OBSERVATIONS UPON THE VASO‐MOTOR REFLEXES
Authors: Vincent Swale , Cameron A. T. 
Published: December 2014

When an animal is deeply under the influence of ether, it is frequently impossible to obtain any vaso‐motor reflexes whatever. When anaesthesia is of a certain degree of completeness, the effect of stimulating the central end of the cut sciatic nerve is a distinct rise. As the effect of the anæsthetic begins to pass off, the rise is followed by a more or less pronounced fall. Respiratory movements are now markedly increased, and the extent of the fall of blood‐pressure appears to be largely proportional to the violence of the respiratory activity. During the actual stimulation there is either a very slight rise, or a more marked rise, or the pressure remains about constant, while the fall usually begins at the moment of cessation of the stimulation.. There is thus a clear indication of the action of two opposing influences, a reflex vaso‐constriction producing a rise, and violent respiration producing a fall.

A similar fall of pressure is brought about by performing rapid artificial respiration by compression on the thorax, and can be induced in the human subject by rapid and deep voluntary respiration.

The effects so far described (except the last) apply to animals anæsthetised with ether, and the same is generally true if chloroform chloral hydrate, or urethane is employed. When the animal is under the influence of morphia, electrical stimulation of the central end of the sciatic may produce a rise of pressure, while mechanical stimulation of the skin produces a fall, but there is every reason to believe that this is after all a quantitative and not a qualitative difference; for weak stimuli tend to give a fall, and strong stimuli a rise, and it is difficult to apply a mechanical stimulation to the skin which can be regarded as the equivalent of a rapidly interrupted electrical stimulation of the sciatic.

When the animal is fully under the influence of curare, stimulating the central end of a cut nerve trunk will in the great majority of cases cause a rise of blood‐pressure. This effect seems, largely at any rate, to be due to cessation of respiratory movements.

The more widely the thorax is opened, the more does the fall of blood‐pressure obtained by stimulation of a nerve (moderate or powerful stimulus and a certain degree of ether anæsthesia, vide supra) tend to become replaced by a rise. The lowering of blood‐pressure when the central end of the sciatic nerve has been stimulated, as also when rapid artificial respiration has been performed by compression of the thorax, and further also when rapid and deep voluntary respiration is carried out by human beings, is probably due to direct mechanical influence upon the heart'
s action, or interference with the return of blood to the heart, or to both of these factors.

The result of a weak stimulation is usually a fall of blood‐pressure, while a pressor effect follows as a rule a stronger stimulus (confirmatory of Knoll and Hunt). This fact probably explains many of the discrepancies encountered between the results of different observers, and even by the same observer in different experiments. The effect of stimulating a few afferent fibres will be comparable to the effect of a weak current. The difference between weak and strong stimuli we have not observed when the thorax is opened.

Cooling the nerve tends to produce vaso‐dilator effects (confirmatory of Howell, Budgett, and Leonard, and of Reid Hunt). This also we have been unable to observe with the thorax opened.

In dogs, stimulation of a recently regenerated sciatic nerve produces a fall of blood‐pressure under circumstances when stimulation of the normal sciatic (control on opposite side) produces a rise (confirmatory of Hunt upon cats).

Stimulation of the skin, kneading of muscle, and manipulation of the intestines all cause a fall of blood‐pressure under certain conditions, and a rise under other conditions. These are, we believe, the same as for the corresponding results with a nerve trunk, though this is not always easy to demonstrate.

The vaso‐motor reflex from the intestine is abolished by full doses of nicotine, is reduced by section of the great splanchnic nerve on both sides, and is abolished by extirpation of the semilunar ganglion.

We have discussed in the body of the paper the bearing of the above results on the condition or conditions sometimes referred to as “surgical shock.”

The evidence derived from our experiments which seems to be in favour of the existence of vaso‐dilator fibres in afferent nerve trunks consists of results in certain animals, in which, no matter what the condition, with thorax opened and curare administered, stimulation of such nerve trunks resulted in a fall of blood‐pressure, and secondly, from the observations made during experiments on the kneading of muscle, in which marked double‐fall effects were produced, easily traceable to separate causes: a vaso‐dilatation, and a respiratory fall.

Whether a rise or a fall of blood‐pressure will result from stimulation of an afferent nerve (terminals or cut central end) depends on the relative effects of the three primal causal factors: (a) reflex vaso‐dilatation, (b) reflex vaso‐constriction, and (c) frequency and depth of respiration. The predominance of one ornmore of these factors is determined by (i.) the strength of the stimulus, (ii.) the number of fibres stimulated, (iii.) the temperature of the nerve, (iv.) the nature of the anæsthetic, (v.) the degree of anæsthesia, and (vi.) the idiosyncrasy of the animal (i.e. other not yet ascertained factors).