Nikolay V. Vasilyev, MD, and Pedro J. del Nido, MD, in the Department of Cardiac Surgery, Boston Children’s Hospital and Harvard Medical School, Boston, MA, are developing novel, prototype cardioports for beating heart, image-guided intracardiac surgery for pediatric patients. The successful implementation of such a device relies on minimizing the size of the cardioport, through which surgical tools are passed, while ensuring sufficient imagery from the cardioport. The researchers at Boston Children’s Hospital engaged Optikos Corporation, Wakefield, MA, to assist in the development and design of new cardioport systems.
Current devices incorporate a 4 mm diameter endoscope with a 70° field of view and a prototype plastic viewing window or bulb to monitor the location and deployment of surgical tools. The cardioport is inserted into the chest cavity through an incision in the chest then into the heart chambers. While in the heart chambers, the cardioport bulb is pressed against the heart’s inner wall. The transparent bulb is needed to displace a volume of blood to overcome the absorption and scatter of light and to be able to image a specific field of view of the heart wall surface. A channel through the bulb allows for surgical tools to be deployed through the port and inserted into the heart. It is of critical importance that the surgeon can see when the deployed tool reaches the heart wall.
The cardioport bulb design consists of a hollow “bulletshaped” transparent plastic shell and a second transparent plastic cylinder the same length of the bulb that intersects the distal end of the bulb to seal off the imaging portion from the tool channel and the blood and media in the chest cavity. (See Figure 1)
A Change in Design Improves Function
Optikos was engaged to revisit the bulb design, conduct optical modeling of the bulb, suggest design changes, source the new bulb, and assemble and test new cardioport prototypes. The initial bulb design introduced blind spots caused by index mismatches between highly curved sections of the plastic bulb and injected saline and native blood within the heart. It was a primary goal of the redesign activity to eliminate these blind spots so that any tools could be continuously visualized as they emerged from the tool channel towards the heart wall.
The original optical bulb was modeled using optical design software to verify the source of the discontinuities and blind spots in the image. The complex shape of the bulb was difficult to model with traditional sequential optical surface modeling. Therefore, a non-sequential modeling method was used to evaluate changes in the 3D surfaces with the bulb. This modeling was able to determine that by altering the surface profile where the tool channel exit the bulb, the blind spots could be eliminated. A revised design was then created that included optimized free-form surface profiles. Small volume prototype molding sources were used to fabricate prototype bulbs.
The next round of device improvement being implemented will replace the imaging endoscope with a miniature camera embedded into the distal end of the cardioport. By converting the system from using a 4 mm endoscope to a CMOS-based camera system; the cardioport diameter will then be reduced from 18 mm to 15 mm. Choices of illumination can utilize lamp-based sources used for endoscopy or white light LEDbased illumination systems. LED systems are attractive because their footprint is small and the overall cost of the system can be much lower than lamp-based systems. Lamp sources are very attractive because they already exist in many, if not all, surgical suites. In both cases, the illumination will be transported to the distal end of the port using a fiber-based illumination scheme.
Aside from optical concerns it is important to mention that the cardioport tool channel must ensure that minimal blood travels up the tool channel and that no air bubbles are introduced by insertion of the tool. Based upon previous work between researchers at Boston Children’s Hospital and Massachusetts Institute of Technology, Optikos was able to redesign and source a tricuspid valve at the distal end of the cardioport to allow tools to enter, but not allow blood to travel back up the tool channel. Based on these successes, Optikos continues to help researchers at Boston Children’s Hospital develop even more compact cardioport and surgical imaging systems.
This article was written by David P. Biss, PhD, Principal Optical Engineer, Optikos Corporation, Wakefield, MA; Stephen D. Fantone, PhD, President/CEO, Optikos Corporation; Nikolay V. Vasilyev, MD, Staff Scientist, Instructor in Surgery, Department of Cardiac Surgery, Boston Children's Hospital, Harvard Medical School, Boston, MA; and Pedro J. del Nido, MD, William E. Ladd Professor and Chairman, Department of Cardiac Surgery, Boston Children's Hospital, Harvard Medical School, Boston, MA. For more information on Optikos, visit http://info.hotims.com/45607-172.