the usefulness of transesophageal echocardiography in the surgical management of older children with...
TRANSCRIPT
The Usefulness of Transesophageal Echocardiography in the Surgical Management of Older Children with Subaortic Stenosis SHIVA SHARMA, M.D., TERESA STAMPER, R.N., PRADIP DHAR, M.D., FREDERICK EMGE, M.D., JAMES BAILEY, M.D., PH.D., KIRK KANTER, M.D., WILLIS WILLIAMS, M.D., and DEREK FYFE, M.D., PH.D. Division of Pediatric Cardiology, Emory University School of Medicine, The Children’s Heart Center, Atlanta, Georgia
Subaortic stenosis is a complex lesion that often presents in older children and adolescents. A clear depiction of the lesion is required for optimization of surgery. Due to the large size of these patients, is not always possible from surface echocardiography. Intraoperative multiplane echocardiography (MTEE) has been performed at our institute i n older children for several different congenital heart lesions including many patients with subaortic stenosis. A retrospective analysis of our experience with MTEE in patients with subaortic stenosis was performed to assess its usefulness in the preop- erative diagnosis and postoperative assessment of repair. Our results show that intraoperative MTEE was useful preoperatively by correcting or confirming suspected diagnosis, and giving addi- tional details of the lesion in many patients. Postoperatively, MTEE was highly useful in the as- sessment of repair. We strongly recommend the use of intraoperative MTEE in older children and adolescents with subaortic stenosis. (ECHOCARDIOGRAPHY, Volume 13, November 1996)
transesophageal echocardiography, congenital subaortic stenosis, left ventricular outflow tract ob- struction
It is well established that congenital subaor- tic stenosis has heterogenous morphology, is of- ten progressive, is prone to recurrence, and is frequently associated with other cardiac le- sions.l-H The surgical management of the lesion entails obtaining complete relief of the obstruc- tion without damage to adjacent structures and correction of associated defects. This in turn re- quires a clear display of the lesion, not only be- fore surgery, but also immediately after repair and before decannulation of cardiopulmonary bypass, a role ideally suited for intraoperative transesophageal echocardiography (TEE). Due to the complexity of this lesion, it is important to image this lesion in as many planes as possi- ble. This is best accomplished by a multiplane
Address for correspondence and reprints: Shiva Sharma, M.D., The Children’s Heart Center, 2040 Ridgewood Dr., NE, Atlanta, GA 30322. Fax: 404-778-5110.
TEE (MTEE) that provides continuously vary- ing imaging planes. Although the role of single and biplane TEE has been well described in the pediatric population, the role of MTEE has not been explored, perhaps due to the large size of the probe. However, patients presenting with subaortic stenosis tend to be older childrenIs and adolescents in whom the larger MTEE probe can be passed. Subaortic stenosis is an in- dication for intraoperative TEE at our institu- tion. A retrospective analysis of our experience was thus performed to evaluate the usefulness of MTEE in the surgical management of subaor- tic stenosis in older children and adolescents.
Methods
The Pediatric Cardiothoracic Surgery Data- base at Emory University Health Sciences Center was searched to identify all patients >
Vol. 13, No. 6, 1996 ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech. 653
SHARMA, ET AL
14 kg who underwent open heart surgery and intraoperative MTEE for subaortic stenosis be- tween October 1993 (commencement of MTEE) and October 1994. The 14-kg limit is based on our positive experience with passage of the probe in children up to this weight. These pa- tients, along with one outpatient who under- went MTEE for suspected subaortic stenosis, form the study group. MTEE was performed with an Interspec 5-MHz Annular Phased Ar- ray Multiplane probe (Interspec, Inc., Ambler, PA, USA) with color flow and continuous-wave Doppler capability. The probe was passed after induction of general anesthesia. Imaging was performed from the mid-esophagus, short gas- tric, and long gastric positions, with variation of scanning plane from 0"-135". In our initial studies the electronic orientation was kept at the top of the displayed image leading to the discordance of image orientation when com- pared with surface echocardiogram. It is now our standard practice to electronically orient all images so that there is concordance with surface echocardiogram display during the study. At preoperative examination, specific attention was paid to the scan plane rotation and the position of the probe that yielded the best views of the left ventricular outflow tract (LVOT). The details of subaortic stenosis, as well as additional new information, if ob- tained, was communicated to the surgeon. Postoperatively, specific attention was focused on the detection of residual lesions and assess- ment of aortic regurgitation.
Subaortic stenosis was defined in accordance with published echocardiographic criteria* as follows: (1) discrete lesions with thin mem- branes; (2) discrete lesions with thick fibromus- cular tissue with broad base; and (3) tunnel le- sions with diffuse tubular narrowing of the ventricular outflow tract by thickened tissue on the septum and anterior mitral leaflet with or without associated aortic annular hypoplasia. At surgery, inspection of aortic valve and subaortic area was accomplished from an oblique aortotomy and the details of the mor- phology were recorded for surgical confirma- tion. The appropriate surgical procedures were performed as indicated for a given lesion. These included simple excision of membranes, resec-
tion of muscle, aortic valve replacement, Konno procedure, or a combination of the above.
Results
Patient Population
Between October 1993 and October 1994, 777 cardiac surgical procedures were per- formed at Egleston Children's Hospital at Emory University. Of these, 468 were open heart procedures and 143 were in patients > 14 kg. From these, 14 patients underwent MTEE for LVOT obstruction. One patient with a sus- pected subaortic membrane that needed evalu- ation with MTEE at time of surgery was in- cluded in the group. Their ages ranged from 4.4 -16 years (10.6 years median) and weight from 16.3 -80 kg (27.7 kg median). There were eight males and six females. The results of the surface echocardiogram, MTEE findings, sur- gical findings, surgical repair, and postopera- tive evaluation are shown in Table I.
Imaging Angles and Position
Transesop hageal
The LVOT lesions were imaged best in the long-axis scans between 100" and 130". The aor- tic valve was also assessed in short-axis scans between 30" and 60". The associated right ven- tricular outflow tract lesions were visualized well from 45"-90". The intermediate planes were also used and served to decrease the need for lateral flexion movements of the probe.
Transgastric
Two transgastric positions were used. The "short" transgastric position was obtained with slight anteflexion and leftward rotation of the probe. The LVOT lesions were best seen with image plane rotations between 90" and 130" from this position. To obtain the "long" trans- gastric position the probe was advanced fur- ther, and with full anteflexion of the probe, a view similar to the subcostal four-chamber view was obtained. In this position, the LVOT long-axis was obtained with scan plane rota- tion between 15" and 45". This represents near
654 ECHOCARDIOGWHY: A J r l . of CV Ultrasound & Allied Tech. Vol. 13, No. 6,1996
TA
BL
E I
Surg
ery
Pati
ent
Age
(Y
rs)
Pert
inen
t Su
rfac
e M
TEE
Find
ings
Po
stop
N
o. 1
2
-
3 4 5 6 7 8 9 10
11
12
13
14
l?L
(kd
C
linic
al
Ech
o w
rnot
atio
nal A
ngle
C
orre
latio
n R
eDai
r M
TEE
Com
men
ts
5.6
22
4
.4
6.3
14
4 3
9.1
1
1.3
28
19.3
46
.8
27
.5
8.5 25
13
5 68
27
29
4.8
17
5
.6
18
15
.4
60
.4
16
80
__
-
__
13
__
13.1
9.9
__
__
-
*S/P
CO
A
.I; *BA
V
*Ill;
.L
V-A
N
01: E
- 13
0"
.BA
V
0111
: E -
130"
*A
bnl.
AoV
: E 0
-45"
.L
V A
N
011: E
80 -
120"
+ Ex
cisi
on
OR
ES-
+ K
onno
V
alve
not
ed
*S/P
mod
ified
Kon
no
.Add
ition
al d
etai
l of a
0.V
. .AS
SOC
LV, A
N
.Add
ition
al d
etai
l *V
SD c
lose
d *I;
.Mild
AI
*BA
V,
*Mod
AI
+ Ex
cisi
on
Mild
AI
Fals
e po
sitiv
e A
VR
Val
ve n
oted
.S
ff C
OA
*S
ff V
SD
*Sff
ao.
val
voto
my
a?
ao.
val
ve
abno
rmal
ity
.S/P
CO
A
*Sho
ne S
yndr
ome
.VSD
*Mus
cula
r pro
tusi
on
into
LV
OT
E 1
20";
.D
yspl
astic
ao.
V
*Qua
dric
uspi
d a0
.V.
E
0 - 4
5:
01: E
90
- 120
" E
30
*VSD
w/R
CC
pro
laps
e E
45-
135"
; *N
O I
RV
OTO
; E 4
5 *I
; 'Win
dsoc
k E
- 11
0" G
30-
90"
011:
E 9
0-13
0"
*Abn
l a0.
V.
0111
: E 1
35"
*BA
V. E
30"
*No
surg
ery
corr
elat
ion
.Cha
nged
dia
gnos
is
*? I,
0M
od-M
*S
upra
valv
e mitr
al
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*A
oV p
rola
pse
*? I
*I;
*Mild
A1
.I1 .Abn
l ao
. V.
0111
; *B
AV
, .M
od A
I 01
11 ring
+ A
VR
Lim
ited
+ M
VR
T
race
AI,
+ R
epai
r N
o A
1
*New
info
rmat
ion
dete
cted
I.
2
*Rul
ed o
ut I
Ass
oc. R
VO
TO
*Sub
aort
ic s
teno
sis
+ E
xcis
ion
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A1
+ Ex
cisi
on
RES
-
+ K
onno
T
race
AI
*Add
ition
al d
etai
l
-
*Hea
rt m
urm
ur
*Sff
11;
*BA
V
-1"A
SD
*I &
111
: E 9
0-12
0"
Val
ve n
oted
C
alci
fied
AoV
. A
VR
+ D
amag
e m
itra
l O
RE
S+
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MR
-
AV
R
Sig.
sub
+ E
xcis
ion
Mild
AI
AS
.Add
ition
al d
etai
l
.SIP
C
OA
*I
1 01
1 & I1
1 E 3
0" - 1
35"
Long
G 0
" - 30
" *L
V b
and
G - 0
" - 30
" *R
VO
TO: E
45
- 90"
*Add
ition
al d
etai
ls
*Sig
nifi
cant
pos
top
*ASS
OC
. RV
OTO
*S
igni
fica
nt p
osto
p fi
ndin
gs
.New
inf
orm
atio
n
.Sff
Tru
ncus
.A
S a?
Sub
AS
.Poo
r
*Poo
r 01
1: E
90
- 120
"
I =
dis
cret
e th
in s
ubao
rtic
ste
nosi
s; I1
= d
iscr
ete
thic
k su
baor
tic s
teno
sis;
111
= t
unne
l su
baor
tic s
teno
sis;
a0.
V.
= a
ortic
val
ve;
BA
V =
bic
uspi
d ao
rtic
val
ve;
AN
=
aneu
rysm
; AV
R =
aor
tic v
alve
rep
lace
men
t; A
S =
aor
tic s
teno
sis;
Ass
oc.
= a
ssoc
iate
d; A
bnl
= ab
norm
al;
MV
R =
mit
ral
valv
e re
plac
emen
t; E
x =
exc
isio
n; E
=
esop
hage
al; G
= g
astr
ic; R
ES
= n
o re
sidu
al d
efec
t; R
VO
T =
righ
t ven
tric
ular
out
flow
trac
t obs
truc
tion.
SHARMA, ET AL.
Figure 1. (Patient 6). A thin discrete subaortic membrane (curved arrow) is seen just below the aor- tic value (AV) in transesophageal view at 1089 Note also the presence of a distinct supravalve mitral ring (SMR) in this patient with Shone syndrome. AML = anterior mitral leaflet; L A = left atrium.
mirror-image rotation of the scanning plane compared to the short gastric position. This is because the degree of anteflexion of the dis- tal probe compared to the shaft is slightly > goo.
Comparison of Surface Echocardiogram and MTEE
Compared to surface echocardiogram, preop- erative MTEE provided significant new diag- nostic information in four patients. It changed the diagnosis in patient 5, ruled out a subaortic membrane in patient 7, and detected subaortic membranes in 2 patients (patients 6 and 14) (Fig. 1). The surgery plan was thus signifi- cantly altered in 4 (28%) of 14 patients. In pa- tient 5, a subaortic membrane was suggested by surface echocardiography, but, due to the large size of the patient the diagnosis was un- clear. MTEE revealed this to be a quadricuspid aortic valve with lack of a central apposition and resultant moderate A1 (Figs. 2A and 2B). No subaortic membrane was found by MTEE, or surgery, and the patient underwent aortic valve replacement. In patient 6, who had Shone syndrome, an unsuspected subaortic mem- brane was revealed by MTEE. In five patients, MTEE findings supplemented surface echocar- diographic findings by providing more focused, additional details of subaortic stenosis. This was exemplified by patient 8, who had a thin membrane that "windsocked" into the aortic valve, and this fact was appreciated by MTEE only (Fig. 3). Patient 14, who had suspected
Fig;ure 2, frame shows luck of central opposition in transesophageal view at 0".
(Patient 5). (A) A quadricuspid aortic ualve i s clearly demonstrated in systolic frame. (B) Diastolic
656 ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech. Vol. 13, No. 6,1996
TEE IN SUBAORTIC STENOSIS
Associated Lesions
Several significant associated cardiac lesions were assessed by MTEE. One patient with Shone syndrome had a supravalve mitral ring displayed in multiple views (Figs. 1 and 5) . One patient (patient 7) with ventricular septal de- fect and aortic valve prolapse had associated right ventricular outflow tract obstruction best demonstrated in transesophageal imaging planes between 30" and 90" (Figs. 6A and 6B) and the transgastric short-axis imaging plane at 45". In this patient, a suspected subaortic membrane was ruled out by MTEE. One pa- tient with a previous Konno procedure had a prominent aneurysm in the left anterosuperior aspect of the left ventricle (Fig. 7). Bicuspid aortic valve association was corroborated in three patients. Another patient with unusual tunnel obstruction (patient 12) (Figs. 8A and 8B) had an associated septal to left ventricular free wall band in mid-cavity with the appear- ance of a "divided left ventricle" (Fig. 10).
Figure 3. (Patient 8). A thin discrete subaortic membrane (curved arrow) appears to "windsock" into the aortic valve is seen in transesophageal view at 108". AV = aortic valve; L A = left atrium.
subaortic stenosis, had a poor surface echocar- diogram due to her large size. An outpatient MTEE revealed a distinct subaortic membrane. All varieties of subaortic stenosis were present in our series. Thin discrete membranes were seen in 4 patients, thick fibromuscular obstruc- tion was noted in 2 patients (Fig. 41, and tunnel obstruction was noted in 4 patients.
Surgery
Surgery observations confirmed MTEE find- ings in all patients except one. Patient 4 had an unusual muscular protrusion from the ven-
Figure 4. (Patient 9). Fibromuscular obstruction with a broad base (open arrow) is noted at some dis- tance below the aortic valve (AV), which also ap- pears abnormal. Seen from transesophageal view at 123".
Figure 5. Supravalve mitral ring (SMR) in same patient as in Figure 1 demonstrated in trans- esophageal four-chamber view, PML = posterior mi- tral leaflet.
Vol. 13, No. 6,1996 ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech. 657
SHARMA, ET AL.
Figure 6. (Patient 7). (A) Ventricular septal defect with prolapse of right coronary cusp (arrow) is seen best from transesophageal view at 112". (B) Prolapse of right coronary cusp (open arrow) and right ventricle out- flow tract obstruction (RVOTO) (closed arrow) is seen from transesophageal view at 66". A0 = aorta.
tricular septum below a previous ventricular Postoperative Assessment septal defect patch. There was color flow tur- bulence noted in this area, along with 2 d s e c velocity by continuous-wave Doppler interro- gation. Subaortic stenosis was suspected. At surgery there was no definite obstruction iden- tified.
- In all patients, postoperative MTEE was
found useful in assessing the adequacy of the repair, ruling out significant residual lesions, and evaluating the degree of aortic and mitral valve regurgitation. In patient 12, a small tear occurred in the anterior mitral leaflet during resection of adherent fibromuscular tissue. This was repaired with pledgetted sutures (Fig. 11). Postoperative mitral regurgitation that occurred was central and mild. In patient 13, following treatment of a regurgitant trun- cal valve by aortic homograft replacement, a large obstructive mass was found wedged be- tween the anterior mitral leaflet and the left coronary cusp (Fig. 9). This necessitated reex- ploration and removal of muscle which was part of the fibromuscular "CUP that came with the aortic homograft. In all other patients, no significant residual obstruction was noted. The degree of aortic regurgitation was not in- creased in any patient.
Discussion Figure 7. (Patient 2). A prominent aneurysm (AN) of the left ventricle is seen anterior to the anterior mitral leaflet (ML) from transgastric two-chamber view at 79" with a slight leftward rotation of the scope.
Many studies have pointed out the complex
ana t~my, ' -~>~ its potential for progressive ob- ~truct ion,~ and its likelihood of recurrence after
nature Of subaortic stenosis with its varied
658 ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech. Vol. 13, No. 6,1996
TEE IN SUBAORTIC STENOSIS
Figure 8. (Patient 12). (A) Multiple levels of muscular obstruction are seen (open arrows). Note adherent tis- sue on anterior mitral leaflet (AML). Seen from transesophageal view at 116". (B) The same multilevel ob- struction (open-headed arrows) is seen from the long gastric position 0". A0 = aorta; AV = aortic valve; LA = left atrium; LV = left Ventricle.
surgical resection.6 Due to these complexities, an imaging modality that provides accurate detailed information pre- and postoperatively would be expected to have high utility. We found MTEE to be an ideal imaging modality for this lesion as discussed below.
planes between 110" and 120°, which aligned the echocardiographic beam along the long axis of the left ventricle. The associated lesions were well seen from both esophageal and gas- tric views. The "long" transgastric view pro- vided sweeps similar to subcostal four-cham- ber views. The transducer in this position was
Feasibility and Preoperative Assessment
In all our patients > 14 kg, the probe could easily be passed and no complications were noted. The probe could readily be manipulated to view the heart from multiple views. The high resolution of the probe and the continu- ously variable imaging planes provided impor- tant new information in many patients. It resulted in an important complete change of diagnosis in one patient by finding a quadri- cuspid aortic valve that mimicked subaortic membrane on surface echocardiogram. We feel this is an important distinction to make be- cause a thin subaortic membrane when pres- ent immediately below the valve can often ap- pear like an additional leaflet. It also proved versatile in accurately evaluating associated lesions of right ventricle outflow tract obstruc- tion as Well as left Ventricle inflow and Cavity lesions. The left ventricle outflow was best scrutinized from transesophageal imaging AV = aortic valve.
Figure 9, (Patient 13). A prominent (open arrow), just below the replaced aortic valve he- mograft, is seen by postoperative multiplane echo- cardiography. View from short transgastric -120".
Vol. 13, No. 6, 1996 ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & Allied Tech. 659
SHARMA, ET AL.
Figure 10. (Patient 12). Prominent left ventricu- lar band (arrow) gives the appearance of a divided left ventricle. Viewed from the short transgastric po- sition at 0".
anteflexed by > 90". Hence, the long-axis views of the ventricles were obtained by "mirror-im- age" rotation of the sector, i.e., the left ventric- ular long axis was best seen at a 35" rota- tion,and the right ventricular outflow tract at a 135" rotation from the long gastric position. To the best of our knowledge this has not been reported before.
Postoperartive Assessment
MTEE played a major role in the postopera- tive assessment of our patients. Since the re- moval of the subaortic obstruction is through the aortic valve and entails resection of mem- branes and muscle that are adherent to the mi- tral valve and perilously close to the conduc- tion axis, the possibility exists of incomplete resection to avoid damage to these structures. Alternatively, in the process of thorough exci- sion, damage may occur to the aortic or mitral valve. Hence, postoperative MTEE is essential to evaluate the repair. This was best borne out by one patient who developed a small tear of anterior mitral leaflet during the removal of adherent tissue. This was recognized and su- tured. In this patient, it was reassuring to see that postoperative MTEE not only demon- strated adequate relief of obstruction, but also
demonstrated that mitral regurgitation was mild. Postoperative MTEE also proved invalu- able in the detection of unexpected muscular subaortic obstruction in one patient. In all other patients it was gratifying to see that there was no significant residual lesion and no increase in aortic regurgitation.
Outpatient Assessment
Although surface echocardiography has been very useful in the diagnosis of subaortic steno- sis in young children,s-12 it has a limited ability to resolve the details and subtleties of LVOT lesions in older children. It is evident from our, and other studies,l-s that subaortic stenosis is frequently seen in older children and adoles- cents. Hence, it appears that MTEE is ideally suited for evaluation of subaortic stenosis. However, our experience in general, with uti- lizing MTEE in adolescents in the outpatient setting has been frustrating. Despite large doses of midazolam, intubation of the esopha- gus has proved difficult. This is probably due to a heightened gag reflex in adolescents. How- ever, in one outpatient MTEE was helpful in revealing a discrete thick membrane. Due to
Figure 11. Postoperative multiplane echocardiog- raphy on same patient as in Figure 8, demonstrat- ing no residual left ventricular outflow tract ob- struction. The site of repair of mitral value tear is shown by the open arrow. Seen from trans- esophageal view at 116".
660 ECHOCARDIOGRAPHY: A Jml. of CV Ultrasound & Allied Tech. Vol. 13, No. 6,1996
TEE IN SUBAORTIC STENOSIS
poor patient acceptability, we have now se- verely limited the use of outpatient TEE at our institution.
Conclusion
With the recent availability of small TEE probes, the role of TEE in children is being defined. 13-15 This retrospective analysis ex- plored the utility of a MTEE probe in a sub- set of children and adolescents with subaortic stenosis. MTEE, due to its continuously vary- ing imaging planes, provided for thorough scrutiny of all aspects of subaortic stenosis. In all our patients it depicted the anatomy clearly. We believe this led to a greater ap- preciation of the lesion by our surgeons and played an important role in planning of the surgical approach. Postoperatively, MTEE proved invaluable in assessing the repair, al- lowing the surgeon to close with confidence, or go back in for residual lesion. We strongly recommend the use of MTEE in the surgical management of older children and adoles- cents with subaortic stenosis.
References
Latson, LA: Fibromuscular subvalvular aortic stenosis. In Garson A Jr, Bricker JT, McNa- mara DG (eds): The Science and Practice of Pe- diatric Cardiology. Philadelphia, Lea & Febiger, 1990, pp. 1346-1352. Wright GB, Keane JF, Nadas AS, et al: Fixed subaortic stenosis in the young: Medical and surgical course in 83 patients. Am J Cardiol
Penkoske PA, Collins-Nakai RL, Duncan NF: Subaortic stenosis in childhood: Frequency of associated anomalies and surgical options. J Thorac Cardiovasc Surg 1989;98:852-860. Vogel M, Smallhorn JF, Freedom RM, et al: An echocardiographic study of the association of ventricular septa1 defect and right ventricular
1983;52:830-835.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
muscle bundles with a fixed subaortic abnor- mality. Am J Cardiol 1988;61:857-860. Freedom RM, Pelech A, Brand A, et al: The progressive nature of subaortic stenosis in con- genital heart disease. Znt J Cardiol 1985;8:137- 143. Leicher DA, Sullivan I, Gersony WM: “Ac- quired” discrete subvalvular aortic stenosis: Natural history and hemodynamics. JACC
Somerville J : Fixed subaortic stenosis-A fre- quently misunderstood lesion. Znt J Cardiol
Cabrera A, Galdeano J-M, Zumalde J, et al: Fixed subaortic stenosis: The value of cross- sectional echocardiography in evaluating dif- ferent anatomical patterns. Znt J Cardiol
Pierli C, Marino B, Picardo S, et al: Discrete Subaortic stenosis: Surgery in children based on two-dimensional and Doppler echocardiog- raphy. Chest 1989;96:325-328. Wilcox WD, Seward J, Hagler D, et al: Discrete Subaortic stenosis: Two-dimensional echocar- diographic features with angiographic and sur- gical correlation. Mayo Clin Proc 1980;55:425- 433. Frommelt M, Snider R, Bove E, et al: Echocar- diographic Assessment of subvalvular aortic stenosis before and after operation. JACC
Reeder GS, Danielson GK, Seward JB, et al: Fixed Subaortic stenosis in atrioventricular canal defect: A Doppler echocardiographic
Stumper OF, Elzenga NJ, Hess J, et al: Trans- esophageal Echocardiography in children with congenital heart disease: Initial experience.
Dan M, Bonato R, Mazzucco A, et al: Value of transesophageal echocardiography during re- pair of congenital heart defects. Ann Thorac Surg 1990;50:637-643. Weintraub R, Shiota T, Elkadi T, et al: Trans- esophageal Echocardiography in Infants and children with congenital heart disease. Circu- lation 1992;86:711-722.
1989;14:1539-1544.
1985;8: 145-148.
1989;24:151-157,
1992;19:1018-1023.
study. JACC 1992;20:386-394.
JACC 1990;16:433-441.
Vol. 13, No. 6, 1996 ECHOCARDIOGRAPHY: A Jml. of CV Ultrasound & Allied Tech. 66 1