It follows, from the irregularities in size of the channels through which it flows, that the blood stream is not uniform in character throughout the entire circuit--indeed, just the opposite is true. From point to point in the branching system of vessels the blood varies in regard to its velocity, its head of pressure, etc. These variations are connected in part with the fixed structure of the system and in part are dependent upon the changing properties of the living matter of which the system is composed." (W. H. Howell.)

If the vascular system were composed of a central pump, projecting at every stroke a given amount of liquid into a series of rigid tubes, the aggregate cross sections of which were equal to the cross section of the main pipe, then the velocity at the openings would be the same as at the source (making allowances for friction). The problem would then be a simple one. In the circulation of the blood no such simple condition obtains. The capillary beds is an enormous area through which the blood flows slowly. From the time the blood is thrown into the aorta the velocity begins to diminish until it reaches its minimum in the capillaries. In no two persons is the initial velocity at the heart the same, nor in the same person is it the same at all times of day. The size of the heart, the actual strength of the muscle, the amount of blood ejected at every beat, and the size and elasticity of the aorta are some of the factors which determine the velocity of blood at the aortic orifice. When to these factors are added the differences in arterial tissue, the activity or resting stage of the various organs, etc., the question becomes exceedingly complicated. In spite of these many disturbing elements, attempts more or less successful have been made to estimate the velocity of the blood in animals. Thus, in the carotid of the horse the velocity was found to be 300 mm. per second (Volkman) and 297 mm. (Chauveau); in the carotid of the dog, 260 mm.

(Vierordt). In the jugular vein of the dog Vierordt found the velocity to be 225 mm. per second. These figures do not represent the actual velocity of the blood in all horses or all dogs, but they do give us some general idea of the rate of flow of the blood. For man it has been calculated that the velocity in the aorta is about 320 mm. per second.

The velocity is not uniform in the large arteries, where at every heart beat there is a sudden increase followed by a decrease as the heart goes into diastole. The farther away from the heart the measurements are made the more even is the flow.

Observations by W. H. Luedde with the Zeiss binocular corneal microscope on the rate of flow in the conjunctival capillaries must modify somewhat our former conceptions. He finds that "The rate varies in the different arteries, capillaries, and veins from a barely perceptible motion to a little more than 1 mm. per second. Further, some parts of the capillary network are ordinarily supplied with blood elements only occasionally.

This is shown by the pa.s.sage of a column of corpuscles along a certain line, followed after an interval of seconds, during which no corpuscles pa.s.s, by another column in the same line as before."

The vessels of the conjunctiva probably are quite like superficial vessels in the skin and mucous membranes. Therefore, we must be free to admit that the circulation in them is not absolutely steady. Luedde found further that in syphilitics there were tortuosities, irregularities, minute aneurysmal dilatations and even obliterations of capillaries. Some of the changes occurred as early as one month after infection.

The rate in the capillaries of man is estimated to be between 0.5 mm.

and 0.9 mm. per second. As the blood is collected into the veins and the bed becomes smaller, the velocity increases until at the heart it is almost the same as in the aorta. That the velocity could not be exactly the same is evident from the fact that the cross section of the veins, which return the blood to the right auricle, is greater than is the cross section of the aorta.

The volume of the bed is subject to rapid and wide fluctuations, which are dependent on many causes, both physiologic and pathologic. The call of an actively functionating organ or group of organs causes a widening of a more or less extensive area, and the velocity necessarily varies.

In states of great relaxation of the vessels there may be a capillary pulse. In order to force blood at the same rate through dilated vessels as through normal vessels, there must be more blood or there must be a more rapid contraction of the central pump. What actually happens, as a rule, is an increase in the rate of the heart beat. There are conditions--such, for example, as aortic insufficiency--where actually more blood is thrown into the circulation at every beat, so that the rate is not changed.

It has been calculated that the average amount of blood thrown into the aorta at every systole of the heart is from 50 to 100 c.c. This is forcibly ejected into a vessel already filled (apparently) with blood.

In order to accommodate this sudden accession of fluid, the aorta must expand. The aortic valves close, and during diastole the blood is forced through the vascular system by the forcible, steady contraction of the highly elastic aorta. Other large vessels which branch from the aorta also have a part in this steady propulsion of blood. From seventy to eighty times a minute the aorta is normally forcibly expanded to accommodate the charge of the ventricle. It is not difficult to understand the great frequency of patches of sclerosis in the arch when these facts are borne in mind.

What relationship the viscosity of the blood has to the rate and volume of flow is not fully understood. As yet there is not much known about the subject, and no one has devised a satisfactory means of measuring the viscosity. It is thought by some that an increased viscosity a.s.sists in producing an increased amount of work for the heart.

=Blood Pressure=

Blood pressure is the expression used for a series of phenomena resulting from the action of the heart. As every heart beat is actual work done by the heart in overcoming resistance to the outflow of blood, this force is approximately measurable in a large artery such as the brachial. It has been determined that the pressure in the brachial artery is almost equal to the intraventricular pressure in the left ventricle. In animals it is easy to attach manometers to the carotid artery and to measure the blood pressure accurately. Formerly the method consisted in attaching a tube and allowing the blood to rise in the tube. The height to which the blood rose measured the maximum pressure.

This is a crude method and has been replaced by the U-tube of mercury with connection made to the artery by saline or Ringer's solution. This apparatus is familiar to all physiologists.

In man the measurement is most conveniently made from the brachial artery. There is some difference in the pressure in the femoral and the brachial and some use both arteries. However, the difficulty of adjusting instruments to the upper leg, the great force which must be used to compress the femoral artery and the relative inaccessibility of the leg as compared to the arm, make the leg an inconvenient part for use in blood pressure determinations. It is not to be recommended.

Blood pressure is a valuable aid in diagnosis and of material help in many cases in prognosis, but it is not infallible neither can it be used alone to diagnose a case. Blood pressure is only one of many links in a chain of evidence leading to diagnosis. It has been badly used and much abused. It has been condemned unjustly when it did not furnish _all_ the evidence. It has been made a fetish and worshipped by both doctors and patients. A sane conception of blood pressure must be widely disseminated lest we find it being discarded altogether.

Blood pressure consists of more than the estimation of the systolic pressure. The blood pressure picture consists of (1) the systolic pressure, (2) the diastolic pressure, (3) the pulse pressure which is the difference between the systolic and diastolic pressure, (4) the pulse rate. Expressed in the literature it should read thus: 120-80-40; 72. That tells the whole story in a brief, accurate form. This is recommended in history reporting. It must be ever kept in mind that a blood pressure reading represents the work of the heart at the _moment when it was taken_. Within a few minutes the pressure may vary up or down. There is no normal pressure as such, but an average pressure for any group of people of the same age living under similar conditions. The habit of speaking of any systolic figure as normal should be broken. A pressure picture may be normal but a systolic reading, whatever it may be, is not accurately designated as normal. This distinction is worth insisting upon.

=Blood Pressure Instruments=

There are several instruments which are in common use for the purpose of recording blood pressure in man.

Historically, the determination of blood pressure for man began with the attempt of K. Vierordt in 1855 to measure the blood pressure by placing weights on the radial pulse until this was obliterated. The first useful instrument, however, was devised by Marcy in 1876. He placed the hand in a closed vessel containing water connected by tubing with a bottle for raising the pressure and by another tube with a tambour and lever for recording the size of the pulse waves. He maintained that when pressure on the hand was made, the point where oscillations of the lever ceased was the maximal pressure, the point where the oscillations of the recording lever was largest, was the minimal pressure.

This pioneer work was practically forgotten for twenty-five years. It was not until 1887 that V. Basch devised an instrument which was used to some extent. This instrument recorded only maximum pressure. It consisted of a small rubber bulb filled with water communicating with a mercury manometer. The bulb was pressed on the radial artery until the pulse below it was obliterated and the pressure then read off on the column of mercury. V. Basch later subst.i.tuted a spring manometer for the mercury column. Potain modified the apparatus by using air in the bulb with an aneroid barometer for recording the pressure. These instruments are necessarily grossly inaccurate. Moreover, they do not record the diastolic pressure.

In 1896 and 1897 further attempts were made to record blood pressure by the introduction of a flat rubber bag encased in some nonyielding material, which was placed around the upper arm. Riva-Rocci used silk, while Hill and Barnard used leather. The latter used a bulb or Davidson syringe to force air into the cuff around the arm and palpated the radial artery at the wrist, noting the point of return of the pulse after compression of the upper arm, and reading the pressure on a column of mercury in a tube.

Except that the width of the cuff has been increased from 5 cm. to 12 cm., this is the general principle upon which all the blood pressure instruments now in use are based. Most of the apparatuses make use of a column of mercury in a U-tube to record the millimeters of pressure. As the mercury is depressed in one arm to the same extent as it is raised in the other arm the scale where readings are made is .5 cm. and the divisions represent 2 mm. of mercury but are actually 1 mm. apart.

The cuff was made 12 cm. in diameter because it was shown (v.

Recklinghausen) that with narrow cuffs much pressure was dissipated in squeezing the tissues. Janeway has shown that with the use of the 12 cm.

cuff accurate values are obtained independently of the amount of muscle and fat around the brachial artery. In other words if an actual systolic blood pressure of 140 mm. is present in two individuals, the one with a thin arm, the other with a thick arm, the instrument will record these pressures the same where a 12 cm. arm band is used. We need have no fear of obtaining too high a reading when we are taking pressure in a stout or very muscular individual. Janeway also was the first to call attention to the fact that the diastolic or minimal pressure was at the point where the greatest oscillation of the mercury took place. This is difficult to estimate in many cases as the eye can not follow slight changes in the oscillation when the pressure in the cuff is gradually reduced. Practically this is the case in small pulses.

The Riva-Rocci instrument was modified by Cook. (See Fig. 13.) He used a gla.s.s bulb containing mercury into which a gla.s.s tube projected. The bulb was connected by outlet and tubing to the cuff and syringe. The gla.s.s tube was marked off in centimeters and millimeters and for convenience was jointed half way in its length. The instrument could be carried in a box of convenient size. This instrument is fragile and more c.u.mbersome, although lighter in weight, than others and is very little used at present.

[Ill.u.s.tration: Fig. 13.--Cook's modification of Riva-Rocci's blood pressure instrument.]

Stanton's instrument (Fig. 14) is practically Cook's made more rigid in every way but without the jointed tube. The cuff has a leather casing, the pressure bulb is of heavy rubber, the gla.s.s tube in which the mercury rises is fixed against a piece of flat metal and there are stopc.o.c.ks in a metal chamber introduced between the bulb and mercury with which to regulate the in- and out-flow of air. The pressure can be gradually lowered conveniently without removing the pressure bulb.

[Ill.u.s.tration: Fig. 14.--Stanton's sphygmomanometer.]

The most accurate mercury manometer is that of Erlanger. (Fig. 15.) The instrument is bulky and is not practicable for the physician in practice. The principle is that used by Riva-Rocci. There is an extra T-tube introduced between the manometer and air bulb connecting with a rubber bulb in a gla.s.s chamber. The oscillations of this are communicated to a Marey tambour and recorded on smoked paper revolving on a drum. There is a complicated valve which enables the operator to reduce the pressure with varying degrees of slowness. The mercury is placed in a U-tube with a scale alongside it. The instrument is expensive and not as easy to manipulate as its advocates would have us believe. Hirschfelder has added to the usefulness (as well as to the complexity) of the Erlanger instrument, by placing two recording tambours for the simultaneous registering of the carotid and venous pulses. In spite of its complexity and necessary bulkiness, very valuable data are obtained concerning the auricular contractions.

[Ill.u.s.tration: Fig. 15.--The Erlanger sphygmomanometer with the Hirschfelder attachments by means of which simultaneous tracings can be obtained from the brachial, carotid, and venous pulses.]

One of the best of the mercury instruments is the Brown sphygmomanometer. In this (Fig. 16) the mercury is in a closed, all-gla.s.s tube so that it can not spill under any sort of manipulation.

It is in this sense "fool-proof." The cuff, however, is poorly constructed. It is too short and there are strings to tie it around the arm. I have found that this causes undue pressure in a narrow circle and renders the reading inaccurate. In the clinic we use this mercury instrument with a long cuff like that provided by the Tycos instrument.

[Ill.u.s.tration: Fig. 16.--Desk model Baumanometer.]

The Faught instrument (Fig. 17) is larger than the Brown, but is less easily broken and is not too c.u.mbersome to carry around. The subst.i.tution of a metal air pump for the rubber makes the apparatus more durable.

[Ill.u.s.tration: Fig. 17.--The Faught blood pressure instrument. An excellent instrument which is quite easily carried about and is not easily broken.]

The v. Recklinghausen instrument is not employed to any extent in this country. It is both expensive and c.u.mbersome, and has no advantages over the other instruments.

Several other instruments have been devised and new ones are constantly being added to the already large list. With those employing mercury the principle is the same. The aim is to make an instrument which is easily carried, durable, and accurate.

In all the mercury instruments the diameter of the tube is 2 mm. One would suppose that there would be noticeable differences in the readings of the different mercury instruments depending upon the amount of mercury used in the tube. By actual weight there is from 35 to 45 gms.

of mercury in the several instruments. After many trials, no noticeable differences in blood pressure readings can be made out between a column weighing 35 gm. and one weighing 45 gm.

There is, however, the inertia of the mercury to be overcome, friction between the tube and the mercury, and vapor tension. The mercury is therefore not as sensitive to rapid changes of pressure in the cuff as a lighter fluid would be. The mercury must be clean and the tube dry so that there is no more friction than what is inherent between the mercury and gla.s.s. In making readings on a rapid pulse the oscillations of the mercury column are apt to be irregular or to cease now and then, due to the fact that the downward oscillation coincides with a pulse wave, or an upward oscillation receives the impact of two pulse waves transmitted through the cuff. Instruments have been devised to obviate this difficulty, but they have not come into favor. They are usually too complicated and at present can not be recommended.

[Ill.u.s.tration: Fig. 18.--Rogers' "Tycos" dial sphygmomanometer.]

An instrument devised by Dr. Rogers (the "Tycos") has met with considerable popularity. (Fig. 18.) This is not an instrument which operates with a spring and lever. The instrument is composed essentially of two metal discs carefully ground and attached at their circ.u.mferences to the metal casing below the dial. There is an air chamber between these discs through the center of which air is forced by the syringe bulb. When air is forced into the s.p.a.ce between these two discs, they are forced apart to a very slight extent, with the highest pressures only 2-3 mm. of bulging occurs. From data gathered after extensive use for five years these discs were not found to have sprung. A lever attached to a cog which in turn is attached to the dial needle magnifies to an enormous extent the slightest expansion of the discs. Every dial is handmade and every division is actually determined by using a U. S.

government mercury manometer of standard type. No two dials therefore are alike in the s.p.a.cing of the divisions of the scale but every one is calibrated as an individual instrument. There is no doubt in the author's mind that for the general pract.i.tioner the instrument has some advantages over the mercury instruments. It reveals the slightest irregularity in force of the heart beat. The oscillation of the dial needle is more accurately followed by the eye than is that of the column of mercury. The needle pa.s.ses directly over the divisions of the scale, while with usual mercury instruments the scale is an appreciable distance (sometimes .5 cm.) from the column of mercury at the side.

(Fig. 19.) The diastolic pressure is more easily read on the "Tycos."

It is where the maximum oscillation of the needle occurs as the pressure is slowly released from the cuff. Although it does not appear that this instrument, if properly made and standardized, could become inaccurate, nevertheless it is advisable to check it every few months against a known accurate mercury manometer instrument.

[Ill.u.s.tration: Fig. 19.--Detail of the dial in the "Tycos" instrument.]

[Ill.u.s.tration: Fig. 20.--Faught dial instrument.]

[Ill.u.s.tration: Fig. 21.--Detail of the dial of the Faught instrument.]

Another perfectly satisfactory dial instrument is the Faught (Figs. 20 and 21). The general plan of this differs in some minor points from the "Tycos." I have compared the two and have found no difference in the readings. Both can be recommended.

[Ill.u.s.tration: Fig. 22.--The Sanborn instrument.]

One or two other cheaper dial instruments are on the market. The Sanborn seems to be quite satisfactory. (Fig. 22.) It is cheaper than the other dial instruments. There is this much to be said, no instrument using a spring as resistance to measure pressure can be recommended.