Intravascular ultrasound assessment of regional aortic wall stiffness, distensibility, and compliance in patients with coarctation of the aorta

Intravascular ultrasound assessment of regional aortic wall stiffness, distensibility, and compliance in patients with coarctation of the aorta J i n p i n g Xu, MD, a Takah i ro Shiota, MD, a Ryozo Omoto , MD, b Xiaodong Zhou, MD, a Shune i Kyo, MD, b Masah i ro Ishii , MD, a MaryJ . Rice, MD, a and DavidJ. Sahn, MD a Portlana~ Ore., and Saitama~Japan

B a c k g r o u n d = Impaired aortic pulsatility has been demonstrated by angiography in children and in studies of experi- mental animals with coarctation of the aorta.

Objectives: The purpose of this study was to assess regional aortic stiffness, distensibility, and compliance before and after balloon dilation in patients with coarctation of the aorta.

Methods and Results: Intravascular ultrasound examination was performed in 13 pediatric patients with the diagnosis of coarctation of the aorta to yield aortic diameter. Area transverse sections at both systolic and diastolic period were measured at three aortic levels: the proximal, distal, and coarctation segments. Balloon dilation was also performed in eight of 13 patients. By using pressures measured in the same areas, an aortic stiffness index (13) was calculated as In(Ps/Pd)/(Ds-Dd), where In is natural logarithm, Ps is systolic pressure, Pd is diastolic pressure, Ds is systolic diameter, and Dd is diastolic diameter. Aortic distensibility and an estimation of aortic compliance were also calculated. The [[3 stiffness index of the coarctation and the proximal segments of the aorta were significantly greater than that of the distal segment of the aorta (p < 0.01 ). The aortic wall stiffness 13 index did not acutely change after successful balloon dilation, but the disten- sibility and compliance of distal aorta were nonetheless significantly decreased after balloon dilation (p < 0.01, p < 0.05) as a function of changes of pulsatility of flow.

C o n c l u s i o n s : Abnormal proximal aortic stiffness may be a strong contributing factor that promotes the genesis of hypertension in patients with coarctation even after successful repair or balloon angioplasty. (Am Heart J 1997; 134:93-98.)

Abnormal aortic pulse propagat ion and altered elastic

properties have been demonstrated by angiography

combined with direct pressure measurements in chil-

dren and experimental animal models with coarctation

of the aorta. 1, 2 Repair of coarctation of the aorta may

not prevent late development of elevated b lood pres-

sure and associated premature morbidity and mortality. 3

One recent study found that increased arterial stiffness

may participate in the genesis of hypertension in

patients with coarctation of the aorta after successful

repair. 4 This process has not been well studied in coarc-

tation of the aorta. The mechanism of relief of obstruc-

tion by bal loon dilation is known to involve intimal and

From the aClinical Care Center for Congenital Heart Disease, Oregon Health Sciences University, and bSaitama Medical School.

Received for publication Aug. 5, 1996; accepted April 9, 199Z Reprint requests: David J. Sahn, MD, The Clinical Care Center for Congenital Heart Disease, Oregon Health Sciences University, 3181 S.W. Sam Jackson Park Rd., UHN- 60, Portland, OR 97201-3098. E-matl: [email protected] Copyright © •997 by Mosby-Year Book, Inc. 0002-8703/97/$5.00 + 0 4/1182545

medial disruptions and dissections. 5 However, whether

the dilation procedure alters arterial stiffness or distensi-

bility in the balloon dilated segment has not been deter-

mined. To our knowledge, no previous studies have

examined the aortic stiffness, distensibility, or compli-

ance of the different segments of aorta in patients with

coarctation and compared them before and after suc-

cessful bal loon dilation or surgical repair.

Aortic distensibility has been extensively surveyed by

using pulse wave velocity, 6 Doppler echocardiography, 7

ultrasonography,8, 9 and magnetic resonance imaging. 1°, 11

However, one problem with these noninvasive methods

of measuring aortic distensibility is that b lood pressure

is indirectly recorded in the brachial artery rather than

inside the aorta, which can distort the data in patients

with coarctation of aorta, although the distortion would

be of less importance in nonocclusive arterial disease. 12

Intravascular ultrasound imaging provides an accurate

on-line method to assess arterial dimensions in vitro and in vivo on a beat-to-beat basis. 5, 13, 14 The high res-

olution images obtained from intravascular ul trasound

94 Xu et al,

provide cont inuous cross-sectional images of the artery

and thereby a l low determinat ion of instantaneous

changes in arterial luminal dimensions35 In addi t ion to

in v ivo arterial distensibility, intravascular u l t rasound

imaging also al lows visualization of morpho log ic char-

acteristics and structure of the aortic vessel walls bo th

before and after invasive ba l loon dilation procedures ,

possibly providing important information unavai lable

with o ther imaging techniques. 5, 14

The pu rposes of this s tudy w e r e to (1) assess the

aortic wal l stiffness, distensibility, and c o m p l i a n c e of

the aorta at three different l e v e l s - - t h e proximal , coarc-

tation, and distal s egment s o f the a o r t a - - i n chi ldren

with aortic coarcta t ion wi th intravascular u l t rasound

imaging; (2) de t e rmine if es t imates o f arterial stiffness,

distensibility, or comp l i ance are a l tered by ba l loon

dilation; and (3) invest igate poss ible mechan i sms for

pers is tent hype r t ens ion in chi ldren after successful

repair of coarcta t ion of the aorta.

Methods Patients

Thirteen patients who had coarctation of the aorta were

included i n the study. All had discrete obstructions or a short segment coarctation except for one patient with a long seg-

ment of coarctation. The patients ranged in age from 5 months to 16.5 years with a mean age of 7 years, and they ranged in weight from 6 kg to 54 kg with a mean weight of

25.4 kg. There were seven boys and six girls. Eight patients (five with native coarctations and three with recurrent coarc-

tations) were referred for balloon dilation. Diagnosis for seven patients was isolated coarctation of the aorta. The

remaining six patients had associated cardiac defects that included a bicuspid aortic valve in two, ventricular septal

defect in two, valvular aortic stenosis in three, and patent ductus arteriosus in one. Informed consent was obtained

before every procedure. Because it is unusual for patients with normal aorta to undergo an aortic intravascular ultra- sound study, we include data from only four subjects without

aortic coarctation.

Procedures The technique of the angioplasty procedure was similar to

that described by others. 16, 17 Balloon dilation was consid-

ered successful when the coarctation segment increased in diameter by >30% and the residual peak systolic pressure gradient across the coarctation was <20 mm Hg. 17, 18

For intravascular ultrasound imaging studies, we used 4.8F, 20 MHz or 6.2F, 12.5 MHz intravascular ultrasound catheters (Boston Scientific Corp.) and a Diasonics or Hewlett Packard intravascular ultrasound scanner or an Aloka SSD-550 intravascular imaging system with 6F, 20 MHz and 8F, 15 MHz intracatheter transducers. The scanning catheters were

introduced through a 5F, 7F, 8F, or 9F sheath, and real-time

cross-sectional images of the aorta were performed on pull-

back in all 13 patients before dilation, in eight patients (five

with native coarctation, three with recurrent coarctation)

after dilation, and in all subjects without aortic coarctation.

During scanning, the catheter was kept as central and coaxi-

al within the vessel lumen as possible to improve imaging

resolution and enhance our capability of seeing all wall

areas, especially in the dilated segments. From the introduc-

tion of the ultrasound catheter, images were continuously

recorded on videotape for later off-line analysis.

Fluoroscopic spot images were used to localize the intravas-

cular transducer compared with the angiographic landmarks.

The ultrasound images themselves also provided localizing

information, as did the pressure measuring catheters in the

aorta. Aortic pressures were measured by filled-fluid

catheters using slow pullbacks with intermittent fluoroscopic

spot views nonsimultaneously but within 5 minutes of imag-

ing of the aortic segments.

Calculation of aortic stiffness 13 index, distensibility, and compliance

Dimensions, area, and expansion of the aorta in three seg-

ments (the coarctation, the proximal, and the distal seg-

ments) were measured with intravascular ultrasound images.

For the small number of subjects without aortic disease or

who had undergone intravascular ultrasound study without

aortic coarctation, three equivalent sections of the aorta were

also evaluated. Systolic (As) and diastolic (Ad) aortic area

were manually selected and measured with digitized elec-

tronic calipers (Fig. 1). Pre- and post-balloon dilation sys-

tolic pressure (Ps) and diastolic (Pd) aortic coarctatiofl pres-

sures were measured during cardiac catheterization. The

following indexes were calculated: (1) A stiffness 13 index

was calculated as In (Ps/Pd)/(Ds-Dd), 4, 9 where Ds is maxi-

mal systolic vessel diameter, Dd is minimal diastolic vessel

diameter, and In is natural logarithm. (2) Distensibility of the

aorta was calculated as [(As-Ad)/Ad]/(Ps-Pd). TM 19 (3)

Compliance of the aorta was also calculated as (As-Ad)/(Ps-

pd).20, 2i Ultrasonic measurements were made at the narrow-

est cross-sectional image of the aorta, corresponding to the

coarctation site, and at levels of the proximal and the distal

segments approximately 1 to 2 centimeters above or below

the coarctation segment. The means of four consecutive mea-

surements were used for data analysis. All patients were sta-

ble and in normal sinus rhythm during the study.

Statistics Data are expressed as mean + SD. The data between

groups were compared with analysis of variance. Student's

two-tailed t tests were used for comparison of paired data

obtained before and after angioplasty. The relation between

two variables was evaluated by linear regression analysis and

American Heart Journal July 1997

Xu etal. 95

Examples of measurement of lumen area and diameter. Maximal (left) and minimal (right) areas and diameter of aorta are displayed. Lumen borders are noted by circle.

Findings

Coa Prox Dis Control Mean + SD Mean + SD Mean + SD Mean _+ SD

(Range) (Range) (Range) (Range)

Stiffness [3 index 6.19 _+ 5.68* 4.21 _+ 2.76* 2.07 + 0.77 1.67 _+ 0.77 (2.23-20.28) {1.51-10.90) (0.56-3.30) (0.75-2.77)

Distensibility Z60 ± 4.99" 6.93 _+ 1.92 * 15.98 _+ 11.24 13.77 _+ 3.38 (mm Hg -1 • 10 -3} (0.01-15.0) (4.0-10.0) (6.0-51.0) (6.75-1Z65)

Compliance 4.34 + 3.33 *T 6.78 _+ 4.86¢ 15.20 + 13.07 15.18 -+ 9.29 (cm 2 • mm Hg-I ° 10 -3) (0.01-11.0) (1.0-14.0) (4.76-45.8) (4.05-35)

Because there was no significant difference in any index of aortic properly among three locations (proximal arch, upper descending, and distal descending aorta} for the control sub- Tects without coarctation, averaged data over the three corresponding locations of the aorta are presented. Coa, Coarctation aorta; prox, proximal aorta; dis, distal aorta; control, control subTeels without aortic coardation. *p < 0.01 compared with distal aorta. tp < 0.01 compared with proximal aorta. ~p < 0.05 compared with distal aorta.

by calculation of Pearson correlation coefficients. A value of p < 0.05 was considered statistically significant.

Results Comparison of aortic elastic properties between three sections of aorta

Intravascular ultrasound images provided clear images of dynamic regional changes in aortic lumen size in all subjects. Table I details the elastic properties of the aorta in the three locations for both patients with coarc- tation and control subjects without coarctation. There were no significant differences in any index of aortic property among the three locations (proximal, coarcta- tion, and distal aorta) for the control subjects without

coarctation. The stiffness ~ indexes of the coarctation

segment and the proximal segment of aorta were signif-

icantly greater than that of the distal segment of aorta.

(bo thp < 0.01), whereas there was no significant differ-

ence between the coarctation segment and the proximal

segment of the aorta (p > 0.05). Similarly, the distensi-

bility of the coarctation and the proximal segments of

aorta were significantly lower than the distal segment

(bo thp < 0.01), whereas there was no significant differ-

ence between the coarctation segment and the proximal

segment (p > 0.05). However, there was significant dif-

ference in aortic compliance between the coarctation

segment and proximal aorta (p < 0.01), and the aorta at

both of these regions was significantly less compliant

American Heart Journal Volume 134, Number 1

9 6 Xu et al.

R p Va lue

13 Index vs distensibility 0.7044 <0.01 13 Index vs compliance 0.7349 <0.01 Distensibility vs compliance 0.3771 >0.05

Findings

Before BD, Af ter BD, m e a n -+ SD m e a n -+ SD p

( range) ( range) Va lue

SP proximal to 113.9 + 8.7 112.9 _+ Z5 NS coarctation (ram Hg) (100-126) (100-123)

SP distal to 81.1 _+ 12.7 102.8 _+ t4.8 <0.01 coarctation (mm Hg} (61-100) (74-122)

PG (mm Hg) 33.1 +10.6 10.0-+8.7 <0.001

(20-49) (0-26) Diameter of coa (cm) 0.59 + 0.21 0.88 -+ 0.29 <0.001

(0.3-0.95) (0.55-1.4) Area of coa (cm 2) 0.32 + 0.20 0.67 -+ 0.41 <0.01

(0.10-0.70) (0.35-1.5) [8 Index of coa 8.14 -+ 6.62 4.68 +2.44 NS

(2.23-20.28) (2.10-9.54) 13 Index of prox 5.41 -+ 2.94 4.65 _+ 2.23 NS

{1.78-10.90) (1.51-8.80} [3 Index of dis 2.19 -+ 0.69 2.46 _4- 0.72 NS

(1.12-3.30} {1.03-3.28) Distensibility of coa Z97 -+ 5.98 Z46 + 5.21 NS

(ram Hg -1 • 10 -3) (0-15.0) (2.86-19.0) Distensibility of prox 6.65 + 1.99 Z66 -+ 2.34 NS

(ram Hg -1 • 10 -3) (4.0-10.0) (5.0-11.0) Distensibility of dis 14.34 -+ 4.04 10.05 -+ 2.75 <0.01

(ram Hg -1 ° 10 -3) (8.0-20.0) (6.0-13.3) Compliance of coa 2.81 -+ 2.65 3.74 + 4.63 NS

(cm 2 ° mm (0.0-6.67) (1.92-15.0) Hg-1 • 10-3}

Compliance of prox 4.15-+4.10 4.28-+4.10 NS (cm 2 • mm (1.0-13.33) (1.0-13.33) Hg -3 * 10 -3)

Compliance of dis 12.72 _+ 12.1 18.33 + 6.94 <0.05 (cm 2 ° mm (4.76-40.0) (3.85-24.0) Hg-1 • 10-3)

SP, Systolic blood pressure; PG, pressure gradient; coa, coarctation aorta; prox, proxi- mal aorta; dis, distar aorta.

than the distal aorta (p < 0.01 a n d p < 0.05, respective- ly).. There was no difference in aortic wall properties between the distal aorta in patients with coarctation and the subjects without aortic coarctation.

Table II lists the statistical correlations of the relation between the distensibility, the stiffness 13 index, and the compliance of the coarctation segment. There was a significant relation between stiffness [3 index and dis- tensibility or compliance, but there was no significant

relation between the distensibility and compl iance-- probably because of the variability of diastolic area of the segments.

Comparison of aortic elastic properties before and after balloon dilation

Balloon dilation of coarctation or of recurrent coarcta-

tion was performed in eight patients (five with native coarctations and three with recurrent coarctations). Dilation was successful in the group of eight patients, with a resulting residual peak pressure gradient of <20 mm Hg. Table III shows the hemodynamic data and regional aortic elastic properties before and after balloon dilation. All patients with coarctation showed clear evi- dence of an intimal flap on the intravascular images, except for one who was being dilated for mild rec0arc- tation. The aortic cross-sectional area at the coarctation site significantly increased after balloon dilation from a mean of 0.32 -+ 0.20 cm 2 to a mean of 0.67 _+ 0.41 cm 2 (p

< 0.01), the coarctation diameter increased from a mean

of 0.59 + 0.21 cm to a mean of 0.88 + 0.29 cm (p < 0.001), and the peak pressure gradient decreased from

33.1 + 10.6 mm Hg to a mean of 10.0 + 8.7 mm Hg (p < 0.001) (Table III). However, there was no significant dif-

ference in aortic cross-sectional area between before and after dilatation in proximal and distal aorta. The stiff- ness [3 index of coarctation segment, proximal and distal aorta did not significantly change after balloon dilation

(all p > 0.05), as shown in Table III. Aortic distensibility and compliance of coarctation segment and proximal aorta did not significantly change after balloon dilation

(p > .05), whereas both aortic distensibility and aortic compliance for the distal segment significantly decreased after balloon dilation (p < 0.01, p < 0.05), mainly as a result of increases in both distal pressure and pressure pulsatility. There was no significant correlation between the residual gradient across the stenosis and the 13 index; distensibility; or compliance of proximal, coarctation, or distal segments of the aorta.

Discussion Regional aortic wall elastic properties

This study demonstrated significantly increased stiff- ness and decreased distensibility and compliance of the segment proximal to the coarctation and of the coarcta- don itself compared with the segment distal to the coarctation in the descending aorta. Similarly, in vitro studies of aortic segments from human beings or ani- mals with coarctation have shown that the proximal aorta is significantly less contractile to potassium, norepineph- rine, and prostaglandin F2 than either sections of

American Heart Journal ju)y 1997

Xu et al. 9 7

aorta below the coarctation or proximal and distal aor- tic ring preparations from control subjects. 22 These

observations may be a result of alterations of the com- position of the arterial wall proximal to and within the

coarctation, with increased scleroprotein content (colla- gen and elastin) and reduced number of smooth mus- cle cells compared with the distal segments. 23 Arndt et al., 24 using angiography, studied the pressure-diameter

relation of the intact thoracic aorta at various distances from the aortic root in normal cats and found that the aortic distensibility decreased as distance from the root increased. Mohiaddin et al. 1° also reported that the regional compliance was greatest in the ascending aorta, lower in the arch, and lowest in the descending aorta in healthy volunteers studied by magnetic reso- nance imaging. In this study, however, we found that

the proximal coarctation aorta was stiffer than the distal aorta in patients with coarctation. These regional differ-

ences in aortic stiffness, distensibility, and compliance suggested a widespread structural alteration of the aor- tic wall in patients with coarctation.

Mechanisms of persistent hypertension after the repair of the coarctation

This study also demonstrates that abnormal arterial stiffness; distensibility, and compliance in the proximal aorta and coarctation segments persist even after suc- cessful balloon dilation. Previous studies have shown that structural changes of large arteries may occur in coarctation of aorta, and the continued increased stiff-

ness and reduced distensibility and compliance of the proximal aorta, even after successful balloon dilation,

are not surprising because they probably reflect a persis- tence of structural changes in the vessel present before repair. Gardiner et al. 25 previously reported that arterial reactivity is significantly altered in the precoarctation vascular bed of healthy young adults despite successful repair of coaretation in childhood and that abnormal vascular responses were not related to age at operation. These abnormal reactivity patterns were found even in subjects who had undergone repair in the neonatal peri- od and were present even in persistently normotensive patients who had no clinical or echocardiographic evi- dence of residual obstruction. This finding is consistent with other evidence of persisting cardiovascular abnor- malities long after successful coarctation repair in early life such as abnormal left ventricular mass and persistent hypertension. 26-28 The cause of these abnormalities has

been unclear, but it has been suggested that structural alteration of the aortic wall or mild persistent narrowing at the coarctectomy site may be responsible.

Differences in surgical technique, abnormalities of the

renin-angiotensin system, persistent increase in sympa- thetic tone, and mild persistent pressure gradients have all been suggested as possible mechanisms for hyperten- sion after coarctation repair. Kimball et al. 28 suggested hypertension at rest may be a result of a hyperdynamic, hypercontractile state. Pulse-wave propagation is abnor- mal in these aortas, enough so that even without

obstruction residual gradients may be generated by blood being pumped between segments of different mechanical properties. 29 Our results suggest that a wide- spread and persistent increased aortic stiffness, reduced

distensibility, and compliance of the aorta may con- tribute to the persistent hypertension even after success- ful repair of aortic coarctation. This finding may suggest that aortic wall stiffness, distensibility, and compliance are important variables to follow up clinically in patients years after coarctation repair. Ong et al. 4 suggested that the combination of increased stiffness in the precoarcta- tion arterial bed with mild residual narrowing at the site

of coarctation repair would account for the elevation of exercise blood pressure in the upper body observed in patients after coarctectomy with good repairs.

Three indexes of aortic elastic properties used in this study

Although aortic wall elastic properties have been stud- ied largely by measurements of distensibility and compli- ance,6, 10-12, 19-21 the indexes depend on blood pressure.

Previous studies have shown that aortic distensibility and compliance depend on the distending pressure and decrease as the pressure increases. 3°, 31 Michelfelder et al. 21 recently reported that the abdominal aortic compli-

ance did not significantly change, but distensibility decreased after balloon dilation or surgical repair of coarctation. In this study we found that both compliance and distensibility Of the distal aorta decreased after bal- loon dilation. However, the [8 index of the abdominal aorta did not significantly change after successful bal-

loon dilation. This result seems reasonable because the inherent wall properties of the distal or postcoarctation segment of aorta would not change immediately after the dilation. The ]3 index value has been viewed and demonstrated as being least influenced by changes in systolic pressure. 9, 22 Our findings supported the idea

that the [8 values obtained can be regarded as a more reliable measure of intrinsic wall elasticity within the physiologic pressure range of the subjects we studied. The distensibility and compliance changes we observed for the distal segment were more likely related to changes of systolic, diastolic, and pulse

American Hearl Journal Volume 134, Number 1

98 Xu et al.

pressure than to acute alterations in aortic wall characteristics.

Conclusions In this Study we used a method that could accurately

evaluate in vivo regional elastic properties of the great arteries with intravascular ultrasound imaging com- bined with pressure measurements during cardiac catheterization. 32 In patients with coarctation of the aorta, the coarctation and proximal segments exhibited markedly abnormal elastic properties even at an early age, including increased stiffness 13 index and decreased distensibility and compliance. These aortic elastic property indexes continued to be abnormal even after successful balloon dilation.

References 1. Gupta TC, Wiggers CJ. Basic hemodynamic changes produced by

aortic coarctation of different degrees. Circulation ]951 ;3:]741. 2. O'Rourke MF, Cartmill TB. Influence of aortic coarctation on pulsatile

hemodynamics in the proximal aorta. Circulation 1971 ;XLIV:28]-92. 3. Moron BJ, Humphries jO, Rowe RD, MeJlits ED. Prognosis of surgica@

corrected coarctation of the aorta: a 20-year postoperative appraisal. Circulation 1973;47:119-26.

4. Ong CM, Canter CE, Gutierrez FR, Sekarski DR; Gddring DR. Increased stiffness and persistent narrowing of the aorta after success- ful repair of coarctation of the aorta: relationship to left ventricular mass and blood pressure at rest and with exercise. Am Heart j 1992; 123:1594&00.

5. Harrison jK, Sheikh KH, Davidson CJ, KissJo KB, Leithe ME, HimmeJstein St, et al. Balloon angioplasly of coarctation of the aorta evaluated with intravascular ultrasound imaging. J Am Coil Cardio11990;15:906-9.

6. Gribbin B, Picketing TG, Sleight P. Arterial distensibility in normal and hypertensive man. Clin Sci 1979;56:413.

Z Schieken RM, Moskowitz WB, Bodurtha J, Mosteller M, Eaves L, Nance W. Aortic stiffness: a new Doppler echocardiographic mea- sure predictive of systolic blood pressure in children, j Am Coil Cardid 1988;] 1:1297-300.

8. Goldberg BB. Suprasternal ultrasonography, lAMA 1971 ;2i5:245-50. 9. Kawasaki T, Sasayamo S, Yagi S-I, Asakawa T, Hirai T: Non-invasive

assessment of the age related changes in stiffness of major branches of the human arteries. Cardiovasc Res 1987;21:678-8Z

10. Mohiaddin RH, Underwood SR, Bogren HG, Firmin DN, Klipstein RH, Rees RSO, et al. Regional aortic compliance studied by magnetic res- onance imaging: the effects of age, training, and coronary artery dis- ease. Br Heartj 1989162:90-6.

11. Honda T, Yano K, Matsuoka H, Hamada M, Hiwada K. Evaluation of aortic dislensibility in patients with essential hypertension by using cine magnetic resonance imaging. J Vasc Diseases 1994;45:207-12.

12. Stefanadis C, Stratos C, Boudoulas H, Dourouklis C, Toutouzas P. Distensibility of the ascending aorta: comparison of invasive and non- invasive techniques in healthy men and in men with coronary arlery disease. Eur Heart J 1990; 11:9904.

13. Mizushige K, Morita H, Senda S, Matsuo H. Evaluation of arterial wall histolog!cal and physical characteristics using intravascular echography. Jpn Circ J 1992;56:565-71.

14. Sohn S, Rothman A, Shiota 1-, Luk G, Tong A, Swensson RE, et at.

Acute and follow-up intravascular ultrasound findings after balloon dilation of coarctation of the aorta. Circulation 1994;90:340-Z

15. Hodgson J McB, Graham SP, Sheehan H, Brands D. Percutaneous intravascular ultiasound imaging: validation of a real time synthetic aperture array catheter. Am J Card Imaging 199] ;5:65-71.

16. Beekman RH, Rocchini AP, Dick M JP, Snider AR, Crowley DC, Serwer GA, Spicer RL, Rosenthal A. Percutaneous balloon angiopJasty for native coarctation of the aorta, j Am Coil Cordial 1987;1 O: ] 078-84.

1Z LocklE, Bass JL, Amplatz K, Fuhrman BP, Castaneda-Zuniga W. Balloon dilation angioplasly of aortic coarctatians in infants and chil- dren. Circulation 1983;68:109-] 6.

18. Morrow WR, Vic GW III, Nihill MR, Rokey R, Johnston DL, Hedrick TD, et aL Balloon dilation of unoperated coarctatian of the aorta: short- and intermediate-term results. J Am Coil Cardid 1988; 11 : 133-8.

19. SavoJainen A, Keto P, Hekali P, Nisula L, Kaitila I, Viitasalo M, et ak Aortic distensibility in children with the Marfan syndrome. Am j Cardiol 1992;70:69] ~3.

20. WrightjS, Cruickshank JK, Kontis S, Dare C, Gosling RG. Aortic com- pliance measured by non-invasive Doppler ultrasound: description of a method and its reproducibiJ>/. Clin Sci 1990;78:463-8.

21. Michelfelder E, Kudomirsky A, Lloyd TR, Beekman RH, Krauzowicz B, Vermilion RP, et aL Echocardiographic assessment of aortic compli- ance and distensibility before and after coarctation of the aorta repair [abstract]. J Am 8oc Echo 1994;7:$13.

22. Hirai T, Sasayama S, Kawasaki T, Yagi S-I. Stiffness of systemic arteries in patients with myocardial infarction. A noninvasive me~hod to predict severi V of coronary atherosderosis. Circulation 1989;80:78-86.

23. Sehested J, Baandrup U, Mikkelsen E. Different reactivity and structure of the prestenotic and poststenotic aorta in human coarctation. Circulation 1982;65:1060-5.

24. ArndtJO, Stegall HF, Wicke Hj. Mechanics of the aorta in viva. Circ Res 1971128:693-704.

25. Gardiner HM, Celermajer DS, Sorensen KE, Georgakopoulos D, Robinson J, Thomas O, et al. Arterial reactivity is significantly impaired in normotensive young adults after successful repair of aortic coarcta- tion in children. Circulation 1994;89:1745-50,

26. Carpenter MA, Dammann JF, Watson DD, Jedeikin R, Tompkins DG, Belier GA. Left ventricular hyperkinesia at rest and during exercise in normolensive patients 2 to 27 years after coarctation repair, j Am Coil Cardio11985;6:879-86.

2Z Leandro j, Smallhorn JF, Benson L, Musewe N, Balfe jW, DyckJD, et al. Ambulatory blood pressure monitoring and left ventricular mass and function after successful surgical repair of coarctation of the aorta. J Am Coil Cardio11992;20:197-204.

28. Kimball TR, Reyndds jM, Mays WA, Khoury P, Claytor RP, Danids SR. Persistent hyperdynamic cardiovascular state at rest and during exer- cise in children after successful repair of coaratation of the aorta. J Am Col] Cardiol ]994;24:194-200.

29. Wagner K, Tacy TA, Whitehead KK, Lee ET, Neches WH, Cape EG. Vessel compliance mismatch can increase workload of the left ventri- cle in absence of obstruction after repair of aortic caarctation. Ped Res 1996;39:39A.

30. Bergel DH. The static elastic properties of the arterial wall. J Physiol 1961;156:445-5Z

31. Dobrin PB. Mechanical properties of arteries. Phys Rev 1978;58:397-460. 32. Stock JH, Relier MD, Sharma S, Pavcnik. D, Shiota T, Bahn DJ. Trans-

balloon intravascular uitrasound imaging during balloon angioplas V in animal models with coarctation and branch purmonary stenosis. Circulation. 1997; 95: 2354-Z

American Heart Journal July 1997