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Appendix 4: Coronary Flow Regulation 435
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in the coronary venous ATP concentration, and the latter correlates with the decline of venous PO vessel metabolic signal F
(Farias III et al., 2005). In the network ow analysis, the terminal
2
i
is either set to be constant in all terminal vessels or
, j
mterm
optimized in each terminal vessel to provide a set level of terminal ow.
Oxygen Deman d and Target Terminal Flow If the secondary contribution of dissolved oxygen on the total oxygen content are considered negligible, the oxygen mass balance is specied by:
M ¼ q
where M is myocardial oxygen consumption for a single terminal arteriole, q the ow in the terminal arterioles, H saturation, S
is the venous oxygen saturation, and cois the oxygen carrying capacity
v
c0HDSa S
ðÞ ð6:100Þ
term
is the hematocrit, Sais arterial oxygen
D
v
term
of RBCs. Hence, the following holds:
M
c
0HDSa
S
ðÞ
v
The values of c
¼
q
term
, and Saare directly measurable and assumed to be constant
0,HD
(independent of M ). The venous oxygen saturation S
is a function of the oxygen
v
ð6:101Þ
consumption M, being dependent on oxygen mass balance, ATP release and trans­port, and the effect of sympathetic inputs on myocardial oxygen consum ption. It is thus affected by the combined action of a feedback pathway signal that is determined by the level of plasma ATP in coronary venous blood, and by adrenergic open-loop (feedforward) signal that increases with exercise (Pradhan, Feigl, Gorman, Brengelmann, & Beard, 2016). Data has been measured by (Farias III et al., 2005) and (Gorman et al., 2010; Gorman, Tune, Richmond, & Feigl, 2000). Based on this relationship between M and S
, the ow in terminal vessels q
v
can be directly
term
related to the oxygen consumption M.
Flow Regulation Time Constant The time constant of the coronary vessel response to changes in pressure and ow (approximately 1.5 folds of t
the time
50
required to establish half of the complete response) is found to be in the range of 15 s to minutes (Dankelman et al., 1992; Hoffman & Spaan, 1990; Mosher, Ross Jr., McFate, & Shaw, 1964; Tsoukias, Kavdia, & Popel, 2004). This response time constant is signicantly higher than the cardiac period (~1 s). The stabilized system response can thus be considered as the time average over a cardiac cycle. Hence, the levels of regulated vessel radius (Eq. 6.89), of the active tensi on (Eq. 6.92) and of the active stiffness (Eq. 6.86) are formulated as functions of the time-averaged trans­vascular pressure.
is
Boundary Conditions The coronary ow is determined by the myocardiumvessel interaction (MVI) which consists of the combined effect of the intramyocardial uid pressure (IMP) and the shortening-induced intramyocyte pressure P with the myocardial relative depth (MRD) from the LV pressure at the endocardium
. IMP varies
SIP
436 6 Network Analysis of Coronary Circulation: II. Pulsatile Flow
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to zero at the epicardium. Waveforms of the inlet pressure, Pin(t), outlet pressure, P
(t), LV pressure, PLV(t), and intramyocyte pressure, P
out
the ow analysis (Fig. A.5). The P
(t) signal is interpolated for different transmural
out
(t), are input signals to
SIP
locations based on predictions from simulation of the unregulated ow in an entire coronary network which included arterial and venous trees and four identical representative capillary networks, at relative myocardial depths (MRD) of 0.125,
0.375, 0.625, and 0.875 (Algranati et al., 2010). P
(t) waveform is taken from
LV
predictions based on a distributive LV mechanical model under resting heart rate (Kiyooka et al., 2005) of 75 BPM. Several considerations guided the choice of the
P
(t) signa l for the sub-endocardial 400 vessel network. The rst is the pressure drop
in
from the aorta to the trunk vessel (order 6) of the subtree. On the other hand, there is a pressure increase due to the added intramyocyte pressure, P during contraction (Rabbany et al., 1989). Finally, P ow perfusion in the terminal order 1 vessels in the range of measured ow of
0.4–2.0 10 these considerations, P
3mm3
/s in systole and diastole (Tillmanns et al., 1974). Based on
(t) is chosen to be 122/90 mmHg (with averagePin¼100) in
in
(t) must provide for sufcient
in
systole/diastole and the signal shape is adopted from (Algrana ti et al., 2010). P
(t) which develops
SIP
out
assigned for each terminal vessel to be between the previously predicted
sub-epicardium and sub-endocardium signals P
subepi out
depending on the transmural location of the vessel. In the absence of data on P under higher metabolic demands (i.e., higher q the same under changes of q
. The tissue pressure PT(t) is derived based on the
target
), the level for each vessel is kept
target
and P
subendo out
(Fig. 6.23),
out
earlier analysis of unregulated coronary ow.
is
Time-Varying Vessel Radius For ow analysis, Eq. (6.84) allows the calculation of the requisite vessel radius, R(t), along the cardiac cycle. In the embedded and tethered microvessel, the radius variations are likely small enough to retain just the rst term in the Taylor series expansion of R(t). In this case, the following can be obtained:
RtðÞ’R
reg
þ
dR
ΔPtðÞΔPðÞ ð6:102Þ
dΔP
where ΔP(t) is the time-varying trans-vascular pressure along the cardiac cycle.
Solution of Network Flow Due to vessel elasticity and interaction with surround­ing myocardium (MVI), the ow equations are highly nonlinear. Hence, network ow solution is an iterative solution of the system of ODEs (Eq. 6.62) subject to the respective boundary conditions. The matrices A and B are modied after each iteration which are continued to reach the desired convergence and periodicity conditions to within specied tolerances.
The numerical framework is used to rst solve the passive network ow, followed by active regulation. Two different schemes are used to solve the autoregulated ow. In cases where metabolic regulation is absent (i.e., only myogenic and/or ow mechanisms are activethe solution for a passive vessel network is used as the
Appendix 4: Coronary Flow Regulation 437
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Fig. 6.23 The assigned pressure boundary conditions. Pin, the input pressure to the order 6 trunk
subendo
vessel; P
of 0.875; P
of 0.125. Both P (Algranati et al., 2010). PLV pressure in LV chamber; PSIP intramyocyte pressure caused by their shortening. Reproduced from Namani et al. (2018) by permission
, the output pressure at the terminal order 1 vessels in a normalized myocardial depth
out
subepi
, the output pressure at the terminal order 1 vessels in a normalized myocardial depth
out
subendo out
and P
subepi
are adapted from previous analysis of unregulated coronary ow
out
initial guess to adjust each vessel diameter following the respective model equations, according to its predicted pressure and ow rate). When the metabolic regulation is active, the solution from passive network ow is iterated under a genetic algorithm search for the distribution across all the j terminal arterioles of the metabolic signals
i
F
(Eq. 6.99) which yield terminal ow rates close to q
, j
meta
, up to within
target
specied tolerance. The solution of a reference case is carried out with parameters listed in Table 6.3. It served as a baseline for the sensitivity analysis to compare predictions under a range of parameter levels.
Simulations are carried out to verify the ow periodicity condition, i.e., smooth­ness of the transition between nodal pressures from the end of one cardiac cycle to the start of the next. The smoothness tolerance is set to 0.075 mmHg. The conver­gence of the computational results is estimated based on the network ow solution, where the net inow/outow deviation at each time point (from the requisite zero level) is calculated at each vessel mid-node and at each coronary bifurcation. This is carried out for both the passive and regulated network ow under both steady as well as dynamic ow conditions. Convergence is satised when all ow deviations are <1% of the inow to the respective node and vessel junctions.
Model Comparison with Flow Characteristics
Since detailed quantitative validation data are presently not available for individual vessels in vivo, model predictions are compared with global response patterns. To that end, four sets of simulations are performed under the following conditions:
438 6 Network Analysis of Coronary Circulation: II. Pulsatile Flow
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(Case 1) Full myogenic activation; (Case 2) Myogenic and shear activation with no metabolic activation; (Case 3) Myogenic, shear, and full metabolic activation of terminal order 1 vessels; and (Case 4) Myogenic, shear, and optimized metabolic activation in terminal order 1 vessels aimed to achieve target ow level s in the terminal arterioles. Case 1 is an arrested heart with the sole effect of pressure on ow control. The shear and metabolic signals are deactivated by setting all F
F
in Eq. (6.97) to zero. Case 2 is also an arrested heart with the effects of pressure
meta
and shear on ow control. The metabolic signal is deactivated by setting all F
and all
τ
meta
in Eq. (6.97) to zero. Case 3 represents the full highest ow capability of the system in a beating heart. In Case 4, the optimal metabolic signals, F
1 vessels are targeted to obtain the least deviation of terminal perfusion level, q from the set targe t ow level (q
target
).
i
of all terminal order
, j
mterm
j
term
The following model predictions are compared with observations: dispersion of the perfusion, transmural perfusion heterogeneity, the magnitude of the metabolic ow reserve (MFR), and autoregulatory response of the system ow.
Perfusion Dispersion Coronary perfusion in the passive (unregulated) state has been shown to be highly heterogeneous (Austin Jr., Aldea, Coggins, Flynn, & Hoffman, 1990; Huo et al., 2009). Regulation is found to decrease ow dispersion (Austin Jr. et al., 1990). The predicted ow level and its dispersion (CV) are evaluated here under various regulation mechanisms with full (F (F
¼ 0) metabolic activation, and under optimized metabolic activation for
mterm
several levels of metabolic demand (i.e., target terminal vessel ow,q
mterm
¼ 1) and no
) and a
target
number of input perfusion pressures.
,
Transmural Perfusion Heterogeneity The effect of network transmural location on perfusion and its dispersion is analyzed as (1) terminal arterioles ow q
term
under various regulation mechanisms and reference perfusion pressure, and (2) optimized ow under a few levels of metabolic demand and perfusion pressures. Transmural heterogeneity is also studied in terms of the metabolic ow reserve (MFR, see section below), the network total ow under different regulation mechanisms, and autoregulation of the network ow.
Metabolic Flow Reserve (MFR) MFR is the ow reserve which results from ow regulation alone, without the effects of heart rate and activation waveform. It is estimated from the total ow in the active vessel network under myogenic, shear, and metabolic activations. The minimum ow is found under myogenic and shear activation and zero metabolic activation. The maximum ow is found under full metabolic activation of all order 1 terminal vessels. MFR is estimated as a function of the average input pressure (P waveform and outlet pressure signal (P
) in the range of 45 to 180 mmHg under the same
in
).
out
Flow Autoregulation Autoregulation in coronary circulation under constant met­abolic demand produces approximately constant network ow under an increase in perfusion pressure. It is studied with the metabolic regulation optimized to provide four q
levels of 1.20, 1.50, 1.80, and 2.25 103mm3/s. As one test of model
target
Appendix 6 439
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validity, the subtree total ow Q is analyzed under changes in perfusion pressure for four levels of requisite terminal ow q also for the passive state, and under full (F
. For comparison, the ow is analyzed
target
¼ 1) and no (F
mterm
¼ 0) metabolic
mterm
activation.
Effect of MyocardialVessel Interaction (MVI) The vessel loading by the myo­cardium (MVI) has three passive components (Algranati et al., 2010; Young et al.,
2012): (1) chamber deriv ed intramyocardial tissue pressure; (2) intracellular pressure
in the contracting myocytes, and (3) effect of vessel tethering to the surrounding myocardium. The effects of MVI on the ow are analyzed in terms of the MFR (Namani et al., 2018), the signicance of each regulation on the total network ow and on ow autoregulation (e.g., effect on the distribution of metabolic activation for various levels of metabolic demand and input perfusion pressure).
Order Dependence of the Metabolic Diameter Regulation The network vessels increase their diameters in response to higher metabolic demand. The change in diameters is, however, non-uniform, re ecting the likewise non-uniform effect of the metabolic regulation on the different vessel orders.
Appendix 5
Effects of different regulations and of the transmural network location on the average terminal arterioles ow rates under a reference average input perfusion pressure of 100 mmHg
Sub-endocardial Sub-epicardial
(103mm3/s) q
q
term
Flow control Passive 2.78 0.69 0.25 3.34 0.81 0.24 Myogenic 0.29 0.10 0.35 0.51 0.23 0.46
Myogenic + Shear(all F
Myogenic + Shear + Full Metabolic Activation(all
F
¼ 1)
mterm
¼ 0) 0.65 0.19 0.29 0.91 0.31 0.34
mterm
Mean SD CV Mean SD CV
2.13 0.61 0.29 2.50 0.70 0.28
(103mm3/s)
term
Appendix 6
Effects of the target terminal ow q network location, on the terminal arterioles ow rate and dispersion (coefcient of variation, CV), and on the terminal arterioles metabolic signal F mized metabolic activation
, of the input pressurePin,and the transmural
target
, under opti-
mterm
440 6 Network Analysis of Coronary Circulation: II. Pulsatile Flow
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mterm
/s) F
3
mm
-3
(10
term
q
(mmHg)
in
P
mterm
/s) F
3
mm
-3
(10
term
Mean SD CV Mean SD CV Mean SD CV Mean SD CV
q
(mmHg)
in
P
Sub-endocardium Sub-epicardium
/s)
3
mm
3
target
q
(10
1.20 75 1.14 0.26 0.23 0.97 0.08 0.08 75 1.24 0.27 0.22 0.94 0.12 0.12
1.20 100 1.34 0.14 0.11 0.55 0.24 0.44 100 1.44 0.17 0.12 0.33 0.27 0.82
1.20 135 1.48 0.19 0.13 0.19 0.23 1.23 135 2.25 0.74 0.33 0.05 0.16 3.23
0.20 0.10 0.47 0.25 0.53 135 2.19 0.58 0.23 0.49 0.27 1.43
1.80 135 2.05
1.80 100 1.91 0.37 0.19 0.91 0.14 0.16 100 2.03 0.32 0.16 0.53 0.25 0.47
1.80 75 1.18 0.34 0.28 1.00 0.00 0 75 1.34 0.40 0.29 1.00 0.00 0
1.50 165 2.15 0.60 0.28 0.00 0.00 0 165 4.29 1.58 0.37 0.00 0.00 0
1.50 135 1.78 0.15 0.08 0.35 0.25 0.72 135 2.34 0.67 0.29 0.10 0.22 2.12
1.50 100 1.67 0.25 0.15 0.80 0.20 0.25 100 1.72 0.21 0.12 0.64 0.25 0.40
1.50 75 1.18 0.34 0.28 1.00 0.00 0 75 1.32 0.35 0.26 0.99 0.05 0.05
1.20 165 2.15 0.60 0.28 0.00 0.00 0 165 4.29 1.58 0.37 0.00 0.00 0
2.25 75 1.18 0.34 0.28 1.00 0.00 0 75 1.34 0.40 0.29 1.00 0.00 0
1.80 165 2.25 0.20 0.26 0.07 0.18 2.69 165 4.29 1.58 0.37 0.00 0.00 0
2.25 100 2.08 0.51 0.25 0.98 0.06 0.06 100 2.33 0.48 0.21 0.94 0.13 0.14
2.25 135 2.53 0.31 0.12 0.66 0.24 0.36 135 2.82 0.46 0.16 0.35 0.31 0.86
2.25 165 2.74 0.26 0.09 0.28 0.25 0.91 165 4.29 1.58 0.37 0.00 0.00 0
Values of 0 and 1 for the metabolic signal CV imply that the metabolic regulation is exhausted
References 441
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Appendix 7
Effect of myocardium–vessel interaction (MVI) on the metabolic ow reserve (MFR) in sub-endocardial and sub-epicardial networks under various levels of the mean input pressureP
P
(mmHg)
in
60 2.43 3.81 3.25
75 2.87 3.44 3.16
90 3.05 3.11 3.06
100 3.13 2.64 2.74
120 3.19 2.28 2.39
135 3.03 2.02 2.13
150 2.76 1.77 1.91
165 2.54 1.53 1.66
180 2.36 1.36 1.45
MFR is the ratio of network ow under full metabolic activation (all F activation (all F
mterm
in
Sub-endocardium Sub-epicardium
MFR (MVI) MFR (no MVI) MFR (MVI)
¼ 1) to no metabolic
¼ 0). Reproduced from Namani et al. (2018) by permission
mterm
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