BLOOD PRESSURE. This term refers to arterial tension or to the pressure of the blood in the blood vessels. It depends upon the cardiac strength, the peripheral resistance, the elasticity of the vessel walls and the volume of blood. Three factors are to be considered — the maximum or systolic pressure, the mini mum or diastolic and the difference between maximum and minimum or pulse pressure. The pressure is indicated by the height of a column of mercury in a capillary tube or by air pressure upon a spring. The former method is altogether the more reliable, the air machines requiring correction from time to time by com parison with a mercurial apparatus. The ap paratus is called a sphygmo-manometre, which signifies measurer of the pulse.
Measurement of the pulse or blood pressure is a procedure of comparatively recent recog nition. Its importance was observed in Europe as early as 1828 but it was not until 1876 that the first sphygmo-manometre was made in this country, being that of Marcy, which for a considerable period was used principally in physiological investigations and experiments. The second period began in 1896 with the ap paratus of Riva-Rocci, which consists of an inflatable rubber bag or cuff, five centimetres in width, surrounded by a band of firm cloth which is wound around the arm. To this is attached a rubber tube connected with a reser voir of mercury, from which proceeds an up right glass capillary tube by the side of which is a wooden scale graduated in millimetres. The rubber bag is placed over the brachial artery above the bend of the elbow, secured by the band wound around the arm and inflated by means of a rubber bulb and tube until the pulse at the wrist has disappeared. As soon as the column of mercury comes to a rest, the point on the scale is noted and this is the 'maximum or systolic pressure. The air in the bag is then gradually released until the pulse clearly reappears and this becomes the mini mum or diastolic pressure.
Many mercurial manometres have been de vised since Riva-Rocci's, but they are only mod ifications of his. Among the mercurial instru ments which are worthy' of mention and reliable are those of Cook, Stanton, Janeway and Nich olson. One modification, and an important one,
too, consists in, a larger and wider cuff, and they are now made from 12 to 14 centimetres in width. Less than this width gives unreliable readings, especially in the upper register. The connections in any apparatus should be of non distensible rubber tubing. The instrument must register both systolic and diastolic pressure, should not require more than two or three minutes for the operation, should be as light and compact as possible and not too expensive. The expense of most of the instruments has been such that many physicians have been dis inclined to buy and use them and so familiar ize themselves with an important means of diagnosis.
In examining for blood pressure it is well to have the patient in the sitting rather than the reclining position. The cuff should be ap plied at the level of the heart and therefore upon the brachial artery above the elbow. The cuff must be bandaged loosely about the arm to permit free inflation of the rubber bag. Breathing should be natural, as deep breathing raises the pressure during expiration and low ers it in inspiration. The cuff being adjusted and the valves of the apparatus open or closed as may be required on the machine which is being used, the bulb for pumping the air upon the reservoir of mercury is compressed and the mercury forced into the capillary tube. When the rubber bag has been sufficiently dilated to obliterate by its pressure the column of blood in the brachial artery, which is indicated by the disappearance of the pulse at the wrist, the column of mercury in the capillary tube• is to be noted when it comes to a rest and the read ing taken from the scale at its side. This is the maximum or systolic pressure. The air in the bag is then slowly released and when the pulse reappears and becomes regular again this will be the diastolic pressure. The differ ence between the two is the pulse pressure. This is the method by palpation, the finger of the physician being kept constantly at the site of the radial artery.