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At Issue: venturi vs. peristaltic pumps
At Issue posed the following questions to a panel of experts: "Some new phacoemulsification systems offer both venturi and peristaltic pump effects. What is the advantage of having both types? Which pump do you use for routine cataract surgery, and why? When would the other come in handy?"
Greater safety, more efficiency
Priscilla E. Perry Arnold, MD: The most common pump system used for phacoemulsification surgery is the peristaltic pump, and that is the system with which 1 have the most experience. The advantage for this pump type is that rise time for vacuum-building is more gradual and controlled so that there can be a greater measure of safety in some situations. The disadvantage is that it may require more time and manipulation to perform the case, and that nuclear fragments are not captured, held and evacuated as quickly.
The venturi pump, in contrast, has a more rapid rise to maximum pressure. This can be desirable for more efficient evacuation of the nuclear material. The disadvantage is that in a situation of more rapid change of vacuum, surgical complications such as posterior capsule capture and rupture might be more likely. The venturi pump type is generally understood to be more preferable for vitrectomy surgery.
Newer phacoemulsification units have combined a capability for more rapid vacuum rise with options for fine pressure control typical of peristaltic pump dynamics, allowing surgeons a range of options for each procedure.
The machine I currently use, the Advanced Medical Optics Sovereign with WhiteStar technology has outstanding pump control. This is typical of current systems. Along with improvements in tubing and tip design, and surgical techniques such as bimanual phaco, surgeons can safely use much greater levels of vacuum than was ever possible previously for cataract surgery. The ultimate benefit for the patient is more efficient surgery using less intraocular energy.
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Peristaltic for safety
Steven A. Arshinoff, MD, FRCSC: There are two basic vacuum pump types, the peristaltic (or scroll), which is flow dependent, and the venturi (diaphragmatic, rotary vane or scroll), which is vacuum dependent. The fundamental difference between the two pump types has to do with which factor, flow rate or vacuum, is independently set, and which is then dependent, and not independently adjustable.
Flow equations are all variations of Ohm’s equation, first proposed to describe the flow of electricity: E = IR, where E = force (voltage in electricity, vacuum in phaco machines); I = flow (current for electricity, flow rate for phaco machines); and R = resistance (of the circuit in electricity, of the aspirate in phaco machines).
To make things more complicated, in real phaco machines:
E = vacuum + bottle height (total force = pull + push); I = aspiration flow rate + wound leakage (total outflow from the system) and R = viscosity of the aspirate/aspiration port area.
So, for peristaltic pumps, the desired flow rate is set, and we have no control over the viscosity of the aspirate, as the tube length, port area, etc., are fixed. Therefore, vacuum developed by the machine is proportional to the viscosity of the aspirate (E = f [R]). As a result, when a lens fragment is aspirated, immediately upon its aspiration, it is followed by aqueous with a dramatic drop in viscosity, and the vacuum falls immediately almost to zero, assuring a stable anterior chamber and making the machine very safe for cataract surgery.
On the other hand, for venturi machines, the vacuum is set, making the flow rate inversely proportional to the resistance of the aspirate (I = f [1/R]). In this case, once a lens piece is aspirated, if the vacuum is not rapidly released by the surgeon, the flow rate can climb rapidly almost to infinity. Venturi machines can therefore generate more “holding power,” but they are inherently unsafe by design, using much more fluid per case. For these machines, surgeons often hang the irrigation bottle on the ceiling to compensate for unexpected posterior capsule trampolining observed with occlusion breaks.
Now in reality, modern phaco machines are designed with electronic sensors to allow users of peristaltic machines to generate some fixed vacuum, thus more pulling power, and to drop the vacuum of venturi machines as soon as occlusion breaks occur, making venturi machines safer.
The bottom line is that I have a tremendous preference for the peristaltic machines, due to their inherent safety, and other design features that I cannot go into in this limited space. This strong preference extends to all cataract cases.
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Venturi for efficiency
I. Howard Fine, MD: A venturi pump is an active pump that behaves very much like a vacuum cleaner and can attract material remote from its tip, having vacuum without the tip being occluded. A peristaltic pump, on the other hand, requires tip occlusion for there to be active vacuum, and as a result it does not draw material as easily and quickly to the tip. Therein lies the advantage in the eyes of some surgeons; it appears to be safer, even though it is slower.
The advantage of having two systems with that capability in one machine resides in the fact that at different portions of the procedure, you can use one rather than the other, depending on how close you are working to the capsule or to the iris.
However, the new fluidics available on most machines allow us to mimic either peristaltic or venturi systems, and the vacuum rise time in most peristaltic pumps is fast enough. The Bausch & Lomb Millennium, with a scroll pump, actually does mimic both a venturi, being vacuum based, and a peristaltic, being flow based.
In a peristaltic pump, one can independently set both aspiration flow rate and vacuum. In a venturi, one can adjust only vacuum, and the flow that accompanies that vacuum is obligatory or dependent upon the vacuum. Both systems have their devotees. Both systems work well.
Most people today use peristaltic pumps, but increasingly with the added safety of fluidics in most phacoemulsification systems, the difference between the two is not as obvious as was true previously. However, I would say that venturi systems certainly are a little more efficient for divide and conquer surgeons because, with grooving, the material can be cleared quite quickly.
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New fluidic responsiveness
Robert P. Lehmann, MD: My current cataract system of choice is the Alcon Infiniti. This system has effectively eliminated compliance, and therefore the fluidic responsiveness is unlike any other system I have experienced. Thus, during nucleus removal the responsive peristaltic action gives me improved ability to attract and hold material as it is emulsified. With safe access to higher vacuum, many procedures truly become phaco-assisted nucleus aspiration.
The Infiniti system maintains a rock-solid chamber and working environment and allows access to levels of high flow and vacuum that for the first time can now be used safely. I am even finding that one can use lower flow levels with higher vacuum settings using the unique dynamic rise capability. This offers me the ability to adjust the time it takes to reach my maximum preset vacuum level.
I now have the option to choose either linear control of aspiration and vacuum or linear control of both simultaneously. During cortical cleanup, I prefer this venturi-like response for rapid but safe removal and even capsule polishing.
My experience with AquaLase goes back three years, initially using a system that coupled with the Legacy. This unique liquefaction technology is now an integral part of the Infiniti system. It utilizes warmed, pulsed fluid delivered through a smooth polymer tip to remove lens material. It is remarkable to experience the improved capability of AquaLase for soft to moderate density cataract removal with the Infiniti System. I believe this technology will add another level of safety as regards elimination of thermal incision issues while maintaining corneal and capsular integrity.
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Venturi for flexibility
Louis D. Nichamin, MD: My preferred pump design continues to be that of a venturi design. I have not yet encountered a peristaltic system that can provide the speed and efficiency that I can achieve with venturi fluidics. This is particularly true in light of technique developments that have taken place in recent years. In my hands, nuclear disassembly by a chop technique is most efficiently achieved with this system. I also find that a venturi system provides the best fluidics for I&A since occlusion does not have to be as complete in order to sequester material and build vacuum. Finally, my practice encompasses a significant volume of vitreoretinal surgery, and, as such, the same pump and machine are used for both anterior and posterior procedures.
I, like most surgeons, trained on a peristaltic system and became very comfortable, if not reliant, upon its “feel” and slower rate of vacuum build. My initial transition to nonperistaltic surgery (an early diaphragm pump) was, admittedly, a bit tricky. Today, however, venturi systems are considerably more refined, and with the technique advances that most phaco surgeons now employ, this transition can take place much more easily and smoothly. I certainly would admit that today’s peristaltic systems are much more efficient and perform well with newer sensing controls and power modulation capabilities; higher flow rates are achievable without the dangerous surge problems that we once encountered. It is interesting to note that some of the new peristaltic systems are no longer being marketed from the standpoint of being safer than venturi pumps, but rather now capable of competing at the same flow rates and fluidic efficiency.
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Peristaltic for stability
Robert H. Osher, MD: Peristaltic pumps have an excellent track record in anterior segment surgery. Because the volume of the anterior and posterior chamber is small, flow in and out of the eye must be carefully controlled. Peristaltic pumps are flow-based and allow a vacuum limit to be set. Venturi pumps, on the other hand, are one-dimensional and allow the surgeon to control vacuum alone. Flow will be variable dependent on the level of applied vacuum.
I am a strong proponent of the peristaltic pump, which has reached new performance heights in the Alcon Infiniti. During the past several months, my chambers have been rock-stable, in part, as a result of the newer less compliant design of the fluidic system, which reduces surge by half and increases chamber stability significantly. A faster and more precise response decreases the time it takes to achieve the vacuum limit. This enables me to achieve occlusion sooner, yet using a flow with which I am comfortable. It also provides unique real-time pressure sensing on the irrigation side. Even low-end vacuum and flow control can be achieved, which is important to my technique of slow-motion phacoemulsification.
Other surgeons at the Cincinnati Eye Institute have been impressed with the versatility of the Infiniti, which even offers simultaneous linear vacuum and linear flow rate control. The system also provides “venturi emulation” with the responsiveness of venturi but with the ability to set the maximum aspiration flow rates. For the past decade, I have used the Alcon Legacy and have been satisfied with the safety and reliability of the peristaltic pump. I will continue this preference with the new Infiniti.
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