Steady flow velocity measurements in a pulmonary artery model with varying degrees of pulmonic stenosis

J Buwzechan~cs Vol 19. Eio Pnntcd I” Grcal Bnram

2. pp

I:9-146.

00’1-9290

1986

c

STEADY FLOW VELOCITY MEASUREMENTS PULMONARY ARTERY MODEL WITH VARYING OF PULMONIC STENOSIS A. P. YOGANATHAN, J. BALL, Bio Fluid Dynamics

Laboratory,

Y.-R.

Woo,

E. F. PHILPOT

School of Chemical Engineering, Atlanta, GA 30332, U.S.A.

and

Georgia

H.-W.

86 13 00 +

1986 Perpamon

00

Press Ltd

IN A DEGREES

SUNG

Institute of Technology,

R. H. FRANCH Department

of Cardiology,

Emory

University

Medical

School, Atlanta, GA 30322, U.S.A.

and

D. J. SAHN Department

of Pediatric Cardiology,

U.C. San Diego Medical

School, San Diego, CA92103,

U.S.A.

Abstract-Velocity and flow visualization studies were conducted in an adult size pulmonary artery model with varying degrees of valvular stenosis, using a two dimensional laser Doppler anemometer system. Velocity measurements in the main, left and right branches of the pulmonary artery revealed that as the degree of pulmonic stenosis increased, the jet tyv flow created by the valve hit the distal wall of the LPA farther downstream from the junction of the bifurcation. This in turn led to higher levels of turbulent and disturbed flow, and larger secondary flow motion in the LPA compared to the RPA. The high levels of turbulence measured in the main and left pulmonary arteries with the stenotic valves, could lead to the clinically observed phenomenon of post stenotic dilatation in the MPA extending into the LPA.

INTRODUCTION Since most cardiovascular circulation,

the majority

diseases affect the systemic of medical and bioengineer-

ing research has been concentrated on the flow of blood in the left side of the heart (i.e. systemic circulation). However, clinical problems such as pulmonary hypertension and pulmonary artery branched stenosis, as well as many congenital cardiovascular problems such as tetralogy of Fallot, valvular pulmanic stenosis, transposition of the great vessels and truncus arteriosus occur on the right side of the heart. In fact, congenital cardiovascular problems afflict approximately 1-2 y0 of the babies born in the United States. In addition, the pulmonic circulation differs in several significant characteristics from the systemic circulation. For example, the mean transmural pressure of the large pulmonary arteries is only 4.00 x lo4 dyn cmW2 (30 mmHg) as opposed to 1.60 x 10’ dyn cm-* (120 mmHg) in the systemic arteries. Furthermore, the branching patterns are quite different in the two systems. In the pulmonic circulation there are many more bifurcations, which are symmetric in nature, and occurring only after a few diameters (1.5-S) from the parent vessel. In order to understand some of the hemodynamic characteristics and problems of the pulmonic circulation, the fundamental fluid dynamic characteristics of the main pulmonary artery and its two branches need to be studied in detail.

Rrceiced I8 December

1984;

in reciscdform

30 July

1985.

Pulmonic stenosis, transposition of the great vessels and truncus arteriosus are serious congenital defects. Pulmonic stenosis occurs when the pulmonary valve leaflets thicken and dome causing a decrease in the cross-sectional area of the opening of the valve. This results in the valve acting like a nozzle, and increasing the intensity of the jet downstream of the valve. According to Sahn and Anderson (1982), pulmonic stenosis is the most common form of obstruction in the pulmonic circulation. Often several other defects such as a ventricular septal defect and poststenotic dilation occur with this congenital defect. This defect is relatively easy to diagnose. For example, in an asymptomatic patient, the defect can be discovered by a physician hearing and recognizing a ‘typical’ systolic murmur associated with it. However, the classification of the severity of the defect can be difficult, especially using noninvasive techniques on very young children and postoperative patients. Lima er al. (1983) have begun using a Doppler technique to solve this classification problem. Their technique is called noninvasive two-dimensional Doppler echocardiography. They have found that the method provides a good measure of the severity of the stenosis, which therefore allows better treatment of this congenital defect. The geometry of the pulmonary artery and its two branches is an asymmetric bifurcation and is very different from any geometry found in the systemic circulation. Very few studies have been conducted in asymmetric bifurcations to investigate the flow patterns in such geometries. One preliminary in ho investigation, finding that the velocity profiles in the 129

130

A. P. YOGANATHAN

main pulmonary artery are relatively flat proving no turbulence is present, has been conducted by Schultz et al. (1969). Furthermore, Talukder (1975) and Talukder and Nerem (1978) have performed flow visuahzation and velocity measurement studies in models mimicking several asymmetric bifurcations present in the systemic circulation. In general, they have concluded from their work that the fluid flow patterns formed in a bifurcation are a strong function of the cross-sectional areas of the branches, the angle between the branches, the location of the junction of the two branches, the flow rates in the branches and the flow rate in the main tube. Ku (1983) has conducted a detailed study of pulsatile flow through a carotid artery bifurcation model (an asymmetric bifurcation model) using flow visualization and velocity measurement techniques. He studied the resulting secondary flow patterns in an attempt to explain the phenomena causing the disease atherosclerosis, which is characterized by a ‘distortion of the intimal layer of large and medium-sized muscular arteries (e.g. coronary,carotid, popliteaIs)and the large elastic arteries such as the abdominal aorta and iliac vessels’. He proposed that atherosclerosis resulted from ‘oscillatory shear stresses and increased particle residence times’ occurring in this type of geometry. To our knowledge no in vitro studies have, however, been conducted in a pulmonary artery model. The object of this paper is to present flow visualization and velocity measurement results obtained in a pulmonary artery model, under steady flow conditions. Furthermore, the effects of pulmonic stenosis on the flow fields of the main, left and right pulmonary arteries are also described.

EXPERIMENTAL APPARATUS AND METHODOLOCY

Pulmonary artery model andflow loop

Geometrical dimensions of the human pulmonary artery were obtained by making detailed measurements from cineangiograms and arteries harvested from cadavers (see Fig. 1). Three sizes of the model, small, medium and large, were constructed to represent the pulmonary artery of a small child (3-6 yr), an adolescent (I 2 yr) and an adult. Results obtained with only the adult size model will be presented and discussed in this article. The adult model was constructed out of glass (for the flow visualization studies) and plexiglass (for the velocity measurement studies). The dimensions and a schematic drawing of the adult (i.e. large) pulmonary artery model are shown in Fig. 2. The small and medium size models were only constructed out of glass, for qualitative flow visualization studies. Pericardial trileaflet valves were used in this study to represent mild to severely stenotic pulmonary valves. The pericardial trileaflet valve was used because its performance paralleled that of a stenotic valve by ballooning outward and having a relatively symmetric opening. Three sizes of valves (19, 21 and 25 mm

et al.

sewing ring diameters) were used with the small, medium and large size models, respectively. The valves were made stenotic by sewing the three commissures together with thin black polyester thread. This decreased the cross-sectional opening area of the valve, thus increasing the degree of valvular stenosis. The mild, moderate and severely stenotic aduh pulmonary valves used had valve leaflet opening areas of approximately 3.0, 1.0 and 0.4 cm*, respectively. A smooth orifice ring (25 mm diameter) was used to represent a normally functioning adult pulmonary valve (5.0 cm2 opening area). The steady Row apparatus consists of: (a) model of the pulmonary artery; (b)six pressure taps; (c)two Brooks model 12-1110 rotameters (flow range from 5 to 50 I.min-’ and accuracy of f 2% full scale); (d) two gate valves; (e) a straight tube with a 3.1 cm diameter to ensure a fully developed entrance flow; (f) an immersible centrifugal pump (Little Giant); and (g) a large plastic bucket. Figure 3a shows a schematic diagram of the system indicating the locations of each part. A blood analog fluid (45% by weight aqueous glycerine solution) with a viscosity of 3.5 CP was used in the study. Flow visualization studies

The flow visualization studies were conducted using a 7 mW He-Ne laser as the light source. Amberlite* particles were added to the fluid and continuously pumped around the flow loop as tracer particles. The particles had a diameter of 70-100 p, and a density of about 1.07 gcme3. A detailed description of the experimental technique has been published previously (Yoganathan er al., 1982; Philpot, 1984). A schematic diagram of the flow visualization set-up is shown in Fig. 3b. All the Bow visualization studies were conducted over a (total) flow rate range of 167-500 cm’ s- ‘, and a 50/50split between the left and right pulmonary artery branches. The flow rates of 167 and 500 cm3 s- ’ correspond to peak systolic flow rates at cardiac outputs of about 2.5 I. min-’ (41 cm3 s-‘) and 6.0 I. min-’ (100cm3 s-l), respectively. All the flow visualization photographs were obtained in the x-y plane (see Fig. 2). The flow visualization experiments were easier to perform than the Iaser Doppler anemometer (LDA) measurements. Therefore, the flow visualization experiments were performed initially to provide a good qualilative description of the flow field. Furthermore, by visually observing the flow field it was possible to decide where to conduct the detailed quontitafice LDA studies. Velociry measurement studies In vitro velocity and turbulence measurements were made with a 55X modular three beam (i.e.

*Trademark of Rohm and Haas Corp., PA.

:/

Fig. 1. Pulmonary

artery of an adult cadaver,

131

Fig. 4. Steady flow visualization

patterns created by the different ‘pulmonic’ valves used.

132

Steady flow velocity measurements

133

WB : 26 5

I j-

Fig. 2. Schematic

of adult pulmonary

artery model, showing measurements.

0 29

the locations

and planes of the velocity profile

I 2 3 4. 5. “7

Fig. 3. (a) Schematic

of steady

mm

flow system.

Pressure taps Valve Rotameter Model Pump S&c;i$d tube

A. P. YOGANATHAN et al.

134

"Glass rod Loser

Fig. 3. (b) Schematic of flow visualization set up.

two-dimensional) DISA* LDA system. A detailed description of the apparatus and methodology of the LDA system has been published previously (Woo, 1984). The LDA system had a Bragg cell for flow directionality, and used two frequency counters for processing the Doppler signal. The frequency counters were digitally interfaced to a PDP 1l/O3 minicomputer via a buffer for on-line data collection and analysis. The optical setup used in the LDA experiments had a probe volume of 0.34 mm in length and 0.06 mm in width. The two velocity components measured were perpendicular to each other, and formed a 45” angle with the axial velocity component. These two velocity components were decomposed and appropriately combined to obtain the axial and radial velocity components during the data processing operations (Woo, 1984). Experiments were conducted at a total flow rate of 500cm3 s-’ for the normal, mild and moderately stenotic valve cases, and at a flow rate of 417 cm’ s- ’ for the severely stenotic case. It was not possible to maintain the higher flow rate for a prolonged period of time (greater than 1 h) with the severely stenotic valve, without physically damaging it. For all four valves, the flow was split 50/50 between the left and right pulmonary arteries. Velocity measurements were conducted at different locations along the z and y axes as shown in Fig. 2. Velocity profiles were measured in both the x-y and x-z planes. Both axial (U) and radial (v) velocity components were measured. The location. MA, in the main branch is as close to the valve as possible, and MB is as far away as possible. The locations, RA and LA, are the closest positions to the bifurcation. In the area enclosed by MB, RA and LA no measurements could be obtained due to optical difficulties encountered with the three beam LDA system. The movement of the traverse table, on which the LDA optics are mounted, does not correspond to the movement of the measuring volume in the flow channel due to refraction and curvature influences. An extensive computer program

written in Fortran was developed to calculate the correct table movement corresponding to the probe volume position in the flow model (Woo, 1984; Woo and Yoganathan, in press). RESULTS AND DlSCUSSION

Flow visualiiation studies

Examples of the flow visualization photographs obtained in the x-y plane (see Fig. 2) are shown in Fig. 4, and the general overall trends are schematically shown in Fig. 5. Flow through the fully opened pulmonic valve (i.e. 25 mm diameter orike ring) was undisturbed and evenly distributed across the main pulmonary artery (see Fig. 4a). No regions of flow separation, secondary motion or jet type flow were observed. The tlow fields in the two branches were also reiatively uneventful, and contained only very small regions of secondary flow.

Severely

stenotic

p Regions axial

of flow

LOW

Fig. 5. Schematic diagrams of the steady flow patterns ‘DISA Electronics, Franklin Lakes. NJ.

created.

Steady flow velocity measuremenis

For the cases of the mild, moderate and severely stenotic pulmonic valves certain general trends were observed. For example, it was generally observed that the fluid exited from the valve as a central jet. The jet broadened progressively as it traveled down the main pulmonary artery by entraining the surrounding fluid until it reached the origin of the two branches. The jet then narrowed, by-passed the origin of the two branches and hit the distal end of the pulmonary artery. The jet subsequently broke up into two smaller jets which flowed along the distal walls of the respective branches until the flow in each branch was completely restored. The sizes of these two jets and the distance down the arm at which flow was completely restored depended greatly on the location of the junction of the two branches. For example, when the junction of the two branches of the pulmonary artery model (known as Model A) was located directly opposite the valve, the majority of the central jet went into the right pulmonary artery causing the flow to be restored at a longer distance down the right arm than down the left arm. This flow pattern has only been clinically observed in the presence of the congenital defect, the transposition of the great vessels. However, when the junction of the two branches was located directly above the right wall of the main pulmonary artery as in the model shown in Fig. 2 (known as Model B), the central jet almost completely by-passed the right branch and went into the left branch. This caused the size of the jet going into the right branch to be small and the distance at which the flow was fully restored to be considerably short in the right branch, and to be relatively long in the left branch. This Bow behavior has been clinically observed in patients with pulmonic stenosis by usand others (Muster et ol., 1976; Chen et ol., 1969; Muster er al., 1982). A series of experiments wereconducted in Model B, with varying flow rates in the range of 167-500 cm3 s-l (i.e. Reynolds numbers in the range of approximately 2500-8000). In all cases, no major qualitatice differences were observed in the flow fields. Several studies were also conducted, varying the volumetric splits between the left and right branches in the range of 40160 to 60140. As with the Reynolds number studies, no major quolitotil;e differences were observed. Therefore, most of the flow visualization work was concentrated on experiments performed in Model ‘B’ at a total flow rate of 500cm3 s-‘, and a volumetric split of approximately SO/SO between the two branches. However, the size of the jet formed and the location where the jet hit the distal end of the pulmonary artery varied with the degree of stenosis as shown in Figs 4 and 5. As the valve became more stenotic, the diameter of the jet at the base of the main pulmonary artery and at the junction of the two branches decreased. For example, the diameter of the jet at the base of the main pulmonary measured 1.73 cm, 1.61 cm and 1.41 cm, and at the junction of the two branches measured 1.1 cm, 0.80 cm and 0.7 cm using the mild, moderate,

135

and severely stenotic ‘pulmonic’ valves, respectively, at a volumetric flow rate of 500 cm’ s- ’ and a SO/50 split. Furthermore, as the valve became more stenotic, the jet hit the model further down the left branch before breaking up into two smaller jets, causing the flow fields to become more disturbed. For example, using the mild, moderate, and severely stenotic ‘pulmonic’ valves at a volumetric flow of 500cm3 s-i, (a SO/SO split), the jet hit the distal (far) wall of the left branch at a distance of 1.46 cm, 1.64 cm and 1.81 cm, respectively, from the junction of the two branches. As the jet hit the model further down the left branch, the distance down the two branches that the flow became completely restored changed. Because the right arm did not ‘see’ the break up of the jet and thus, did not ‘see’ the movement of the jet down the left branch, its flow field was not drastically affected by an increase in the amount of stenosis contained in the valve. The flow continued to be restored at a fairly short distance down the right arm. However, the left branch did ‘see’ the break up of the jet, and its flow was considerably affected by the movement of the jet. The movement down the left branch caused more disturbed flow and thus, the distance at which the flow was completely restored occurred further down the branch or, in other words, a larger region of disturbed flow was formed (in the left branch). For instance, the first signs of recovered flow were observed at a distance ofabout 1.4 cm, 1.6 cmand 1.8 cm in the right armand at a distance of 1.6 cm, 2.0 cm and 2.3 cm in the left arm using the mild, moderate, and severely stenotic ‘pulmanic’ valves, respectively, at a volumetric flow rate of 500 cm3 s- ‘. Therefore, as the jet hit the model further down the left branch, the size of the regions oflow axial velocity increased, extending further down into the left branch. The sixes of these regions are compared qualitatively and are shown schematically in Fig. 5. It should be noted that the previously described fluid flow patterns were not observed with a ‘normally functioning pulmonic valve’, as shown in Figs 4 and 5. For this case, the flow was relatively undisturbed. The photographs taken showed only twodimensional fluid flow behavior, but by physically observing the fluid flow through the glass models, a three-dimensional sketch could be described. For example, in both the left and right branches regions of secondary flow were observed. In these secondary flow regions the particles seemed to travel down the wall in the pattern of a helix, which was probably caused by the particles hitting the wall and traveling towards the jet and then being pushed back towards the wall by the jet. In the right arm the particles traveled down the branch in a counterclockwise direction, while in the left branch the particles traveled down the branch in a clockwise direction. when looking in the direction of the axial flow down the particular arm. It was also observed that the directions of the secondary flow helical patterns were not a function of the location where the jet impinged on the distal (far) wall of the left branch. Semblences (i.e. outlines) of these helical

136

A. P. YOGANATHAN

patterns could be distinguished in most of the photographs (see Figs 4b, c and d). More detailed photographs can be obtained elsewhere (Philpot, 1984). Velocity measurement

studies

Examples of the velocity measurements results are shown in Figs 6-17. For the case of the ‘fully opened normal pulmonic valve (i.e. 25 mm diameter orifice ring), the velocity measurements showed no high velocity jet type flow and no regions of flow separation (see Figs 6 and 7~). The flow was evenly distributed across the main pulmonary artery (MPA). The maximum axial velocity measured was 140cms-‘. No secondary flow motion was observed in the MPA. The turbulent intensity was not high, with a maximum root mean square (rms) axial velocity of 35 ems- ‘. In the left pulmonary artery (LPA), the flow was also evenly

PULMONARY

NORMAL

U-EL

et al.

distributed

with

a

maximum

axial

velocity

of

140 cm s- I. The turbulent intensity was very low in the

LPA, with a maximum rms axial velocity of 20 cm s- I. Higher velocities were observed along the far (distal) wall of the right pulmonary artery (RPA). The maximum axial velocity measured was 150 ems- ‘. The turbulent intensities in the RPA were slightly higher than those observed in the MPA and LPA, with a maximum rms axial velocity of 37 cm s- * (see Fig. 7a). Radial velocity measurements in the x-z plane showed regions of secondary flow in both branches, which resulted in a double helical flow pattern symmetric to the plane of bifurcation (x-y plane) in both the RPA and LPA. For the case of the mildly stenotic valve, the fluid that emerged from the valve formed a jet type flow field in the central part of the MPA (see Fig. 8). However,

ARTERY

VALVE

(M/S)

MA:21

5

mm

0 25 0 3125

Fig. 6. Schematic of axial velocity profiles with the ‘fully

opened

normal

valve’

(i.e.

ring)

in the

x-f

plane.

Steady flow velocity

137

measurements

r

I”-RVS

lCM/Sl

Rf

/U-RMS

1

(CM/SJ IJ-VEL

IM/Sl

RE /i

/ I

V-VEL

ia)

I?/ RO

1 b)

xz - plane

-IO

-05

i’.I/Sl

0

RB’

05

10

R/R0 XZ-pIam (cl

(d) Fig. 7. Velocity profiles in the RPA, with

the jet was not profound, with a maximum axial velocity of 220 cm s- ‘. Small regions of flow separation were observed around the jet. The rms axial velocity measurements showed that high turbulent intensities occurred at the edges of the jet, which corresponded to the locations of high velocity gradients. The maximum rms axial velocity measured was 65 ems-’ in the MPA (see Fig. 17a). The flow across the LPA was more evenly distributed than that across the RPA. No regions of flow separation were observed in either branch. The maximum axial velocity measured was 220 cm s- ’ in the RPA and 150 cm s- ’ in the LPA as shown in Fig. 8. The flow pattern did not change in either branch as the flow traveled from planes RA (LA) to RD (LD). Secondary flow was observed in both the LPA and RPA. The secondary flow motion in the RPA was stronger than that observed in the LPA. The maximum radial velocities measured in the X-Z plane were -25 ems- ’ and 75 cm s-’ in the LPA and the RPA, respectively (see

the ‘fully

opened

normal

valve’.

Figs 9d and 1Od). Unlike the situation with the fully opened valve, the pattern of the secondary flow was no longer symmetric in the plane of the bifurcation. Nevertheless, it still formed a double helical flow pattern. The intensity of the secondary flow decreased rapidly as the flow traveled downstream in the RPA, especially along the distal (far) wall. No secondary flow motion was observed along the plane RD near the far wall. In the LPA, the strongest secondary flow motion was observed along the near wall. The intensity of the secondary flow also decreased as the flow traveled downstream in the LPA. The secondary flow motion practically ceased along the plane LD, 42 mm downstream of the branching point in the LPA. The maximum axial rms velocity measured was 30 cm s- 1 in the LPA, and 70 cm s- ‘ in the RPA (see Figs 9a and 1Oa). The turbulent intensity decreased as the flow traveled downstream in the RPA. The maximum rms velocity measured along piane RD was 40 cm s- ‘. In the LPA, the turbulence did not seem to decay as

138

A. P.

MILLY STEWTIC

YOGANATHAN

et al.

V4LVE

/

_--

Fig. 8. Schematic of axial velocity profiles (x-y plane) obtained with the mildly stenotic valve.

rapidly as the flow traveled from plane LA to LD. The maximum rms axial velocity measured along plane LD was 25 ems-‘. For the case of the moderately stenotic valve, a very high velocity jet was observed in the MPA. The maximum velocity measured was 650 cm s- *,as shown in Fig. Il. The size of the jet was considerably smaller than that observed with the mildly stenotic valve. It resulted in larger regions of flow separation and/or stagnation around the jet. The region of reverse flow extended 8 mm from the wall at the widest part, with a maximum reverse velocity of - 100 cm s-t. The size of the jet did not change as the flow traveled downstream from plane MA to MB. Small radial velocities (on the order of 30 ems-’ measured along plane MA indicated that the jet tended to go upwards. This

upwards tendency, however, disappeared along plane MB. Since this upwards tendency was not strong, and disappeared in a short distance (5 mm), the jet still remained in the central part of the MPA. Very high axial rms velocities were observed, at the locations of high velocity gradients which corresponded to the edges of the jet. The maximum rms axial velocity measured was 2 IO cm s- ’ (see Fig. 18b). The flow in the two branches was observed to cling to the far (distal) walls of the respective branch. The maximum axial velocity measured was 370 cm s- ’ in the LPA, and3OOcms-t in the RPA, as seen in Fig. 11. The flow was very unevenly distributed in the LPA. A large region of flow separation was observed adjacent to the near wall of the LPA, which extended 7 mm from the wall at the widest part. The maximum reverse velocity

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(CM/S)

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profiles in the RPA with the mildly stenotlc valve.

(CM/S1

‘8”

8 L

and turbulence

00

0

25

50

75

00

140

A. P. YOGANATHAN

et al.

MODERATELY STEKGTIC VALVE

Fig. 11. Schematic of axial velocity profiles (x-y plane) obtained with the moderately stenotic valve.

was - 110 cm s- ‘. The flow appeared to recover quite rapidly. The measurements along the plane LC indicated that the flow had already recovered from separation. Strong secondary flow was observed on the upper part of the LPA with a maximum radial velocity of - 110 cm s- I (see Fig. 12d). The intensity of the secondary flow decreased as the flow traveled downstream. The maximum radial velocity measured along the plane LD was -60 cm s- ‘. In the lower part of the LPA, a less profound secondary flow motion was observed. The maximum radial velocity in this part of the LPA changed from - 25 cm s- ’ to 40 cm s- ’ as the flow progressed from planes LA to LD. This indicated that the helical flow motion in the lower part of the LPA had a smaller length scale than that measured

observed in the upper part. The turbulent intensity in the LPA was high with a maximum rms axial velocity of 125 cm s- ’ (see Figs 12a, b). The turbulence did not decrease rapidly as flow traveled downstream. Along the plane LD, 42 mm downstream of the branching point, the maximum rms axial velocity was still elevated (85 cm s- ‘). No regions of flow separation were observed in the RPA (Figs 11 and 13~). The measurements along the plane RA indicated that there was strong secondary motion, which resulted in a double helical flow pattern (see Fig. 13d). The maximum radial velocity measured was 120 cm s-r in the upper part of the RPA and in the lower part of RPA. Although the 80cms-’ upper helix was stronger than the lower one near the

:

2

142

A.

PULMONARY

SEVERELY STENOTiC

P.

YOGANATHAN

et al

ARTEW

VAI_~E

Fig. 14. Schematic of axial velocity profiles (x-y plane) obtained with the severely stenotic valve.

branching point, it disappeared rapidly. Along the plane RC, almost no helical motion was observed in the upper part of the RPA. The lower helix, on the other hand, persisted much longer. A radial velocity of SOcms-’ was still present along the plane RD in the lower part of RPA. The turbulent intensity in the RPA was high near the branching point. A maximum rms axial velocity of 130 cm s- ’ was measured along plane RA. However, the turbulence decayed much faster than that in the LPA. Along the plane RD. 24 mm downstream of the branching point. the maximum rms axial velocity was observed to be Scms-r. For the case of the severely stenotic valve, a very narrow jet was observed in the MPA as shown in Fig. 14, with extremely high axial velocities up to

960cms-’ (please note that the experiments conducted with this valve were at a flow rate of 417 cm3 s-t, which was lower than the flow rate used for other valves), The jet was located towards the right side of the MPA with a large region of flow separation adjacent to the upper and the left side wall of the MPA. The region of flow separation extended 12.5 mm from the wall at the widest part. The maximum reverse velocity measured in the separated region was -2OOcms- t. The jet diverged a little as it traveled downstream from plane MA to MB. No secondary flow motion was observed for this valve in the MPA. Very high rms axial velocities were observed at the edges of the jet. The maximum rms velocity measured was 225 cm s- t (see Fig. 17~).In the LPA, the forward flow

(a)

XY-

lCM/S)

plow

LB

R/R0

(b)

X2-

Fig. 15. Velocity and turbulence

R/R0

I-RMS

Left

-10

-21

severety

H/R0

-05

05

valve

(c

1

XL-plane

0

stenottc

10

-,o R/R0

-05

V-VEL

(d)

X2-

0

05

plane

(M/S)

profiles in the LPA with the severely stenotic valve.

plane

branch

LB

10

Fig. 16. Velocity

I

profiles in the RPA with the severely stenotic valve

(CM/S1

and turbulence

JFRMS

A. P. YOGANATHAN et al. Main Mildly stenotlc valve

I-RMS

175 150 125 ICC 75 5c 25 C 175 150 125 100 75 50 25 0 175 150 I25 100 75 50 25 0 IO

(CM/S)

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150

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R/R0

(a)

XZ-plane

(bl

R/R0

0

05

IO

XZ - plane

(c)

Fig. 17. RMS axial velocity profiles in the MPA for the mild, moderate and severely stenotic valves.

was jet like and clung to the far wall of the LPA (see Fig. 14). The maximum axial velocity of the jet was 380cm s- I. A large region of flow separation was observed adjacent to the near wall of the LPA. It extended 11.3 mm from the wall at the widest part (larger than the radius of the LPA). The maximum reverse velocity measured was - 130 cm s- ‘. The flow did not recover from separation until 42 mm downstream of the branching point. The jet type flow first converged a little as flow traveled from the planes LA to LC, since it was pushed by the momentum from the MPA jet. It then diverged as the flow traveled from planes LC to LD. The results of the radial velocity measurements (see Fig. 15d) showed that the secondary flow motion was symmetric to the plane of the bifurcation, which resulted in a double helical flow pattern. The intensity of the secondary flow motion did not decay as the flow progressed from plane LA to LD. The maximum radial velocity measured was -lOOcms-‘. Very high turbulent intensities were observed in the LPA. The maximum rms axial velocity measured was 165 ems- ‘ (see Fig. 15a). The turbu-

lence did not decay rapidly as flow traveled downstream in the LPA. Along the plane LD, the maximum rms velocity still was high with a value of llOcms_ ‘. Although the Row in the RPA tended to cling to the far wall, it was much more evenly distributed across the cross-sectional area than that observed in the LPA (see Fig. 14). No reverse flow was observed in the RPA. The maximum axial velocity measured was 175 cm s- ‘. The flow redistributed itself as it traveled downstream. Along plane RD, the maximum axial velocity had decreased to 100 cm s- I. The secondary flow motion in the RPA was not very strong, as seen in Fig. 16d. A helical flow pattern was, however, observed in the upper part of the RPA with a maximum radial velocity of 60 cm s-t. The turbulent intensities in the RPA (see Figs 16a, b) were not as high as those observed in the MPA and LPA as indicated by the lower rms axial velocities. The maximum rms axial velocity measured was 80 cm s- t. The turbulence did not decay rapidly as the flow traveled downstream. The maximum rms velocity measured along the plane RD was still high (65 cm s- ‘).

Steady Sow velocity measurements As indicated by the results of the velocity measurement studies, jet type flow fields were observed downstream of the stenotic pulmonic valves in the MPA. The higher the degree of valvular stenosis, the higher the velocity of the jet, The jet became narrower and with larger regions of flow separation and/or stagnation as the degree of stenosis increased. Higher reverse flows in the regions of flow separation were also observed. As the jet traveled downstream in the MPA, it hit the distal (far) wall of the LPA, and subsequently broke up into two smaller jets, which flowed along the far walls of the LPA and RPA. For the case of the mildly stenotic valve, the LPA jet diverged as it traveled downstream from planes LA to LD. This implied that the MPA jet hit the distal wall of the LPA upstream of the plane LA. For the moderately stenotic valve, the LPA jet first converged slightly between planes LA and LB, and then began to diverge between planes LB and LD. This indicated that the MPA jet hit the far wall of the LPA between planes LA and LB. For the case of the severely stenotic valve, the LPA jet first converged as it traveled from planes LA to LC, and then diverged from planes LC to LD. This indicated that the MPA jet hit the far wall of the LPA near plane LC. From these results, it can be deduced that with increasing degree of valvular stenosis, the MPA jet hit further downstream of the LPA (i.e. from the bifurcation) as the momentum of the MPA jet increased. The exact same observation was made during the flow visualization studies. The distribution of the flow across the LPA was strongly dependent on the degree of stenosis, since it was directly affected by the MPA jet. For the mildly stenotic valve, the flow across the LPA was quite evenly distributed. As the degree of valvular stenosis increased, jet like flow started to appear in the LPA along the far (distal) wall, with a region of flow separation adjacent to the near wall. The higher the degree of stenosis, the higher the velocity of this jet, which in turn led to higher turbulent intensities, a larger region of flow separation adjacent to the near wall, and stronger secondary flow motion in the entire L?A. The axial velocity profiles obtained in the RPA did not change much with the varying degrees of valvular stenosis. This indicated that the flow distribution across the RPA was mainly determined by the geometry of the bifurcation. The turbulent intensities and the strength of the secondary flow motion were, however, dependent on the momentum transferred from the MPA. The turbulent intensities and secondary flow motion obseryed with the moderately stenotic valve were higher than those observed with the mildly stenotic valve. The turbulent intensities and secondary flow motion observed with the severely stenotic valve were, however, not as strong as those observed with the moderately stenotic valve. There are two factors that could be attributed to this phenomena: one is the lower volumetric flow rate used for the severely stenotic valve case than that for the moderately stenotic valve; the other more important factor is that with the severely stenotic valve the MPA jet hit

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further down the arm of the LPA. Therefore, a larger portion of the momentum possessed by the MPA jet was transferred to the LPA rather than to the RPA, which in turn led to lower turbulent intensities and secondary flow velocities in the RPA than those observed with the moderately stenotic valve. The highly elevated levels of turbulence observed in the MPA and LPA with the varying degrees of pulmonic stenosis, could lead to the clinically observed development of post stenotic dilatation of the MPA extending into the LPA (Roach, 1977). Furthermore, the increased velocity fluctuations in the MPA, LPA and RPA could lead to intrinsic pulsations in the pulmonary arteries, as clinically observed in patients in the valvular pulmonic stenosis (Gay and Franch, 1961). The force of impingement of the MPA jet (created by the stenosed pulmonic valve) as it hits the distal wall of the LPA, could lead to the clinically observed phenomenon known as the ‘tent pole’ effect of the pulmonary artery bifurcation.

Acknowledgements-This work was supported by the Whitaker Foundation, and a research investigatorship award from the American Heart Association-Georgia Affiliate. The paper was written while Dr. Yoganathan was an Alexander Von Humboldt Fellow at the Helmholtz Institute in Aachen, West Germany.

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