Fig. 16.—Desk model Baumanometer.
The Faught instrument (Fig. 17) is larger than the Brown, but is less easily broken and is not too cumbersome to carry around. The substitution of a metal air pump for the rubber makes the apparatus more durable.
Fig. 17.—The Faught blood pressure instrument. An excellent instrument which is quite easily carried about and is not easily broken.
The v. Recklinghausen instrument is not employed to any extent in this country. It is both expensive and cumbersome, and has no advantages over the other instruments.
Several other instruments have been devised and new ones are constantly being added to the already large list. With those employing mercury the principle is the same. The aim is to make an instrument which is easily carried, durable, and accurate.
In all the mercury instruments the diameter of the tube is 2 mm. One would suppose that there would be noticeable differences in the readings of the different mercury instruments depending upon the amount of mercury used in the tube. By actual weight there is from 35 to 45 gms. of mercury in the several instruments. After many trials, no noticeable differences in blood pressure readings can be made out between a column weighing 35 gm. and one weighing 45 gm.
There is, however, the inertia of the mercury to be overcome, friction between the tube and the mercury, and vapor tension. The mercury is therefore not as sensitive to rapid changes of pressure in the cuff as a lighter fluid would be. The mercury must be clean and the tube dry so that there is no more friction than what is inherent between the mercury and glass. In making readings on a rapid pulse the oscillations of the mercury column are apt to be irregular or to cease now and then, due to the fact that the downward oscillation coincides with a pulse wave, or an upward oscillation receives the impact of two pulse waves transmitted through the cuff. Instruments have been devised to obviate this difficulty, but they have not come into favor. They are usually too complicated and at present can not be recommended.