ANALYSIS OF RECORDS 387 



generated at dilTcreiit phases of the eardiac eycle. The space 

 between eaeh line is generally 1 mm. = 1/10, ()()() volt (Einthoven's 

 standard). The lines are engraved across the ividth of the eyHn- 

 drical lens. When illuminated they produce shadows forming 

 lines along the length of the record. The vertical lines, shortened 

 to ticks at the foot of the record illustrated, are a measure of 

 time — in the case given = one-thirtieth of a second. 



They are produced by the interruption of the focused beam of 

 light by a serrated wheel (Fig. 92) so that for a short interval no 

 light falls on the whole (or on part of the sensitised surface) as it is 

 travelling past the slit. In consequence, a sharp line falls on the 

 record. 



Before a record can be taken, it is necessary to know the resist- 

 ance of the subject's body and the magnitude of the " skin- 

 current." The latter factor is a relatively large and fairly constant 

 potential difference caused by the glandular activities of the skin. 

 It has to be counterbalanced by sending an equal current through 

 the fibre in the opposite direction. The resistance of the body to 

 the passage of a current is very rarely considered in routine 

 clinical electrocardiography. 



The analysis of electrocardiograms (Fig. 93) is liy no means 

 simple. Considerable uncertainty exists as to the exact interpreta- 

 tion of certain units in the trace. If Einthoven's symbols PQRST 

 are used it is generally agreed that P is pre-systolic and that Q 

 (positive E.M.F.) indicates that the wave of contraction does not 

 start at the base of the ventricle but a short distance from it. 

 R is no doubt the wave of negativity produced by the contraction 

 of the ventricles. The upstroke of R is inscribed just before 

 ventricular systole starts. S is the second phase or positive 

 reaction of the ventricles. The space between S and T represents 

 the time during which the whole ventricle is excited, and T probably 

 indicates the arrival of the wave of negativity at the apex, culmi- 

 nating at the moment that the ventricles begin to relax. The 

 QRS complex is a composite picture consisting of the algebraic 

 sum of the electrical elTects in both ventricles. Other interpreta- 

 tions have been given. 



It has been suggested for ease in analysis, that it is advisable to 

 compound the records from all three leads into one diagram. This 

 so-called monocardiogram represents the algebraic sum of all the 

 potential differences at every point of the cardiac cycle. 



Consider again the sheet of muscle mentioned above. The line 

 joining AB and projected to the periphery will be the electrical axis 

 when the point of stinnilation lies on it, i.e. the electrical axis is the 

 resultant direction of the electromotive changes. It is obvious 



25—3 



