Although it is nearly 70 years since ':'Taller (1887) first recorded the human electrocardiogram, there is still a divergence of opinion among various workers as to the genesis of the electrocardiogram. It is generally agreed, however, that one of the principal factors involved is the order in which the various parts of the heart are excited. The finer details of the manner in which-the excitation wave spreads both in the auricles and in the ventricles are still not clear and therefore various assumptions have had to be made in regard to this for the analysis of the electrocardiogram.
The purpose of this study was to investigate certain aspects of the spread of excitation in cardiac tissues as it relates to the genesis of the electrocardiogram. This entailed an investigation of some fundamental properties of these tissues. In Part I of the thesis, observations on the monophasic action potentials from different species of mammalian hearts, recorded by means of a Ling -Gerard intra- cellular microelectrode are described. This part is in general a correlative study of the monophasic action potential and the electrocardiogram obtained from the same animal.
Structurally, the heart is complex in that (1) it is a syncytium, (2) there are several muscle bands running in different directions which form the walls of the atria and the ventricles and (3) a specialised conducting tissue is widely distributed in the ventricular walls and sometimes in the atria. An investigation of the spread of
- 2 - excitation in the heart would have to take into account these structural characteristics of the pathway. ,Part II describes observations on the structural aspects of the subject.
It is also difficult to really understand the spread of excitation in the heart until some knowledge has been ob- tained about the rate of conduction in the various cell types making up the excitable tissues of the cardiac musculature. As will be clear from the review in Part III, this property has never been studied adequately. Particular attention has therefore been paid to this aspect of the subject in the present study and this investigation is described in Part III.
Various theories of the genesis of the electrocardio- gram have been advanced as our knowledge of the manner of spread of excitation in the heart has increased. The de- velopment of ideas regarding the spread of excitation in the mammalian heart has provided a general background to the present studies and it has therefore been briefly reviewed. ThÈ. literature on the subject is so extensive that to give a complete review is neither possible nor relevant to the present study. Only those papers which in one tray or other relate to the observations made in this investigation have been included in the survey. Early work has been reviewed by Gaskell, (1900), Erlanger, (1913) and by Davies & Francis, (19)66). The literature on the genesis of the electro- cardiogram has been extensively reviewed by Katz, (1928, 1947).