677
JACC Vol. 31, No. 3
March 1, 1998:677– 83
EXPERIMENTAL STUDIES
Double-Helix Coil for Occlusion of Large Patent Ductus Arteriosus:
Evaluation in a Chronic Lamb Model
RALPH G. GRABITZ, MD, FRANZ FREUDENTHAL, MD, MATTHIAS SIGLER, MD,
TRONG-PHI LE, MD,* CHRISTOPH BOOSFELD, PHD,† STEFAN HANDT, MD,‡
GÖTZ VON BERNUTH, MD
Aachen, Bonn and Cologne, Germany
Objectives. We sought to evaluate the efficacy and tissue reaction of a new miniature interventional device for occlusion of large
patent ductus arteriosus (PDA) in a neonatal lamb model.
Background. A variety of devices are used to close PDAs by
interventional measures. Spring coils found to have a high
cumulative occlusion rate have thus far been limited to smaller
PDAs because of the physical limitation of grip forces.
Methods. Memory-shaped double-cone stainless steel coils with
enhanced stiffness of the outer rings by a double-helix configuration were mounted on a titanium/nickel core wire. A snap-in
mechanism attaches the coil to the delivery wire, allowing intravascular coil retrieval and repositioning. The system was placed
through a 4F or 5F Teflon catheter. A chronic lamb model (n 5 8)
of PDA (>5 mm) was used in which ductus patency was secured
by a protocol of repetitive angioplasty procedures. The animals
were killed after 1 to 181 days, and the ductal region was examined
by inspection as well as by light and electron microscopy.
Results. Placement of the coils within the PDA was possible in
all lambs. Before final detachment, the coils were retrieved or
repositioned, or both, up to 12 times. In all but one animal the
ductus was closed within 6 days after the procedure. The coils
caused no infections or aortic and pulmonary artery obstruction.
Histologic and electron microscopic studies revealed endothelial
coverage of the implants but no foreign body reaction or local or
systemic inflammation or erosion of the implant.
Conclusions. The device effectively closed large PDAs in our
model and may overcome the previous limitations of coils. Clinical
trials are indicated.
(J Am Coll Cardiol 1998;31:677– 83)
©1998 by the American College of Cardiology
Several percutaneous transcatheter techniques for closing the
persistently patent ductus arteriosus (PDA) in older children
and adults have been described (1– 8). The Rashkind occluder
is the only system thus far with widespread acceptance. However, size and a relatively large delivery system, as well as high
costs and a late incidence of residual shunting, limit its
application.
Spring coils to occlude arteries were introduced by Gianturco et al. (9) in 1975. These coils, once delivered, are not
retrievable into the delivery catheter and therefore carry the
risk of improper placement or undesired embolization. Preformed Nitinol snares may be used to improve delivery (10).
New developments allow the retrieval of certain coils after
positioning and until final release (11,12). These devices can be
used with small introduction catheters but because of their
physical properties (increasing the diameter of the reconfigured coil decreases its stiffness by threefold, leading to pullthrough and embolization), they were found to be effective
only in occluding small PDAs (13–15).
The present report focuses on the use of selectively enhanced stiffness of the outer rings of coils to allow safe
positioning of the device in medium to large PDAs. The new
coil on trial relies on the strong memory effect of certain metals
(American National Standards Institute [ANSI] 301/4) to form
biconical, “double-disk” coils in which rings inside the PDA
cause its mechanical and thrombotic closure (12,16). To evaluate practicability, efficacy and medium-term biocompatibility
of the new device in occluding PDAs, we used a new chronic
lamb model with high shunting and a minimal inner ductal
diameter .5 mm.
From the Department of Pediatric Cardiology and Interdisciplinary Center
of Clinical Research on Biomaterials, Aachen University of Technology, Aachen;
*Division of Cardiology, Children’s Hospital, University of Bonn, Bonn;
†Produkte für die Medizin GmbH, Cologne; and ‡Department of Pathology,
Aachen University of Technology, Aachen, Germany. This study was supported
in part by Grant 01 KS 9503/9 from the German Ministry of Research and
Technology, Bonn, Germany. Some coils and catheters were generously donated
by Produkte für die Medizin (pfm) GmbH, Cologne, Germany. Dr. Freudenthal
owns patents on aspects of the device described in this report and may receive
royalties if and when it is marketed. Dr. Boosfeld is a consult to pfm GmbH.
Manuscript received February 17, 1997; revised manuscript received August
25, 1997, accepted November 21, 1997.
Address for correspondence: Dr. Ralph G. Grabitz, Department of Pediatric
Cardiology, Aachen University of Technology, Pauwelsstrasse 30, 52074 Aachen,
Germany. E-mail: grabitz@alpha.imib.rwth-aachen.de.
©1998 by the American College of Cardiology
Published by Elsevier Science Inc.
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Methods
Occlusion device and delivery system. The PDA occlusion
device consists of 1) A double-cone memory-shaped spring coil
with enhanced stiffness limited to the larger distal loops by a
selective double-helix configuration of the primary wire strand
0735-1097/98/$19.00
PII S0735-1097(97)00025-4
678
GRABITZ ET AL.
DOUBLE-HELIX COIL FOR CLOSURE OF LARGE PDA
Abbreviations and Acronyms
ANSI 5 American National Standards Institute
PDA 5 patent ductus arteriosus
(stainless steel [DIN 1.4310, 'ANSI 301/4] wire strand
0.20 mm in diameter, 0.80-mm primary coil, 6- to 12-mm
reconfigured secondary diameter, minimal inner diameter
,1.5 mm, overall number of loops 8 to 13 [pfm GmbH,
Cologne, Germany]) (Fig. 1); 2) a pusher system: a) a pusher
wire (a coiled stainless steel wire, 90 cm long, with a modified
base to fit into hold-back mechanism of the safety handle) that
is advanced over the b) core wire (nickel/titanium alloy [Nitinol], 110 cm long, 0.35 mm in diameter, a modified tip to
control the coils and a modified base to fit into the hold-back
Figure 1. Top, Macroscopic appearance of two coils, nearly identical
in appearance, with primary strand diameter 0.2 mm, primary coil
diameter 0.8 mm, reconfigured coil diameter 10 mm (in contrast to the
double-cone shaped configuration of coils used in the study); the coil
on the right is constructed as a double-helix coil. Bottom, Effect on
stiffness is demonstrated when the coils are each pulled at 0.11 N.
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JACC Vol. 31, No. 3
March 1, 1998:677– 83
mechanism of the safety handle); 3) a safety handle (stainless
steel, engraved metric scale, two safety mechanisms and one
distal ring on its shaft [OccluGrip, pfm, Cologne, Germany]);
and 4) a Teflon introduction catheter (60-cm long, 4F or 5F in
diameter [1.0/1.3 mm], including tip marker [inner gold ring]).
The system itself is described in detail elsewhere (12,17).
Briefly, the coil is stretched and mounted on the distal end of
the core wire. The proximal end of the core wire is secured into
the safety grip, into which the pusher wire is screwed as well,
allowing the latter to be moved independently of the core wire
or vice versa. The whole system is introduced through the
delivery catheter like a conventional guide wire. Advancing the
pusher wire or withdrawing the core wire will deploy the ductal
coil from the tip of the core wire, allowing it to regain its
original shape. As long as at least 0.5 cm of coil is left on the
core wire, the whole system can be withdrawn into the delivery
catheter. Final release is accomplished by a positive felt action
of pushing the coil from the core wire. Because the purpose of
the stiffer outer loops is to avoid pull-through from the aortic
side into the pulmonary artery, the loops have to be released
first, characterizing the system as a transvenous device by
design.
Animal studies. The animal experiments were conducted
according to the guidelines of the German Animal Protection
Law and were approved by the state agency supervising animal
experimentation. Catheterization of the animals was performed as a sterile procedure under anesthesia with halothane
(0.3% to 0.6%) and N2O and using portable, digital, monoplane fluoroscopy (Philips, The Netherlands). A color Doppler
echocardiograph (Kontron, France) was used to acquire echocardiographic data. No animal received antibiotics or antithrombotic agents, except for a heparinized flush solution
(2 U/ml of normal saline solution). Before and after the
investigations the lambs were cared for by the ewe.
Lamb model of persistent PDA. Eight neonatal lambs
(Morino Mixed) 3 to 30 h old and weighing 3.2 to 6.4 kg
(median 4.7) were anesthetized and artificially ventilated. By
means of a 6F vascular sheath in the left-sided jugular vein, a
pediatric valvuloplasty catheter (balloon diameter 6 to 8 mm,
length 20 mm; Dr. Osypka, GmbH, Grenzach, Germany) was
passed over a guide wire through the right heart across the
ductus arteriosus. The ductus was dilated up to 6 to 8 mm over
10 min. On postpartum day 5 or 6, the procedure was repeated,
and an angiogram 10 min after dilation demonstrated ductal
patency and dimensions. If control angiography revealed circumscript narrowing on the aortic or pulmonary orifice or the
minimal inner diameter of the ductus was ,5 mm, the balloon
angioplasty was repeated, as previously described (Fig. 2).
Coil delivery. Four to 53 days (mean 20) after the last
dilation the lambs were again anesthetized and artificially
ventilated. A sheath was placed into the femoral artery and
jugular vein. The location and size of the PDA were again
demonstrated by aortogram. By the venous route and the right
heart, a 4F or 5F end-hole delivery catheter (nylon or Teflon)
was placed through the PDA into the descending aorta. Two
rings of the occluding coil were freed from the core wire and
JACC Vol. 31, No. 3
March 1, 1998:677– 83
GRABITZ ET AL.
DOUBLE-HELIX COIL FOR CLOSURE OF LARGE PDA
679
Figure 2. Angiographic appearance
of the PDAs used for double-helix
coil closure at day of implantation
(lateral or left lateral oblique projection; anterior is left, cranial is top; in
some instances, a scaling catheter in
the aorta is visible, where the smaller
distance between marks counts for
10 mm and the larger for 20 mm).
extruded into the aorta, regaining their original shape and
forming the distal, outer disc. The whole system was then
withdrawn into the aortic ampulla of the duct, where additional loops were released. Withdrawing the system further
across the PDA into the pulmonary artery, the final loops
forming the proximal disk were released and, once a satisfactory position was achieved, the occlusion coil was freed from
the core wire. Alternatively, the first two distal loops were
primarily configured and placed into the midportion of the
ductus; because of their enhanced stiffness, they were not
dislocated by the shunt flow across the ductus. The softer
middle and proximal loops were then deployed into this ring to
occlude the ductus mechanically and through thrombus forma-
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tion (Fig. 3). An additional aortogram and angiogram of the
main pulmonary artery documented positioning of the coils 30
to 60 min after final placement of the device. The catheters,
wires and sheath were removed, and anesthesia was discontinued. The animal was returned to the pen to be cared for by the
ewe.
The lambs were investigated clinically on days 1, 2 and 4
after ductal closure. Additionally, color Doppler echocardiography (transthoracal or transesophageal, or both) and clinical
investigation were performed under sedation or anesthesia on
day 6 to evaluate the occlusion of the duct and flow disturbances in the pulmonary artery and descending aorta. One to
181 days (mean 64) after coil implantation, a second left and
680
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DOUBLE-HELIX COIL FOR CLOSURE OF LARGE PDA
JACC Vol. 31, No. 3
March 1, 1998:677– 83
Figure 3. Implantation of double-helix
coil (lateral projection; anterior left,
cranial top) in lamb 5. Top left, Native
aortogram showing the PDA. Top center and right, Distal loops of coil with
enhanced stiffness are placed inside
the lumen of the ductus, and softer
loops are added to the center. Bottom
left, Conglomerate of softer loops is
held by the stiff outer loop in place,
obstructing the ductal lumen. Bottom
center and right, Coils are released
and stay in place. Aortogram demonstrates adequate placement and closure (on motion pictures, a small residual shunt remained on the final
angiogram on the day of implantation).
right heart catheterization through the jugular vein and femoral artery, including angiography, was performed under general anesthesia before the animals were killed and the ductal
block removed. For gross and microscopic examination, the
specimens were fixed in formalin, alcohol or glutaraldehyde.
The aortic and pulmonary portions were scanned by raster
electron microscopy, and the middle portion was prepared for
light microscopy. Some portions were embedded in methyl
methacrylate and sectioned crosswise at 1-mm intervals; the
rest was submitted for routine paraffin embedding after careful
coil removal to avoid damage to this section.
Results
Lamb model of the PDA. In all animals, dilation of the
PDA was possible and produced persistent ductal patency.
Angioplasty had to be repeated a third time in one animal and
a fourth time in another. Four to 53 days (mean 19) after the
last dilation at the time of coil implantation, the PDA had a
tubular appearance, and the minimal inner diameter ranged
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between 5.2 and 7.8 mm (mean 6.3) and the overall length
between 6.5 and 15 mm (mean 10).
Coil placement. Placement of coils in the PDA was possible in all lambs. To achieve complete obstruction of the PDA
in the first animal, two coils had to be implanted at the same
time in a crossover technique from the aorta and the pulmonary artery. This procedure was necessary to provide enough
loops to obstruct the ductal flow entirely.
The whole system had to be pulled back into the delivery
catheter between 1 and 12 times for repositioning or exchange.
In one case the distal ring was loosened by mistake, and the
coil was unintentionally freed, leading to its embolization into
the left pulmonary artery. Retrieval by a snare into an 8F
catheter was possible without difficulty.
Ductal occlusion and follow-up. The final aortogram 30 to
60 min after implantation revealed a complete occlusion in two
cases. Clinical follow-up and color Doppler echocardiography
on day 6 after implantation confirmed a complete occlusion of
the ductus in four additional lambs, and no turbulent flow in
the aorta or pulmonary artery was seen. One animal had to be
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10.7
15.0
6.5
6.3
7.8
5.0
28.1
66.0
9.0
10.4
21.0
6.5
19.0
53.0
4.0
4.7
6.4
3.2
Mean
Max
Min
8
7
8
7
8
10
10
10
10
10
2
2
3
2
2
16
23
9
9
17
10.2
6.6
6.5
7.4
7
9
18
4
5
11
5.3
4.5
5
4.9
3.2
4/M
5/M
6/F
7/F
8/F
Diam 5 diameter; Explant 5 explantation; F 5 female; ID 5 inner diameter; Implant 5 implantation; M 5 male; Max 5 maximum; Min 5 minimum (minimal); PDA 5 patent ductus arteriosus; Wt 5 weight.
22.3
46.0
6.4
64.4
181.0
1.0
98
13
7
62
52
Tubular
Tubular
Tubular
Tubular
Tubular
9
13
9
10
6.5
7.8
6
5.5
5
5.5
13
10
8
9
8
11 3 9
937
736
736
736
235
165
110
120
120
15
14
6.2
6.5
13
12
11.8
12.6
66
27
4
2
10
10
10
8
21
58
2
10
7
29
23
3.7
6.4
2/F
3/M
#606523
#606900
1pre
9510/7001
#606528
#606514
#606507
#606502
#606503
11 3 9
12 3 12
210
240
28.5
8.4
6.4
21.4
18
Minute shunt
2nd coil for
definitive
closure
Closed
Closed
Closed
Closed
Closed
11.8
46
1
181
37.8
101
Slight stenosis of aortic
orifice
Tubular
Tubular, very long
9
7.5
160
8
12 3 10
#605910
4.5
1/M
53
Implant
ID
Overall Coil
Overall
No.
Length if
Coil Size
of
Straightened Min ID Length
(mm)
Loops
(mm)
(mm)
(mm)
681
Duration
Birth PDA Diam at of Last
Age at
Wt at
Days After
Wt Last Dilation Dilation
No. of Implant Implant Last Dilation
Lamb (kg)
(mm)
(min)
Dilations (days)
(kg)
at Implant
Transcatheter occlusion of the PDA is now a well established method of treatment that started with the pioneering
work of Porstmann et al. (18) and was continued with the
Rashkind PDA occluder (7,8), probably the most extensively
investigated device. High costs, size limitations, a large transvascular sheath, a late incidence of residual shunting and risks
of left pulmonary artery stenosis have led to a search for
alternatives in experimental models (19 –21) and clinical trials
(11,17,22,23). High cumulative occlusion rates have been accomplished with various coils in different protocols in PDAs of
small to moderate size; however, attempts to occlude large
PDAs with coils have either not succeeded (13,15,24 –27) or
have led to increased side effects (28).
Table 1. Summary of Coil Implantation in Lambs
Discussion
PDA
killed prematurely 1 day after coil placement because of
recurrent arterial bleeding at the femoral puncture site. Premortem angiography demonstrated a small residual shunt
across the PDA. In one animal a hemodynamically insignificant shunt persisted clinically and on color Doppler echocardiography, and a standard coil was added 3 months after
primary implantation, leading to subsequent complete ductal
closure.
Seven to 181 days after primary implantation the final
angiograms confirmed persistent ductal occlusion in all seven
remaining animals. Postmortem macroscopic examination of
the ductal orifices showed little if any protrusion of the outer
rings of the coils into the lumen of the aorta or the pulmonary
artery. Depending on the duration of implantation, the coils
were partially or completely covered by thin, shiny tissue (Fig.
4). Further details are summarized in Table 1.
Scanning electron microscopic and histologic studies. At
higher magnification, most of the coils were covered by a
monolayer of cells resembling endothelial cells, starting as
early as 13 days after implantation (Fig. 5). When different
intervals after implantation were compared, endothelial coverage was more pronounced at longer intervals of implantation.
A foreign body reaction was not noticeable at any time (13,
62, 80, 98 and 181 days after implantation). Segmental areas of
the media showed the destruction of elastic fibers and the
replacement by fibrous tissue with focal calcification that was
not related to the implant (Fig. 6).
General Description
Follow-Up
After
Wt at
Implant Explant
(days)
(kg)
Figure 4. Aortic ductal orifice 101 days after coil implantation in lamb
1, demonstrating adequate ingrowth.
Closed
GRABITZ ET AL.
DOUBLE-HELIX COIL FOR CLOSURE OF LARGE PDA
Comment
JACC Vol. 31, No. 3
March 1, 1998:677– 83
682
GRABITZ ET AL.
DOUBLE-HELIX COIL FOR CLOSURE OF LARGE PDA
JACC Vol. 31, No. 3
March 1, 1998:677– 83
Figure 5. Scanning electron micrograph in lamb 5, 13 days
after coil implantation. The surface is covered partially
with a fibrin net on which single endothelial cells are
growing. Scale 5 0.1 mm; ;36,000, reduced by 35%.
Technical aspects. Larger and stiffer coils are needed for
use in larger PDAs to withstand high flow and avoid embolization. For a given material and diameter of the primary
strand, increasing the diameter of a reconfigured coil will result
in a threefold decrease in its stiffness. Enlarging the diameter
of the primary strand would cause a fourfold increase in the
stiffness of the coil but make it very difficult to control the
small-diameter loops in the center of the double-cone coil.
Shaping the primary strands of the outer loops in a doublehelix configuration leads to a fourfold increase in the stiffness
of this selected portion of the coil because the double-helix
portion will act as two coil springs at the same place.
Chronic lamb model of PDA. Dilating the ductus after a
protocol of angioplasty procedures provided a chronic model
of a large tubular PDA without operation (19) or implantation
of stents (29), thus avoiding unwanted interactions with the
device. Despite the PDA large size, we encountered no acute
problems with overcirculation of the lungs, most likely due to
the immediate onset of a right to left shunt on the day of birth,
which allowed gradual adaptation parallel to the physiologic
decrease in pulmonary resistance. Our dilated ducts are similar
in shape to the long PDAs found in humans and are thought to
be difficult to close because of high flow, a large minimal inner
diameter and lack of tapering. Their long-term patency is at
least comparable (if not superior) to other animal models of
PDA (30,31).
Functional results. Appropriate placement of coils was
possible in all lambs. In one animal, a coil embolized because
of entrance into the pulmonary artery by mistake, but it was
successfully retrieved by a snare. The overall success rate for
cumulative PDA closure with appropriate follow-up rose to
100% after implantation of an additional standard coil in one
animal. However, because of the small number of animals
tested, the 95% confidence interval for failure still extends to
43% (32).
Histopathologic examination. No clinical, bacteriologic or
histologic signs of inflammation were found in any of the
lambs. Raster electron microscopy revealed endothelial cover-
Figure 6. Central ductal portion in lamb 4, 98
days after coil implantation. Fixation with
methyl methacrylate, toluidin blue stain. Black
areas show coil implantation. No histiocytic
reaction is visible. Arrows indicate area with
calcification beneath the implant. 350, reduced
by 35%.
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JACC Vol. 31, No. 3
March 1, 1998:677– 83
GRABITZ ET AL.
DOUBLE-HELIX COIL FOR CLOSURE OF LARGE PDA
age of the device that was visible as early as 13 days after
implantation. In contrast to a previous study in piglets utilizing
the same alloy (12), we found no foreign body reaction to the
implant. This finding could be explained either by species
differences or by thermal modeling of the new device in the
present study, which was done without oxygen to avoid scaling
on the surface of the coils.
Clinical implications and conclusions. Coils can be deployed through 4F or 5F catheters, reducing potential hazard
to vessels and permitting application in very small infants.
Attempts to minimize the risk of inadvertent embolization
have recently led to the introduction of retrievable coils
(12,17,24). Reduced costs and a comparatively easy procedure,
together with a high cumulative PDA closure rate, have led to
widespread clinical trials limited to small PDAs. This limitation is almost solely due to physical properties because larger
coils lack appropriate stiffness. Our new coil device offers
increased stiffness of the outer rings without loss of any of the
advantages attributed to retrievable coil systems. Like other
coils it is made of a ferruginous alloy, leading at least theoretically to limited nuclear magnetic resonance imaging compatibility. Our device may be difficult to place in very short ducts
because despite being safely anchored on the aortic side
through the enforced loops, the softer parts may pull through
to the pulmonary side and not leave enough material to
obstruct the narrowest portion. The cost of the present device
will most likely range between the recently commercially
available detachable coils and the Rashkind double umbrella.
The device can be manufactured in different shapes and sizes
and has the potential to overcome the previous limitations of
coils for use in the occlusion of large PDAs. Clinical trials of
the device are warranted.
We thank the Departments of Electron Microscopy and Experimental Animal
Research, Medical Faculty, Aachen University of Technology, for their technical
support and expertise.
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