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Appendix 5: Elliptical Tube Representation of Coronary Veins (Kassab... 355
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
1  H
B ¼ 1 þ 6:98
0:4
¼
X
0
D
mother
X
is the minimal fractional blood ow required to draw erythrocyte into the
0
daughter branch, B is the nonlinearity of the relation between FQ
D
mother
D
ð5:20Þ
ð5:21Þ
and FQB, and
E
A is the difference between the relations derived for the two daughter vessels. For FQ
< X0, FQEis equal to zero and for FQB> 1 X0, FQEequals to 1. A, B, and X
B
have units of μm1and diameter D has a unit of μm.
Appendix 4: Compliance of Entire Coronary Arterial Tree (Huo & Kassab, 2009)
Table 5.1 Diameter-compliance and normalized diameter-compliance in different diameter-
dened Strahler orders of pig heart
Orders
diameter range
9–11 500 μm Orders
5–880–500 μm Order 8 8.3 10
0–4 80 μm Order 4 6.2 10
Approximate
Diameter-compliance (cm/mmHg) at 100 mmHg
6.3 10
10–11 Order 9 2.3 10
Order 7 4.5 10 Order 6 2.4 10 Order 5 1.3 10
Order 3 3.9 10 Order 2 2.5 10 Order 1 1.9 10 Order 0 1.4 10
Normalized diameter-compliance Reference
4
2.4 103mmHg
4
5
2.0 103mmHg
5
5
5
6
2.1 103mmHg
6
6
6
6
1
1
1
Hamza et al.
(2003)
Giezeman et al.
(1994)
Kassab, Le, and
Fung (1999)
0
Appendix 5: Elliptical Tube Representation of Coronary Veins (Kassab et al., 1994)
At low normal venous pressures, the cross section of a vein can be approximated by an ellipse. Relative to a set of rectangular Cartesian coordinates x and y with origin located at the center, the parametric equations of the ellipse with a semi-major axis a and a semi-minor axis b are as follows:
356 5 Network Analysis of Coronary Circulation: I. Steady-State Flow
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x ¼ a cos θ; y ¼ b sin θ ð5:22Þ
The cross-sectional area is given by:
Area ¼ 1=2Hxdyþ ydxðÞ
Area ¼ 1=2
Area ¼ πab
An equivalent circle of radius R
Z
2π

ab cosþ ab sin
0
will have the same area if the following holds:
e
2
πR
¼ π ab ð5:24aÞ
e
dθ
ð5:23Þ
or
¼ abðÞ
R
e
1=2
¼ ab=aðÞ
1=2
ð5:24bÞ
Kassab et al. (1994) showed that (a/b) varies from 1.25 to 1.96 for vessels between orders 1 and 12. Hence R
, varies between 0.714a and 0.895a .
e
The circumferential length of the ellipse is given by:
Circumference ¼Hdx
Z
2π
Circumference ¼
0

2

a2sin2θ þ b2cos2θ
þ dy
1=2
2
1=2
dθ
ð5:25Þ
which is an elliptical integral involving a and b. It may be argued that the cross­sectional shape of a vein is sensitive to internal pressure, especially if the transmural pressure is negative, when the compression of the wall causes elastic instability and buckling, whereas the circumferential length remains constant in the buckling process. Hence the circumferential lengt h is a more stable parameter than the major and minor axes a and b. It is, however, difcult to measure this length, and its value depends on the smoothness of the wall and any irregularity.
The parameters relevant to the ow can be derived from the Navier–Stokes equation. For a steady longitudinal ow of a Newtonian viscous uid in a long cylindrical tube of elliptical cross section subjected to a constant pressure gradient. In analogy to the exact solution of ow in a circular cylinder, the velocity prole (u):
hi
u ¼ 2U 1 x=aðÞ
2
y=bðÞ
2
ð5:26Þ
Equation (5.26) satises the Navier–Stokes equation and the boundary condition that u is zero on the elliptical wall described by Eq. (5.22). U is the mean velocity over the cross section. With Eq. (5.26), the Navier–Stokes equation yiel ds the following:
References 357
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dP=dx ¼4μUa2þ b


2
2b2
= a
ð5:27Þ
where μ is the coefcient of viscosity of the uid, x is the length along the longitudinal axis of the tube, and dP/dx is the pressure gradient. Then the volume rate of ow (Q) is given by:
Q ¼ Area U ¼ π ab U ¼π=4μ a


3b3
= a
2
2
þ b
dP=dx ð5:28Þ
The blood ow conductance is given by the following coefcient:
Conductance ¼ π=4μLa


3b3
= a
2
þ b
2
ð5:29Þ
where L is the length of the tube. The resistance to ow is given by the inverse of conductance as:
Resistance ¼ 4μL=π a

2

2
= a
3b3
þ b
ð5:30Þ
Equations (5.26)–(5.30) show that a and the ratio b/a are the most important parameters of venous blood ow in which the Womersley number is <1.
Finally, if the cross section is very narrow, the normal cross section may be better approximated as a rectangular slit rather than an ellipse. If h represents the thickness of the slit and the tube is rigid, then dP/dx is related to the mean ow velocity by the equation
2
dP=dx ¼μUh
F ð5:31Þ
in which F is a number that depends on the structure of the slit, red cell dimensions, and hematocrit. In this case, the elasticity of the tube becomes very important to hemodynamics.
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Chapter 6
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Network Analysis of Coronary Circulation: II. Pulsatile Flow
6.1 Introduction
The blood ow in the coronary arteries is very pulsatile (i.e., time-dependent) with zero or even reversing (negative) ow in systole. The two major determinants of coronary ow pulsatility are the: (1) Pulsatile aortic pressure due to cyclic cardiac contraction, and (2) Myocardial/vascular interaction where the heart cyclically compresses the coronary vasculature within the myocardium. In order to understand coronary blood ow, each of these two effects must be understood in turn. We will start with the former in this section and the latter is described in a subsequent section.
The theory of pulsatile blood ow is initiated by Leonhard Euler in 1775 and Thomas Young in 1808. Later, numerous attempts have been made at the study of pulsatile blood ow in the vascular system, e.g., Zamirs Womersley-type analysis (Duan & Zamir, 1992, 1995; Zamir, 1998, 2000), and Hughesone-dimensional theory (Hughes & Lubliner, 1973). Basically, there are three major approaches to simulate the pulsatile blood ow and pressure waves in the vascular system:
1. Womersleys solution of pulsatile blood ow (Avolio, 1980; Duan & Zamir,
1995; Gan & Moodie, 1989; Gan & Yen, 1994; Gao, Huang, & Yen, 2000;
Huang, Tian, Gao, & Yen, 1998; Womersley, 1955, 1957),
2. One-dimensional wave propagation solution (Formaggia, Gerbeau, Nobile, &
Quarteroni, 2001; Hughes & Lubliner, 1973; Olufsen, 1999, 2000; Stergiopulos,
Young, & Rogge, 1992; Vignon & Taylor, 2004);
3. Three-dimensional solution of pulsatile blood ow (Formaggia et al., 2001).
The latter two methods require appropriate outow boundary conditions to simulate the pulsatile blood ow wave. Different outow boundary conditions have been developed to satisfy the 1D or 3D equations, such as the pure resistive load boundary conditions (Stettler, Niederer, & Anliker, 1981), the Windkessel model (Judd, Redberg, & Mates, 1991; Lee, Chambers, Akizuki, & Downey,
1984) the nonreecting outlet boundary conditions (Formaggia et al., 2001), or the
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impedance boundary conditions (Formaggia et al., 2001; Olufsen, 1999, 2000; Vignon & Taylor, 2004). All boundary conditions may be classied as lumped parameter model boundary conditions, meaning that a distributive vascular tree is idealized as a discrete or lumpedequivalence that approximates the outlet ow or pressure outl et conditions. Despite the usefulness of lumped models, however, they are generally limited to the global aspects of coronary blood ow and fail to reveal the ow and pressure wave distribution through the vascular system. For example, lumped models cannot be used to predict the signicant spatial distribution of coronary blood ow, e.g., transmural distribution of blood ow in the myocardium. Womersleys solution avoids those shortcomings because it assumes that pressure and ow are decoupled. Hence, this approximation can be applied throughout a complex vascular network such as the entire coronary arterial tree.
In this chapter, we will extend the boundary value formulation initiated in Chap. 5 to pulsatile ow analysis that accounts for pulsatile inlet boundary condition (i.e., aortic pressure) in diastolic heart as well as cyclic boundary conditions external to the coronary vessels from the contracting surrounding myocardium (i.e., myocardial–vessel interaction). The analysis will be compared to available experi­mental data to refute or verify the models.
6.2 Pulsatile Flow in Passive Hearts
To validate the pulsatile ow analysis outlined below, experiments are performed on isolated, arrested hearts (Huo & Kassab, 2006). Briey, the isolated heart is placed in a cold (0
C) saline bath as shown in Fig. 6.1a. The major coronary arteries are cannulated under saline to avoid air bubbles. The coronary arteries are perfused with a piston pump that provides a continuous sawtooth pulsatile ow. The sawtooth pulsatile ow produced by the piston pump is conrmed to be similar to that measured in vivo at a heart rate of 90 beats/min (Fig. 6.1b). Therefore, the piston pump is used to simulate the in vivo aortic pressure. The wave amplitude of the pulsatile pressure is in the range of 50–100 mmHg with a mean pressure of around 75 mm Hg. In order to mimic the prole and range of the in vivo pressure wave, tubes of appropriate resistance and compliance arranged between the piston pump and pressure transducer are selected. It is found that the phase difference is very small (<20 ms), which is within the range of relative error of the ow transducers.
The discussion below is divided into three subsections. First, low-frequency ow
(ω!0) is compared with steady-state ow. Second, the pressure produced by the piston pump is applied as the inlet pressure boundary condition to calculate the pressure and ow distribution in the entire coronary arterial tree by using the current mathematical model in conjunction with a Fourier transform. The mathematical predictions are in turn compared with the experimental results. Finally, the effects of various parameters (e.g., frequency, viscosity) on pulsatile ow are examined for the entire coronary arterial tree.