20 



METHOD OF ESTIMATING BLOOD-PRESSURE. 



cury in the open limb of the tube. A writing-style, /, placed transversely on the free 

 end of the float, inscribed the movements of the float and, therefore, of the mercury 

 upon a cylinder, r, caused to revolve at a uniform rate. This apparatus registered 

 the height of the blood-pressure, as well as the pulsatile and other oscillations 

 occurring in the mercury. Volkmann called this instrument a kymograph or 

 " wave-writer." The difference of the height of the column of mercury, c, d, in 

 both limbs of the tube indicates the pressure within the vessel. If the height of 

 the column of mercury be multiplied by 13'5, this gives the height of the corre- 

 sponding column of blood. Setschenow placed a stop-cock in the lower bend, h, 

 of the tube. If this be closed so as just to permit a small aperture of communica- 

 tion to remain, the pulsatile vibrations no longer appear, and the apparatus indicates 

 the mean pressure. By the term mean pressure is meant the limit of pressure, above 

 and below which the oscillations occurring in an ordinary blood-pressure-tracing 

 range. [Briefly, it is the average elevation of the mercurial column.] 



In a blood-pressure-tracing, such as fig. 102, each of the smaller waves corresponds to a 

 heart-beat, the ascent corresponds to the systole and the descent to the diastole. The large 



Fig. 102. 

 Blood-pressW curve of the carotid of a dog obtained with a mercurial manometer. - x = line 

 of no pressure zero line, or abscissa; y-y' is the blood-pressure-tracing with small waves, 

 each one caused by a heart-beat, and the large waves due to the respiration. A millimetre 

 scale shows the height of the pressure in millimetres of mercury. 



undulations are due to the respiratory movements. It is clear that the heart-beat is expressed 

 as a simple rise and fall (fig. 102), so that the curve of the heart-beat obtained with a mercurial 

 kymograph diners from a sphygmographic curve. 



Faults of a Mercurial Manometer. -A perfect recording instrument ought to indicate the 

 height of the blood-pressure, and also the size, form, and duration of any wave-motion com- 

 municated to it. The mercurial manometer does not give the true form of the pulse-wave, as 

 the mercury, when once set in motion, executes vibrations of its own, owing to its great inertia, 

 and thus the finer movements of the pulse- wave are lost. Hence a mercurial kymograph is used 

 for registering the blood -pressure, and not for obtaining the exact form of the pulse-wave. 

 Instruments with less inertia, and with no vibrations peculiar to themselves, are required for this 

 purpose. x 



[Method. Expose the carotid of a chloralised rabbit, and isolate a portion of the vessel 



