Power, phaco time reduced in second half of surgery

Steven H. Dewey, MD, described his observation at the annual meeting of the American Society of Cataract and Refractive Surgery.

"The vacuum rise time is the most important factor distinguishing the two," said Dr. Dewey, in private practice in Colorado Springs, CO. "While the magnitude of vacuum may be ultimately the same, it is this acceleration that appears to be the key difference.

Dr. Dewey studied the efficiency of the two vacuum systems using the phaco system. He explained that the single fluidics cassette used for every case has the ability to switch between peristaltic and venturi vacuum systems with only the touch of a button.

In his study, he performed a clean hemisection using a horizontal chop. In half of the cases, he used peristaltic vacuum first, then switched to venturi. He reversed the order for the other half of the cases. He then reviewed all of the surgeries to determine if a clean chop had been performed and the appropriate vacuum had been used.

Although the cataracts included in the study were dense, he said, a limitation of the study was randomization because the pumps make a different sound depending on which system is activated, preventing him from being masked to the pump in use. He also said that a true hemisection is difficult to perform because the nucleus usually breaks along the Y suture. This resulted in many cases having an asymmetric cleavage or even trisection instead. In gathering the data, he also forgot in many cases to have the vacuum mode changed.

"There is no automation that tells us we're halfway through, and many times I simply got caught up in the efficiency of the surgery," he said.

He collected the data from the video overlay (Surgical Media Center, AMO). This system allows for recording of the video feed directly into a laptop computer. The data from the surgical case also are recorded, and a summary can be obtained at any time point in the case.

Dr. Dewey used identical bent 20-gauge, 30° bevel phaco tips (Dewey Radius Tips, MicroSurgical Technology) for each case.

He set the venturi vacuum at 275 mm Hg with 100% rise. Peristaltic vacuum was set using proprietary technology (Fusion Fluidics), with a maximum vacuum at 350 mm Hg, dropping to 275 mm Hg with occlusion. Rise time for the peristaltic system was at 75%. Bottle height was 100 cm for each.

"These were the values we felt were comparable," when each vacuum system was calibrated independently, Dr. Dewey said. "I was really looking for good followability and good chamber stability across all grades of nuclei."

Flow was a slightly different issue. Peristaltic vacuum flow was set at 32 ml/minute.

"One of the real differences between the two vacuum systems is the inability to control aspiration, or flow, with venturi vacuum," he said. "This typically results in a higher flow rate with venturi than with a peristaltic system."

He pointed out that in his experience, a 19-gauge needle is unnecessary with current phaco technology, and created a bit more chamber instability than a smaller-gauge needle.

"As there is more flow with venturi, it becomes important to orient the irrigation sleeve even more precisely to avoid directing the irrigation toward the corneal endothelium," he said. "In addition, having a smaller-gauge needle acts as a flow restrictor for the case and improves chamber stability."

During the cases, Dr. Dewey did not select different vacuums based on the different cataract densities. He showed a video of a case with a clean hemisection. Using the venturi system on the aspiration graph, no measurable aspiration was seen, despite the presence of substantial aspiration.

"Using the peristaltic vacuum, there is control over the aspiration rate," he said.

In a video demonstration of a very dense cataract, he showed how the venturi system was more efficient, but really in the aggregate, only in the second half of the surgery.