l62 METHODS FOR MEASURING THE BLOOD-PRESSURE. 



fluctuations, can only be determined approximately. Tigerstedt considers Volk- 

 mann's figure much too high. He determined the quantity of blood expelled by 

 the left ventricle with each systolic contraction in the rabbit by introducing in 

 the continuity of the aorta an instrument resembling a current-meter. From 

 animal experiments he estimates that in man only 69 cubic centimeters are ex- 

 pelled at each ventricular contraction. 



Place calculated as follows: A man uses about 500 liters of oxygen in 24 

 hours. In order that the venous blood, which contains on the average 7 volumes 

 per cent, less of oxygen than arterial blood, may take up this quantity of oxygen, 

 about 7000 liters of blood must be driven through the lungs in 24 hours. Allowing 

 100,000 heart -beats for the 24 hours, only 70 cubic centimeters are propelled with 

 each systole. 



Other more recent investigators also have calculated that the quantity of 

 blood expelled with each systole is equal only to $ of the capacity of the dead 

 ventricle, or 60 cubic centimeters. 



METHODS FOR MEASURING THE BLOOD-PRESSURE. 



A. In Animals. i. Hales' Tube. Stephen Hales, in 1727, first fastened a long 

 glass tube in the lateral wall of a vessel and determined the blood-pressure by 

 measuring the height of the vertical column of blood in the tube. 



Hales' tube was fitted at its lower extremity with a short copper tube, bent 

 at a right angle and directed toward the heart ; it therefore really represented a 

 so-called Pitot's tube. Pitot, in 1731, used a similar tube to determine the 

 velocity of the current in rivers. The water entering the horizontal portion of 

 the tube, which is directed up-stream, rises in the vertical portion, which projects 

 above the water, to a level proportional to the velocity of the current. This 

 level represents the "velocity-altitude" and it indicates that the water flows with 

 a velocity equivalent to that attained by a body falling freely from a height equal 

 to the velocity-altitude. If a Pitot tube (Fig. 70, II, o p x) be introduced into a 

 closed tube through which flows a fluid under pressure, and an ordinary manom- 

 eter (x y) be introduced at the same time, the latter will register only the tension 

 of the wall; but in a Pitot tube the fluid will rise to a higher level, for this column 

 of fluid indicates not only the tension of the blood, but also its velocity-altitude. 

 In arteries, however, the latter is extremely small as compared with the former. 



2. Poiseuille's Hematodynamometer. Poiseuille, in 1828, used a U-shaped man- 

 ometer-tube filled with mercury, which he inserted laterally by means of a rigid 

 connecting piece into the wall of the vessel. A I shaped tube may also be used 

 to connect the blood-vessel with the manometer, the short continuous extremities 

 being inserted into the open vessel (Fig. 65, I, a a) and the vertical limb being 

 connected with the manometer (M) by means of a leaden tube. 



3. Ludwig's Kymograph. Carl Ludwig, in 1847, placed a float (Fig. 65, I, d s) 

 on a column of mercury (as James Watt had already done for the manometer of 

 the steam-engine) . To the float was attached a vertical wire carrying a writing- 

 contrivance, which records not only the height of the blood-pressure, but also the 

 variations in the pulse-waves on the drum (C) , which is made to rotate by clock- 

 work. A. W. Volkmann gave the name of kymograph (wave-tracer) to this 

 instrument. The difference between the levels of the mercurial columns (c d) 

 in the two parts of the tube indicates the pressure within the vessel (the height 

 of the column of mercury multiplied by 13.5 gives the pressure-altitude of the 

 corresponding blood-column) . Setschenow added a stopcock at the center of the 

 lower bend of the tube (at b) . When this stopcock is turned so as to leave only 

 a narrow orifice of communication, the pulse-waves cease to manifest themselves 

 and the instrument records only the mean pressure. In this form the instrument 

 is the most reliable for this purpose. 



The pulsatory variations in pressure are recorded by the kymograph as simple 

 elevations (Fig. 65, III) and, therefore, they do not in the least correspond to 

 the curves obtained with the sphygmograph. After the mercury has once been 

 set in motion by the pulse-beats, it simply undergoes movements up and down 

 by virtue of its own oscillations and all the finer shades of the pulse are completely 

 obliterated. For this reason the kymograph can be used only for recording the 

 blood-pressure, and never for pulse-tracings. 



In order to determine the mean pressure from a long blood-pressure tracing 

 presenting numerous elevations and depressions, the planimeter is employed. This 

 instrument is carried over the entire outline of the surface occupied by the curve 



