Transballoon Intravascular Ultrasound Imaging During Balloon Angioplasty in Animal Models With Coarctation and Branch Pulmonary Stenosis
Background Intravascular ultrasound (IVUS) studies performed after balloon dilation provide a method for evaluating the adequacy of angioplasty and the nature of associated changes in vessel walls. Previously, IVUS studies required the use of separate scanning catheters inserted independently before and after balloon angioplasty. We tested a 0.035-in, 30-MHz IVUS transducer wire that images from within commercially available 5F balloon dilation catheters.
Methods and Results Seven stenoses were created in the left pulmonary artery (n=3) and in the aortic isthmus (n=4) in six lambs (weight, 3.4 to 12.5 kg). The balloon catheter selected was advanced across the stenotic area and the IVUS wire advanced in the guide lumen to the center of the balloon. Continuous IVUS images were obtained through balloons before, during, and after dilation. Transballoon imaging confirmed balloon location within the stenotic segment. Luminal diameters of stenotic and adjacent vessel segments before and after angioplasty by IVUS showed good correlation with angiographic measurements (r=.93, P≤.001). After successful dilation, imaging during deflation allowed the assessment of vascular elastic recoil, mural dissection, and luminal size without requiring changes in balloon position. Repeat dilation could be undertaken and the inflation pressure and technique modified on the basis of the observed results.
Conclusions This transballoon IVUS system provides important on-line information about lumen diameter and wall structure for evaluation of angioplasty without the need for catheter changes, providing a method to possibly reduce the likelihood of excessive wall damage and to potentially reduce the number of angiograms required to accomplish and confirm results.
Balloon angioplasty was introduced initially in 1982 as an investigational procedure for nonsurgical treatment of coarctation of the aorta and recurrent coarctations.1 2 3 4 Subsequent clinical studies in infants and children5 6 have resulted in it becoming an accepted procedure for many children with aortic coarctation, as well as for a wide variety of other vascular stenoses. Disruption of the intimal and medial vessel-wall layers during balloon dilation for coarctation produces tears and dissections,3 7 8 9 10 11 12 13 14 which may be necessary for successful relief of the obstruction. It is possible that these disruptions may set the stage for changes leading to aneurysm formation.
Intravascular ultrasound (IVUS) has been found to be helpful in characterizing the effects on the vessel wall during,10 11 immediately after,10 11 12 13 14 and on follow-up of balloon angioplasty10 12 for coarctation of the aorta. Currently, IVUS is performed via the introduction of a long sheath past the coarctation site, through which the IVUS probe and its housing catheter are passed. The purpose of the present study was to test a commercially available IVUS wire probe (Wise-Wire; Meditech, Inc), which fits within the guide lumen of commercially available 5F balloon dilation catheters. This probe has the advantage of allowing transballoon intravascular imaging before, during, and after balloon angioplasty without the need for additional wire exchanges or sheaths. Our study involved testing the IVUS transducer and its imaging capabilities during balloon dilation of surgically created pulmonary artery stenoses and coarctations in a newborn lamb model.
Guidelines established by the Department of Animal Care at Oregon Health Sciences University were followed for the care and study of all sheep. The first group consisted of three lambs with an average age of 13.6 days (range, 10 to 16 days) and an average weight of 5.8 kg (range, 5.1 to 6.2 kg). Anesthesia was induced with intravenous diazepam and ketamine, then maintained with ventilated isoflurane 1.5% to 2%. Through a left lateral thoracotomy, the left pulmonary artery (LPA) was dissected free. An LPA band was placed with the use of Vicryl (braided polyglycolic acid suture) mesh, tied with a 7-0 chromic suture, and tightened to approximate half the size of the proximal LPA diameter. The chest was closed, and the lamb recovered. Approximately 5 weeks later, the lambs (average age, 53.7 days [range, 51 to 57 days]; average weight, 16.9 kg [range, 15.5 to 17.0 kg]) underwent cardiac catheterization. Anesthesia was performed in the same manner as above. A 7F sheath was introduced into the femoral vein by use of the vascular cutdown technique. An LPA angiogram was performed in the 30° left anterior oblique, 25° cranial projection with the use of 20 mL of RenoCal-76 contrast (Bracco Diagnostics, Inc) at 25 mL/s. A PE-MT 5F 8 mm×3 cm (Meditech; Boston Scientific Corp) or Accent DG 6F 10 mm×4 cm (Cook Group) balloon dilation catheter was advanced over a guidewire to the LPA. The IVUS wire, when removed from its housing (Spy Catheter; SciMed Corp), is 0.035 inches in diameter, with the transducer located at the distal tip of the wire. The wire rotates when attached to a motorized unit, which is controlled by a Hewlett-Packard SONOS 100 Intravascular Imaging System. This wire was advanced through the lumen of the balloon dilation catheter to the catheter tip. A Tuohy-Borsht adapter (CR Bard, Inc) was attached to the end of the catheter, enabling us to flush fresh saline solution through the wire lumen. IVUS scanning was performed at 30 MHz at 25 to 30 frames per second. Images were recorded on 0.5-inch Super VHS tape for later review.
The IVUS wire could be advanced freely within the lumen of the balloon dilation catheter for imaging. Proper alignment of the stenotic area within the center of the balloon was confirmed by IVUS imaging and fluoroscopy. The balloon was inflated to pressures of 1 to 8 atm, then deflated with a carefully debubbled mixture of saline and contrast (1:1 dilution). Before and between inflations, the balloon lumen was flushed, and air bubbles were removed carefully.
Continuous IVUS scanning was performed before, during, and after balloon inflation to measure proximal and distal vessel diameters, stenosis diameter, and balloon diameter; to detect the presence of intimal dissections and tears; and to evaluate vascular recoil during balloon deflation (Fig 1⇓). After the procedure, the lambs were euthanatized with pentobarbital solution (Beuthanasia-D; Schering-Plough Animal Health).
Four acute coarctations were created in three lambs (average age, 23 days [range, 7 to 35 days]; average weight, 8.8 kg [range, 3.4 to 12.5 kg]). Anesthesia was performed in the same manner as above. Through a left lateral thoracotomy, the aortic isthmus was dissected free. An acute aortic coarctation was created with the use of cotton umbilical tape, clamped and tied with 7-0 or 8-0 prolene (polypropylene suture), to a diameter approximating half the diameter of the distal aorta. Simultaneously, a 7F sheath was inserted into the femoral artery via the vascular cutdown technique. An angiogram proximal to the coarctation was performed in the lateral projection with the use of 20 mL of contrast at 20 mL/s. Systemic heparinization was performed with 100 U/kg heparin. A PE-MT 5F balloon dilation catheter (6 mm×2 cm or 8 mm×3 cm) or Cordis PTA 5F 10 mm×8 cm balloon dilation catheter was advanced across the coarctation, and the IVUS wire was advanced through the lumen of the catheter.
Multiple balloon inflations with IVUS imaging were performed, as described above. Cut-film angiograms were performed with scale markers used to identify distances. The diameters of the vessels before and after balloon dilation in the area of narrowing were measured, as well as proximal and distal vessel diameters. After the procedure, the lambs were euthanatized by use of pentobarbital solution.
All luminal diameter measurements are expressed as mean data. Regression analysis was used to compare angiographic and IVUS findings. A value of P<.05 was considered significant.
Correlation of Measurements
For the three animals in group 1, the pulmonary artery banding we performed did not produce the degree of stenosis anticipated, resulting in only a mild stenosis. As a consequence, the maximal balloon diameter that allowed adequate imaging at 30 MHz was not large enough to allow dilation of the stenosis. Thus, IVUS and angiographic measurements were compared only at the stenosis site in this model.
For two of the three lambs in group 2, IVUS measurements were compared with angiographic measurements obtained from the aorta just proximal to the coarctation, the coarctation site, and the aorta distal to the coarctation site. In addition, in one of these animals, angiographic measurements were obtained from the same locations after balloon angioplasty.
In assessing the relationship between the vessel diameters obtained by use of IVUS and those obtained angiographically, a significant relationship was found (Fig 2A⇓). By linear regression, Diameter by IVUS in Millimeters=(Diameter by Angiography in Millimeters×0.87)+1.84 mm (SEE=0.98 mm, r=.93, r2=.86, P≤.0001). The mean difference between angiographic and IVUS diameters was 0.57±0.99 mm (Fig 2B⇓).
Balloon Inflation During IVUS Imaging
Balloon angioplasty was attempted with a variety of balloon dilation catheters, including 5F 6 mm×2 cm (n=2), 5F 8 mm×3 cm (n=6), 6F 10 mm×4 cm (n=1), and 5F 10 mm×8 cm (n=1). In group 1, in which the vessel luminal diameter was larger than the available balloon-catheter size, balloon inflation was performed to assess the quality of imaging through the balloon during inflation and deflation and to observe vessel distention. In all studies, transballoon imaging was helpful in verifying balloon position in the narrowed area. In the group 2 animals, in which the stenoses were tight enough to be dilated, IVUS was used to observe the balloon inflation to its maximal diameter. With deflation of the balloon catheter, vascular recoil and the results of the dilation could be judged immediately. Intimal and medial dissections were observed in two of the three studies in which successful balloon dilation was performed (Fig 1B⇑ and 1C⇑).
The new system we have employed uses a miniaturized IVUS transducer that can be placed in a variety of balloon catheters with a minimum lumen of ≥5F, allowing immediate visualization of vascular wall structures from within the balloon dilation catheter. Previous studies using similar methods have used larger transducers10 12 13 14 or experimental IVUS transducers that were built into the catheter.10 11 The disadvantage of those approaches was in the size of the arterial or venous sheath required for catheter introduction, as well as the shared balloon lumen, which may prolong balloon inflation and deflation cycle times.11 The Wise-Wire can be advanced within the catheter guidewire lumen, which allows for advancement of the transducer to any point within the catheter. In addition, the wire may be withdrawn and reused in the same patient if a larger balloon-catheter diameter or different balloons are required.
A major advantage of this system is that it allows the immediate identification of loss of waist, intimal flaps, medial dissections, vascular elastic recoil, and vessel diameter without the need for catheter exchanges or angiograms. In addition, the system does not interfere with fluoroscopic imaging. Compared with images obtained with the wire tip protruding from the catheter end hole, the balloon produces little distortion except with balloon folding during deflation or if air is introduced. This may potentially reduce the number of cineangiograms required during and after balloon angioplasty, as well as the overall catheterization-procedure duration.
One of the problems identified in these preliminary data was the overestimation of the diameter by IVUS compared with angiography. One explanation for this finding may have been that IVUS may measure the internal luminal diameter from an oblique or tangential plane where a vessel turns or branches, distorting and enlarging the diameter compared with angiographic measurements made perpendicular to the axis of the vessel. Fluoroscopic localization of the IVUS tip can assist in identifying locations where this may be more likely (eg, the aortic isthmus).
In using a system like this, care must also be taken to avoid bubbles within the catheter during balloon dilation. The balloon should be evacuated of any air and filled with debubbled, diluted contrast. In addition, we found that IVUS imaging was facilitated by the addition of a Tuohy-Borsht adapter to the end of the balloon dilation catheter, which allowed for catheter flushing while the IVUS wire was in place.
A higher transducer frequency is associated with better resolution at a smaller depth from the imaging core than a lower-frequency transducer. This system uses a 30-MHz transducer, which provides excellent vascular imaging to a depth of 10 to 12 mm. This will provide adequate imaging of vascular structures for those infants who require balloon angioplasty with 5F balloon dilation catheters. This system can also be used at 20 MHz, which along with miniaturization of even lower-frequency transducers, may provide intraballoon IVUS imaging through larger-diameter balloons for those patients with larger vessel diameters.
Wires used clinically during balloon dilation often extend distally to stabilize balloon position and are quite stiff. However, to assist in stabilization of the balloon dilation catheter during angioplasty, this IVUS wire system in its present form would require modification. Development of a wire-within-a-wire system may allow for intravascular imaging through an echolucent segment of wire. A balloon-catheter modification to include an additional guidewire lumen would also enable IVUS imaging during balloon angioplasty. Alternatively, a standard wire could be used during angioplasty that would be removed and replaced with the IVUS wire to evaluate the results without having to move the balloon. Scanning could be performed through a deflated or, optimally, a partially inflated balloon.
This experimental study defines potential uses of a transballoon IVUS system that adequately measures vessel diameter, assesses vascular elastic recoil, and identifies vascular wall disruption or dissections without moving the balloon catheter. The ability to assess these changes without repeated wire and catheter exchanges lowers the potential risk of transmural dissection through intimal tears. Also, the balloon may be inflated progressively and repeatedly while observing results between inflations. The availability of such a system provides the groundwork for IVUS to be used to identify vascular changes that may assist in determining the “ideal” degree of vascular injury.10 11 12 Other potential applications include the evaluation of aortic distensibility, recoil, vascular integrity, and vessel diameter during and on follow-up of balloon angioplasty, to assist in expanding the use of IVUS in patients with congenital heart disease.
We thank Pat Renwick for her professional assistance in anesthesia and overall animal care, the Dotter Institute for their expertise during cardiac catheterization, and Michelle Ann Cheney for her editing and preparation of this manuscript.
- Received December 31, 1996.
- Revision received March 18, 1997.
- Accepted March 18, 1997.
- Copyright © 1997 by American Heart Association
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