Congestion may be defined to be excess of blood in the vessels of a part, and is to be regarded not as an independent disease, except in a very small proportion of cases, but as one stage or one manifestation of some other disease. It is, however, of such importance and of so frequent occurrence as to require a separate notice. Congestions are divided into two groups—viz. active, due to dilatation of the arteries of the affected part; and passive, due to some cause interfering with the return of blood by the veins. In active congestion the flow of blood is usually increased, and the excess is chiefly in the arteries. Familiar instances are the phenomenon of Blushing (q.v.) and the effect of heat, especially moist heat, in increasing the redness of any part to which it may be applied. But the most important active congestions are those which occur in connection with Inflammation (q.v.), of which they constitute the first stage. In passive congestion the flow of blood is always diminished, and the excess is chiefly in the veins. It may be due either to direct obstruction to the return of blood through the veins, or to general enfeeblement of the circulation.
Congestion from venous obstruction is easily illustrated by tying up the arm, as is done before opening a vein, when the veins are compressed more than the arteries. If the ligature is kept on for a sufficient time, the veins swell, the fingers become red, and then livid, and the whole limb is swollen. Cold applied to the surface of the body acts similarly on it, and contracts the veins more rapidly than the arteries, which lie deeper; and the purple colour of the hands and face after exposure to cold shows the congested state of the capillaries. 'Congestions,' says Dr C. J. B. Williams, 'are caused in internal organs by an obstruction of the veins leading from them. Thus, congestion of the brain may be produced by a tight cravat or by a tumour pressing on the jugular veins. Efforts of straining, coughing, holding the breath, and asthmatic paroxysms which impede the flow of blood through the lungs, cause congestion in various parts. Tubercles in the lungs cause congestion of that organ. Obstruction to the transit of blood through the liver causes congestion in the abdomen, hemorrhoids, &c.'
Congestion from weakness of the circulation includes a numerous class of cases. Here gravitation comes into play very much more forcibly than in the healthy body in determining the distribution of the blood to different parts and organs, though it always produces some effect. Diseases of the valves of the heart are almost always accompanied by congestion of other internal organs at some stage of their progress. In extreme debility, certain fevers, &c., there is general congestion of the parenchymatous organs—the lungs, liver, &c.—and the blood gravitates to the lowest parts, giving rise to what is termed hypostatic congestion of the posterior parts of the lungs, the skin of the back, &c. In other cases the effect is more local, as when the feet swell after long standing, in consequence of over-distension of the veins. Congestive affections of this kind are often mistaken for inflammation, and instead of being treated by tonics, are treated by depletion, which, although affording temporary relief, usually aggravates the evil.
Active congestion, when it requires treatment, must be dealt with in the same manner as inflammation. With regard to passive congestion, the end to which treatment must be directed is the removal of its cause. This can be effected in some cases by measures having a local effect, either completely, as by the loosening of a ligature, the disuse of too tight articles of clothing, or by the removal of a tumour compressing veins, or partially, as by elevation of the head in affected brain, and the recumbent position in congestion of the hemorrhoidal or uterine vessels. Uniform gentle pressure, by supporting the weak vessels, and friction, by increasing the onward movement of the blood in the veins, are often of great use in superficial parts. Where the heart-power is at fault, strengthening remedies must be given—ammonia, cinchona bark, digitalis, &c. The American Witch-hazel (q.v.) when administered internally seems to have the power of diminishing the size of dilated veins, and is sometimes useful in varicose veins, hemorrhoids, &c. Various remedies are supposed to have a special power of removing the congestion of certain organs; thus, mercurials are recommended for congestion of the liver; digitalis and cantharides for congestion of the kidneys; squills, benzoin, and the balsams for bronchial congestion.