CHAPTER V.
BLOOD PRESSURE IN ITS CLINICAL APPLICATIONS
It is well to bear constantly in mind the point made over and over in this work, that blood pressure is only one of many methods of acquiring information. He who worships his sphygmomanometer as a thing apart and infallible will sooner or later come to grief. Judgment must be used in interpreting changes in blood pressure just as judgment is essential in properly evaluating any instrumental help in diagnosis. One must not forget the personal equation which enters into even accurate instrumental recording in medicine and surgery.
In this chapter there will be no attempt to quote largely from what others have said or thought. Every one has his own opinion as to the value of certain methods after he has worked with them for a long time. The ideas here expressed, except in cases where no opportunity has offered to make personal studies, are those gathered from personal experience.
Blood Pressure in Surgery
Careful estimation of the blood pressure in surgical cases has, at times, great value. In all surgical diseases the most important fact to know is not the systolic pressure, but the pulse pressure. If the pulse pressure keeps within the range of normal, does not drop much below 30 mm. in an adult, then so far as we can tell the circulation is being carried on. When the systolic pressure is gradually falling and the diastolic remains the same, the circulation is failing and unless the pulse pressure can be established again the patient will die. Again we see the value of the pulse pressure.
All prolonged febrile diseases tend to produce a lowering of the blood pressure picture. The diastolic does not fall to the same extent as the systolic so that there is a pulse pressure smaller than normal. This is to be expected from what we know of the general depression of the circulation in fevers. The blood pressure reading is only a graphic record of what we have long known, and enables us from day to day accurately to measure the general circulation.
Head Injuries
It was claimed that in fracture of the skull or in concussion much could be gained by frequent estimations of the blood pressure. This seemed probable in the light of experiments on compressing the brains of dogs by the use of bags inserted through trephine openings (Cushing). In the clinic, however, it has not been found of any material value. It has a value in differentiating a simple fracture, let us say, from a case of uremia which is picked up on the street with a bump on the head. There the high pressure usually found would at once direct attention to the kidneys and the newer methods of blood examination would at once settle the question. Naturally uremics may also have skull fracture. There the diagnosis would be complicated. A decompression done at once would be indicated. If the skull fracture happened in a uremic, the decompression would probably do no harm. In fact, there are some who advise decompression for uremia.