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]
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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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    appreciable decrease, in the rate of sinus beating under vagal stimulation, the block was increased, as his curves clearly show. In these descriptions of Gaskell’s experiments there is no evidence to show that during vagal stimula- tion the tissue of the bridge is capable of conducting impulses more effectively than before the nerve stimulation was applied, the improvement of con- duction which he here describes is not the direct outcome of vagal stimulation, but is secondary to a change of rate. Two figures which he uses as illus- trations show no change of sinus rate, and in both these nerve stimulation produces a greatly enhanced block. It is here to be observed, however, that in his subsequent remarks, Gaskell refers to instances in which, though the rate was not reduced, the block became decreased. Since these instances are not illustrated, it does not seem to us entirely clear that the block was relieved actually during the stimulation, though this is the actual statement in the text. It seems to us that Gaskell is probably here describing the effects immediately following nerve stimulation. Such effects are perfectly clearly illustrated and described in other places. The usual effect of vagal stimulation, when a partial block has been established, is a lowered rate of sinus beating and the passage of the bridge by each impulse during stimulation ; immediately stimulation is withdrawn there is a rebound from inhibition, and the beats of the sinus, now accelerated, pass the bridge regularly and for considerable periods of time. From Gaskell’s description of the tortoise auricle therefore, it is quite clear that vagal stimulation may decrease the conduction power of the bridge ; it is equally clear that the original block usually becomes reduced as an immediate after effect of stimulation ; it is not quite clear if, in excep- tional cases, he saw an actual increase in the bridge’s capacity to conduct during vagal stimulation. Gaskell’s observations are summed up in his sentence :—‘‘ Although the initial effect of the vagus is to depress some function, its final and most enduring power is to exalt, intensify and repair that function.” In Engelmann’s experiments® a clamp was applied to the auricle, and the interval between the contractions of the proximal and distal segments was measured. These experiments afford little information from our present standpoint, as little or no change of interval was observed during stimulation, and for the most part vagal stimulation produced much slowing of the heart. Garrey* used the turtle’s heart. He produced intra-auricular block by applying a clamp between the right and left auricle and obtained results with vagal stimulation, which he describes as invariable. When he stimulated the left nerve and obtained no alteration of basal rate, the block was always increased; when he stimulated the right nerve and obtained little reduction of rate, a similar effect was observed ; when on stimulating the right nerve, the basal rate was much reduced, the degree of block diminished. Garrey’s experiments, and his discussion of them, particularly emphasise the double influence of the vagus, its direct and depressing effect
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