Mastering Phacoemulsification in Difficult Situations Tanuj Dada, Jagat Ram, Prema Padmanabhan, Haripriya Aravind
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Basic Principles of Phacoemulsification and Fluid Dynamics1

Uday Devgan,
Sourabh Patwardhan
Since the time of the inspiration of Charles Kelman in the dentist's chair (while having his teeth ultrasonically cleaned), incremental advances in phacoemulsification technology have produced ever-increasing benefits for patients with cataract. Though the central concept of using the ultrasonic energy to emulsify lens matter remains the same, the newer fluidics and mechanics has made the surgery much safer and controlled. The 5- minute procedure has years of work and innovations behind its success. Newer machines use software based controls to achieve excellent fluidics as per requirement of the surgeon according to the stage of surgery. Despite these new machines it is advisable for all the phaco surgeons to know the basic physics and controls of the machine. It is recommended the surgeon should at least be well versed with machine he or she is using.
When introduced in 1960s by Dr Kelman, the science of phacoemulsification was in infancy with number of drawbacks that one expects in any new technique initially, but with application of advanced bioengineering technique it has now become a state-of-art technique used as the first option for cataract extraction in many parts of the world. Initially Dr Kelman devised a method to prolapse the nucleus into the anterior chamber for emulsification. This technique required a generous anterior capsulotomy that he performed with a large cystotome in a “Christmas tree” or triangular fashion. The cystotome was then used to impale the nucleus and bring it forward to the anterior chamber in a tire iron maneuver under air. Once accomplished, the nucleus was emulsified in the chamber in a one-handed manner. Advent of CCC, viscoelastics and IOLs added significantly along with various chopping maneuvers and state-of-art fluidics and power delivery systems.
In this chapter, the phaco machines and phacodynamics is described so as to correlate various physics related aspects of the machine with the surgical maneuvers.
 
THE BASIC PHACO MACHINE
All phaco platforms share the same basic structure and concepts. The phaco machine aims to balance fluidics within the eye, while delivering ultrasonic energy and vacuum in order to emulsify and aspirate the cataract through a small incision.
The three main functions of the phaco machine are: (i) to provide irrigation into the eye, (ii) to create vacuum/aspiration to remove the cataract, and (iii) to deliver ultrasound energy in order to emulsify the nucleus. These three functions correspond to the three phaco foot-pedal positions.
The phaco foot pedal (Figure 1.1) is the primary instrument used to control the phaco machine during cataract surgery.
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Figure 1.1: Phaco foot pedal position and function
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This foot pedal traditionally works by depressing it toward the floor with the dominant foot (the right foot for most surgeons). Each foot pedal position is additive to the previous positions, so that while the pedal is in position 2 (vacuum/aspiration) it is also providing the full function of position 1 (irrigation). Similarly, once the pedal is in foot position 3 (ultrasound energy), it is also providing the function of position 2 (vacuum/aspiration), as well as position 1 (irrigation).
 
FOOT POSITION 1: IRRIGATION (FIGURE 1.2)
It is important to realize that during phacoemulsification, we are working in a very small space of the anterior and posterior chambers, comprising well under one cubic centimeter of space together. During the surgery we must always maintain the stability and structure within the eye, particularly to prevent collapse of the anterior and posterior chambers which can lead to severe complications.
The irrigation function of the phaco machine is meant to provide a source of fluid infusion into the eye during the surgery. By depressing the foot pedal to position 1, the infusion is turned on. There is no linear control of the infusion—the infusion is either turned on or turned off.
The height of the infusion bottle determines the relative infusion pressure and flow rate during the surgery. To keep the eye inflated during surgery, we need to make sure that the fluid inflow rate is greater than the fluid outflow rate.
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Figure 1.2: Irrigation of fluid into the eye is the function of phaco foot position 1
 
FOOT POSITION 2: VACUUM/ASPIRATION OF FLUID (FIGURE 1.3)
Phaco foot position 2 is the control of the relative aspiration and vacuum level of the fluid from the eye. There is a linear control of vacuum and flow, so that the top of foot position 2 provides less vacuum or flow than the middle or bottom range of the same foot position 2. This is similar to the gas pedal in a car, where the car's throttle is opened more as the gas pedal is further depressed.
To create the vacuum and the aspiration flow of fluid, the phaco machine must have a fluid pump. The most common types of fluid pumps are peristaltic and venturi.
The vacuum and aspiration levels that are created draw the fluid out of the eye and into a waste fluid collection via the outflow tubing. The regulation of vacuum and aspiration is controlled by the foot pedal, with more depression of the pedal resulting in higher levels. There are two primary sources of fluid outflow during phacoemulsification: the outflow from the phaco probe created by the fluid pump, and the leakage of fluid from the incisions.
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Figure 1.3: Vacuum and aspiration of fluid from the eye is the function of phaco foot position 2
 
Irrigation-aspiration Handpiece
The irrigation-aspiration (I-A) handpiece (Figure 1.4) has a silicone sleeve that fits snuggly around the aspirating tip. Irrigation is delivered through the openings of the sleeve. The tip differs from the phaco tip in being smooth and rounded with a single aspiration port on the side of the tip and not at the end. Its size is also comparatively smaller than phaco tip. The sleeve may be turned to orient the irrigation port in any direction.3
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Figure 1.4: Different I-A tips
The irrigation ports in the silicone sleeve should be kept perpendicular to the metallic aspiration port as this helps direct the infusion fluid along the iris plane. This reduces iris flutter during the surgery.
A variety of I-A tips are available: straight, 30°, 45° or 90° angulation; 0.2 mm, 0.3 mm, 0.5 and 0.7 mm lumen diameters. Most frequently used is the 0.3 mm tip. During use for irrigation-aspiration, the foot-pedal is on position 2.
 
FOOT POSITION 3: ULTRASOUND ENERGY (FIGURE 1.5)
The bottom-most position of the foot pedal is position 3, which controls the delivery of ultrasound energy into the cataract. There is linear control of the ultrasound energy level so that further pedal depression results in more ultrasound energy, such as would be needed for a denser cataract.
Note that if the pedal is in position 3, we are already engaging the full function of both positions 1 and 2. The irrigation is on, and the vacuum and aspiration level is at its highest preset level. Ultrasound energy should only be applied once the tip of the phaco probe is in contact with part of the cataract.
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Figure 1.5: Foot pedal position 3 controls the delivery of the ultrasound energy into the eye
When we look at the phaco probe closely, we see that there are three lines attached: (i) the infusion tubing carrying fluid into the eye, (ii) the outflow tubing that removes the fluid via flow that is created by the phaco machine's fluid pump, and (iii) the line that carries the electrical signals to control the ultrasound energy at the tip of the phaco probe. These three lines correspond to the three phaco foot pedal positions.
 
BASIC CONCEPTS OF FLUIDICS
Due to the small volume of the anterior and posterior chambers, the control of fluidics during phacoemulsification surgery is important to ensure efficient removal of the cataract while preventing complications due to tissue collapse.
The basic concept of fluidics is that the inflow of fluid must be greater than the outflow of fluid. By keeping a constant infusion pressure and limiting the outflow, we can ensure that the eye stays inflated and stable during surgery. If we allow the outflow to exceed the fluid inflow, even for just a fraction of a second, we experience surge within the eye and this can cause chamber instability, collapse of the eye, and aspiration of the posterior capsule. The primary rule for phaco fluidics is to keep the inflow greater than the outflow (Figure 1.6).
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Figure 1.6: Keep inflow greater than outflow to ensure stability of the eye during surgery
 
MODULATING PHACO FLUID FLOW: POISEUILLE'S EQUATION (FIGURE 1.7)
The basic equation that governs all fluid flow during phacoemulsification surgery is Poiseuille's equation:
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Figure 1.7: Poiseuille's equation shows that smaller bore tubing (A) requires higher vacuum and results in a lower flow, as compared to larger bore tubing (B) which can achieve a high flow with less vacuum required. The change in flow is exponentially related to the radius of the tubing
In this equation, F = flow, ΔP = pressure gradient, r = radius of the tube, η = viscosity of fluid, and L = length of the tube. We are concerned with the relative relationship and not the exact values; therefore, for simplicity we can simplify this formula. The viscosity of the fluid is relatively constant, as is the length of the tubing. And the values of π and 8 are constant. This leaves us with a simpler equation:
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Flow is proportional to the change in pressure times the radius of the tubing to the fourth power. Because the value for tubing size is exponential, a small change to the radius results in a large change in the relative flow. This is clearly illustrated in a common sense situation of drinking with straws.
 
MODULATING FLUID INFLOW (FIGURE 1.8)
The source of fluid inflow is the bottle of balanced salt solution that is hanging on the phaco machine. The two factors that determine the rate of inflow are: the change in pressure and the radius of the inflow tubing. The change in pressure can be modulated by raising or lowering the height of the bottle relative to the patient's eye: the higher the bottle, the higher the infusion pressure. The inflow tubing has a large radius in order to maximize the flow and make sure that we keep our inflow greater than the outflow. Similarly, the size of the infusion channel within the phaco probe (or other infusion instrument) is kept as large as possible so as not to cause a bottleneck effect.
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Figure 1.8: Fluid inflow can be modulated by changing the bottle height and therefore the pressure gradient, as well as changing the radius of the inflow tubing
 
MODULATING FLUID OUTFLOW (FIGURE 1.9)
For fluid outflow, there are two sources of fluid leaving the eye: (i) the fluid that is removed via the phaco probe as a result of the vacuum level generated by the fluid pump, and (ii) fluid leakage from the incisions.
The rate of the fluid outflow via the phaco needle is determined by the radius of the needle and tubing, as well as the change in pressure generated by the phaco machine's fluid pump. The rate of the fluid outflow loss via the incisions depends on their size and the relative fit of the instruments within these incisions.
Some degree of fluid leakage from the incisions is helpful to allow cooling of the phaco needle and to prevent thermal injury during surgery, particularly in early in the learning stages of phacoemulsification. With the use of advanced phaco power modulations, more experienced phaco surgeons tend to move toward tighter incisions which can give more stable fluidics.
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Figure 1.9: Fluid outflow rate as determined by Poiseuille's equation
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The composition, nature, and size of the inflow and outflow tubing are different, and this is important for safe and efficient phaco surgery.
 
FLOW BALANCE AND TUBING COMPLIANCE (FIGURE 1.10)
Surge is the situation when the outflow of fluid from the eye exceeds the inflow, even for just a fraction of a second. When this occurs, the chamber tends to collapse and the posterior capsule can be sucked into the phaco probe in an instant, resulting in a ruptured posterior capsule and vitreous loss.
In order to maintain this flow balance, where the inflow is always greater than the outflow, we can use different sized tubing. If we look at the inflow tubing we notice that it is significantly different than the outflow tubing.
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Figure 1.10: Comparison of inflow and outflow phaco tubing. Upper: Small, rigid outflow tubing. Lower: Large, flexible inflow tubing
 
INFLOW VS OUTFLOW TUBING
The inflow tubing is large bore with walls that are thin, and the tubing is very flexible. The purpose of this tubing is to provide a high flow of fluid under low pressure situations. The maximum pressure achieved within this inflow tubing is determined by the height of the infusion bottle, and this level is not very high.
The outflow tubing is smaller bore with thick walls, and the tubing is very rigid and relatively non-compliant. Because the flow varies exponentially with the radius of the tubing, the smaller bore outflow tubing can help ensure that the outflow is less than the inflow. The outflow tubing has rigid, thick walls in order to have a low compliance which helps prevent surge. The maximum pressure achieved within the outflow tubing is determined by the fluid pump of the phaco machine and can easily exceed 500 millimeters of mercury.
This high vacuum level can cause collapse of the outflow tubing if its walls are too thin and of high compliance (Figure 1.11). When the outflow tubing collapses, and then rebounds back to its normal state after the vacuum level drops, this energy release causes an immediate and dangerous surge of fluid out of the eye. This collapse of tubing due to high vacuum levels occurs most commonly during occlusion of the phaco probe; and then once the occlusion breaks, the tubing rebounds and the surge occurs. This is called post-occlusion surge and is one of the main causes of posterior capsule rupture during cataract surgery.
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Figure 1.11: Compliant tubing can collapse and cause surge during cataract surgery
 
PHACO NEEDLE SIZING (FIGURE 1.12)
The size of the phaco needle is important for phaco fluidics because it affects the outflow rate. The important thing to remember from Poiseuille's equation is that the flow is proportional to the radius of the tube to the fourth power. This means that a small change in the size of the phaco needle can result in a very large change in the flow. Comparing two common size phaco needles, 0.9 mm versus 1.1 mm, with all other factors equal it is surprising to see that the flow through the larger 1.1 mm needle is more than twice that of the 0.9 mm needle. As the needle size decreases, the flow drops exponentially.
If we switch from a 1.1 mm phaco needle to a 0.9 mm needle, with all other phaco parameters unchanged, the relative flow will decrease by more than half to 45% of the relative flow through the 1.1 mm needle. In order to achieve the same flow while decreasing the needle size, a very substantial increase in the pressure gradient is required.6
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Figure 1.12: Flow rate varies exponentially with the size of the phaco needle radius/diameter
Once we determine the proper tubing size and phaco needle size for our needs, we can then select the other parameters of the phaco machine. Remember that the tubing size and phaco needle size are definitely variables that play an important role in the fluidics.1,2
The primary variables that are adjustable on the phaco machine are bottle height, vacuum level, and aspiration flow rate. There is also a choice of fluid pumps: peristaltic and venturi.
 
PERISTALTIC AND VENTURI FLUID PUMPS
With modern day techniques of phaco-assisted aspiration of cataracts, one of the most important concepts in our phaco platforms is the creation of vacuum levels during surgery.
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Figure 1.13: The peristaltic pump uses rollers to compress the phaco outflow tubing in a peristaltic manner, thereby creating flow. The compression of the rollers on the tubing with the rotation of the pump physically moves fluid and creates a continuous “milking” action on the fluid column
The creation of fluid flow and vacuum levels within the phaco machine is required to provide the driving force for the outflow of the emulsified cataract and balanced salt solution during surgery.
The vacuum level is dynamic and changes during the various parts of surgery; lower vacuum levels are required during nucleus sculpting, whereas higher vacuum levels are needed for chopping and fragment removal. Our machines let us select a maximum vacuum level, typically specified in millimeters of mercury, and we can have relative control of the vacuum level up to this limit with our foot pedal position, depending on the type of fluid pump.
Two primary vacuum pumps are used in phacoemulsification platforms — peristaltic and venturi. Although they work in different ways, each has its advantages and disadvantages.
 
PERISTALTIC PUMP (FLOW BASED)
Peristaltic pump was popularized by the heart-lung machine. In these pumps, a pressure differential is created by compression of the aspiration tubing in a rotatory motion (Figure 1.13). When the rotational speed is low, vacuum develops only when the aspiration port is occluded. On occlusion, vacuum builds up to preset value in a stair-stepped pattern. By increasing the rotational speed, as in the newer generation machines, a linear build up of vacuum occurs even without occlusion of the tip. It can thus be made to simulate—a venturi or a diaphragmatic pump.
Systems with peristaltic pumps have two aspiration controls: aspiration flow and vacuum limit. The aspiration 7flow control determines the speed at which the pump turns; the faster the pump turns, the greater the resulting flow rate. By comparison, the vacuum limit is simply a safety setting that stops the pump when the vacuum reaches the set limit. Peristaltic systems can have either linear flow or linear vacuum (vacuum limit) modes. (The availability of these modes for each machine function [e.g. phacoemulsification, irrigation/aspiration, and vitrectomy] depends on the device manufacturer and model). In the linear flow mode, the flow rate is controlled by the foot pedal, and the vacuum limit is constant. This allows the surgeon to adjust the speed with which fluid and objects move toward the tip. In the linear vacuum mode (sometimes called the variable vacuum mode), the pump speed remains constant, but the vacuum level at which the pump shuts off varies depending on the depth to which the foot pedal is depressed (i.e. as the pedal is depressed further, the vacuum limit allowed before pump shutoff increases).
Advantages of a peristaltic pump are that vacuum build up occurs only on occlusion of the aspiration port and also the fluidics of the peristaltic pump are more controlled with little or no deflation of the anterior chamber on sudden removal of occlusion. Vacuum level and flow rate may be controlled independent of each other.
There is a large safety margin in this pump as it is slower in building up vacuum. The peristaltic system is a more forgiving system as there is no inadvertent pull on the ocular structure since vacuum builds up only on occlusion. Peristaltic pump allows both zero and high vacuum phaco.
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Figure 1.14: The vacuum level is created within a rigid drainage cassette, to which the phaco aspiration tubing is connected. Since there is no milking of the aspiration line, the phaco tubing can be made rigid with low compliance
But the vacuum build up is in a stair-stepped pattern and true linear aspiration is not seen, however newer pumps do simulate a linear build up of vacuum. Because of the stair-stepped pattern of the vacuum build up, there could be more pulsations in the anterior chamber. The vacuum build up is directly related to the density of occlusion which in turn would depend upon the bevel angle of the titanium tip and the tissue density.
The surgeon has to mechanically approach the nuclear or cortical matter to first achieve occlusion for vacuum to build up in order to aspirate the tissue. However, the pulse mode has significantly improved the followability of the tissue, even in the peristaltic pump.
 
VENTURI PUMP
A venturi pump uses compressed gas to create inverse pressure. Vacuum generated is related to gas flow which in turn is regulated by a valve (Figure 1.14). Vacuum build up occurs linearly in a consistent manner from zero to a preset value. The build up is almost instantaneous on pressing the foot-pedal. Due to this there is an increased risk of iris trauma and posterior capsular rents which make these pumps unsafe, particularly so for beginners. 8But there is a good followability of the tissue. Nuclear and cortical material can be attracted toward the probe on depressing the foot-pedal.
On systems using either a venturi or diaphragm pump, the only aspiration control is vacuum. This vacuum setting is the actual negative pressure applied to the collection container and aspiration tubing. For a given vacuum setting, the flow rate is determined by the dimensions of the tubing, fluid viscosity, and the degree of occlusion (i.e. typically, the flow rate will be proportional to the applied vacuum). These systems have only a linear (or variable) vacuum mode. In this mode, the applied vacuum is controlled by the foot pedal. With this type of pumping mechanism, adjusting the vacuum directly affects the flow rate.
Many surgeons prefer venturi pumps as the vacuum build up is quick and there is a good followability of the tissue. Nuclear and cortical material can be attracted toward the probe on depressing the foot-pedal.
But this pump has the least safety margin and is not forgiving to the surgeon.3 The rise time is too fast. Incidence of ocular tissue damage has been reported to be much higher with this pump as compared to the peristaltic pump.
 
Differences
Due to these differences, operating with these two fluid pumps is somewhat different. To create the preset maximum vacuum level with a peristaltic pump, there must be complete occlusion of the phaco needle with cataract material. To create the preset vacuum level with a venturi pump, the surgeon simply needs to depress the foot pedal. For surgeons doing split infusion bimanual cataract surgery, the peristaltic pump allows a maximum flow rate to be set so that the limited inflow from the smaller-bore infusion instruments is not outstripped. Currently in the United States, the most commonly used fluid pumps are of the peristaltic design (Table 1.1).
 
HYBRID PUMP
The primary example of the hybrid pump is the Sovereign peristaltic pump or the Concentrix pump (Bausch & Lomb Surgical). These pumps are interesting in that they are able to act as either a vacuum or flow pump dependent upon programming. They are the most recent supplement to pump types and are generally controlled by digital inputs, creating incredible flexibility and responsiveness.
 
HOW TO TACKLE SURGE?
Surge is sudden collapse of the anterior chamber after the occlusion is broken either due to aspiration of the lens matter or slip of matter from occlusion. The mechanism is shown in Figure 1.11. During occlusion maximum preset vacuum is achieved as pedal is depressed fully. Due to occlusion there is no active flow in the aspiration line. The pump keeps on trying to move fluid from system. Due to compliance of the vacuum tubes, there is partial collapse of these tubes with reduction in volume of fluid in tube. When the occlusion is broken there will be sudden aspiration of fluid from anterior which may not be compensable by irrigation channel leading due volume deficit in anterior chamber and collapse (See Figure 1.11). Additionally, there may be expansion of tubing if it is not properly reinforced which adds to the insult. This collapse can lead to damage to endothelium as well as posterior capsule making it most dreaded complication in fluidics.
Table 1.1   Different pumps
Peristaltic pump
Parameter
Occlusion
Function
Flow rate
Present
Changes rise time. More flow rate quicker rise
Absent
Followability
Vacuum
Present
Holding lens material
Absent
No effect directly. But preset vacuum determines maximum flow rate achieved. Higher vacuum preset more flow rate achievable
Vacuum pump
Vacuum
Present
Holds material
Absent
Followability
9
 
Principles of Avoiding Surge
  1. Prevent rapid vacuum rise by decreasing flow rate and decreasing preset vacuum level. It gives extra protection against surge by increasing time to achieve maximum vacuum and more safety margin of irrigation.
  2. Improve irrigation by increasing bottle height, using pressure irrigation or using sleeve with larger irrigation ports.4
  3. Prevent compromise of irrigation by tight wounds by proper wound size and use of microflow tips which have alternate grooves for the irrigation despite compression.
  4. Avoid leaky wound which adds to the irrigation requirement.
  5. Alternate flow during occlusion as in ABS (Aspiration bypass system) of Alcon which avoids a ‘No Flow’ situation even in maximum occlusion due to a small 0.18 mm opening on the side of tip. 5
  6. Tubings with lesser compliance avoids collapse during high vacuum (See Figure 1.13).
  7. Reducing vacuum just before occlusion breaks or immediately after it breaks. This can be done manually as experienced surgeon can judge the stage of occlusion break. Now newer software based microprocessor controlled fluidic systems can achieve the same by sampling the pressures of system rapidly and reducing the vacuum as the occlusion breaks and flow starts (Figure 1.15).
Auditory feedbacks: Various auditory feedbacks are also preset in the phaco machine. These are shown in Table 1.2. Auditory electronic sounds may be an additional feature of some machines, e.g. beeping is indicative of I/A mode.
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Figure 1.15: Surge control mechanisms at various levels
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Table 1.2   Different auditory feedbacks in the phaco machine
Sound
Mode
Cause
Metallic click
Entering irrigation
As the plunger which was pinching the irrigation tubing is snapped into an open position
Buzzing
Engaging power
Not ultrasound! Harmonic overtones of handpiece and needle
Humming
a. Tip occluded
b. Tip not occluded
c. Tip occluded but no aspiration
Machine venting to keep the vacuum
Obstruction in the aspiration line
Low preset vacuum: may need to be increased
User modification may be possible with regard to type and intensity of the sound.
 
OPTIMIZING PHACO FLUIDIC SETTINGS
The challenge of cataract surgery is in large part due to the small confines of the working space. The anterior and posterior chamber combined, typically comprise less than 1 cubic centimeter of space and provide very little room for error. The function of the phaco fluidics is to balance the inflow and outflow of fluid in order to maintain the working space, bring cataract material to the phaco tip, and prevent collapse of the eye. Optimizing the phaco fluidic settings is instrumental to the efficiency and safety of phacoemulsification surgery.
With a typical peristaltic phaco machine platform, the most common type in the US market, there are only a few parameters that are adjustable: the bottle height, the flow rate, the maximum vacuum level, and the phaco needle size.
Perhaps the most important parameter is the selection of phaco needle size. Recall that the difference in flow between a larger bore needle and a small bore needle varies exponentially due to Poiseuille's equation. In summary, the smaller bore phaco needles are suited for high-vacuum, low-flow fluidics, while the larger bore needles are better suited for high-flow, low-vacuum fluidics. The analogy of drinking a milkshake via a small bore cocktail straw versus a larger bore drinking straw works well to illustrate this point.
The flow rate for a peristaltic machine is typically given in cc of fluid per minute. This is determined by the rate at which the peristaltic rollers milk the fluid along the outflow tubing. With the phaco needle unobstructed the maximum flow rate is achieved and in large part, determines the speed at which things happen in the eye. Upon occlusion of the phaco needle with cataract material the flow rate declines and approaches zero.
The bottle height determines the inflow rate of fluid into the eye. Very much like a water-tower in a small town, the height of the fluid above the eye creates a forceful infusion of fluid via gravity: the higher the infusion bottle, the greater the inflow pressure and inflow rate.
With an unobstructed phaco needle, the flow rate is at the maximum, but the vacuum level is very low—very far from the maximum vacuum level that the surgeon has selected. The vacuum level in a peristaltic-based system is only achieved upon occlusion of the phaco tip. The higher the vacuum, the greater the holding power —and the holding power is used to fixate the cataract while we mechanically chop it. The effect of the vacuum level varies with the bore of the phaco needle due to the effect of surface area. The larger the cross-sectional surface area of the phaco needle, the greater the holding power given the same amount of vacuum. The vacuum level determines the “holding power” or “grip” of the phaco tip onto nuclear pieces.
 
OPTIMIZING YOUR SETTINGS
In order to optimize the phaco fluidic settings, it is important to match the parameters to the technique and the surgeon's preference.11
The first decision is the selection of phaco needle size, with the most common sizes being the smaller-bore 0.9 mm needle and the larger bore 1.1 mm needle size. If your preference is a quicker procedure with rapid nucleus removal, the larger 1.1 mm needle size is preferred since it will give a significantly greater flow rate. If your preference is a slower but more controlled procedure, then the smaller-bore 0.9 mm needle is more suited to your technique.
The bottle height determines the inflow of fluid into the eye. In order to help prevent surge, it is important to keep the inflow of fluid greater than the outflow of fluid at all times. The inflow of fluid comes from only one source, the bottle of balanced salt solution, while the outflow of fluid comes from two sources, the suction via the phaco needle and the leakage from the incisions. If, at any time, the outflow out-strips in the inflow, the eye will collapse and there is a high likelihood of posterior capsule rupture. It is often advantageous to start with a high bottle height to ensure a sufficient inflow of fluid, and then to taper it downward to minimize the posterior displacement of the lens-iris diaphragm due to the infusion pressure. If you sometimes notice corneal striae and anterior chamber instability during your surgery, you may benefit from increasing the bottle height.
For phaco chop, holding power of the nucleus is important in order to securely fixate it while using the chopper to mechanically disassemble the nucleus. This requires a relatively high vacuum, such as 200 to 250 mm Hg with the 1.1 mm needle, or 300 to 400 mm Hg with the 0.9 mm needle. Once the nucleus has been broken into smaller fragments, the speed at which the fragments are attracted to the phaco tip is determined by the peristaltic flow rate, with 20 cc/min being very slow and 50 cc/min being very fast. The same vacuum and flow rate settings can be used for the entire nucleus removal procedure during phaco chop.
For divide-and-conquer, there are two distinct parts of nucleus removal: sculpting of the nucleus and then quadrant removal, and different fluidic settings are required for each. For grooving and sculpting of the nucleus, the work is being done by the ultrasonic energy and thus the flow and vacuum settings are quite low— just enough to aspirate the nuclear material removed from each forward stroke of the phaco probe. A vacuum level of less than 100 mm Hg and a flow rate of less than 30 cc/min are sufficient for this purpose. For quadrant removal, a moderate amount of holding power is required to bring each quadrant into the phaco tip. Using a higher vacuum level of 200 to 300 mm Hg and a flow rate of 30 to 50 cc/min, depending on the needle size, is typically sufficient for this purpose.
With knowledge of the concepts behind the variables, it is easy to tailor the fluidic settings to the surgeon and technique. Understanding the concepts behind the phaco fluidic settings is instrumental in optimizing the parameters for increasing the efficiency and safety of your phaco technique.
 
FUNDAMENTALS OF ULTRASONIC PHACO POWER
 
MECHANISM OF ACTION OF PHACO6,7
The concept of phacoemulsification involves the use of a probe tip which vibrates rapidly to break up lens material into fragments. Fragments are aspirated from the eye via the center of this probe tip which is hollow. An outer sleeve provides for passage of infusion fluid. Fluid enters the eye via infusion ports in this outer sleeve. The infusion fluid constantly replaces any aspirate removed from the eye to maintain stable intraocular pressure.
Mechanisms responsible in breaking the lens material include:
  1. Cavitation: At the cessation of the forward stroke, the tip has imparted forward momentum to the fluid and the lens particles in front of it. On the tip being retreated, the fluid cannot follow thereby creating a void in front of the tip. The void is collapsed by the implosion (cavitation) of the tip thereby creating additional shock waves.
  2. Jackhammer effect: A mechanical impact of the tip against the lens.
  3. An acoustical wave: Transmitted through fluid in front of the tip
  4. There is an impact of fluid and lens particles being pushed forward in front of the tip.
The stroke of the phaco needle creates a mechanical impact as the metal phaco needle hits the cataract material. It also creates cavitation and implosion as a microvoid is created just in front of the phaco needle. A fluid and particle wave is propagated into the cataract material and finally, heat is created as a by-product. It is important to avoid choosing phaco power settings that cause excessive heat build-up as this can burn the cornea and damage the delicate ocular structures.12
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Figure 1.16: The phaco pinch test
The phaco pinch test (Figure 1.16) is a simple way to determine if your ultrasound power settings are likely to cause an incision burn in the eye. During wet lab testing, program your selected settings into the phaco machine, remove the protective silicone sleeve from the phaco needle, grasp the needle between your fingers, and push the foot-pedal all the way down. If your settings cause excessive heat build-up, the needle will get hot and may even burn your fingers. But it is better to singe your fingertips than fry your patient's cornea.
During surgery, the phaco machine keeps track of the average phaco power, given as a percentage of maximum, as well as the total time during which phaco ultrasonic power was delivered. These are displayed as “U/S AVE” which stands for “ultrasound average” and “EPT” which is “elapsed phaco time”.
We can measure and compare the amount of phaco energy that we use in surgery by calculating the APT: Absolute Phaco Time. This is done by multiplying the “U/S AVE” by the “EPT”, which the phaco machine does for us automatically, and it displays the “APT”.
It makes sense that if you deliver 15 seconds of energy at 100% power, it is about the same as 30 seconds at 50% power, or 60 seconds at 25% power. This is because for each of these three examples, the APT (Absolute Phaco Time) is 15 seconds.
It is important to give as little ultrasonic phaco energy as possible during the cataract surgery. The ultrasonic energy can easily damage the corneal endothelial cells, and excessive phaco energy can cause pseudophakic bullous keratopathy and corneal decompensation. The most important way to decrease the APT is to use a mechanical method of nucleus disassembly such as phaco chop. This is far more efficient than techniques like divide-and-conquer, resulting in less energy delivery as well as a quicker procedure.
To decrease the APT maximally, we need to decrease the phaco time and we need to decrease the average phaco power. The average phaco power can be decreased by limiting the foot pedal depression in position three or by decreasing the maximum phaco power level on the machine.
The phaco time can be decreased by only applying the ultrasonic power when cataract pieces are at the phaco tip and are not aspirated by the vacuum forces alone. Additionally, phaco time can be reduced by delivering smaller pulses or bursts of phaco energy instead of continuous ultrasound. This method of breaking up the ultrasonic energy into smaller packets of pulses and bursts is called “phaco power modulation”.
With optimized ultrasonic phaco power parameters, it is possible to remove cataracts with less than 1 second of absolute phaco time, giving immediate clear corneas and happy patients (Figure 1.17).
zoom view
Figure 1.17: Less than 1 second of absolute phaco time: (A) Optimized phaco setting(B) Ultra low APT: absolute phaco time
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REFERENCES
  1. Davison JA. Performance comparison of the Alcon Legacy 20000 1.1 mm TurboSonics and 0.9 mm Aspiration Bypass System tips. J Cataract Refract Surg 1999;25(10):1386–91.
  1. Davison JA. Performance comparison of the Alcon Legacy 20000 1.1 mm TurboSonics and 0.9 mm MicroTip. J Cataract Refract Surg 1999;25(10):1382–5.
  1. Georgescu D, Payne M, Olson RJ. Objective measurement of postocclusion surge during phacoemulsification in human eye-bank eyes. Am J Ophthalmol 2007;143(3):437–40.
  1. Yaguchi S, Kageyama T. In vitro evaluation of pressure fluctuations with differing height of the infusion bottle in phacoemulsification. Jpn J Ophthalmol 2000 Nov 1;44(6):690–1.
  1. Payne M, Georgescu D, Waite AN, Olson RJ. Phacoemulsification tip vacuum pressure: comparison of 4 devices. J Cataract Refract Surg 2006;32(8):1374–7.
  1. Packer M, Fishkind WJ, Fine IH, Seibel BS, Hoffman RS. The physics of phaco: a review. J Cataract Refract Surg 2005;31(2):424–31.
  1. Tognetto D, Sanguinetti G, Sirotti P, Brezar E, Ravalico G. Visualization of fluid turbulence and acoustic cavitation during phacoemulsification. J Cataract Refract Surg 2005;31(2):406–11.