Fig. 14.—Sphygmograph.
A, An ivory button which is pressed on the skin over the radial artery by a metal spring. B, A continuous screw which works against the cogwheel C. By rotating B, the lever D is raised to a position in which its point scratches the travelling-plate E (covered with blackened paper). F, A box containing clockwork which moves E. G, A screw by means of which the pressure of the spring is adjusted to the force of the pulse.
Fig. 15.
A, A cardiogram, or tracing of the impulse of the heart, recorded on a blackened plate borne on the end of a vibrating tuning-fork; a-b, the systole of the auricles; b-e, the ventricular systole. From c-e the heart is shrinking as blood leaves it by the aorta and pulmonary artery. B, C, D, E, F, Sphygmograms. B is shaded to show the portion of the pulse-wave which corresponds to the systole of the heart. C, High-tension pulse of vigorous health. D, Low-tension pulse. E, Dicrotic pulse of fever. F, Hog-backed pulse of hardened (atheromatous) arteries.
Much may be learned from the pulse with regard to the condition of the vascular system, although it is impossible to balance the effects of the several factors which go to the production of its various modifications. The character of the pulse depends upon the vigour with which the heart is beating, the efficiency or otherwise of the cardiac valves, the quantity of blood in circulation, the suppleness of the arterial walls, the degree to which they are contracted, the resistance offered by the smaller vessels. Departures from the normal may take the direction of unduly high tension or of unduly low tension. In place of the sudden rise and more or less gradual fall, with the slightest possible roughness due to secondary waves, which constitutes a healthy pulse, the rise may be shorter, its subsidence prolonged. This is a high-tension or hard pulse. The pressure in the arteries is unduly high, or the walls of the vessels are not as elastic as they should be. Considerable pressure is needed to obliterate such a pulse—i.e., to prevent it from passing on beneath the finger. As the converse of this condition, the difference between the beginning of the pulse and its end may be very marked, the vessel suddenly dilating and as suddenly collapsing. But little pressure is needed to stop such a low-tension pulse from passing beneath the finger. Usually it has a distinct secondary or dicrotic wave. Some tactile education is needed by the finger that aspires to read the pulse. It was hoped that the personal equation would be of less importance if mechanical records were substituted for statements as to the impression produced upon the observer. Various forms of sphygmograph (σφυγμός, pulse) have been invented for this purpose. The form commonly used ([Fig. 14]) consists of a metal spring which is adjusted so that a button beneath its free end presses on the radial artery at the wrist. The force with which it presses is regulated by a screw. At each pulsation its free end is lifted by the distension and rounding of the artery. Its movement is transmitted by means of a continuous screw, attached to it vertically, to a cogged wheel, which in its turn raises a lever. The end of the lever scratches blackened paper fastened on a plate moved by clockwork. Records made in this way are useful for future reference. They are not, however, so valuable as it was anticipated that they would be. The form of the tracing depends to so large an extent upon the amount of pressure exerted by the spring, and the amount of pressure must be adapted to the vascular tone in every case. Some of the most interesting tracings are obtained from old people affected with atheroma of the arteries. This is a condition in which, owing to old-standing inflammation of the subepithelial coat of the vessels, the arteries have lost their suppleness. They are hard and inelastic. Instead of showing the normal steep face of the pulse-wave rising abruptly to its highest point, the tracing rises vertically for a short distance, and then slopes upwards. The wave is flat-topped or hog-backed.
All pulses are dicrotic, although the dicrotism may not be sufficiently pronounced to be felt with the finger. The notch which divides the primary from the secondary wave is produced by the closure—that is to say, by the falling down of the aortic valve. The wave from the commencement of its ascent to the dicrotic notch corresponds to the period during which blood is passing from the heart into the aorta. This part of the tracing represents systole of the ventricle after the semilunar valve has been forced. It is the push given to the bottom of the column by the additional 3 ounces of blood thrust into the aorta. The effort of the ventricle then comes to an end. The pressure beneath the semilunar valve is less than that above it. The valve closes. If the blood were contained in an open tube, the wave would now end, save for secondary oscillations, due to inertia of the fluid. But the arterial system is practically closed owing to the fineness of the tubes into which it ultimately divides. Its walls are elastic. They distend, taking up the pressure and returning it again in the second half of the wave. In fever, after the consumption of alcohol, and in other conditions in which the finest bloodvessels are dilated, the division between the two parts of the wave is very marked. Dicrotism is plainly felt. We have used the expression “finest vessels” rather than “capillaries,” because the ascription to the capillary vessels of all peripheral resistance has led to misunderstanding. Resistance is offered throughout the whole vascular system, with the exception of the largest veins. It is greatest in the small arteries, capillaries, and small veins. It is so adjusted as to fall to zero just before the blood reaches the heart.