This pioneer work was practically forgotten for twenty-five years. It was not until 1887 that V. Basch devised an instrument which was used to some extent. This instrument recorded only maximum pressure. It consisted of a small rubber bulb filled with water communicating with a mercury manometer. The bulb was pressed on the radial artery until the pulse below it was obliterated and the pressure then read off on the column of mercury. V. Basch later substituted a spring manometer for the mercury column. Potain modified the apparatus by using air in the bulb with an aneroid barometer for recording the pressure. These instruments are necessarily grossly inaccurate. Moreover, they do not record the diastolic pressure.
In 1896 and 1897 further attempts were made to record blood pressure by the introduction of a flat rubber bag encased in some nonyielding material, which was placed around the upper arm. Riva-Rocci used silk, while Hill and Barnard used leather. The latter used a bulb or Davidson syringe to force air into the cuff around the arm and palpated the radial artery at the wrist, noting the point of return of the pulse after compression of the upper arm, and reading the pressure on a column of mercury in a tube.
Except that the width of the cuff has been increased from 5 cm. to 12 cm., this is the general principle upon which all the blood pressure instruments now in use are based. Most of the apparatuses make use of a column of mercury in a U-tube to record the millimeters of pressure. As the mercury is depressed in one arm to the same extent as it is raised in the other arm the scale where readings are made is .5 cm. and the divisions represent 2 mm. of mercury but are actually 1 mm. apart.
The cuff was made 12 cm. in diameter because it was shown (v. Recklinghausen) that with narrow cuffs much pressure was dissipated in squeezing the tissues. Janeway has shown that with the use of the 12 cm. cuff accurate values are obtained independently of the amount of muscle and fat around the brachial artery. In other words if an actual systolic blood pressure of 140 mm. is present in two individuals, the one with a thin arm, the other with a thick arm, the instrument will record these pressures the same where a 12 cm. arm band is used. We need have no fear of obtaining too high a reading when we are taking pressure in a stout or very muscular individual. Janeway also was the first to call attention to the fact that the diastolic or minimal pressure was at the point where the greatest oscillation of the mercury took place. This is difficult to estimate in many cases as the eye can not follow slight changes in the oscillation when the pressure in the cuff is gradually reduced. Practically this is the case in small pulses.
The Riva-Rocci instrument was modified by Cook. (See Fig. 13.) He used a glass bulb containing mercury into which a glass tube projected. The bulb was connected by outlet and tubing to the cuff and syringe. The glass tube was marked off in centimeters and millimeters and for convenience was jointed half way in its length. The instrument could be carried in a box of convenient size. This instrument is fragile and more cumbersome, although lighter in weight, than others and is very little used at present.
Fig. 13.—Cook's modification of Riva-Rocci's blood pressure instrument.
Stanton's instrument (Fig. 14) is practically Cook's made more rigid in every way but without the jointed tube. The cuff has a leather casing, the pressure bulb is of heavy rubber, the glass tube in which the mercury rises is fixed against a piece of flat metal and there are stopcocks in a metal chamber introduced between the bulb and mercury with which to regulate the in- and out-flow of air. The pressure can be gradually lowered conveniently without removing the pressure bulb.