Aspiration Fluidics and Vitreous Cutting
What’s the role of fluid dynamics in vit-ret surgery?
STEVE CHARLES, MD, FACS, FICS
Pars plana vitrectomy requires both infusion and aspiration; there are many parallels between infusion and aspiration fluidics because the same physical principles influence them. Resistance to fluid flow is determined by the internal diameter of a lumen or port and the length of the tubing, cannula, or tool, as well as by the cutter port opening and closing, cyclically obstructing the port.
THE ROLE OF RESISTANCE
Fluidic resistance is proportional to the fourth power of the diameter (Hagen-Poiseuille equation) and is linearly related to the length. The impact of diameter is very significant because of the fourth-power relationship, and it is clinically relevant because of the transition in recent years from 20-gauge, which has a 0.89-mm outside diameter, to 23-gauge with a diameter of 0.75 mm, or 25-gauge, with a diameter of 0.5 mm.
The resistance of the cutter inner needle and the infusion port limit flow far more than the 84 in of connected tubing. Ohm’s law — E=IR, or voltage = current × resistance — is mathematically equivalent to Ohm’s law for fluid flow — pressure gradient = flow times resistance, or P = F × R.
|Steve Charles, MD, FACS, FICS, is clinical professor of ophthalmology at the University of Tennessee College of Medicine in Memphis. Dr. Charles reports significant financial interest in Alcon. He can be reached via e-mail at email@example.com.
Port-based flow limiting is a term I coined to encompass both the flow limiting resulting from smaller diameter cutters and that resulting from higher cutting rates. Higher cutting rates cyclically interrupt flow through the port, causing an increase in fluidic resistance.
High cutting rates and higher fluidic resistance at the port are beneficial for all cases and all tasks because they decrease pulsatile vitreoretinal traction on both detached and attached retina and reduce the risk of iatrogenic retinal breaks (Figure). I refer to the amount of fluid that passes through the port during an open-close cycle as “pulse flow.” High cutting rates produce low pulse flow with many small-volume pulses and with fewer remote effects — in other words, less pulsatile vitreoretinal traction.
PULSE FLOW AND CUT RATES
Small pulse flow does not give the vitreous enough time to accelerate and produce remote effects because of the relationship of force = mass × acceleration, or F=MA. Higher cutting rates do not cut collagen fibers better because the velocity of the cutter does not increase with higher cutting rates on pneumatic cutters.
Figure. High cutting rates are beneficial because they reduce iatrogenic retinal breaks.
In addition, port-based flow limiting decreases surge and therefore iatrogenic retinal breaks after sudden elastic deformation of dense epiretinal membrane or scar tissue through the port. High cutting rates, in addition to producing desirable port-based flow limiting, reduce the travel of uncut vitreous collagen fibers through the port.
Twenty-five-gauge provides more resistance than 23-g because fluidic resistance is proportional to the fourth power of the diameter. Surgeons often incorrectly believe that 25-g vitrectomy is “inefficient” or produces insufficient flow rates, when in fact it is safer because of less pulsatile vitreoretinal traction.
Port-based flow limiting relies on the same physical principles as high-vacuum, low-flow phaco, which is now the standard of care for phaco surgery. High-vacuum, low-flow phaco produces better anterior-chamber stability and decreased fluid surge after occlusion break, which is directly analogous to the advantages of port-based flow limiting for posterior vitrectomy, ie, less pulsatile vitreoretinal traction and less surge after dense tissue suddenly deforms through the port.
THE ROLE OF VITRECTOMY TECHNIQUE
Phaco technique is largely based on using aspiration to move lens material centrally away from the lens capsule to prevent capsular defects and vitreous loss. In marked contrast, the vitreous cutter port should be moved to the vitreous, rather than the vitreous pulled to the port due to excessive flow rates. Phaco surgeons performing pars plana vitrectomy must consciously focus on moving the port to the vitreous because their phaco experience teaches them the opposite approach.
Higher flow rates from larger-diameter cutters or greater duty cycles are not more or less efficient; efficiency is defined as the volume of vitreous removed per volume of infusion fluid. Similarly, efficiency is not a function of cutting rate; efficiency is entirely driven by technique and keeping the port constantly immersed in vitreous collagen and gel, instead of infusion fluid producing efficiency.
I refer to the optimal technique as continuous engage-and-advance vitrectomy. The current emphasis on efficiency and faster operating times can result in the unintended consequence of pulling the cutter back while aspirating, greatly increasing vitreoretinal traction.
Higher cutting rates are also safer because the collagen fibers travel a shorter distance before they are sheared, leading to a reduction in vitreoretinal traction.
PHYSICS OF VITRECTOMY
Vitreous is a very complex tissue with low homogeneity. The physical properties vary widely from patient to patient and disease to disease, and they change dramatically as the vitrectomy progresses. Vitreous hyaluronan acts as a non-Newtonian, pseudoplastic fluid similar to viscoelastic agents in the anterior chamber resisting deformation into the cutter port.
Early in the vitrectomy, surgeons often believe that “nothing is happening,” when in fact hyaluronan is being removed. They may react by unsafely increasing flow rates, often by decreasing the cutting rate or insisting that 25-g systems are ineffective.
Hyaluronan acts as a dampening agent, reducing vitreoretinal traction from pulsatile flow through the port. Hyaluronan is diluted as the vitrectomy progresses, decreasing the dampening effect.This is clearly an issue because vitreous cutting close to the retina is typically performed after the core vitrectomy.
Furthermore, infusion fluid changes the electrochemical properties of the vitreous, dramatically decreasing its viscosity. It is of interest that vitreous viscosity is reduced by a factor of five in minutes after removing it from the eye or enucleating an animal eye.
Pneumatic cutters are much lighter and more compact than electric cutters, improving dexterity based on the Weber Fechner law. Lighter cutters also decrease hand fatigue. Although this is commonly misunderstood, disposable tools actually reduce per-case costs because they eliminate cleaning, rinsing, drying, wrapping, sterilization, storage, replacement, and spare parts costs.
Cleanup of any tool with a lumen, which includes cutters, scissors, forceps, and cannulas, has the potential of creating inflammation similar to toxic anterior-segment syndrome (TASS), which is caused by biological materials from previous patients, enzymes used in ultrasonic cleaning, and water impurities from the autoclave.
In addition, vitreous cutters, scissors, and forceps, especially in smaller forms (23-, 25-, and 27-gauge) have fragile cutting and gripping surfaces, which the cleaning and sterilization process can easily damage.
I developed the InnoVit dual-actuation scheme to eliminate the spring used to open the port after the pressure pulse on the diaphragm closes the port. Elimination of the spring increased cutting rates, as well as cutter velocity, at the time of closure.
The InnoVit utilized a limited-angle rotary cutting scheme rather than an axial or guillotine cutting action.
The UltraVit on the Alcon Constellation system uses a diaphragm-based, dual-actuation, axial cutting design. Duty cycle is defined as the percentage of port open time vs total time. A lower duty cycle results in more port-based flow limiting and therefore fluidic stability and less pulsatile vitreoretinal traction. A higher duty cycle produces greater flow and more pulsatile vitreoretinal traction, suitable only for core vitrectomy.
The Alcon Constellation UltraVit, with which I am most familiar, cuts at 7,500 cuts/minute and has variable duty cycle control, enabling control of port-based flow limiting independent of cutting rates. Flow rates do not decrease with increased cutting when using the dual-actuation UltraVit; reduced flow rates at high cutting rates cannot be avoided with single-actuation cutters.
Vacuum response to a foot pedal command for decreased vacuum is far more important from a safety perspective than a command to increase vacuum. Many factors drive response time, including vacuum chamber size in the cassette, proportional valve(s), embedded controllers, and use of a real-time operating system.
The Constellation Vision System has two proportional vacuum valves and one proportional pressure valve controlling a very small aspiration chamber that a peristaltic pump empties continuously. The Constellation real-time digital flow control system utilizes sensing of the fluid level in the aspiration chamber, which in turn controls the peristaltic pump aspiration rate, providing flow sensing.
This technology produces rapid, nonpulsatile control, unlike the slower pulsatile flow control produced by a peristaltic pump system. Plus, it has a flow-limiting mode. These systems should be able to increase safety near the retinal surface, especially with mobile retina. The port-based flow limiting produced by higher, 7,500-cuts-per-minute cutting rates and smaller 23- and 25-g lumens, and now variable duty cycle control, is instantaneous, while console-based flow control must interact through the two-way pass of the fluidic signal through 84 in of compliant tubing.
Peristaltic pumps, which directly control flow, are not optimal for vitrectomy because transorifice pressure rises rapidly if dense ERM, scar tissue, or lens material occludes the port. The surge occurring when dense material suddenly goes through the port can cause retinal breaks.
In addition, peristaltic pumps inherently produce a low-frequency pulsatile vacuum as the rollers compress. They do not produce contact flow rates, as some surgeons incorrectly state. Peristaltic pumps, with their console-based flow control, must interact through two-way pass of the fluidic signal through 84 in of compliant tubing with very slow response time, driven automatically by sensors in the console or surgeon foot pedal command. RP
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