flow of blood issues from the capillaries (see page 25). Thus being dependent upon a fairly constant factor (peripheral resistance), the diastolic pressure varies very little in the larger arteries, where it is a comparatively fixed factor as compared with the systolic pressure.

In

All other factors remaining unchanged, a high peripheral resistance means a high diastolic pressure and a low peripheral resistance means a low diastolic pressure. other words, vasoconstriction raises and vasodilatation lowers diastolic pressure.

The Pulse Pressure, Range or Amplitude.-These synonymous terms are employed to designate the blood-pressure variation in the arterial tree, occurring at any given point during a cardiac cycle. It represents the actual head of pressure driving the blood toward the periphery and is the measure of the force exerted by the ventricle

Systolic=130 Range=45

Mean=107

Diastolic 85

FIG. 2.-Normal pulse tracing: showing relation of systolic, diastolic, pulse pressure and mean. Pulse pressure equals 45.

over and above that required to overcome peripheral resistance (diastolic pressure). It becomes evident, therefore, that the pulse pressure may at any time be determined by subtracting the diastolic pressure from the systolic pressure and that without these observations the pulse pressure cannot be ascertained (see Fig. 2).

The determination of the pulse pressure is of great importance in clinical medicine, particularly in the prognosis

of many cardiovascular diseases, as it in all probability indicates alterations in heart-muscle efficiency not only in cardiac disease itself, but also in acute and chronic infections, cachectic states, etc. It is also used clinically to determine the actual velocity of blood flow.

The Mean Pressure.-There seems to be a degree of confusion and uncertainty in the minds of physicians and students regarding the terms mean and average as applied to blood-pressure findings. These terms are frequently employed synonymously, although as far as I have been able to ascertain they should not be used interchangeably even when coincidently they may be found to be the same. I would emphasize this because, in a number of recent articles and in at least one recent text-book these terms have been loosely employed. Thus "by mean pressure is understood the average pressure (italics mine) at a given point; the mean pressure is not, however, the arithmetical mean between the systolic and diastolic pressure, because the pressure may rise for only a moment to the systolic level, falling very gradually to near the diastolic level and remaining near it throughout the greater part of the average cycle."

Whatever the accepted usage of terms, average and mean may come to be, it is plain that they are used so ambiguously that they interfere with a proper understanding of a definite nomenclature.

Strictly speaking, we may obtain an average only by adding the sum of any number of similar units and dividing this sum by the number of units in the series, while we can obtain a mean only as between two similar units. We 1 Norris, 1914 edition.

cannot therefore, determine the mean between the systolic and the diastolic pressure unless it be of two isolated observations, any more than plums could represent the

!

FIG. 3.-Woley's chart showing effect of age on blood-pressure, giving mean, high and low average.

mean between peaches and pears (see average pressure figures in Woley's table, Fig. 3). The only possible way of obtaining the mean pressure, either systolic or diastolic, in a series of observations, as for example to determine

the mean systolic and the mean diastolic pressure for a given age, would be to pair all the observations of the systolic pressure, making each pair the extremes, after having eliminated at each calculation the most widely separated pair. The result of the means obtained in this way would be a series of figures approximating each other, but not necessarily identical. This series of figures could then be added and an average mean obtained, which would in no way except by accident resemble the average. The same process carried out upon the diastolic pressure observations would produce an average mean diastolic pressure and from these a general mean pressure might, if advisable, be determined, and such a figure would not, except accidentally, be similar to half the pulse pressure added to the diastolic of any observation in the series.

The Average Pressure. In sphygmomanometry, we depend on the average pressure either systolic, diastolic or pulse pressure as the most practical way of estimating the limits of normal variation. This will be referred to in the chapter upon normal pressures and their variation (see page 112).

The value and clinical significance of the mean pressure is yet to be determined.

THE CAUSES OF ARTERIAL PRESSURE AND THE FACTORS INVOLVED IN ITS MAINTENANCE

We believe the arterial pressure to be dependent upon five separate, distinct and individually variable factors.

These several factors are found to vary continually under normal conditions while in pathologic states they may and do undergo great and permanent changes, which are

reflected, as either transient or permanent variations in blood-pressure.

Arterial blood-pressure at any given time is the sum of these factors plus their reciprocal relations.

Not only may one vary independently of the other, but they are capable also of most complicated interaction.

1. The Heart Energy.-The heart is a force pump of intermittent action and is the most important factor in the circulation as it is the fountain head of all energy. But of itself it is powerless to maintain the circulation without the supplementary and supporting factors above mentioned. Thus while the heart supplies the force which causes the flow of blood, it is the peripheral resistance and arterial elasticity which are chiefly concerned with the maintenance of an adequate blood-pressure during the diastolic pause, while the heart is resting, which aggregates according to Brunton1 on the average in the normal circulation, thirteen hours out of the twenty-four.

The left ventricle during systole forces the column of the blood into the arterial system while during diastole the blood is distributed forward throughout the arterial tree.

Any increase in the rapidity in the discharge of blood from the heart (increase in volume output per minute) will, the other factors remaining constant, cause a rise in systolic blood-pressure. Conversely any diminution in the pulse rate or volume output will cause a reduction in systolic blood-pressure. On the other hand a compensatory relation between the heart rate and the volume output may permit either or both (if inversely) to be altered without any appreciable change in blood-pressure.

1 Lauder Brunton, Brit. Med. Jour., Nov. 5, 1910.