Clinical electrocardiography / by Sir Thomas Lewis.

  • Thomas Lewis
Date:
1913
Catalogue details

Licence: Public Domain Mark

Credit: Clinical electrocardiography / by Sir Thomas Lewis. Source: Wellcome Collection.

    23/136 (page 11)
    that it gives an excursion of 1 centimetre ; the curve obtained has an outline similar to those shown to the right hand of Fig. 6. The string moves when the current enters it and •eventually takes up a position 1 centimetre away. But in arriving at the new position it describes a curve. The characters of this curve are important. Fig. 6 shows six electrocardiograms from the same subject, and six responses to an E. M. F. of 1 millivolt. The curves differ because they were taken with different resistances in circuit, and con¬ sequently with different string tensions.* From above downwards the added resistances were increased and the string was therefore slackened. All the curves were taken with the string at such a tension that 1 millivolt gave 1 centimetre of excursion (see deflections to right of strips). In the first place, the movement of the string in response to 1 millivolt should be dead beat ” ; that is to say, there should be no over-shooting ; over-shooting tends to produce distortion of the electrocardiograms by increasing the amplitude of the initial deflections. Over-shooting is shown in Fig. 5 (at &) at a stage when the tension of the fibre was too great. In the second place, the movement should be of sufficient rapidity. The slacker the string, the more slowly does it respond ; the deflection times for the six strips of Fig. 6 are -013, -023, -028, 045, 060 and -070 seconds, respectively, from A to F. Now the initial electric changes which result from the heart beat are rapid, and if the quickest movement of which the string is capable is too slow to follow these changes, an accurate electrocardiogram is not obtained. Undue slackness of the string produces distortion of the curve .and this distortion is well illustrated by the series of curves shown in Fig. 6. As the string is slackened, a time arrives when the steepest deflections (R, S) are shortened * Adding resistance to the main circuit decreases the sensitiveness of tho instrument and the string must be slackened to compensate this decrease.