It has been indicated already that the pressure of the blood may be communicated to the disk of an elastic manometer either by means of liquid or of air. A given series of fluctuations of blood-pressure may yield decidedly different curves according to the method of " transmission" employed to obtain them; and the controversies as to the true form of the endocardiac pressure-trace turn upon the question whether such "transmission by air" or "transmission by liquid" yield the truer curve. The objections to the former. method depend upon the readier compressibility of air; the objections to transmission by liquid depend upon its greater inertia.

The General Characters of the Ventricular Pressure-curve.-Whatever kind of elastic manometer and of transmission be used, the curve

FIG. 22.-Magnified curve of the course of pressure within the right ventricle of the dog, the chest being open; to be read from left to right. Recorded by the elastic manometer, with transmission by air (von Frey).

obtained shows certain characters which are recognized by all as properly belonging to the changes of pressure within the ventricle, whether right or left. These general characters, moreover, persist after the opening of the chest. They are as follows (see Figs. 22, 23, 24): The muscular contraction of the systole begins quite suddenly, and produces a swift and ex

FIG. 23.-Magnified curve of the course of pressure within the left ventricle and the aorta of the dog, the chest being open; to be read from left to right. Recorded simultaneously by two elastic manometers with transmission by liquid. In both curves the ordinates having the same numbers have the following meaning: 1, the instant preceding the closing of the mitral valve; 2, the opening of the semilunar valve; 3, the beginning of the "dicrotic wave," regarded as marking the instant of closure of the semilunar valve; 4, the instant preceding the opening of the mitral valve (Porter).

tensive rise of pressure, marked in the curve by a line but slightly inclined from the vertical. In the same way the fall of pressure is nearly as sudden and as swift as the rise, and perhaps even more extensive. The systolic rise begins at a pressure a little above that of the atmosphere; the diastolic fall continues, toward its end, perhaps, with diminishing rapidity, till a point is

reached often below the pressure of the atmosphere. The pressure then rises, perhaps continuing negative for a longer or shorter time, but presently becoming equal to that of the atmosphere. Near this it continues, perhaps with a gentle upward tendency, until, near the end of the ventricular diastole, the rise becomes more rapid to the point at which the succeeding ventricular systole is to begin.

It is the course of the pressure between its rapid rise and its rapid fall which has been the most disputed. The observers who employ manometers with liquid transmission, have so far found that the high swift rise at the outset of the systole is soon succeeded by a sudden change. According to them the pressure within the manometer now exhibits fluctuations of greater or less extent which are due, partly at least, to the inertia of the transmitting liquid; but, with due allowance made for these, the cardiac pressure is seen to maintain itself at a high point throughout most of the systole until the rapid fall begins. During this period of high pressure, the height about which the fluctuations occur may remain nearly the same; or this height may gradually increase, or gradually decrease, up to the beginning of the rapid fall. As is shown by Figure 23, this course of the systolic pressure causes its curve to bend alternately downward and upward between the end of its greatest rise and the beginning of its greatest fall; but between these two points the general direction of the curve approaches the horizontal, and therefore entitles this portion of it to the name

FIG. 24.-Magnified curve of the course of pressure within the left ventricle of the dog, the chest being open; to be read from left to right. Recorded by the elastic manometer with transmission by air. The ordinates have the following meaning: 1, the closure of the mitral valve; 2, the opening of the semilunar valve; 3, the closure of the semilunar valve; 4, the opening of the mitral valve (von Frey).

of the "systolic plateau," a name which becomes more truly descriptive when appropriate means are taken to eliminate the fluctuations due to inertia. The best of the manometers with air transmission yields a curve of the pressure within the ventricle which presents a different picture (Figs. 22 and 24). The steeply rising line may diminish its steepness somewhat as it ascends, but its rapid turn at the highest point of the curve is succeeded by no plateau. The line simply describes a single peak, and begins the descent which marks the rapid fall of pressure recognized by all observers. In these peaked curves

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this descent is often steepest in its middle part. Such a peaked curve would indicate, of course, that there is no such thing as the maintenance, during any large part of the systole of the ventricles, of a varying but high pressure. The experienced observer who is the chief defender of the peaked curve holds the plateau to be a product either of too much friction within the manometer tubes, or of a faulty position of the cannula within the heart, whereby communication with the manometer is, for a time, cut off. The able and more numerous adherents of the plateau, on the other hand, attribute the failure to obtain it to the sluggishness of the instrument employed, or to an abnormal condition of the heart. Recent comparative tests of elastic manometers, and other studies, would seem to show that the curves obtained by liquid transmission, and which exhibit the plateau, afford a truer picture of the general course of the pressure within the ventricles than the peaked curves written by means of air.

The Ventricular Pressure-curve and the Auricular Systole.-It is striking testimony to the smoothness of working of the cardiac mechanism, that the curve of intra-ventricular pressure rarely gives any clear indication of the beginning or end of the auricular systole. This event may be expected to increase the pressure within the ventricles; and, in the curve, the very gentle rise which coincides with the latter and longer part of the ventricular diastole passes into the steep ascent of the commencing ventricular systole by a rounded sweep, which indicates a more rapidly heightened pressure within the ventricle during the auricular systole. As a rule, no angle reveals an instantaneous change of rate to show the beginning or end of the injection of blood by the contracting auricle (see Figs. 22, 23, 24). Occasionally, however, a slight "presystolic" fluctuation of the curve may seem to mark the auricular systole.1

The Ventricular Pressure-curve and the Valve-play. It is also exceedingly striking that no curve, whether it be pointed or show the systolic plateau, gives a clear indication of the instant of the closing or opening of either valve, auriculo-ventricular or arterial (see Figs. 22, 23, 24). These instants, so important for the significance of the curve, can, however, be marked upon it after they have been ascertained indirectly. A method of general application would be as follows: Two elastic manometers are "absolutely graduated" by causing each of them to record a series of pressures already measured by a mercurial manometer. The two elastic manometers can then be made to mark upon the same revolving drum the simultaneous changes of pressure in a ventricle and in its auricle, or in a ventricle and its artery. The pressure indicated by any point of either curve can then be calculated in terms of millimeters of mercury. That point upon the intra-ventricular curve which marks a rising pressure just higher than the simultaneous pressure in the auricle or artery, may be taken to mark the closing of the cuspid valve. or the opening of the semilunar valve, as the case may be. By a converse process, the moment of opening of the cuspid valve, or of closing of the semi1 von Frey and Krehl: op. cit., p. 61.

lunar, may also be ascertained. The practical difficulties in the way of applying this method to the ventricle and auricle are much greater than to the ventricle and artery. By another application of the principle just described, a "differential manometer" has been devised for the purpose of registering as a single curve the successive differences, from moment to moment, between the ventricular and auricular pressures, or the ventricular and arterial pressures (see Fig. 25). To this end, two elastic manometers are fastened immovably together, and their two elastic disks, instead of bearing upon separate levers, are made to bear upon a single one, which has its fulcrum between the disks, and is a lever not of the third order, but of the first, like a common balance.

L

D

S

FIG. 25.-Diagram of the differential manometer: A, artery: V, ventricle; D, drum of kymograph, revolving in the direction of the arrow, and covered with smoked paper; L, recording lever in contact with the revolving drum; S, a spring by which the movement of the lever worked by the disks is transmitted to the recording lever. (The working details of the instrument are suppressed or altered for the sake of clearness.)

As the lever or beam of the balance turns from the horizontal as soon as the scales are pressed upon by unequal weights, so the lever of the differential manometer turns as soon as the disks are unequally affected by the pressures within the ventricle and the auricle, or the ventricle and the artery. As, however, the pressures upon the scales are from above, while those upon the disks are from below, the disk which tends to "kick the beam" is the one acted upon by the greater pressure, instead of by the less, as in the case of the scales. The manometric lever marks its oscillations as a curve upon the kymograph by the help of a second or "writing lever" connected with it. The persistence of exactly equal pressures, no matter what their absolute value, in the two manometers would cause a horizontal line to be drawn by the writing lever. This would serve as a base-line. The differential manometer is a valuable instrument, although it is evident that where such minute differences of space and time are recorded as a curve by such complicated mechanisms, the sources of error must be numerous and difficult to avoid.1

The methods which proceed by the measurement of differences of pressure may sometimes be controlled, or even replaced, by an easier method, as follows: If two manometers simultaneously record on the same kymograph the pressure1 K. Hürthle: Pflüger's Archiv für die gesammte Physiologie, 1891, Bd. 49, S. 45.

curves of the ventricle and the auricle, or of the ventricle and the artery, any very sudden change of pressure, produced in auricle or artery at the opening or shutting of a cardiac valve, will produce a peak or angle in the curve of pressure of the auricle or artery. By the rules of the graphic method the point in the pressure-curve of the ventricle can easily be found which was written at the same instant with the peak or angle in the auricular or arterial curve. That point upon the ventricular curve, when marked, will indicate the instant of opening or shutting of the valve in question. In the pressure-curve obtained from the aorta close to the heart, there is a sudden angle which clearly marks the instant when the opening of the semilunar valve leads to the sudden rise of pressure which causes the up-stroke of the pulse (see Fig. 23). Again, the fluctuation of aortic pressure which we shall learn to know as the "dicrotic wave" begins at a moment which many believe to follow closely upon the closure of the semilunar valve. That moment may be indicated by a notch in the aortic curve. So, too, the rise of pressure within the auricle produced by its systole may suddenly be succeeded by a fall, the beginning of which must mark the closure of the cuspid valve, which closure thus may correspond with the apex of the auricular curve.

In Figure 23, ordinate 1 indicates the closing, and ordinate 4 the opening, of the mitral valve. These two points were found by help of the differential manometer. Ordinate 2 indicates the opening, and ordinate 3 the closing, of the aortic valve. These two points were marked with the help of the curve of aortic pressure, also shown in Figure 23, each ordinate of which has the same number as the corresponding ordinate of the ventricular curve. In the arterial curve, 2 marks the beginning of the systolic rise, and 3 the beginning of the dicrotic wave, which latter point is treated by the observer as closely corresponding to the closure of the aortic valve. In Figure 24 each ordinate has the same number, and, as regards the valveplay, the same significance, as in Figure 23. Ordinate 1 corresponds to the apex of a peak in the auricular curve (not here given) which represents the end of the auricular systole. Ordinate 2 corresponds to the beginning of the systolic ascent in the aortic curve (not here given). Ordinate 3 was found by comparing, by means of two elastic manometers, the simultaneous pressures in the ventricle and the aorta. Ordinate 4 corresponds, on the auricular pressure-curve, to a point which marks the beginning of a decline of pressure believed by the observer to succeed the opening of the cuspid valve. In both the figures given of the ventricular curve, and in such curves in general, the points which mark the valve-play occur as follows: The closure of the cuspid valve corresponds to a point, not far above the line of atmospheric pressure, where the moderate upward sweep of the ventricular curve takes on the steepness of the systolic ascent. The systole of the auricle is of little force, and the blood injected by it into the distensible ventricle raises the pressure there but little; that little, however, is more than the relaxing auricle presents, and the cuspid valve is closed. Somewhere on the steep systolic ascent occurs the point corresponding to the rise of the ven