordium. To increase the resistance in the input circuit of our recording apparatus we have connected the string galvanometer to the balanced plate circuit of a one-stage vacuum-tube amplifier

The order of ventricular excitation in human bundle-branch block

Abstract Leads in which one electrode (the exploring electrode) is placed close to the heart and the other (the indifferent electrode) at a distance from it are semidirect leads. In experiments on dogs serial semidirect leads were taken from the surface of a pad of gauze soaked in warm normal salt solution and laid upon the exposed heart. The galvanometer connections were made in such a way that relative negativity of the exploring electrode yielded an upward deflection in the completed curve. In animals in which bundle-branch block had been produced, the chief upstroke of the ventricular complex occurred early in those semidirect leads in which the exploring electrode was close to the surface of the contralateral, and late in those leads in which this electrode was close to the surface of the homolateral ventricle. In the former case was usually negative; in the latter case, usually positive. In precordial leads the exploring electrode is placed on the precordium and the indifferent electrode on the left leg. Serial precordial leads in cases of clinical bundle-branch block of the common type yield results similar to those obtained by semidirect leads in experimental left branch block. In the rare type of bundle-branch block they yield results similar to those obtained in experimental right branch block. The common type of bundle-branch block is left branch block; the rare type, right branch block. The chief ventricular upstroke of semidirect leads, in which the exploring electrode is placed close to the ventricular surface, corresponds to the intrinsic deflection of direct leads from the ventricles in which a single contact is placed upon the muscle.

The excitatory process observed in the exposed human heart

Abstract 1. 1. A study of the arrival of the excitation process at a number of points on the surface of the human heart with reference to the beginning of R in Lead II was made. It was found that the order of excitation differs in some respects from that found by Lewis and Rothschild in the dog. The earliest points were on the anterior surface of the right ventricle near the atrio-ventricular border. Our results suggest that the conducting tracts in the right ventricle of man may differ from those of the dog. 2. 2. The curves produced by stimulation of various points on the surface of the human heart were recorded in the three standard leads taken simultaneously. These curves indicate that: 1. (a) Ventricular premature contractions of right ventricular origin are represented in the electrocardiogram by ventricular complexes in which the chief initial deflection is upward in Lead I. Ventricular premature contractions of left ventricular origin are represented in the electrocardiogram by ventricular complexes in which the chief initial deflection is downward in Lead I. 2. (b) The clinical electrocardiograms at present ascribed to block in the right branch of the His-bundle indicate block in the left branch, and vice versa . 3. (c) In so-called left ventricular preponderance the electrocardiogram is dominated by right ventricular effects and vice versa .

A clinical type of paroxysmal tachycardia of ventricular origin in which paroxysms are induced by exertion

Abstract There is a type of paroxysmal ventricular tachycardia in which the abnormal mechanism is induced by emotion and exertion. The attacks may be long, but more often short attacks separated by periods of extrasystolic arrhythmia occur in rapid succession. In the majority of the cases there is no other evidence of cardiac disease. Patients with this disorder are usually seriously incapacitated. Small doses of quinidine are often strikingly beneficial but do not always prevent the occurrence of attacks. In one of the cases studied by the authors a long paroxysm of ventricular tachycardia was frequently interrupted temporarily by short attacks of paroxysmal auricular tachycardia.

The significance of electrocardiograms characterized by an abnormally long QRS interval and by broad S-deflections in lead I

Abstract Serial precordial leads have been used in an attempt to locate the conduction defect responsible for electrocardiograms characterized by a QRS interval measuring 0.12 second or more and by narrow R-deflections and broad S-deflections in Lead I. In Lead III there is a narrow Q or S deflection synchronous with R in Lead I and a broad upward deflection synchronous with S in the same lead. In cases in which the standard electrocardiogram is of this type precordial leads from the right side of the precordium show a very late chief upstroke; precordial leads from the left side of the precordium show an early chief upstroke approximately synchronous with the peak of R in Lead I. These curves are strikingly similar to those obtained by the same method of leading after section of the right branch of the His bundle in dogs. For this reason it is believed that electrocardiograms of the kind mentioned represent right bundle-branch block in man. There is much less difference in frequency between clinical right and clinical left branch block than has heretofore been supposed.

The potential variations produced by the heart beat at the apices of Einthoven's triangle

Abstract A method is described by means of which it is possible to determine the potential variations produced by the heart beat at any one or all of the apices of Einthovens equilateral triangle. It is consequently possible to determine the potential variations produced by the heart beat at any point of the body by leading from this point to the left leg, or the left or right arm, and subtracting the effect produced by the potential variations of this extremity from the recorded curve. It is possible by this method to free curves obtained by leading from points on or near the heart to points at a distance from it, such as points on the left leg, from the influence exerted by potential variations at the distant electrode.

The electrogram of cardiac muscle: An analysis which explains the regression or T deflection☆

Abstract 1. 1. A method for recording electrograms from the uninjured frogs auricle has been described which approximate those obtainable from a simple strip of muscle. 2. 2. Starting with the observation that a potential difference exists between active and resting muscle, an analysis of the processes of activation and recovery has been made. Based upon this analysis, a graphical method for the construction of a complete theoretical electrogram has been devised. Any assumptions regarding the properties of the excitation process may be made and the appropriate theoretical electrogram plotted. When a theoretical and an actual electrogram accurately correspond, the properties of the excitation process in the muscle which produced the actual curve are presumed to be similar to those assumed in constructing the theoretical one. 3. 3. The regression, like the accession deflection, is expressed in a diphasic curve, the central portion of which may be parallel to the isoelectric line.

The accuracy of Einthoven's equation☆

Abstract The accuracy of Einthovens equation does not depend upon the resistance of the string galvanometer. In a properly standardized electrocardiogram the deflection at any instant is an accurate measure of the potential difference that would have existed between the body points to which the galvanometer terminals were attached had the galvanometer not been connected with the body.

The effect of certain pure digitalis-like glucosides on the frog's heart

Abstract Certain glucosides of the digitalis group have been found to reduce the duration of electrical systole (Q-T) of both the auricles and ventricle of the frog. The refractory period is shortened pari passu with this reduction of electrical systole. It was not possible to bring out any differences in the mode of action of the various glucosides tested by the methods used.