Article Date: 4/1/2009

Aspiration Flow Control: A "Speed Bump" in Vitrectomy?

Aspiration Flow Control: A "Speed Bump" in Vitrectomy?

Learn how this added control enhances safety and how these surgeons expect to use it.

Dr. Packo: Cataract surgeons rely on flow control as they bring nucleus material to the port, but flow control is a new concept for retina surgeons. Do you think we need flow control in retina surgery?

SAFETY MEASURE

Dr. Dugel: In my opinion, aspiration flow control is a major advancement. I look upon it as a speed bump or safety device in my vitrectomy. If you apply constant vacuum to tissue that has a constant viscosity, you will have constant flow. In that sense, you can control vacuum, and the aspiration flow control may not be necessary, but that is not how we operate inside the eye. We move from vitreous to water to fibrous tissue to nucleus material, so the viscosity of the fluid is changing all the time. Imagine a port that is blocked with fibrous tissue or nucleus material, and there is no flow. Then, as the tissue deforms and goes into the port, suddenly there is a surge of flow. This is what the cataract surgeons call an occlusion break,1,2 and I have experienced this, as well. When there is a sudden surge of flow, that is not a safe state. Aspiration flow control has an exquisite pump system that takes charge of the vacuum and equalizes the flow to avoid a surge. That is extremely important to me.

"If you apply constant vacuum to tissue that has a constant viscosity, you will have constant flow. … But that is not how we operate inside the eye. We move from vitreous to water to fibrous tissue to nucleus material, so the viscosity of the fluid is changing all the time."

Pravin U. Dugel, MD

FLOW LIMIT PARAMETERS

Dr. Packo: Anyone who has had to hit the reflux button on the pedal or had a retinal incarceration in a sclerotomy would have benefited from flow control. With this technology, what flow limit should we set when we are nibbling membranes from the retinal surface?

Dr. Dugel: Quite honestly, I do not know. We have not gained the broad experience yet to comment on exactly where the setting is. Surgeons have been "programmed" to use specific settings, but I think soon our settings will be situational, depending on whether we go from nucleus material to detached retina and so forth.

Dr. Packo: We now can control many parameters that we could not control before. Over the next few years, surgeons will determine which of these parameters they really need to control. That may vary from surgeon to surgeon.

Dr. Dugel: Aspiration flow control may become important when we do multigauge surgery. If you are also doing a lensectomy and must enlarge the port, for instance, surge protection certainly would help, particularly with the IOP control.

DEFINING MODES

Dr. Murray: I think we will simplify these settings by defining different modes of surgery, such as core, vitreous hemorrhage, tight tissue and retina surface. We will incorporate the parameters in a targeted way, based on what we are going to do and allow the machine to select parameters that we have predefined.

Dr. Pollack: Although the Constellation Vision System enables us to control more functions, I think the utility of the system may be that it will simplify our surgical experience by eliminating the need to adjust multiple variables as surgical conditions change. While we will retain the ability to enter different mode settings, I suspect this may no longer be necessary.

Reflux, Diathermy, Auto Gas Fill
Dr. Packo: Push-prime technology, which enables us to drive fluid into the vitrectomy machine, also gives us an extension of what we do with reflux in the eye. Have you had experience in using the Constellation reflux function?
Dr. Murray: From my perspective, handling fluidics is critical, and being able to reflux is causing me to rethink that process. Previously, with reflux, when I engaged the wrong tissue, I hoped the tissue would disengage and if not, I would be tugging away. With the new technology, I feel more comfortable that I will be able to control tissue in a significantly better way.
Another unique feature is that the bottle cannot run dry without your knowing it. Of course, that should never happen, but unfortunately, I think many of us have experienced that. So it is important for me to be able to stabilize the IOP, to know that I will not lose fluid flow during surgery without notification, and that I can control the influx and egress of fluid through reflux.
Dr. Packo: Previously, I never toggled between pushing fluid into the eye and aspirating. Stirring up blood on the retinal surface and then aspirating it away is an example of where this may help. Have you had occasion to try that, Dr. Dugel?
Dr. Dugel: There are two different functions, pulse (micro) reflux and proportional reflux. We are accustomed to pulse, and we use that frequently. We are still learning about proportional reflux, but I think it will be advantageous. Instead of drawing blood off the retinal surface, we can "windshield wipe" it proportionally.
Several of this machine's functions, such as reflux and diathermy, are proportional, which gives the surgeon more control. I rarely use diathermy, but when I do use it, I want it to be proportional diathermy and at a lower frequency to confine my treatment.
Dr. Ho: Proportional diathermy makes sense. It will allow for greater efficacy in diathermizing retinal breaks and creating seals, for example.
Dr. Packo: Auto gas fill is another feature that I think is more than just user-friendly. Sterile transfer of the gas and accuracy in the gas collection has been cumber-some and often confusing to staff. How has this been addressed in the new technology?
Dr. Dugel: It is a matter of safety. It is nice to have an entirely sterile situation and be able to fill the syringe and not lose any gas. But imagine being in an unfamiliar OR with a circulator nurse who is not familiar with your surgery, and relying on her to dilute your gases or get the gases in a sterile manner. There is a dimension of safety that this affords.
Dr. Packo: It is also nice to be able to know you have gas in the tank, as opposed to finding out someone accidentally left it on overnight. This transfer system makes that scenario basically impossible. It also provides a cost benefit. We have to purge the current system to remove the air space and dilute it with pure gas before we prepare to mix it. I have seen surgeons who are accustomed to running cc after cc through the syringe, and in fact, one purge gets you close to 99% of what you want to achieve. The cost savings is not insignificant. It makes a difference, particularly with the more expensive C3F8, of going from a $10 per unit cost to the patient, to as low as about $1.50 based on how much you are purging. At the end of the day, those dollars add up.

To attain high flow rates with the previous vitrectomy system, cut rate had to be decreased as vacuum was increased. Consequently, many surgeons use one combination of variable vacuum and cut rate settings to perform relatively high-flow core vitrectomy, and a second set of dual proportional settings to perform low-flow vitrectomy over detached retina. What is beneficial is that we no longer need to decrease cut rate — and the margin of safety that higher cut rates impart — to attain higher flow rates. With Constellation's duty cycle set to 50% open, we can maintain 5000 cpm if the flow is 1 cc/min. or 10 cc/min. Because we no longer need to decrease cut rate to achieve high flow rates, it might be safest to maintain a constant 5000 cpm. Because 50% duty cycle provides a great range of available flow rates, it may be reasonable to keep this constant too. This means that with a single set of settings and linear control of flow with the vitrectomy pedal, we can safely manage a range of conditions such as mobile retina, dense blood and thick post-traumatic fibrous membranes.

Dr. Packo: Lensectomy is a great example of major differences in tissue deformability and how this machine will help us with surge control. We may need to dial that back on a more micro level. Just the difference between vitreous and a diabetic patient's membrane may be enough for us to pay attention to surge control and variability. RP

References
  1. Zacharias J, Zacharias S. Volume-based characterization of postocclusion surge. J Cataract Refract Surg. 2005;31:1976-1982.
  2. Georgescu D, Payne M, Olson RJ. Objective measurement of postocclusion surge during phacoemulsification in human eye-bank eyes. Am J Ophthalmol. 2007;143:437-440.


Retinal Physician, Issue: April 2009