THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 295 



held, because a movement in the opposite direction is prevented by the immediate 

 approximation of the flap to the mouth of the cannula. Quite similarly, the 

 diastolic minimum may be determined very easily by inverting the glass cannula 

 so that its outlet is now directed toward the heart. The mercury is then drawn 

 downward until it approximates the lowest pressure prevailing in the heart. 

 The mean pressure may be obtained by interposing in the system of connecting 

 tubes a stop-cock which is closed more and more until the mercurial column 

 eventually shows only the smallest possible oscillations. 



As the pressure in the auricles is low, the manometer tube should be filled 

 with water instead of mercury. The displacement of the water, however, should 

 always be given in millimeters of mercury, because blood pressures in general 

 are usually adjusted to this standard. The exact height of the pressure registered 

 by the manometer at any one moment is obtained by measuring the distance 

 (H) between the zero line, or abscissa, and the level of the curve recorded by the 

 writing needle of the float. This distance must then be multiplied by two, be- 



FiG. 150. — Diagram to Show the Adjustments Necessary for DETERMrNiNO the 

 Zero- LINE op the Manometer (M). 

 Its central limb (A) is brought upon the same horizontal line as the level of the water 

 in the glass bulb (B) when held at the level of the blood vessel (C). 



cause the tube is U-shaped, i.e., while the column of mercury moves upward in 

 its distal limb, it moves downward in its central limb. The float, of course, 

 indicates solely the movement of the distal limb of the mercury, i.e., one-half of 

 the total movement of the mercurial column. Another factor must also be taken 

 into'consideration, namely the specific gravity of the fluid against which the blood 

 pressure is exerted. As mercury possesses a specific gravity which is 13.5 times 

 greater than that of blood, the height of the column of mercury (H) must be 

 divided by 13.5. The figure so obtained must then be subtracted from the pre- 

 ceding value. The complete formula for calculating the blood pressure is as 

 follows : 



It need scarcely be mentioned that the zero line must be accurately determined 

 beforehand by temporarily connecting the manometer with a glass bulb containing 

 water. When the level of the water is approximated to that of the mercury in 

 the central limb of the instrument, the float is adjusted at zero. The blood- 

 vessel in which the pressure is to be ascertained, must, of course, be approximated 

 to the level of the mercury in the central limb of the manometer (Fig. 150). 



