From these facts it would seem safe to construct an equa

tion: Blood-pressure quotient

pressure = normally about 0.3.

=

pulse pressure ÷ systolic

The S. D. R. Index or Energy Index.-Barach1 bases his assertions in determining the energy index upon the assumption that systole gives us the energy factor in the heart and diastole the energy in the peripheral resistance. From the pulse rate we know how many pressure impulses to each minute there are in the arterial tree. For example if the systolic pressure is 120, the diastolic pressure 70 and pulse rate 72 per minute, the exertion in one minute would be:

If the assumption of Barach is correct, it follows that the knowledge of the systolic pressure alone gives but a partial clue to what is going on in the circulation. In an extensive series of observations by Barach upon the blood-pressure of healthy young men, this contention seems to be proved.

The conclusion reached is that the highest normal energy index is close to 20,000 mm. Hg. per minute. In a series of ten cases of recognized cardiovascular and renal involvement, five of which afterward had apoplectic strokes, the figures of the total energy or the S. D. R. index were all above 20,000, the highest being 50,400. Barach concludes his article by stating that we may obtain by this indication a definite idea of the cardio

1 Jour. A. M. A., 1914, Vol. lxii, 7, p. 525.

vascular energy expanded and the strain under which the heart and blood-vessels are laboring.

The Diastolic Index.-MacWilliams and Melvin1 in taking blood-pressure observations, find it desirable to determine what they call the diastolic index. This is based upon the discovery that in certain cardiovascular cases there was a decided difference in the diastolic pressure, and consequently in the pulse pressure, when the reading was made on the way up from that made on the way down, and that this finding gave valuable data as to the tendency of the case to vasomotor instability. They found in some cases a difference as great as 20 mm. Hg. between the up and down diastolic readings.

1 J. A. Mac Williams and G. S. Melvin, Brit. Med. Jour., Nov. 7, 1914, 2818.

CHAPTER VIII

VENOUS AND CAPILLARY BLOOD-PRESSURE

While the pioneers in the study of blood-pressure did not neglect the study of capillary blood-pressure, they failed to attach any special importance to this observation, probably because at that time the dynamics of the circulation were but little understood and also because of these crude methods for estimating capillary blood-pressure.

Regarding the physiology of capillary blood-pressure, we know that, in comparison with the pressure within the arterial tree, capillary pressure is low, and the flow uniform and continuous owing to the length of the conducting vessels, which, together with arterial elasticity and ramification, absorb the rhythmic impulses of the heart.

Conditions Influencing Capillary Pressure.-The anatomic situation of the capillaries places them in a position where alterations in their pressure, may be affected by both arterial and venous pressure. Further, the superficial location of large areas of the capillaries subjects them to the influence of external agencies.

A high venous pressure, by impeding the free flow of blood from the capillaries into the veins, will result in an increase in capillary pressure. The reverse is also true. Capillary pressure is influenced by the proximity of this system to the larger venous trunks, and again by the character of the tissues in which the capillaries are located,

the capillary pressure being higher in dense firm tissues than in soft areolar tissues.

The relative importance of capillary blood-pressure has until recently been rather underestimated, although now it is receiving more deserved attention, because we more fully realize the close relation between the pressure in the smallest arterioles and in the true capillaries, and the maintenance of normal peripheral resistance (Chapter II, page 45) which, as has been shown, is the prime factor, excepting the heart itself, in maintaining normal systolic and diastolic pressure.

The actual pressure in the capillaries stands midway between that of the arteries and that of the veins, but it usually more closely approximates the venous than the arterial pressure.

Methods of Measuring Capillary Pressure.—Various means have been devised for demonstrating capillary bloodpressure, the simplest being to press upon a superficial area with a clean microscope slide until whitening of the tissues beneath occurs. The degree of pressure required to accomplish this decoloration will roughly measure the capillary pressure. Formerly a means of applying graduated weights was employed to some extent, but this has been discarded as uncertain and unreliable.

The most practical improvement in apparatus for the measurement of capillary blood-pressure was made by Adolf Basler and by Warren Lombard' who separately made devices for this purpose. The instrument of Basler, the "Ochrometer" appears to be the most practical and productive of most accurate results. Its important features 1 Zentralbl. f. Physiol., 1911, xxv, p. 157.

briefly are: two small tin compartments, the tops of which consist of clear glass windows, arranged for the introduction of two adjacent fingers of the subject to be tested. One of the compartments contains a very thin-walled small rubber balloon which may be so inflated that it will subject the finger to a very delicate pressure, which pressure is measured in millimeters of mercury. The pressure in the balloon is gradually raised until slight blanching of the finger occurs, this point being the measure of capillary blood-pressure. A reading device composed of branching tubes and prisms allows the two fingers to be observed simultaneously for purposes of comparison. This is simply a refinement of the older methods for compressing and measuring the amount of pressure necessary to produce change in color in the part under pressure.

The Normal Capillary Pressure and Its Modifications.— Employing Basler's instrument, Landerer1 found that the normal capillary pressure varied between 17 and 25 mm. of mercury, although it may at times rise as high as 70 (in firm tissue and branches of arterioles). The figures given in the "American Text-book of Physiology" are between 24 and 54 mm.

Landerer also made an effort to itemize those diseases in which the capillary pressure was either lowered, elevated or unchanged. His results were generally so variable that they possess very little clinical value, the exception being in cases of high arterial pressure, when he found a more or less constant reduction in capillary tension. This was to be expected, if we believe that high arterial pressure is due, in part at least, to arterial and arteriole tightening or narrow1 Rudolph Landerer, Zeitsch. f. klin. Med., 78, Nos. 1 and 2.