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tions and the nervous mechanism. The volume of the blood to the chemical changes and nervous mechanism and the viscosity of the chemical composition.

One should not be led, in the consideration of this subject into the belief that these several factors are entirely separable. As a matter of fact they are all so interrelated that it is utterly impossible to decide where the effect of one ceases and that of another begins; since the normal circulation is made up of the sum of activities and of these factors, which all may and which do vary constantly, under both normal, and in pathologic conditions. While it might be theoretically possible for one factor alone to produce discernible alterations in the circulation, it is improbable that it ever does, as all are so closely related one to the other any definite alteration in one of them must result in change in others. We are, therefore, forced to a consideration of the subject as a whole, while acknowledging and endeavoring to isolate the predominating effect of one or more, in relation to the pathologic changes found in any given case.

CHAPTER II

TERMS AND DEFINITIONS EMPLOYED IN THE

STUDY OF ARTERIAL PRESSURE

Before undertaking a study of the clinical relations of blood-pressure, with its many and complex variations of both physiologic and pathologic origin, it is essential to have a knowledge of the different terms now in common use in the study of blood-pressure, to be familiar with the several accepted methods of study and to have a practical conception of the relations of the several events in the cardiac cycle to the various factors concerned in the maintenance of normal arterial pressure. A brief review of these matters may be conveniently considered here.

BLOOD-PRESSURE

Beginning with a broad general significance, when the term blood-pressure was applied to almost any condition of arterial pressure in any part of the circulatory system. The word has gradually become restricted, so that at the present writing, unless qualified by defining adjectives, the term has become so indefinite, that it should not be employed. The term, to have any clinical significance, must be qualified by the proper adjective, which will indicate what particular part of this general subject is under consideration, thus: systolic blood-pressure; diastolic bloodpressure; pulse pressure; mean pressure; average pressure;

venous pressure; capillary pressure; intraventricular pressure, etc.

Definition. When unqualified, the term blood-pressure is taken to mean the systolic blood-pressure as measured in the brachial artery, though it is advisable to be more specific in all discussions and writings upon the subject of pressures, and to apply carefully the proper qualifying words in order to afford more definite scientific and therefore more accurate information.

In the chapter on physiology it has been said that arterial pressure is subject to many and various modifying factors and that we know now that the arterial pressure is in a state of constant rhythmic fluctuation due to the intermittency of the cardiac systoles, and also that the arterial pressure in normal individuals at all times is dependent upon the proper correlation of five distinct and separately determinable factors (see page 34).·

Our present nomenclature of arterial pressure includes a number of terms some of which will be briefly defined and discussed.

First.-Arterial pressure (either end or lateral) which is divided into:

Second. (a) Systolic or maximal pressure (measured by end pressure) and

Third. (b) Diastolic or minimal pressure (measured as lateral pressure) from which we determine.

Fourth. The pulse pressure, range or amplitude.
Fifth.-Mean arterial pressure.

Sixth.-Average arterial pressure.

The Arterial Pressure.-This is found to differ according to the manner of its measurement; thus the pressure which

is exerted against the vessel wall while the current of blood is flowing unopposed by direct obstruction, is known as the lateral pressure; whereas the pressure exerted by the blood against any obstruction which reduces the lumen of the vessel is known as the end pressure. The latter exceeds the former when measured at any given point in the arterial tree and the difference between the two is the measure of the dynamic power of the heart which is involved in actually carrying on blood flow at this point.

In clinical sphygmomanometry, the methods employed yield the end systolic pressure and the lateral diastolic pressure, and it is these pressures that are to be inferred when encountered in blood-pressure discussion.

The Systolic or Maximal Pressure. This term indicates the maximum force exerted by the systole of the heart. It is the transmitted intraventricular pressure. This pressure measured in the brachial artery while it is less than, nevertheless tends to approximate, the intracardiac pressure, and the lateral pressure in the aorta.1 Thus when we say that the systolic pressure is equal to 125 mm. Hg., we mean that the end pressure in the artery at the point of obstruction by the sphygmomanometer cuff, is capable of sustaining a column of mercury of 125 mm. over and above the atmospheric pressure.

The Absolute Pressure. To determine the absolute systolic pressure, it is necessary to add to the observed systolic pressure in mm. Hg., the actual barometric pressure as measured by the same scale, at the time of estimation. This point should not be forgotten, although such an

'Louis M. Warfield, Int. Med. Jour., xix, No. 10, October, 1912.

estimation is rarely employed except in physiologic and experimental investigations.

Accuracy of the Systolic Pressure Estimation.-The difference between the systolic pressure, determined by sphygmomanometry in the brachial artery, as compared with that found in the aorta by direct measurement in animals is so slight that, for practical purposes, it is ignored. Therefore, when the systolic pressure is ascertained in the brachial artery, it represents a satisfactory clinical measure of the pressure throughout the arterial system except in very small vessels.

The systolic blood-pressure may be divided into two distinct portions, which have a valuable clinical and pathologic significance of the measure of the force exerted by the heart. It is compounded of the pressure required to overcome the minimal pressure in the aorta (the measure of peripheral resistance) plus the force necessary to cause arterial flow and to sustain the circulation in normal equilibrium (heart work).

Diastolic or Minimal Arterial Pressure. This term refers to the pressure found in the smaller arterials and transmitted backward to the general arterial tree. It is, therefore, the measure of peripheral resistance and represents the amount of pressure that the left ventricle must exert before it can reëstablish the forward movement of blood in the arterial system.

As the blood goes farther and farther from the heart toward the smaller arterials situated on the confines of the arterial tree, the systolic and diastolic pressures tend to approximate each other, until when the capillaries are reached the rhythmic fluctuation is lost, and a uniform

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