Development and initial experimental evaluation of a prosthetic aortic valve for transcatheter placement. Work in progress

Nonsurgical pulmonary valve replacement: Why, when, and how?

Percutaneous transcatheter interventions for valve replacement or implantation is one of the most exciting developments in the field of interventional cardiology. Valvular stenosis has been treated by balloon dilatation with early and late results; however, treatment for valvular regurgitation has remained surgical until now. Most new designs have been investigated for implantation of valves in the left or right ventricular outflow tracts. Patients with surgery on the right ventricular outflow tract for congenital heart disease constitute the most common group for reoperations during late follow-up. Surgical pulmonary valve replacement can be performed with low mortality; however, it sets up a substrate for future operations. Also, the risk of cardiopulmonary bypass, infection, bleeding, and ventricular dysfunction remains. A transcatheter technique is likely to have more acceptance and may expand the indications for early intervention for right ventricular outflow tract dysfunction.

Future of interventional cardiology in pediatrics

A trend toward use of less invasive, nonsurgical approaches to the treatment of congenital heart disease has developed. Although transcatheter valve replacements and ventricular septal defect closures have already been performed in children, many other techniques and devices are being developed for the palliation of children with a single ventricle. Several groups have already established catheter-based procedures for the creation of Fontan communications, and others have performed nonsurgical pulmonary artery banding. Additionally, fetal catheter-based interventions are being developed for the treatment of severe congenital heart disease in utero. Because this trend toward catheter-mediated treatment is certain to continue, care must be taken to regulate safely the introduction of novel techniques and devices into clinical use in pediatric cardiology.

Percutaneous aortic valve replacement: an experimental study. I. Studies on implantation

OBJECTIVE

The purpose of this preliminary study was to devise a new surgical procedure for minimally invasive aortic valve implantation with a transluminal technique.

METHODS

The new collapsible heart valve was prepared by mounting a porcine aortic valve, taken from a freshly slaughtered pig, into a self-expandable nitinol stent by means of a suture technique. The outer diameter of the valved stent ranged from 15 to 23 mm, and the length ranged from 21 to 28 mm. Before implantation in vivo, these valved stents were tested in an in vitro circulatory system. Only in vitro-tested valved stents with a pressure gradient of less than 7 mm Hg and regurgitation of I degrees or less were used for transluminal aortic valve implantation in vivo. Six of these valved stents were implanted in the descending aorta and 8 in the ascending aorta of anesthetized pigs. The catheter delivery system (22F) was extraperitoneally inserted through the left iliac artery or the infrarenal aorta. Measurements for transvalvular gradient, valvular opening and closure, blood-flow characteristics, regurgitation, and macroscopic analysis were performed at baseline and after the observation period (164 +/- 48 minutes).

RESULTS

This preliminary study contained 14 animals. One animal died of ventricular fibrillation. Technical failure occurred in 2 pigs as a result of stent twisting. At the end of the observation period, the 11 successfully implanted valved stents demonstrated low transvalvular gradients (mean end-systolic Deltarho(max) of 5.4 +/- 3.3 mm Hg for the descending aorta group, 5.4 +/- 1.2 mm Hg for the supracoronary group, and 5.4 +/- 1.1 mm Hg for the subcoronary group), which did not differ from their in vitro gradients. Two-dimensional echocardiography demonstrated complete valvular closure and opening in 5 of 5 cases. Angiography indicated only a physiologic jet of regurgitation (0 degrees ) in 8 animals and mild (I degrees ) regurgitation in 3 animals. Color Doppler ultrasonography indicated no regurgitation in 5 of 5 cases and minor paravalvular leakage in 1 case.

CONCLUSION

Aortic valved stents can be successfully implanted without thoracotomy by using a transluminal catheter technique. Long-term function of the valves remains to be established.

Radiopaque polymeric materials for medical applications. Current aspects of biomaterial research

The aim of this review is to give an overview and some insight into different radiopaque polymeric materials that are currently used as medical implants or inserts. The advantages and limitations of each radiopaque polymeric material are summarized. The main method used to make medical implants radiologically visible is based on blending polymers with conventional radiopaque agents, blends which usually are a physical mixture of acrylic derivatives and inorganic salts. Other methods reported involve either the formation of single-phase radiopaque polymer salt complexes somehow preventing the release of the radiopacifying element by entrapment of the complex in a crosslinked network, or radiopaque polymerized monomers characterized by a radiopacifying element associated with the monomer unit prior to polymerization. In the near future, research will certainly concentrate on biocompatible radiopaque polymers with covalently bound opaque elements leading to stable polymers with properties equivalent to the nonopaque, parent polymer.

Development of an intraluminal device for the treatment of aortic regurgitation: prototype and in vitro testing system

OBJECTIVES

Development of an intraluminal device to reduce aortic regurgitation could provide a strategy intermediate between medical treatment and aortic valve replacement. An initial prototype and a testing system have been designed.

METHODS

Aortic valves obtained from heart transplant recipients were explanted and assessed in a mock circulatory loop with resistive and capacitive elements, including pressure-flow characteristics, similar to those of a normal arterial system. Normal heart function was simulated by a pulsatile ventricular-assist device. Pressure on each side of the valve and flow through the valve were recorded, allowing the calculation of regurgitant fractions and transvalvular gradients. Six solid geometrically differing obturators were tested.

RESULTS

All six designs resulted in significant reduction in aortic regurgitation (p < 0.0001), ranging from 15% to 38% improvement compared with control values. Small increases in transvalvular gradients (from 0 to 7 mm Hg) were also noted.

DISCUSSION

Initial results suggest that an intraluminally placed obturator can reduce aortic regurgitation without creating clinically significant transvalvular gradients. These initial in vitro experiments demonstrate the feasibility of an intraluminal device for the treatment of aortic valve disease, and further investigation is warranted.

Monodisk: device for percutaneous transcatheter closure of cardiac septal defects

The Monodisk, a device to facilitate percutaneous closure of cardiac septal defects, was developed and tested in vitro and in vivo. Atrial septal defects (8-10 mm) were created in five dogs using transcatheter techniques. The defects were then closed with the Monodisk. The device was easily and successfully fixed in place in all dogs. The efficacy of the device was determined radiographically. No shunting of contrast medium was observed in any case. Animals were followed for 6 months. Results showed good biocompatibility and several advantages of the device: it requires only a simple one-step placement procedure; it utilizes a small delivery system; it is easy to place and detach; it is self-centering and stable; and it can be repositioned or retrieved prior to detachment. Its possible clinical applications include correction of atrial septal defects, ventricular septal defects, patent ductus arteriosus, as well as creating an aortopulmonary window.