This term is applied to conditions in which the vessels, and especially the capillaries, contain an excess of blood. Two forms are distinguished according as, on the one hand, the blood is too freely admitted by the arteries, or, on the other, meets with some obstacle to its passage by the veins. These are called respectively active or arterial and passive or venous hyperemia.

I. Active Or Arterial Hypermia. Active Congestion

This occurs when an excess of blood is admitted by the arteries into a part. The terms atonic and arterial hyperemia are synonymous with active hyperemia, as are also fluxion and determination of blood.

Causation

Active hypersemia is brought about chiefly by causes which induce dilatation of the arteries, although a local increase of blood pressure, by forcing more blood into the arteries, will have a similar effect.

Local increase of blood-pressure is not of frequent occurrence, because as a general rule the vessels are so completely under the regulation of the nervous system that a rapid accommodation is readily effected by contraction of. the arteries. If the arteries be diseased, however, so that their walls are rendered rigid (as by atheroma) then variations in blood-pressure will not be readily compensated. Hence in people whose cerebral arteries are atheromatous, temporary increase of blood-pressure may cause congestion of the brain.

Collateral hyperemia might be supposed to be due to a local increase of blood-pressure. It occurs when in consequence of obstruction of an artery, or otherwise, the blood supply is diminished in one locality, with the result that there is an increase in the amount of blood in another, generally a neighbouring locality. Doubtless an obstruction in an artery leads to increase of pressure in the arteries proximal to the obstruction, and there may be a resulting hyperemia of neighbouring parts which will assist in establishing an anastomotic circulation. But the problem is not always so simple as this. The arteries are so completely under the command of the nervous system that the site of the collateral hyperemia is frequently at a certain distance from that of the anaemia, and is determined not by increase of blood-pressure but by relaxation of arteries. Thus obstruction of one renal artery will induce collateral hyperemia in the kidney of the opposite side.

Dilatation of arteries so as to lead to active hyperemia is brought about in several different ways. It may be due to purely local causes, or to an action on the vaso-motor nerves or vaso-motor nerve-centres.

As examples of dilatation of arteries from Local causes may be cited cases in which the sudden withdrawal of pressure induces local hyperemia. When the abdomen, for instance, is the seat of a collection of fluid, pressure is exercised on the vessels, and the arteries will relax as completely as possible to allow the blood to overcome the pressure from without. But if the fluid in the abdomen be suddenly removed, the pressure outside the vessels will be greatly reduced, and, till the arteries have time to contract, there will be an active hyperemia. This effect of the sudden removal of pressure is generally, at least in part, obviated by the application of a bandage. On similar principles removal of large ovarian tumours that have pressed on the kidneys and renal arteries sometimes leads to active hyperemia of the kidneys, which may be accompanied by albuminuria.

Other examples of active hyperemia due to local causes are afforded by the. application of warmth to the surface of the body as by poultices, the result being a direct dilatation of the cutaneous arteries. Mechanical irritation of the skin, as by stroking it, usually induces, first a contraction of the arteries, evidenced by paleness, and then a dilatation, shown by a red streak. Similarly, chemical irritants produce dilatation, sometimes preceded by contraction, the dilatation leading to active hyperemia, which, in this case, may be the first phenomenon of inflammation. Again a mere temporary deprivation of blood may induce dilatation of arteries and a local hyperemia. Thus in surgical operations, when the circulation has been suspended by the application of an elastic bandage, the removal of the latter is often followed by an active hyperemia.

Active hyperamiia due to causes influencing the Vaso-motor nerves may be produced by paralysis of the vaso-constrictors or by irritation of the vasodilators. Injury or disease of the sympathetic nerve in the neck is sometimes followed by unilateral congestion of the face, and by unilateral sweating, together with narrowing of the pupil, smallness of the eyeball and ptosis, these all being signs of paralysis of the sympathetic.

Such eases have been described by Weir Mitchell and others as following gunshot wounds, etc., and by Hutchinson as the consequences of fractures of the clavicle; some of the manifestations, at least, may ensue from the pressure of tumours, or aneurysms, or the extension of abscesses, tuberculosis, etc., to the neighbourhood of the sympathetic in the neck.

In certain forms of neuralgia, especially in hemicrania, the phenomena indicate first an irritation and then a paralysis of the vaso-constrictors. Du Bois-Reymond by analysis of his own symptoms in such attacks came to this conclusion. In his case there was redness of the external ear; in other cases redness of the retina has been observed. The temperature as tested by the thermometer, in the external meatus, is usually raised, both in the neuralgic cases and those due to injury.

There are comparatively few cases of active hyperemia which can be directly referred to irritation of the vaso-dilators, but after wounds of nerves, there sometimes occur during the process of healing, severe attacks of pain accompanied by redness and elevation of temperature, which seem only explicable on the supposition that the vaso-dilators are irritated. These phenomena are sometimes followed by trophic ' disturbances, more particularly the " glossy skin " of Paget.

Paralysis of Vaso-motor centres is calculated to induce active hyperemia if the dilatation of the arteries be localized. Thus in a case observed by the author there was a traumatic lesion of the medulla oblongata which injured the vaso-motor centres of the kidneys. The result was an intense hyperemia of both kidneys, visible after death, and manifested during life by an excessive secretion of watery urine. During the few hours of life a large quantity of urine was twice removed by catheter, and after death the bladder was found greatly distended. Some authors ascribe diabetes mellitus to paralysis of the centres in the medulla oblongata, or in the semilunar ganglia. Such a paralysis would induce congestion in the abdominal organs, and more especially in the liver.

Phenomena Of Active Congestion

When a local dilatation of the finer arteries of a part occurs, the most direct result is that the blood. is admitted more freely, and at an accelerated rate. It meets with less resistance in the arteries, and reaches the capillaries and veins at a higher pressure than normal. The part so affected is of a bright red colour, and, if it be an external part, its temperature is raised. In many cases the active congestion is of short duration, but if it continue it leads commonly to more definite changes. There is increased transudation from the vessels, and consequent swelling. The secretion of the part is increased. This has been observed more particularly in the case of the secretion of sweat; unilateral hyperemia and sweating have been ascribed to pressure on the sympathetic by an aneurysm (Gairdner) or a tumour (Ogle and Verneuil). There may also be considerable hypertrophy in consequence of active hyperemia. (See Hypertrophy).

It has been said that active hyperemia may lead to haemorrhage, but experiment seems to prove that even a very great rise in the blood-pressure in the capillaries does not lead to haemorrhage, unless the vessels are badly supported, or else defective in some way. By obstructing the respiration in a dog the arterial pressure may be raised enormously, but there is no rupture of the capillaries, or only in such delicate structures as the retina, brain, or conjunctiva, or in the pleura and pericardium, where, on account of the violent and abortive efforts of inspiration, a partial vacuum is produced, and the vessels are ill-supported. The increase in pressure in a pure active hyperemia is, of course, greatly less than this.

Literature

Weir Mitchell, Morehouse, and Keen, Gunshot wounds and other injuries of nerves, 1864; Hutchinson, Lond. Hosp. Reports, 1866; Weir Mitchell, Injuries of nerves and their consequences, 1872; Denmark, Med. chir. trans., iv.; Paget, Med. Times and Gaz., 1864; Berger (Hemicrania), Virchow's Archiv, vol. liv., 1874; Gairdner, Clin. Med., 1862, p. 557; Verneuil, Gaz. des hopit., 1864.