This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
In fainting there is sudden unconsciousness, which appears to be caused by sudden arrest of the supply of blood to the brain. This arrest may be due to a rapid fall in blood-pressure, either from stoppage of the heart, rapid dilatation of the arterioles, or sudden removal of pressure from the larger vessels. It is possible that these conditions may be associated with spasmodic contraction not only of the vessels of the face and surface generally, but of those supplying the brain itself. The effect of sudden change from a horizontal to an upright posture in producing syncope has already been mentioned (p. 205). Sudden removal of external pressure from the great vessels acts upon both arteries and veins. It removes external support from the arteries, and allows them to yield more readily to the influence of the blood-pressure, and by their dilatation to lessen it. It allows the large veins also to dilate, and blood to stagnate in them. Its influence is readily seen when fluid is removed too suddenly from the abdomen, and external pressure by a bandage not supplied in its place, as in cases of ascites.
Fig. 84. - Diagram to show the effects on the cerebral circulation of rapidly emptying the bladder.
It is seen, perhaps, even more strikingly, where the bladder has been allowed to become distended and is suddenly emptied. The effect of this is shown in Fig. 84. In a the bladder is represented as full, and, the pressure within the abdomen being considerable, the veins are prevented from dilating, the heart is well supplied with blood, and the circulation in the brain is active. In b, the bladder is represented as empty, and the abdominal contents being diminished, so that the intra-abdominal pressure is lessened, not only do the aorta and other vessels become lax from loss of the external pressure, but the veins dilate, the heart is imperfectly supplied with blood, the cerebral circulation fails, and syncope ensues. This occurs more readily just after waking, before the vaso-motor centre has recovered its usual tone, so that one of the most favourable conditions for its occurrence is when a man jumps suddenly into the upright position and empties his bladder immediately on waking. The consequence of this sometimes is that he falls down suddenly, quite insensible, during the act of micturition. I have seen one case in which the tendency appeared to be increased by the practice of opium-eating, probably from the diminished excitability of the vaso-motor centre produced by the drug. It is evident that the danger will be increased if the intervals between the systoles of the heart are prolonged, and it is the combination of the natural tendency to syncope, produced by large doses of digitalis, with that caused by the sudden assumption of the upright posture, and by the rapid emptying of the bladder, which renders micturition in the upright posture so excessively dangerous in persons under the action of digitalis, and leads so frequently to death.
It is evident that fainting may be prevented by increasing the blood-pressure in the brain locally, or throughout the body generally. To increase it locally the head of a fainting person should be allowed to lie level with the body, or a little below it, and on no account raised even by pillows. A fainting fit may indeed often be prevented by sitting with the head hanging between the knees. It may also be prevented or removed by such conditions as raise the general blood-pressure, e.g. a draught of cold water, which causes contraction of the gastric vessels, or a sniif of ammonia or acetic acid, which stimulates the nasal nerves, and causes reflex contraction of the vessels generally. In some parts of India the natives are accustomed to bring persons round from a faint by compressing the nostrils and holding the hand over the mouth, so as completely to stop respiration. The accumulation of carbonic acid in the blood irritates the vaso-motor centre, raises the blood-pressure, and thus probably tends to bring the person round.
In shock there is no unconsciousness, but the failure of the circulation is even more profound than in syncope. Its pathology is not perhaps exactly ascertained, but it probably depends to a great extent on a paralytic distension of the great veins, as in Goltz's experiments. I have found that in shock produced in a similar manner in a rabbit the blood-pressure could be raised from two inches up to two and a half by the inhalation of ammonia.