Fluidics and Cutter Dynamics
Physics matter in deciding on cut rates and duty cycles.
Steve Charles, MD, FACS, FICS
Although fluidics, cutting rates, and the merits of small-gauge surgery have been discussed in many forums, I am often surprised at what I hear leading surgeons state at meetings about these issues. For these reasons, I have attempted in this column to better explain the engineering principles underlying these issues and the clinical implications.
Infusion systems available prior to the development of the Accurus (Alcon, Inc., Ft. Worth, TX) were gravity-based, eg, IV poles. Gravity-based systems have three significant disadvantages: (1) There is no digital readout of infusion pressure; (2) Surgeons cannot mentally convert inches (or centimeters) of water to millimeters of mercury; and (3) IV poles cannot be controlled by the surgeon foot switch, and motorized IV poles are slower than vented gas-forced infusion (VGFI) and the more advanced Alcon Constellation Vision System pressurized infusion system.
The earlier gas-forced infusion (GFI) is better than gravity-based systems because air pressure infusion produces a sensor-based direct digital readout of infusion pressure (not IOP). The VGFI implemented on the Accurus was even better because it allowed a rapid decrease, as well as increase, of infusion pressure via surgeon-foot pedal command because of the console-controlled venting.
The Constellation Vision System has an IOP Compensation System, which compensates for pressure drops in the infusion tubing and cannula. This system determines the fluidic resistance of the infusion tubing and cannula during push-priming using a known pressure, verified by dual pressure sensors, and by measuring actual flow using an ultrasonic sensor. The system calculates Ohm's law and adjusts infusion pressure according to the sensed flow rate to produce the selected IOP during surgery (Figure 1).
Figure 1. Infusion systems help to control pressure between infusion and aspiration.
ASPIRATION FLUIDICS AND VITREOUS CUTTING
Pars plana vitrectomy requires both infusion and aspiration; infusion and aspiration fluidics are influenced by the same physical principles. Resistance to flow is determined by the internal diameter of a lumen or port and the length of the tubing, cannula, or tool, as well as the cutter port opening and closing, cyclically obstructing the port.
Fluidic resistance is proportional to the fourth power of the diameter (the 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 from 20-gauge (0.89 mm) to 23-gauge (0.75 mm) or 25-gauge (0.5 mm) instrumentation.
The resistance of the cutter inner needle and of the infusion port limits flow far more than the 84 inches of connected tubing. Ohm's law — Voltage = Current x Resistance (E=IR) — is mathematically equivalent to Ohm's law for fluid flow — Pressure gradient = Flow x Resistance.
“Port-based flow limiting” is a term I coined to describe the flow limiting resulting from smaller diameter cutters, as well as caused by higher cutting rates and biased closed duty cycles. Higher cutting rates cyclically interrupt flow through the port, thereby increasing fluidic resistance when using single actuation pneumatic cutters.
High cutting rates and therefore higher fluidic resistance at the port is beneficial for all cases and all tasks because it increases fluidic stability, which in turn decreases pulsatile vitreoretinal traction on both detached and attached retina, thereby reducing the incidence of iatrogenic retinal breaks. I refer to the amount of fluid that passes through the port during an open-close cycle as “pulse flow.”
High cutting rates produce smaller pulses with much less pulsatile vitreoretinal traction than occurs using lower cutting rates. Small pulse flow means that the vitreous doesn't have time to accelerate and produce remote effects because of the Force = Mass x Acceleration (f=ma) relationship (Figure 2).
Figure 2. High pulse flow leads to high retinal motion, while low pulse flow results in retinal stability.
Higher cutting rates do not cut collagen fibers better; this is because the velocity of the cutter does not increase with higher cutting rates on pneumatic cutters. In addition, port-based flow limiting decreases surge and therefore iatrogenic retinal breaks after sudden elastic deformation of dense epiretinal membrane, lens material, or scar tissue through the port.
High cutting rates, in addition to producing port-based flow limiting, reduce the travel of uncut vitreous collagen fibers through the port. Using 25-g instruments provides more resistance than 23-g because fluidic resistance is proportional to the fourth power of the diameter (Figure 3).
Figure 3. Using 25-g instruments (left) provides more resistance than using 23-g (right).
It has long been incorrectly taught that cutting rates should be reduced when removing dense ERMs, scar tissue, and lens material or when performing core vitrectomy; in fact, the highest possible cutting rate should be used for all tasks and all cases unless all of the vitreous has been removed, and very dense tissue must be addressed.
Some surgeons 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. Many surgeons in correctly believe that 20-g systems should used for straightforward and complex retinal detachments such as diabetic tractional retinal detachments, PVR, giant breaks, and complex trauma. In fact, 25-g systems are safer and more effective than 20-g or even 23-g systems for all cases.
Port-based flow limiting relies on the same physical principles as high-vacuum low-flow phaco, first implemented on the Alcon MicroFlare ABS and MicroTaper ABS phaco probes. High-vacuum low-flow phaco has become the standard of care because it produces better anterior-chamber stability and decreased fluid surge after occlusion, directly analogous to the advantages of port-based flow limiting for posterior vitrectomy.
VITRECTOMY VERSUS PHACO TECHNIQUE
Phaco technique is largely based on techniques to mobilize lens material 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 using excessive flow rates. Phaco surgeons doing 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 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, as keeping the port constantly immersed in vitreous produces efficiency.
I refer to the optimal technique as “continuous engage and advance vitrectomy.” Emphasis on OR efficiency and faster operating times can result in the unintended consequence of pulling the cutter back while aspirating, which greatly increases vitreoretinal traction (Figure 4).
Figure 4. With “engage and advance” vitrectomy, less traction can be achieved.
Vitreous is a highly complex tissue with marked inhomogeneity; the physical properties vary widely from patient to patient and from disease to disease, and they change dramatically as the vitrectomy progresses.
Vitreous hyaluronan acts as a non-Newtonian, pseudoplastic fluid similar to the viscoelastic agents (OVDs) used in cataract surgery. This semisolid property resists deformation into the cutter port.
Early in the vitrectomy, surgeons often believe that there is little or no flow, when in fact hyaluronan is being removed, and they may react by unsafely increasing flow rates, usually by decreasing the cutting rate. 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, which is an issue because vitrectomy near the vitreous base is per formed after core vitrectomy.
Furthermore, infusion fluid changes the electrochemical properties of the vitreous, dramatically decreasing viscosity; 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, which is a significant advantage because it improves dexterity and decreases hand fatigue. Disposable tools reduce per case costs because they eliminate cleaning, rinsing, drying, wrapping, sterilization, storage, replacement, and spare parts costs.
The cleaning of any tool with a lumen, including cutters, scissors, forceps, and cannulas, has the potential of creating TASS-like (toxic anterior segment syndrome) inflammation from biological materials from previous patients, enzymes used in ultrasonic cleaning, autoclave water impurities, etc. In addition, vitreous cutters, scissors and forceps, especially in smaller form factors (23-, 25-, and 27-gauge) have fragile cutting and gripping surfaces, which are easily damaged by cleaning and sterilization processes.
I invented the InnoVit dual actuation scheme to eliminate the spring used to open the port after the pressure pulse on the single actuation diaphragm actuator closed 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 (guillotine) cutting action. The UltraVit developed for 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. Lower duty cycle results in greater port-based flow limiting and, therefore, fluidic stability and less pulsatile vitreoretinal traction. Higher duty cycle produces greater flow and more pulsatile vitreoretinal traction, suitable only for core vitrectomy.
The Alcon Constellation Ultravit currently cuts at 5,000 cuts/minute and has variable duty cycle control, enabling control of port-based flow limiting independent of cutting rates. Higher cutting rates produce higher flow rates only with the Ultravit dual actuation scheme because pulse flow remains constant.
Rapid response to a foot pedal command to decrease vacuum is far more important from a safety perspective than a command to increase vacuum. Response time is driven by many factors: the size of the vacuum chamber in the cassette, proportional valve(s) response time(s), embedded controllers, and the use of a deterministic, real-time operating system.
The Alcon Accurus had the first real-time operating system and distributed embedded processing, resulting in a 25-millisecond response time, compared to 10 times longer for the closest competitor. The Constellation Vision System has flow control using flow sensing and closed loop feedback control, and it is twice as fast as the Accurus. This technology produces rapid, non-pulsatile control, unlike the pulsatile, slow response time flow control produced by peristaltic pump systems.
In addition to flow control, there is a flow-limiting mode; these systems increase safety near the retinal surface, especially with mobile retina. Port-based flow limiting produced by high cutting rates (5,000), smaller lumens (23-, 25-, or 27-gauge), and variable duty cycle control is instantaneous, while console-based flow control must interact through two-way transmission of the fluidic signal and system response through 84 inches of compliant tubing.
The surgeon reaction time from seeing a retina approach the port to moving a foot pedal is about 400 milliseconds, because of visual and cognitive processing, generation of a motor response, propagation through the spinal cord and leg, and contraction of the lower leg muscles. The surgeon reaction time is more than an order of magnitude longer than the 25 milliseconds it takes an advanced venturi-based system, like the Constellation, to respond to a foot pedal command.
I have shown that approximately 30 times the volume of fluid contained between the tip and the port pass through the port in the time that occurs between the surgeon deciding to lift the pedal and the vacuum actually decreasing at the cutter port.
An understanding of infusion and aspiration fluidics, as well as cutter dynamics, is essential to a rational approach to machine selection, gauge selection, and machine parameters. Use of outdated machine parameters, 20-gauge technology, and even surgical techniques remain significant barriers to improved outcomes. RP