PROPOSED STUD I ES OF ARTERIAL FLL IL) DYNAmics AND ATHEROMA
INTRODUCTION
We have undertaken in this laboratory over the past 15 years a ser4es
of fundamental studies on the disease of arteries, atheroma or arteriosclerosis.
These studies have led to clarification of the locations at which atheroma occurs
and of underlying mechanisms, which appear to include interactions between arterial
fluid dynamics and arterial walY mass transport. Our very recent work, which
involves both fundamental investigations and non-invasive studies of arterial haemo-
haemodynamics in man suggests, moreover, the possibility of influencing arterial
fluid dynamics and mass transport and, hence, atherom. The scientific background
and the proposed research for which support is sought are described in outline.
BACKGROUND
Disease of the circulatory system is the single largest cause of death
in Britain. The most recent figures - for 1982 - show it to have been responsible
Pcr about 49% of the approximately 647,000 deaths from natural causes. The
,.-)ri nc iDal cause of disease of the circulatory system is atheroma or arterioscierosiz,
@.;hich tends to obstruct arteries and as a result to interfere with the blood supply
to parts of the body. Among the major diseases caused by atheroma are coronary
thrombosis and stroke. About 4o% of the 1982 deaths from natural causes were due
to atheroma with many of the subjects being in the age range 40-55 years. By
comparison, about 23% of the deaths were due to neoplasms, that is various forms
of cancer.
Atheroma has been reported to be associated with a number of factors,
including abnormality of the composition of the blood, but the focal occurrence
of the condition, among other findings, implies the operation of local factors
such as spatial variation of arterial fluid dynamics or of the properties of the
arterial wall.
Atheroma affects particularly the thick- rather than the thin-walled
arteries and sites of branching and curvature, with the lesions developing in the C=>
intima (inner coat) (Woolf, 1982). Recent studies support the hypothesis,
proposed initially from this laboratory (Caro et al, 1969; 1971) that atheroma
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Orcurs preferentially in regions in arteries where, although the detailed fluid
mech&nics require clarification, the velocity of blood adjacent to the wall
(wall shear) is expected to be low (Kiaernes et al, 1981; Friedman et a!, 1981.
Zarins et al., 1983).
The mechanisms underlying this distribution of athercma re,--ain unclear.
LL appears, however, that they involve interaction between arterial fluid dynamics
:@nd the mass transport of the arterial wall. Early studies concentrated on trans-
--,cr-. across the intima which vas,found to depend weakly on wall shear (Fry, 1969.,
arc 1, t4erem, I 973), but. there is' uncertainty concerning the integrity- of the
vessels in -.base experiments. Later studies have examined the question of trans-
port of material right across the arterial wall. Both water (Wilens, 1951;
Vargas et al, 1979) and various solutes appear to be transported from the 1"men
towards the adventitia (outer coat of *the wall), seemingly to the lymphatics
and the fine blood vessels (vasa vasorum) (Bratzler et a-1, 1977, Caro et al..
Detailed study of this transport (Caro et al, 1980; Carc & Lever, 1963,
!,Cver Tedgui, 1981) has led to some unexpected and potentially important 'Ind-
@ngs- It has been recognised for some time that the endothelium (layer of cells
lining the inner surface of the wall) offers a high transport res3.stanCe (FrY,
1973; Sirlinger et al, 1974). Our experiments with arteries in situ and in vitro
show strong exclusion particularly of macromolecular tracers frozi the media (middle
muscle-containing coat of the wall)i=plying that it, too, offers a significant
transport resistance. The media could hence act as a barrier to the drainagc
Of ma-teria.1 from the subendothelium (region between the ondothelium and media)
to the adventitial lymphatics and vasa vasorum. Our results show in addition
that the transport properties of the interstitium or extracelluar material of
tire media, the main thoroughfare for the movement of water and water Soluble
substances through the tissue, are influenced by prevailing mechanical stresses.
The exclusion of extracellaar tracers including albumin and sucrose from the media
is increased by increasing smooth muscle tone with noradrenmline and decreased
by the vascdilators sodium nitrite and isocorbide dinitrate (ISDN). The exclusion
of tracers by the medial interstitium is also increased by pressurising arteries
in such a way that there is no convection of water through the wall. In contrast,
the uptake of the tracers was increased by increasing transmural water flux. C=>
UNJ
The complexity of these changes and of the structure of the arterial
wall has encouraged us to undertake in addition mass transport studies in vitro 01\
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3.
using various solutes and isolated components of the arterial interst-;ti=
as the matrix. The diffusion has been examined of both inert solutes, as in
the experiments in intact vessels and lipoproteins. To simulate more closely
conditions in the intact vessel, we have also studied mass transport in the
presence of water flux. Convection of water has been found to deform the
matrix. Furthermore, depending on solute and matrix properties and. other
conditions, the system has been found to behave either like a filter retain-
ing larger solutes or like a gel chromatographic column with the velocity of
larger solutes exceeding that of smaller ones (Parker & Winlove, ,
An extensive programme of fundamental studies is being carried out
in this laboratory to increase understanding of these complex processes. 'This
includes investigations on the mechanical and mass transport properties of
the interstitium (Parker & Winlove, 1984; Winlove & Parker, 1984) and, revert-
ing to our earlier interest, studies on the interactions of fluid dynamics with
transintimal transport.
Shear appears to exert a range of effects on the arterial wall.
It is reported to influence the dimensions of arteries (Kamiya & Togawa, 1980)
and both the morphology of endothelial cells (Nerem & Levesque, 1983) and the
release of chemica.'$ by them (Frangos et al, 1983). We have been concerned to
determine, moreover, whether concentration polarisation occurs of plasma proteins
at the blood: wall interface, a process which would be influenced by the fluid
dynamics immediately adjacent to the wall (Blatt et al, 1970).
In order to study this last question, we have developed a tecbnicue
for measuring the transmural flux of water in excised, seemingly intact arteries.
The average transmural velocity of water at a normal transmural pressure is found
-6 _1 I
to be about 10 cm . However, most transeAdothelial water flux is believed to
occur through the intercellular clefts which represent only a small fraction of
the wall surface area. Solvent velocity in the clefts could, therefore, be of
-1
order I - 10 pm s , sufficient to cause plasma proteins to be swept in and to
accumulate, causing interference with water flux.
Using the excised artery preparation, we have examined the influence
of perfusate albumin concentration on transmural water flux, after taking into
account perfusate oncotic pressure (Caro et al, 1984). Water flux was found to
decrease rapidly at first with increase of albumin concentration, consistent with
albumin binding to the glycocalyx (Mason et al, 1977). However, a progressive
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though lesser fall I of water flux was seen with further increase of perfusate
albumin concentration. An explanation consistent with the latter finding
is concentration polarisation. Further preliminary, unreported results are
also consistent with that explanation. Increase of wall shear in the perfused
-2
artery preparation from zero to about 2 dyn cm , a value well within the
normal range, increased transmura.1 water flux and the change was reversed by
returning wall shear to zero. Shear would be expected to lessen concentration
polarisation by thinning the concentration boundary layer adjacent to the wall
and enhancing diffusion of accumulating solute away from the wall.
These studies reveal the mass transport of the arterial wall to be
highly complicated and to involve numerous interactions. For example, the
blood pressure and endothelia.1 permeability appear to affect transmural
convection and the blood pressure, medial smooth muscle tone and transmura.-
convection appear to influence the transport resistance of the media. Moreover,
vasoactive materials possibly directly (Robertson & Khairallalh, 1972; Baldwin
el. al, 1983) and wall shear, possibly through an effect on concentration
D01a.risation, appear to influence endothelial permeability. In addition,
convection will influence solute transport through the mechanism of solvent
drag.
Such complexity renders the design of experiments difficult. On the
other hand, it may offer opportunities for influencing function by, for
example, pharmacological means. Thus, while the mechanisms associating
atheroma and arterial wall mass transport remain unclear, there is suggestive
evidence that a high medial resistance favours the occurrence of the disease.
The evidence includes the preferential occurrence of atheroma in the intina of
thick-walled arteries and the sparing of thin7valled vessels, and observations
on the distribution of lipoprotein in the arterial wall (Smith & Staples, 1982).
Further fundamental study of these processes is evidently needed.
However, we have also felt it indicated to explore the possibility of influencini,
medial smooth muscle tone and possibly arterial fluid dynamics by pharmacological
means. To this end, we have undertaken some studies in man.
In collaboration with hospital co" eagues, we have carried out a
limited, double-blind non-invasive ultrasound study of the influence of vasoactive
agents on the haemodynamics of the superficial femoral artery (and in a few subjects CD
"
the common carotid artery) in normal man. Using MAVIS, a multi-channel pulsed Doppl-i
blood flow meter with an imaging facility, we have found that a single 20 mg oral
dcse of ISDN (conventionally used to treat angina) causes marked haemodynamic
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5.
changes. As judged by increase of artery width despite a small fall of blood
pressure and a. reduction of pulse wave velocity, there was relaxation of
the arterial smooth muscle and an increase of the vessel compliance. There
was, furthermore, a pronounced increase of the pulsatility of the blood
velocity (amplitude of velocity/time average velocity) and alteration of the
velocity waveform. The changes persisted during the 2 hr duration of the studies.
Highly preliminary studies have also been undertaken with the same
equipment on the effect in non-smokers of smoking a single cigarette. These show
results which contrast strongly with those obtained with ISDN. In addition to the
expected slight increase of blood pressure and heart rate, there was a fall of
arterial compliance and a reduction of the pulsatility of blood velocity.
The mechanisms associated with these changes are no'. fully understood
and fundamental fluid dynamic studies are needed to clarify them. They may,
nevertheless, be of physiological interest. In the light of our studies with
excied arteries, reported above, it is possible that the alterations of arterial
smooth muscle tone influenced arterial wall mans transport. It is possible, in
addition that the alteration of blood velocity including its pulsatility, affected
the mixing of blood and wall shear not only generally but also in regions where wall
shear is low, which are notably prone to the development of atheroma.
PROPOSED STUDIES
The studies described above lead to consideration, among a number of
questions, of the control of arterial vasomotor tone. They lead, moreover, to
consideration of the role of the flexibility or compliance of arteries in determin-
ing the details of arterial fluid dynamics. The pursuit of these questions will
require extensive fundamental fluid dynamic studies and also investigations in man.
The longer term objectives of such work would include assessment of whether means
might be found of influencing the course of atheroma.
Studies in Man
As in the case of our earlier work, the proposed studies in man should
be carried out in a hospital.The broad objective is to increase understanding of
factors which influence arterial wall properties and arterial fluid dynamics. The
technical requirements for such work include a pulsed Doppler device such as
MAVIS and a small part imager; the pulsed Doppler provides inadequate informa-
tion concerning vessel dimensions or the presence of lesions. There will also C=)
LJNJ
be the need for the measurement of arterial blood pressure. In some instances the L-71
-ch.
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6.
measurements will be intra-arterial and made at various locations in the arterial
tree, including at the origin of the aorta to assess the fluid dynamic input
to the arterial system. Ideally measurements will also be made of blood velocity
near the origin of the aorta, say with an ultrasonic aortic veloci=eter. A later
objective will be to try to assess by non-invasive ultrasonic means local arterial
fluid dynamics including mixing. Such studies might be undertaken where flow
separation is prone to occur, say in the carotid sinus or with the use of an
indicator containing micro-bubbles and hence having different properties than
blood in respect of ultrasound.
it is envisaged that the work will involve extension of the studies
with ISDN and other agents. It will be of importance to establish the extent
and time course of the response to the agents and the question of the development
of tolerance. Agents or factors additional to ISDN which it would seem indicated
to study would include calcium antagonists, a blockers, smoking, alcohol,
exercise, psychological stress and agents affecting it, and hypertension, and
other disease-favouring atheroma.
Fundamental Studies
It is intended that there will be extension of the fundamental studies
on the interaction of fluid dynamicsand vasoactive agents with arterial wall
properties and arterial wall mass transport. In addition, the need exists to
carry out fundamental studies of arterial fluid dynamics. Model and probably
theoretical work is proposed of the complex subject of large amplitude pulsatile
flow in elastic tubes, including consideration of mixing and flow separation
in relation, for example, to regions of curvature and branching
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