(Gittings.)

The sounds depend upon the resonating character of the cuff, upon the size and accessibility of the vessel, upon the force of the heart beat, and upon the velocity of the blood.

=The Maximum and Minimum Pressures=

The maximum (systolic) pressure is read at the point where the first audible click is heard after the cuff is blown up and the pressure gradually reduced by means of the needle valve in the hand bulb or on the upright of the gla.s.s containing the mercury. All are agreed upon this point. There has been some dispute as to the place where the diastolic pressure should be read. Korotkov considered that the diastolic pressure should be read at the fourth phase when the loud tone suddenly becomes dulled. Others held that the diastolic pressure should be read at the fifth phase, the absence of all sound. Experiments carried out to determine this point were made by me with the a.s.sistance of Prof. Eyster and Dr. Meek at the Physiological Laboratory of the University of Wisconsin. We arranged apparatus making it possible to hold the pressure in the carotid artery of dogs at maximum or minimum. A femoral artery was then dissected and an instrument devised to compress the artery with a water jacket. The whole was connected up with a kymograph. A time marker was put in so as to record the place where changes in sound were heard while listening below the cuff around the femoral artery. Two sets of records were taken. One with pressure greater than minimum pressure and a falling pressure over the femoral artery (Fig. 29), the other with pressure at zero and gradually raised to minimum pressure (Fig. 30). Both sets of records showed the same result; viz., that at a point corresponding to the sudden change of tone the pressure on the artery corresponded to the minimum pressure. It was therefore concluded that experimentally in dogs the point where diastolic pressure should be read is at the tone change from clear to dull, not at the point where all sound disappears.

[Ill.u.s.tration: Fig. 29.--Tracing of auscultatory phenomena. (See explanation in legend of Fig. 30.)]

[Ill.u.s.tration: Fig. 30.--Figures are to be read from left to right. The top line records the points where sounds were heard, the figures above the short vertical lines refer to tones (see text). Mx. B. P., maximum blood-pressure. M. B. P., minimum blood-pressure. P. B., pressure bulb recorder. It was impossible to lower and raise this bulb by hand without obtaining the great irregular oscillations of the attached lever above the mercury manometer. B. L., base line.]

Erlanger showed some years ago, that with his instrument, the point at which diastolic pressure should be read was at the instant when the maximum oscillation of the lever suddenly became smaller. While checking up the graphic with the auscultatory method using Erlanger's instrument, it was noticed that the disappearance of all sound did not correspond with the sudden diminution of the oscillation of the lever connected with the brachial artery. A series of records were carefully made on patients. It was seen that during the period of the third tone phase the oscillations of the lever on the drum reached a maximum (Fig.

31) and remained at approximately the same height for some millimeters while the pressure was gradually falling. At a point at which the third tone, clear and distinct, became dull, there was an appreciable decrease in the height of the pulse wave. From this point to the disappearance of all sound there was a gradual diminution of the size of the pulse waves.

[Ill.u.s.tration: Fig. 31.--Fast drum. Sudden decrease in size of pulse wave at 4, marking the change from clear sharp tone to dull tone.]

[Ill.u.s.tration: Fig. 32.--Slow drum. Sudden decrease in amplitude at 4.]

For normal pressures the difference between the fourth (dull) tone and the fifth (disappearance of all tone) phase, amounted to 4 to 10 mm.

Occasionally the difference was so little, the change from sharp third tone through fourth dull tone to disappearance of all sound was so abrupt, that one could take the disappearance of all sound as the diastolic pressure, with an error of not more than 2 to 4 mm. This is within the limits of normal error and practically may be used by those who have difficulty in noting the change from third to fourth phase. For high pressures, however, the difference between fourth and fifth phases was never less than 8 mm., and was found as much as 16 mm. The diastolic, therefore, should always be taken at the fourth phase if possible.

It was found that with the dial instrument the greatest fling of the lever corresponded to the third phase and the sudden lessened amplitude of the oscillation was at the fourth phase and was coincident with the change of tone from sharp to dull. Thus the diastolic pressure may be read off on the dial scale by watching the fling of the hand and with some practice one might acquire considerable accuracy. It is better, simpler, and, for most observers, more accurate to use the stethoscope and hear the change of sound.

=The Relative Importance of the Systolic and Diastolic Pressures=

The systolic pressure represents the maximum force of the heart. It is measured by noting the first sound audible over the brachial artery using the auscultatory method. It is the summation of two factors largely; the force expended in opening the aortic valves (potential) and the force expended from that point to the end of systole, the force which is actually driving the blood to the periphery (kinetic). To start the blood in motion, the heart must overcome a dead weight equal to the sum of all the forces holding the aortic valves closed. This sum of factors, called the peripheral resistance, must be reached and pa.s.sed by the force of the ventricular beat before one drop of blood is set in motion along the aorta. This factor of resistance a.s.sumes a great importance.

The systolic pressure is always fluctuating as it depends upon so many conditions, and the calls of the body except during sleep are many and various. In a study of diurnal variations in arterial blood pressure it has been found that--(1) A rise of maximum pressure averaging 8 mm. of Hg. occurs immediately on the ingestion of food. A gradual fall then takes place until the beginning of the next meal. There is also a slight general rise of the maximum pressure during the day. (2) The range of maximum pressure varies considerably in different individuals, but the highest and lowest maximum pressures are practically equidistant from the average pressure of any one individual.[4]

[4] Weyse, A. W., and Lutz, B. R.: Diurnal Variations in Arterial Blood Pressure, Am. Jour. Physiol., 1915, x.x.xvii, 330.

The pressure is lowest during sleep and gradually rises near the end of sleep, so that on awakening the pressure was the same as before sleep.

Physiologically there are many conditions which modify the systolic pressure. Sleep, position, meals, exercise, emotional states cause often wide fluctuations which may be very sudden. It should be constantly borne in mind, that the systolic pressure reading which is made, is the maximum effort of the heart at that moment only.

The diastolic pressure measures the peripheral resistance. It measures the work of the heart, the potential energy, up to the moment of the opening of the aortic valves. It is the actual pressure in the aorta.

The diastolic pressure is not very variable; it is not subject to the same influences which disturb the systolic pressure. It fluctuates as a rule, within a small range. It is not affected by diet, by mental excitement, by subconscious psychic influences, to anything like the extent to which the systolic pressure is affected by the action of these factors. The diastolic pressure is determined by the tone in the arterioles and is under the control of the vasomotor sympathetic system.

Any agent which causes chronic irritation of the whole vasomotor system produces increase in the peripheral resistance with consequent rise in the diastolic pressure. Any agent which acts to produce thickening of the walls of the arterioles, narrowing their lumina, produces the same effect.

Such states naturally result in increased work on the part of the heart, which as a result, hypertrophies in the left ventricle. The increase in size and strength is a compensatory process in order to keep the tissues supplied with their requisite quota of blood. Conversely, paralysis of the vasomotor system produces fall of diastolic pressure which, if long continued, results in death.

The diastolic pressure then is of importance for the following reasons:

1. It measures peripheral resistance.

2. It is the measure of the tonus of the vasomotor system.

3. It is one of the points to determine pulse pressure.

4. Pulse pressure measures the actual driving force, the kinetic energy of the heart.

5. It enables us to judge of the volume output, for pulse pressure which is only determined by measuring both systolic and diastolic pressure, is such an index.

6. It is more stable than the systolic pressure, subject to fewer more or less unknown influences.

7. It is increased by exercise.

8. It is increased by conditions which increase peripheral resistance.

9. The gradual increase of diastolic pressure means harder work for the heart to supply the parts of the body with blood.

10. Increased diastolic pressure is always accompanied by increased pulse pressure, and increased size of the left ventricle, temporarily (exercise) or permanently.

11. Decreased diastolic pressure goes hand in hand with vasomotor relaxation, as in fevers, etc.

12. Low diastolic pressure is frequently pathognomonic of aortic insufficiency.

13. When the systolic and diastolic pressures approach, heart failure is imminent either when pressure picture is high or low.

When all these factors are taken into consideration, it becomes apparent that the diastolic pressure is most important, if not the most important part of the pressure picture.

Up to within a very brief time all the statistical evidence of blood pressure was based on systolic readings alone. This data is most valuable and much has been learned as to diagnosis and prognosis, but it is a ma.s.s of data based on a one-sided picture and can not be as valuable as the statistics which will undoubtedly be published later when all the pressure picture figures can be a.n.a.lyzed.

=Pulse Pressure=

The pulse pressure is the actual head of pressure which is forcing the blood to the periphery. At every systole a certain amount of blood 75-90 c.c. (Howell) is thrown violently into an already comfortably filled aorta. The sudden ejection of this blood instigates a wave which rapidly pa.s.ses down the arteries as the pulse wave. The elastic recoil of the aorta and large arteries near the heart contract upon the blood and keep it moving during diastole. Normally the blood-vessels are highly elastic tubes with an almost perfect coefficient of elasticity. The pulse pressure varies under normal conditions from 30 to 50 mm. Hg. There is a very definite relationship between the velocity of blood and the pulse pressure which is expressed thus; velocity = pulse rate x pulse pressure.[5]

Further it has been demonstrated that under normal conditions and during various procedures--the pulse pressure is a reliable index of the systolic output.[6]

[5] Erlanger and Hooker: An Experimental Study of Blood Pressure and of Pulse Pressure in Man, Johns Hopkins Hosp. Rep., 1904, xii, 145.

[6] Dawson and Gorham: The Pulse Pressure as an Index of Systolic Output, Jour. Exper. Med., 1908, x, 484.

Increased pulse pressure therefore goes hand in hand with greater systolic output. Physiologically this is most ideally seen during exercise. Following exercise the pulse rate increases, the systolic pressure rises greatly, the diastolic slightly or not at all. The pulse pressure therefore is increased. The velocity also is much increased.

The call comes for more blood and the heart responds. In the chronic high pulse pressures there are four correlated conditions which, so far as I have studied them, are always present. These are: (1) An increase in size of the cavity of the left ventricle. The ventricle actually by measurement contains more blood than normal, and therefore throws out more blood at every systole. The volume output is greater per unit of time. (2) There is actual permanent increase in diameter of the arch of the aorta. This is a compensating process to accommodate the increased charge from the left ventricle. (3) There are on careful auscultation over the manubrium, particularly the lower half, breath sounds which vary from bronchial to intensely tubular, depending upon the anatomic placing of the aorta, the shape of the chest, and the degree of dilatation. Often there is very slight impairment of the percussion note as well. (4) There is increase in size of all the large distributing arteries, carotids, brachials, femorals, renals, celiac axis, etc., with fibrous changes in the media, loss of some elasticity, and increase in size of the pulse wave. Increased pulse pressure means increased volume output, but does not always mean increased velocity. The proper distribution of blood to the various organs of the body is regulated by the vasomotor system acting upon the small arteries which contain considerable unstriated muscle. When fibrous arteriosclerosis is present there is loss of elasticity in the distributing arteries and a greater volume of blood must be thrown out by the ventricle at every systole in order that every organ shall have its full quota of blood. A force which is sufficient to send blood through elastic normal distributing tubes becomes totally insufficient to send the same amount of blood through tortuous and more or less inelastic tubes.

It is evident then that pulse pressure is exceedingly important. It can only be determined by measuring both the _systolic_ and _diastolic_ pressure. The pulse rate must also be known in order to compute the velocity. It is essential to have the whole pressure picture for all cases if correct conclusions are to be drawn.

In an irregular heart, especially in the cases due to myocardial disease, it is quite impossible to determine the true diastolic pressure. One can only approximate it and say that the pulse pressure is low or high. As a matter of fact the real systolic pressure can not be determined. For this figure the place on the scale where most of the beats are heard may be taken for the average systolic pressure. No one can seriously maintain that he can measure the diastolic pressure under all circ.u.mstances.

By means of the auscultatory method of measuring blood pressure we are able to determine irregularities of force in the heart beats more easily than by listening to the heart sounds. A pulsus alternans is readily made out. The irregular tones heard over the brachial artery in cases of irregular heart action have been called "tonal arrhythmias."

=Blood Pressure Variations=

A recent study of diurnal variations in blood pressure has shown that while the maximum pressure rises after the ingestion of food and steadily rises slightly throughout the day, the minimum blood pressure is very uniform throughout the day, and is little affected by the ingestion and digestion of meals. When it is affected, a rise or a fall may take place. Throughout the day, it tends to become slightly lower.