Eccentric Device Design

Amplatzer devices designed for closing secundum atrial septal defects, patent ductus arteriosus, and muscular ventricular septal defects are concentrically symmetrical because there are no valves near the edges of the defects they are designed to close. It is noteworthy that perimembranous ventricular septal defects are different in an important way: the aortic and tricuspid valves are close to the defect margins. Previous attempts to close perimembranous ventricular septal defects with the Clamshell and Sideris button devices have been less than optimal. For example, distortion of the aortic valve resulted in aortic insufficiency, and in some cases, the devices embolized (21).

With these challenges in mind, it was considered that the flexibility of shaping the Amplatzer device frame could produce an eccentric, asymmetric device. Subsequently, an Amplatzer Perimembranous Ventricular Septal Occluder (Fig. 4) was designed with a minimal rim of the left ventricular

Left Ventricular Gram

Fig. 4. Amplatzer Perimembranous Ventricular Septal Occluder device. (A) Photograph of the perimembranous device showing the delivery cable attached to the right ventricular disk. The asymmetric left ventricular disk is positioned with the minimal rim of the subaortic portion at the top of the device, thus preventing interference with the aortic valve. (B) Left ventriculogram after device placement. The asymmetric left ventricular disk avoids distortion of the aortic valve. There is no flow through the device immediately after deployment.

Fig. 4. Amplatzer Perimembranous Ventricular Septal Occluder device. (A) Photograph of the perimembranous device showing the delivery cable attached to the right ventricular disk. The asymmetric left ventricular disk is positioned with the minimal rim of the subaortic portion at the top of the device, thus preventing interference with the aortic valve. (B) Left ventriculogram after device placement. The asymmetric left ventricular disk avoids distortion of the aortic valve. There is no flow through the device immediately after deployment.

disk (0.5 mm) to sit beneath the aortic valve, whereas a longer (5.5 mm) inferior left ventricular disk with a short waist (1.5 mm) was designed to keep the right ventricular disk away from the tricuspid valve. Recent animal trials have shown that an eccentric design protected the aortic and tricuspid valves, but at the same time allowed closure of perimembranous ventricular septal defects.

It is noteworthy that an initial difficulty in deploying these eccentric devices was in the reliability of delivering the device in the proper (optimal) orientation. For example, advancing a pigtail catheter from the pulmonary artery through a patent ductus often resulted in the curl of the catheter oriented along the lesser curvature of the aorta. Subsequently, a sharply curved delivery sheath was designed to deliver the device to the left ventricular apex, mimicking this property. Yet, simply advancing the asymmetric device through this sheath did not always result in proper device orientation.

Hence, a sharply curved delivery catheter was designed that forced attachment of the device with the longer left ventricular disk along the lesser curvature of the catheter (Fig. 5).

When this catheter design was used in combination with the sharply curved delivery sheath positioned in the left ventricular apex, the device was easily advanced to the tip of the delivery sheath to assume proper orientation (22). This was confirmed in human trials; complete closure occurred in 96% of patients, and there were no serious complications, although the numbers were small (23).

The aortic disk of the concentric Amplatzer Ductal Occluder sometimes protrudes into the aortic lumen. This is because the ductus arteriosus forms a 65° angle with the descending aorta, and the concentric device has a 90° angle. An eccentric device was designed to allow the aortic disk to hug the aortic wall (Fig. 6), and a similar combination of a sharply curved delivery catheter and sheath resulted in proper orientation of the device (24).

6. DEVICES WITHOUT FABRIC

The polyester baffles and stuffing of Amplatzer devices sewn within the Nitinol wire frame are considered important for reliably producing thrombosis within the fabric spacing and occlusion of defects. However, sewing the material into the frames is time consuming and costly and limits automation of production. Thus, eliminating fabric could greatly simplify production and might even reduce the size of delivery systems. The initial attempt at a fabric-free device was simply to increase the wire count. Standard Amplatzer devices have a 72-wire Nitinol frame. An angled Ductal Occluder Device was developed with 144 wires. In an animal model, this resulted in complete occlusion of an artificially created patent ductus arteri osus (25). This simple design modification also allowed placement through a 6-French guiding catheter. Yet, human use has revealed the potential for recanalization with this design (26).

Hence, it was considered that a more effective solution would be to place wire mesh inserts within the frame of the device (Fig. 6). Initial experimental trials with the newest design placed in both a patent ductus arteriosus and atrial septal defect suggested that this may again reliably produce occlusion of such defects, but could greatly simplify device production. It remains to be determined how much this will reduce delivery system size.

7. SUMMARY

The Amplatzer family of devices is an interesting case study in the development of minimally invasive cardiac devices. They provide a successful means for transcatheter closures of congenital cardiovascular abnormalities. The simple design of such devices has allowed easy modification for numerous different types of abnormal communications. Unique characteristics of this family of devices include ease of delivery, small delivery systems, retrievability, safety, and effectiveness. To the credit of their inventor (Kurt Amplatz, MD), these devices were initially designed for use in children despite a larger adult market.

Over the past 25 yr, there have been several changes in the therapy of children with congenital heart disease. Noninvasive echocardiographic diagnosis of congenital heart disease was the first significant change to reduce the number of cardiac catheterizations. Balloon dilation of congenital narrowing of valves and arteries was the next big change in management.

Medical Device Design Evolution
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