A term much employed in connection with disturbances of the circulation is "compensation," which refers to the ability of the heart to maintain arterial pressure in spite of some condition or lesion which tends to make the arterial pressure low. It is the ability of the heart to compensate for some leakage or other adverse condition. We speak of the lack or failure of compensation when the heart is unable to maintain adequate arterial pressure. The effects of failure of compensation are: (1) General venous and pulmonary engorgement, with lymphatic damming up and a tendency to edema and dropsy. (2) Diminished supply of blood to the organs. (3) Poor aeration of the blood on account of the sluggish pulmonary circulation. The symptoms are: Labored breathing, inability to lie flat, weak and dilated heart, rapid pulse, sluggish peripheral circulation with cold extremities, cyanosis, and perhaps edema or dropsy.

Ordinarily, when a lesion, e. g., a defective valve, would tend to interfere with the heart's ability, there is a natural compensatory hypertrophy of the muscle and a compensatory enlargement of one or other of its cavities, which is spoken of as "dilatation"; so that in spite of quite a marked lesion of the heart, compensation may be maintained. Thus if there is a lesion of the mitral valve which permits leakage, then at each systole some of the blood from the ventricle is forced back through the leaking valve into the auricle, instead of forward into the systemic arteries. In consequence, the heart would not be able to keep up the systemic circulation were it not for the fact that in response to requirement the cavity of the ventricle becomes more capacious, and the muscular walls become hypertrophied, so that the heart can pump more blood at each systole. It thus provides for the needs of the systemic circulation in addition to the leakage. In other words, by dilatation and hypertrophy the heart compensates for the loss by leakage.

Sooner or later, however, the lesion extends beyond any power of natural compensation; or for some other reason, usually a change of rhythm, the muscle fails, and then there is failure of compensation. A condition of threatened failure of compensation may exist when the heart is on the brink of failure, but remains adequate so long as special pains are taken to protect the body from effort. In these cases there is no reserve force, and failure is constantly threatened.

Mackenzie perhaps expresses these ideas better by assuming that the power of the heart may be divided into a working force and a rest force. The rest force is that which meets the needs of the body at rest, while the working force meets the additional requirements when the body is engaged in effort. The beginning of heart weakness would then be evidenced by limitation of the working force. It might show by discomfort or distress in performing some act which formerly gave no distress, e. g., shortness of breath on going up stairs, on running, or on lifting a heavy weight. The working force may be encroached upon to any degree, even to its exhaustion, but if the rest force remains, the patient may still maintain an adequate circulation if put to bed and kept from effort. When the rest force is cut down, there is serious failure of compensation, with the consequences as detailed above.