923

difference from the blood picture in bracken-fed cattlemay be due to the ability of sheep to tolerate brackenfeeding for much longer periods. Red cells have an

appreciably longer life than white cells, and probablycattle do not survive long enough for the changes inthe bone-marrow to affect the red-cell count. Several

groups of earlier workers 6 reported similar haemato-

logical changes in cattle with bracken poisoning, but thecause of the bone-marrow lesions in ruminants is not

yet known.The ruminant differs from simple-stomached animals

in that it depends for its nutrition largely on the metabolicactivity of the micro-organisms in its rumen. These

micro-organisms break down cellulosic material, assimi-late nitrogen from the feed, and synthesise vitamins ofthe B complex and vitamin K. When the mixtureof food particles and micro-organisms passes from therumen into the abomasum, or true stomach, the bacterialcells are broken down and their contents become availablefor absorption through the normal digestive processes.Bracken poisoning in the ruminant may result fromdisturbance of normal microbial activity in the rumenleading to a deficiency in the animal of some nutrientnecessary for the maintenance of normal bone-marrowfunction. Several groups of workers

3-5 have excludedthe possibility of this nutrient being one of the knownvitamins of the B complex, vitamin K, or vitamin C ;but, as Naftalin and Cushnie point out, the dysfunctionmay be caused by lack of some essential amino-acid orof some still unidentified vitamin. An alternative hypo-thesis is that bracken contains a factor (or causes therumen organisms to produce one) with a toxic action onthe bone-marrow. But so far the only noxious principlediscovered in bracken is thiaminase, which probablyplays no important part in the production of brackenpoisoning in ruminants.

6. Schofield, F. W. Rep. Ont. vet. Coll. 1947, p. 127. Guilhorn, J.,Julou, J. Bull. Acad. vet. Fr. 1949, 22, 407; Ibid, 1950,23, 185. Evans, E. T. R., Evans, W. C., Hughes, L. E. Vet.Rec. 1951, 63, 444.

7. Kossman, C. E. Circulation, 1953, 8, 920.8. Shillingford, J., Brigden, W. Brit. Heart J. 1951, 13, 233.9. Wilson, F. N., McLeod, A. G., Barker, P. S. Trans. Ass. Amer.

Phys. 1931, 46, 29.

VECTORCARDIOGRAPHY

THE diagnostic reliability of the electrocardiogram isnow so soundly established that it is easy to forget thatthe absolute limits of normal-particularly of the P-Rinterval, and of the width of the Q R s and the Q R S Tcomplexes-are still not exactly agreed. Variations dueto age, posture, and perhaps race, in addition to thosecaused by instrumental distortion, faulty technique, andinaccurate measurement, constitute the chief difficultiesin the way of establishing statistically acceptable cri-teria.’ The use of newer leads, although invaluable indiagnosis, has increased the problem. There are othertheoretical objections to the present electrocardiogram :the standard and unipolar limb leads and the chestleads record electrical activity only in the frontal planeof the body, although the heart, being a three-dimensionalstructure in a volume conductor, has electrical forces inother planes. The study of these forces as spatial vectorswas first undertaken by Mann in 1920 but made littleprogress because their determination from successivemeasurements of the electrocardiogram was tedious evenwhen aided by optical devices, such as that of Shillingfordand Brigden.sThe conception, put forward by Wilson and his

colleagues,9 of the ventricular gradient has helped bothin understanding the electrocardiogram and in distin-guishing changes in the T wave due to abnormalities ofQRS from those due to local abnormalities of the myo-cardium. The gradient is a vectorial expression of thecombined electrical forces producing the Q R s and T waves.Its magnitude is calculated by planimetry from the

algebraic sum of the areas bounded by the Q R s andT deflections and its direction determined by plottingmeasurements in any two standard leads on Einthoven’striangle or Bayley’s triaxial reference system. Funda-mentally it is a measure of the duration and magnitudeof the electrical forces produced by lack of uniformityin the processes of excitation and repolarisation of theheart muscle. Ashman and his associates 10 have workedout normal values for the ventricular gradient and havefound them to vary within wide limits ; but they havebeen able to establish constant spatial relationshipsbetween the direction of the gradient, the Q R s vector,and the long anatomical axis of the heart, and haveshown that the gradient lies posteriorly to the long axisand the Q R s vector still farther behind. Unfortunatelymeasurement of the ventricular gradient is difficult andtoo cumbersome to be really useful in practice.The adaptation of the cathode-ray oscilloscope by

Shellong and others in Germany, and by Wilson andJ ohnston,11 has made it possible to record automaticallythe time-course of the cardiac vectors. In some waysthe direct vectorcardiograph is a more sensitive instru-ment for studying the myocardium than the electro-

cardiograph, but it has considerable limitations-for

example, in the study of temporal relationships such asthe p-R, Q-T, and T-P intervals-while normal standardsare hard to define. In addition, a standard referenceframe has still to be agreed on. Some favour Wilson’sequilateral tetrahedron with the Einthoven leads andWilson’s central terminal, recording in the frontal plane,supplemented in the sagittal plane by an electrode onthe back ; whereas others prefer more complicatedframes, such as those proposed by Duchosal and otherContinental workers. The Wilson frame is at leastsimple in practice and has the theoretical advantage ofbeing based on the Einthoven triangle. The presentationof frontal and sagittal loops- so that they can be viewedas a single spatial vectorcardiogram is not easy. Stereo-vectorcardiograms have been attempted in several ways,including double photography of model loops and theuse of additional electrodes to define other planes, butthey are unsatisfactory in practice. A special circuitwith unequal resistances producing oblique-plane vector-cardiograms which can be viewed simultaneously onadjacent oscilloscopes has been devised by Burch et al.,12and seems more promising. In an attempt to overcomethe spatial limitations of frontal-plane leads, Trethewie 13has proposed an interesting simplified electrocardio-graphy. By placing an electrode on the manubriosternaljunction and another at the xiphisternum, with twoothers at the same horizontal level in the left midaxillaryline and at the back of the right chest near the midlineof the back, he has been able to record three leads atright-angles to each other. These should detect electricalabnormalities of the heart in both horizontal and verticaldirections of the frontal plane as well as in the sagit-tal plane. Satisfactory electrocardiograms have beenobtained from cases of known heart-disease, but con-firmation from larger numbers is needed. Variations inthe normal values of the leads, especially of the T wavein the horizontal lead, require statistical study if themethod is to be of more than theoretical interest.

It is unlikely that either vectorcardiography, which isstill chiefly a research method, or Trethewie’s simplifiedsystem will replace electrocardiography. In doubtfulcases clinicians, probably rightly, rely on the case-historyand on other methods of examination rather than onstatistical criteria.

10. Ashman, R., Goldberg, M., Byrd, E. Amer. Heart J. 1943,26, 473.

11. Wilson, F. N., Johnston, F. D. Ibid, 1938, 16, 14.12. Burch, G. E., Abitoskov, J. A., Cronvich, M. S. Spatial Vector-

cardiography. New Oreleans, 1953.13. Trethewie, L. R. Simplified Electrocardiography. Melbourne,

1953.