The effect of vagal stimulation on intra-auricular block produced by pressure or cooling / by T. Lewis and A.N. Drury.

  • Thomas Lewis
Date:
[1923]
Catalogue details

Licence: Public Domain Mark

Credit: The effect of vagal stimulation on intra-auricular block produced by pressure or cooling / by T. Lewis and A.N. Drury. Source: Wellcome Collection.

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    varied up to and somewhat beyond 300 mm. Hg; but we were disappointed to find that such grades of compression failed to produce the desired effect, namely, an impaired conduction through the tissues of the bridge. It became obvious that the degree of compression required to produce block of different degrees could not be attained conveniently by means of pneumatic compression; a solid metal clamp was therefore built, which would submit the muscle to much higher pressures. The jaws of this clamp were 1 centi- metre wide, flat, and smooth, one jaw being moved to meet its fellow by a very fine screw having 60 threads to the inch. The method of experiment is as follows :—Dogs of about 10 kilogrammes weight are employed and these are fully anaesthetised with morphia, paraldehyde and ether. The heart is exposed, the tip of the right appendix secured and passed through the jaws of the clamp, until the latter rests at the base of the appendix. The tip of the appendix is then fastened without material tension to a convenient part of the chest wall, and the clamp secured to an adjustable stand. Two pairs of non-polarisable contacts are now placed in the line of the auricular appendix, one pair lying proximal and the other distal to the clamp ; gaps of about a centimetre being allowed between the edges of the clamp and the nearest contacts, to avoid disturbance of the contacts on tightening the clamp. Each pair of non-polarisable contacts is connected to one string of the recording galvanometer. A pair of stimu- lating electrodes is placed on the body of the right auricle, and in line with the two pairs of recording contacts, and into these rhythmic induction shocks are sent, at a rate slightly surpassing the rate of the natural heart beat after section of the two vagi. To these rhythmic shocks, which are maintained throughout the experiment, the auricle responds, and with each response the excitation wave passes to the proximal contacts, across the area involved by the clamp and to the distal or appendicular contacts; the arrival of the wave at the two pairs of contacts is thus recorded, and the corresponding transmission intervals are subsequently measured. By graduating the clamp pressure it is easy to obtain various grades of block between the body of the auricle and the appendix. The first change observed on tightening the clamp is a widening of the transmission interval ; this widening increases until the auricular appendix occasionally fails to respond ; with more severe pressure this failure to respond becomes more frequent, 2: 1 block becomes established, and finally, with heavy pressure, no beats are transmitted. At any of these stages, the effect of vagal stimulation upon the block so induced may be tested. It is to be observed, however, that if pressure is applied and a certain grade of block is obtained, maintenance of this same pressure for unduly long periods of time is accompanied by an increase in the grade of block; the pressure used is much more than is sufficient to bring the blood supply to the appendix to an end, and the muscle will not continue for many minutes to beat in these circumstances. It is necessary, therefore, by preliminary tests, to gain an idea of the pressure needed to
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