Arthur Sheridan Lea Sir Michael Foster.

A text book of physiology online

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uous, so that the pulse-curve presents two notable crests only,
the primary crest and a secondary one, the pulse is said to be
" dicrotic *' ; when two secondary crests are prominent, the pulse is
often called " tricrotic " ; when several, " polycrotic." As a general
rule, the secondary elevations appear only on the descending limb
of the primary wave as in most of the curves given, and the curve
is then spoken of as " katacrotic." Sometimes, however, the first
elevation or crest is not the highest, but appears on the ascending
portion of the main curve : such a curve is spoken of as " anacrotic "
Fig. 64. As we have already seen (§ 116) the curve of pressure
at the root of the aorta, and, indeed, that of endocardiac pressure
may be in like manner " anacrotic " (Figs. 53, 54).



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Chap.ivJ the vascular MECHANISM.



231



Of these secondary elevations, the most frequent, conspicuous,
and important is the one which appears
some way down on the descending limb,
and is marked C on Fig. 68 and on most
of the curves here given. It is more or
less distinctly visible on all sphygmograms,
and may be seen in those of the aorta
as well as of other arteries. Sometimes
it is so slight as to be hardly discernible ;
at other times it may be so marked as
to give rise to a really double pulse
(Fig. 65), t,e. a pulse which can be felt
as double by the finger : hence it has been
called the dicrotic elevation or the dicrotic wave, the notch
preceding the elevation being spoken of as the '' dicrotic notch."




Fio.64. Anacrotic 8PHTO-

MOORAPU TRACING FROM

THE Ascending aorta
(Aneurism).





FlO. 66. Two GRADES OF MARKED DICROTISM IN RADIAL PULSE OF MAN.

(Typhoid Fever.)

Neither it nor any other secondary elevations can be recognized
in the tracings of blood pressure taken with a mercury manometer.
This may be explained, as we have said § 121, by the fact that
the movements of the mercury column are too sluggish to repro-
duce these finer variations. Moreover, when the normal pulse
is felt by the finger, most persons find themselves unable to detect
any dicrotism. But that it does really exist in the normal pulse
is shewn by the fact that it appears, sometimes to a marked
extent, sometimes to a less extent, not only in sphygmograms and
in curves of arterial pressure taken by adequate instruments, but
also and in a most unmistakeable manner in the tracing obtained
by allowing the blood to spirt directly from an opened small
artery, such as the dorsalis pedis, upon a recording surface.

Less constant and conspicuous than the dicrotic wave, but yet
appearing in most sphygmograms, is an elevation which appears
higher up on the descending limb of the main wave ; it is marked
B in Fig. 68, and on several of the other curves, and is frequently
called the predicrotic wave ; it may become very prominent. Some-
times other secondary waves, often called ' post-dicrotic,' are seen
following the dicrotic wave, as at D in Fig. 63, and some other
curres ; but these are not often present, and usually even when
present inconspicuous.

When tracings are taken from several arteries, or from the same
MTterv under different conditions of the body, these secondary
WMves ai« found to vary very considerably, giving rise to many



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232 THE DICROTIC WAVE. [Book i.

characteristic forms of pulse-curve. Were we able with certainty
to trace back the several features of the curves to their respective
causes, an adequate examination of sphygmographic tracings
would undoubtedly disclose much valuable information concerning
the condition of the body presenting them. The problems, how-
ever, of the origin of these secondary waves and of their variations
are complex and difficult; so much so that the detailed interpre-
tation of a sphygmographic tracing is still in many cases and in
many respects very uncertain.

§ 128. The Dicrotic Wave, The chief interest attaches to
the nature and meaning of the dicrotic wave. In general the
main conditions favouring the dicrotic wave are (1) a highly
extensible and elastic arterial wall ; (2) a comparatively low mean
pressure, leaving the extensible and elastic reaction of the arterial
wall free scope to act ; and (3) a vigorous and rapid stroke of the
ventricle, discharging into the aorta a considerable quantity of
blood.

The origin of this dicrotic wave has been and indeed still is
much disputed.

In the first place, observers are not agreed as to the part of
the arterial system in which it first makes its appearance. In
such a system as that of the arteries we have to deal with two
kinds of waves. There are the waves which are generated at the
pump, the heart, and travel thence onwards towards the periphery ;
the primary pulse-wave due to the discharge of the contents of
the ventricle is of this kind. But there may be other waves
which, started somewhere in the periphery, travel backwards
towards the central pump ; such are what are called * reflected '
waves. For instance, when the tube of the artificial model, bear-
ing two levers, is blocked just beyond the far lever, the primary
wave is seen to be accompanied by a second wave, which at the
far lever is seen close to, and often fused into, the primary wave
(Fig. 59, VI. a'), but at the near lever is at some distance from it
(Fig. 59, 1, a'), being the farther from it the longer the interval
between the lever and the block in the tube. The second wave is
evidently the primary wave reflected at the block and travelling
backwards towards the pump. It thus, of course, passes the far
lever before the near one. And it has been argued that the
dicrotic wave of the pulse is really such a reflected wave, started
either at the minute arteries and capillaries, or at the several
points of bifurcation of the arteries, and travelling backwards to
the aorta. But if this were the case the distance between the
primary crest and the dicrotic crest ought to be less in arteries
more distant from, than in those nearer to the heart, just as in
the artificial scheme the reflected wave is fused with a primary
wave near the block (Fig. 59, VI. 6 a. a'), but becomes more and
more separated from it the farther back towards the pump we trace
it (Fig. 59, I. 1. a. a'J- Now this is not the case with the dicrotic



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Chap, iv.] THE VASCULAR MECHANISM. 233

wave; careful measurements shew that the distance between
the primary and dicrotic crests is either the same or certainly not
less in the smaller or more distant arteries than in the larger or
nearer ones. This feature indeed proves that the dicrotic wave
cannot be due to reflection at the periphery or indeed in any way
a retrograde wave. Besides the multitudinous peripheral division
would probably render one large peripherically reflected wave im-
possible. Again, the more rapidly the primary wave is oblite-
rated or at least diminished on its way to the periphery the less
conspicuous should be the dicrotic wave. Hence increased ex-
tensibility and increased elastic reaction of the arterial walls
which tend to use up rapidly the primary wave, should also lessen
the dicrotic wave. But as a matter of fact these conditions, as we
have said, are favourable to the prominence of the dicrotic wave.

We may conclude then that the dicrotic wave like the primary
wave begins at the heart, and travels thence towards the periphery.
But even if this be admitted observers are not agreed as to the
mechanism of its production. The following view is the one
which seems the most satisfactory, though it is not accepted
by all inquirers.

The simultaneous curves of endocardiac and aortic pressure
' Fig. 54 and others) shew us that the dicrotic notch as it is called,
the depression m[imediately preceding the dicrotic wave is, in a
normal beat, coincident with the end of the systole. The curve
of the differential manometer further shews us that this is the
point at which the pressure in the ventricle begins to become less
than in the aorta. We may therefore reason in the following
war. The flow from the ventricle into the aorta ceases because
the systole ceases ; the cessation takes place while the two cavi-
ti^-s are still open to each other, and probably, in most cases at
lea-^t, while there is still more or less blood in the ventricle. The
pressure in the ventricle tends to become less than that in the aorta,
and the blood in the aorta tends to flow back into the ventricle.
But the first effect of this is to close firmly the semilunar valves.
The exjiansion of the aorta, (which in many cases had been lessen-
mg even during the systole owing to the flow through the periphery
of the arterial system being more rapid than the flow from the
vpntricle, but in some cases, in the anacrotic instances, had not,)
le^«*ens with the cessation of the flow ; the aorta shrinks, press-
ing upon its contents. But part of this pressure is spent on the
cUwed semilunar valves, and the resistance off'ered by these starts
a new wave of expansion, the dicrotic wave, which travels thence
^mvards towards the periphery in the wake of the primary wave.
If we admit that the blood is flowing from the ventricle during
the whffle of the systole, we must also admit that the semilunar
riires do not close until the end of the systole, and this being, as
ftbewn hv the curves, just antecedent to the dicrotic wave, we may
MUribute this wave to the rebound from the closed valves. It is



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284 THE DICROTIC WAVE. [Book i.

not necessary that the valves should act perfectly, and the dicrotic
wave may occur in cases where the valves are more or less in-
competent ; all that is required for its production is an adequate
obstacle to the return of blood from the aorta to the ventricle,
and without such an obstacle the circulation could not be carried
on.

§ 129. Moreover it must be remembered that though we may
thus regard the closed valves as so to speak the determining cause
of the dicrotic wave, the wave itself is an oscillation of the arterial
walls, and the remarks made a little while back concerning the
inertia of the walls hold good for this explanation also. Hence
the conditions which determine the prominence or otherwise of
the dicrotic wave are conditions relating to the elasticity of the
arterial walls, and to the circumstances which call that elasticity
into play. For instance, the dicrotic wave is less marked in rigid
arteries (such as those of old people) than in healthy elastic ones ;
the rigid wall neither pxpands so readily nor shrinks so readily,
and hence does not so readily give rise to secondary waves. Again,
the dicrotic wave is, other thmgs being equal, more marked when
the mean arterial pressure is low than when it is high ; indeed it
may be induced when absent, or increased when slightly marked,
by diminishing, in one way or another, the mean pressure. Now
when the pressure is high, the arteries are kept continually much
expanded, and are therefore the less capable of further expansion,
that is to say, are, so far, more rigid. Hence the additional
expansion due to the systole is not very great ; there is a less
tendency for the arterial walls to swing backwards and forwards,
so to speak, and hence a less tendency to the development of
secondary waves. When the mean pressure is low, the opposite
state of things exists ; supposing of course that the ventricular
stroke is adequately vigorous (the low pressure being due, not
to a diminished cardiac stroke but to diminished peripheral
resistance) the relatively empty but highly distensible artery
is rapidly expanded, and falling rapidly back enters upon a
secondary (dicrotic) expansion, and may even exhibit a third.

Moreover the same principles may be applied to explain why
sometimes dicrotism will appear marked in a particular artery
while it remains little marked in the rest of the system. In
experimenting with an artificial tubing such as the arterial model,
the physical characters of which remain the same throughout,
both the primary and the secondary waves retain the same
characters as they travel along the tubing save only that both
gradually diminish towards the periphery; and in the natural
circulation, when the vascular conditions are fairly uniform
throughout, the pulse-curve, as a rule, possesses the same general
characters throughout, save that it is gradually * damped ofif.*
But suppose we were to substitute for the first section of the
tubing a piece of perfectly rigid tubing , this at the stroke of the



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Chap, iv.] THE VASCULAR MECHANISM. 285

pump on account of its being rigid would shew neither primary
HOT secondary expansion, but the expanding force of the pump's
stroke would be transmitted through it to the second, elastic
section, and here the primary and secondary waves would at once
become evident This is an extreme case, but the same thing
would be seen to a less degree in passing from a more rigid, that
is less extensible and elastic section, to a less rigid, more exten-
sible and elastic section ; the primary and secondary expansions,
in spite of the general damping effect, would suddenly increase.
Similarly in the living body a pulse-curve which so long as it is
travelling along arteries in which the mean pressure is high, and
which are therefore practically somewhat rigid, is not markedly
dicrotic, may become very markedly dicrotic when it comes to a
particular artery, in which the mean pressure is low (we shall see
presently that such a case may occur), and the walls of which
are therefore for the time being relatively more distensible than
the rest.

Lastly we may recall the observation made above § 123 that
the curve of expansion of an elastic tube is modified by the pres-
sure exerted by the lever employed to record it, and that hence,
in the same artery, and with the same instrument, the size, form,
and even the special features of the curve vary according to the
amount of pressure with which the lever is pressed upon the
artery. Accordingly the amount of dicrotism apparent in a pulse
may be modified by the pressure exerted by the lever. In Fig. 61
for instance the dicrotic wave is more evident in the middle than
in the upper tracing.

§ 130. Concerning the other secondary waves on the pulse-curve
such as that which has been called the * predicrotic ' wave {B on Fig.
63 and on some of the other pulse-curves) it will not be desirable
to say much here. They have been the occasion of much discus-
sion, especially when considered under the view that the ventricle
rapidly emptied itself at the earlier part of the systole. We will
content ourselves with the following remark. The predicrotic
and the other secondary waves in question are, like the dicrotic
wave, propagated from the heart towards the periphery, they are
seen in sphygmograms taken from the root of the aorta as well as
from more peripheral arteries, and some are also seen in the curves
of ventricular pressure. Some of the features of these secondary
waves may be due to imperfections in the instruments used, to
inertia and the like, but the main features undoubtedly represent
events taking place in the vascular system itself. When we com-
pare the curve of pressure in the aorta with that in the ventricle,
we obeerve that up to the dicrotic notch, (in what may be called
the systolic part of the pulse-curve, the part which corresponds to
tiie systole of the ventricle, in contrast to the diastolic part which
follows and which includes the dicrotic wave), the variations seen
in the aortic curve, the secondary waves of which we are speaking,



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236 THE VENOUS PULSE. [Book i.

are exactly reproduced in the ventricular curve. And it has, with
considerable reason, been urged that both in the aorta (and so in the
other arteries) and in the ventricle they are due to events taking
place in the ventricle, the systole for instance not being equally
sustained.

We may further call once to mind the fact to which we have
already called attention that, while sometimes the curve of ven-
tricular pressure reaches its maximum at the very beginning of
the systole, declining more or less slowly afterwards, at other times
the maximum is reached at the end of the systole, the curve of
pressure being anacrotic; we may add that the maximum may
also occur at any intermediate point. Further, and this is the
matter to which we wish to call attention, the curve of aortic
pressure follows that of the ventricular pressure, both being kata-
crotic or anacrotic as the case may be. As we have urged, the
anacrotic curve is seen when the peripheral resistance is such that,
for some time during the systole, the flow from the aorta towards
the periphery is slower than the flow from tlie ventricle into the
aorta. Such a condition is apt to be met with when the arteries
are more rigid than normal, and under these circumstances the
anacrotic characters of the pulse may become prominent.

§ 131. Venous Pulse, Under certain circumstances the pulse
may be carried on from the arteries through the capillaries into the
veins. Thus, as we shall see later on, when the salivary gland is
actively secreting, the blood may issue from the gland through the
veins in a rapid pulsating stream. The nervous events which give
rise to the secretion of saliva, lead at the same time, by the agency
of vaso-motor nerves, of which we shall presently speak, to a widen-
ing of the small arteries of the gland. When the gland is at rest,
the minute arteries are, as we shall see, somewhat constricted and
narrowed, and thus contribute largely to the peripheral resistance
in the part; this peripheral resistance throws into action the
elastic properties of the small arteries leading to the gland, and
the remnant of the pulse reaching these arteries is, as we before
explained, finally destroyed. When the minute arteries are dilated,
their widened channels allow the blood to flow more easily through
them and with less friction ; the peripheral resistance which they
normally offer is thus lessened. In consequence of this the elasti-
city of the walls of the small arteries is brought into play to a
less extent than before, and these small arteries cease to do their
share in destroying the pulse which comes down to them from the
larger arteries. As in the case of the artificial model, where the
' peripheral ' tubing is kept open, not enough elasticity is brought
into play to convert the intermittent arterial flow into a con-
tinuous one, and the pulse which reaches the arteries of the gland
passes on through them and through the capillaries, and is con-
tinued on into the veins. A similar venous pulse is also some-
times seen in other organs.



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Chap, rv.] THE VASCULAR MECHANISM. 237

Careful tracings of the great veins in the neighbourhood of the
heart shew elevations and depressions, which appear due to the
variations of endocardiac pressure, and which may perhaps be
spoken of as constituting a * venous pulse/ though they have
a quite different origin from the venous pulse just described
in the salivary gland. In such a pulse it is the depression of
the wave, not the elevation, which corresponds to the systole
of the ventricle, the pulse-wave is the negative of the arterial
pulse-wave ; the matter however needs further study. In cases
again of insufliciency of the tricuspid valves, the systole of the
ventricle makes itself distinctly felt in the great veins ; and an
expansion travelling backwards from the heart becomes very
visible in the veins of the neck. This, in which the elevation of
the wave like that of the arterial pulse-wave corresponds to the
ventricular systole, is also spoken of as a venous pulse.

Variations of pressure in the great veins due to the respiratory
movements are also sometimes spoken of as a venous pulse ; the
nature of these variations will be explained in treating of respi-
/ation.



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SEC. 6. THE REGULATION AND ADAPTATION OP
THE VASCULAR MECHANISM,



The Regulation of the Beat of the Heart

§ 132. So far the facts with which we have had to deal,
with the exception of the heart's beat itself, have been simply
physical facts. All the essential phenomena which we have
studied may be reproduced on a dead model Such an unvary-
ing mechanical vascular system would however be useless to a
living body whose actions were at all complicated. The promi-
nent feature of a living mechanism is the power of adapting itself
to changes in its internal and external circumstances. And the
vascular mechanism in all animals in which it is present is capable
of local and general modifications, adapting it to local and general
changes of circumstance. These modifications fall into two great
classes :

1. Changes in the heart's beat. These, being central, have of
course a general effect ; they influence or may influence the whole
body.

2. Changes in the peripheral resistance, due to variations in
the calibre of the minute arteries, brought about by the agency of
their contractile muscular coats. These changes may be either
local, affecting a particular vascular area only, or general, affecting
all or nearly all the blood vessels of the body.

These two classes of events are chiefly governed by the
nervous system. It is by means of the nervous system that the
heart's beat and the calibre of the minute arteries are brought
into relation with each other, and with almost every part of the
body. It is by means of the nervous system acting either on
the heart, or on the small arteries, or on both, that a change of



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Chap, iv.] THE VASCULAR MECHANISM. 289

drcTimstances affecting either the whole or a part of the body is
met by compensating or regulative changes in the flow of blood.
The study of these changes becomes therefore to a large extent
a study of nervous actions.

The circulation may also be modified by events not belonging
to either of the above two classes. Thus, in this or that periphereu
area, changes in the capillary walls and the walls of the minute
arteries and veins may lead to an increase of the tendency of the
blood corpuscles to adhere to the vascular walls, and so, quite
apart from any change in the calibre of the blood vessels, may
lead to increase of the peripheral resistance. This is seen in an
extreme case in inflammation, but may possibly intervene to a less
extent in the ordinary condition of the circulation, and may also
be under the influence of the nervous system. Further, any
decided change in the quantity of blood actually in circulation
must also influence the working of the vascular mechanism. But
both these changes are unimportant compared with the other two
kinds of changes. Hence, the two most important problems for
U8 to study are, 1, how the nervous system regulates the beat of
the heart, and 2, how the nervous system regulates the calibre of
the blood vessels. We will first consider the former problem.



The Development of the Normal Beat

§ 133. The heart of a mammal or of a warm blooded animal

generally ceases to beat within a few minutes after being removed

from the body in the ordinary way, the hearts of newly-born

animals continuing however to beat for a longer time than those

of adults. Hence, though by special precautions and by means of

an artificial circulation of blood, an isolated mammalian heart may

be preserved in a pulsating condition for a much longer time, our

knowledge of the exact nature and of the causes of the cardiac

beat is as yet very largely based on the study of the hearts of

cold blooded animals, which will continue to beat for hours, or

under favourable circumstances even for days, after they have

been removed from the body with only ordinary care. We have

reason to think that the mechanism by which the beat is carried

on varies in some of its secondary features in different kinds of

inimals : that the hearts, for instance, of the eel, the snake, the

tortoise and the frog, differ in some minor details of behaviour,

hotb trom each other and from those of the bird and of the mammal ;

hat we may, at first at all events, take the heart of the frog as

jJtagtnting the main and important truths concerning the causes

iod mechanism of the beat



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240 GRAPHIC RECORD OF HEART BEAT. [Book i.

In studying closely the phenomena of the beat of the heart it becomes
necessary to obtain a graphic record of the various movements.

1. In the frog, or other cold blooded animal, a light lever may be



Online LibraryArthur Sheridan Lea Sir Michael FosterA text book of physiology → online text (page 29 of 148)