Efficiency is usually the last priority to a beginning cataract surgeon. The immediate goal is to remove the cataract and to “stay out of trouble” while doing it. However, there are ways even for the beginning surgeon to optimize the potential for removing the cataract and at the same time to minimize the overall time spent in the eye. Attention to the actual settings that one chooses to remove the nucleus and epinucleus can actually make phacoemulsification easier and facilitate both learning and efficiency.
The term efficiency must be defined. For the purposes of this chapter, I will use efficiency to mean complete nuclear and epinuclear removal using minimal ultrasound power and minimal overall time in the eye. One might believe that the most important factor in maximizing efficiency with settings is the ultrasound power. Surprisingly, this is not the case. It is not the ultra- sound power that is most important, but rather the aspiration flow rate and the vacuum that impact efficiency the most.
B. Aspiration Flow Rate
By aspiration flow rate, or simply “aspiration,” I mean the rate at which fluid and particles come to the ultrasound tip. Higher aspiration means faster flow and faster movement of nuclear and epinuclear pieces to the tip.
C. Ultrasound Power
Once the nucleus is at the tip, ultrasound power is responsible for emulsi- fying it into a small enough piece to fit through the tip and travel through the bore of the ultrasound handle. The surgeon must choose a power level that is just enough to break up the nuclear fragment into small enough chunks to be vacuumed through the tip. The surgeon must also choose between longitudinal ultrasound power and torsional ultrasound. Longitudinal ultrasound delivers an axial cutting force that will tend to push the nuclear piece away from the tip and create a “tunnel-like” cavity into the tissue, much like a jack- hammer. Torsional ultrasound, on the other hand, is produced by side-to-side oscillation of the phaco tip, and therefore delivers the ultrasound force to a larger region of tissue.
Once the ultrasound has “handled” the nuclear fragment, it is the vacuum that determines how quickly the fluid or particle will make its way through the tip. Softer nuclear and epinuclear pieces need less vacuum to pull the tissue through the ultrasound tip. Harder and denser nuclei need higher vacuum levels to draw them through the tip. Vacuum rise occurs when the ultrasound hand piece tip is occluded. Maximum vacuum is generated when the tip is completely occluded. This occlusion may occur with any tissue or substance (eg, nucleus, epinucleus, viscoelastic, iris, and vitreous), but for the purposes of this discussion, occlusion with the nucleus or epinucleus is assumed.
Aspiration and vacuum are integrally related as the flow rate determines how readily a piece will flow to the tip and the vacuum determines how easily the piece will climb up and through the ultrasound hand piece. As a result, aspiration and vacuum are generally titrated up and down together. The 3 parameters (aspiration, ultrasound power, and vacuum) all work together to create “followability” or the way in which successive nuclear fragments smoothly move from their starting positions to the tip, get emulsified at the tip, and then are removed through the hand piece.
E. Fixed Versus Variable Parameters
Each main parameter (ultrasound, aspiration, and vacuum) can be selected as either fixed or variable. If the parameter is fixed, then it will appear at that designated level throughout the excursion of the foot pedal position. For example, if aspiration is set fixed at 33 cc/min, then as soon as the surgeon depresses the phaco foot pedal into position 2, an aspiration flow rate of 33 cc/min will be created in the eye. Similarly, if a fixed vacuum limit of 400 is chosen, then as soon as complete occlusion of the tip occurs, the vacuum limit of 400 is enforced and any “excess” vacuum is vented.
Variable settings allow the particular parameter to increase in a linear fash- ion throughout the foot pedal position. For example, when the aspiration is set on 35 cc/min and is variable, the surgeon will depress the foot pedal into position 2 and get some aspiration, but will not reach the full 35 cc/min until the foot pedal is at the “bottom” of position 2. Likewise, if ultrasound is set on 60% and variable, the full 60% power will not be attained until the foot pedal is fully depressed.
The purpose of fixed versus variable settings is to give the surgeon maximum control of the speed and efficiency of tissue removal. In general, the more parameters on fixed settings, the faster things happen. On the other hand, for the beginning surgeon, variable settings may allow the surgical steps to occur at a slower, more “controlled” pace. In addition, for eyes with some sort of compromise (especially weak zonules, pseudoexfoliation, and history of trauma), variable parameters give the surgeon finer control over forces generated within the eye.
III. BASIC PHACOEMULSIFICATION SETTINGS
Every surgeon will develop his or her own settings for all parts of the phacoemulsification. Table 12-1 is a sample of parameters used for the beginning surgeon. Settings for irrigation and aspiration of cortex are outside the scope of this chapter’s discussion and will be covered elsewhere.
All parameters (ultrasound, vacuum, and aspiration) are usually kept at mid-range during the sculpting phase of the phacoemulsification. Since at this point the nucleus is one large piece, high levels of aspiration will cause excessive movement of the piece toward the tip. Instead, the settings during sculpting are focused on emulsifying moderate amounts of nucleus and efficiently moving those pieces through the hub of the phacoemulsification tip. Aspiration is set just high enough to engage the nucleus and keep it at the tip. Ultrasound is set just high enough to emulsify the tissue so that the tip can continue moving across the nucleus. Vacuum is adjusted just high enough to keep the tissue that has been emulsified moving through the hand piece.
During the chopping phase, success depends upon the nucleus being held tightly onto the tip. For that reason, the vacuum limit must be relatively high. On a machine with a peristaltic pump, the aspiration flow rate will drive the maximum vacuum level (achieved only when there is full occlusion), and so the aspiration limit must be set relatively high as well. Table 12-1 summarizes the parameters typically used by surgeons who are transitioning to chopping techniques.
C. Nuclear Section Removal
For purposes of this discussion, I will use the term section to refer to any nuclear section, whether it is a small chopped section, a quadrant, or even a heminucleus. The beginning surgeon will likely be performing a divide-and- conquer technique, so the “section” being removed would be a quadrant. In a chopping technique, the “section” would likely be smaller than a quadrant. Regardless of the size of the piece, the principles (with respect to phacoemulsification settings) of section removal are similar.
Since sections are fairly large and the goal during section removal is to bring the section to the tip and hold it there, aspiration flow rate must be relatively high. High vacuum will quickly remove these fragments from the eye once they are emulsified (and possibly chopped into small fragments).
The surgeon has 3 choices as far as section removal is concerned: continuous, burst, and pulse. All will have similar parameters and similar settings for aspiration, ultrasound power, and vacuum. However, the 3 modalities differ significantly in terms of how the parameters are controlled by the surgeon. In continuous mode, the ultrasound power is delivered without interruption, as long as the surgeon keeps the foot pedal in position 3. As the foot is depressed lower in position 3, the power increases until the maximum preset limit is reached. In burst and pulse modes, the ultrasound power is delivered to the eye in discrete “packets” of energy. These packets are more or less controlled by the surgeon (depending on the mode).
In pulse mode, the packet of energy is determined by the ultrasound power, which is delivered for a predetermined duration. The surgeon may choose the rate of delivery of these packets by manipulating the pulse rate. The higher the pulse rate, the more rapidly the ultrasound packets are delivered. The lower the pulse rate, the less frequently the packets are delivered. Therefore, when the surgeon depresses the foot pedal enough to generate irrigation, aspiration, and ultrasound (“position 3”), full aspiration flow rate and pulse rate are generated.
Similar to pulse mode, a packet of ultrasound energy (the surgeon determines the power level) is delivered in burst mode. However, in this mode, the surgeon also sets the duration of the ultrasound packet, or burst width. As the burst width is increased, a longer ultrasound packet is delivered to the eye. In addition, the surgeon directly controls the frequency with which ultrasound packets are delivered to the eye by increasing his or her pressure on the foot pedal. In position 3, both vacuum and packet frequency increase as the foot is depressed lower until maximum vacuum and continuous phacoemulsification are achieved.
3. The difference between pulse and burst modes
Although the actual settings may not appear to be very different in pulse versus burst mode, the 2 modalities result in a fairly different surgical experience. Pulse mode allows for efficient but slightly slower removal of the nucleus. One reason that the surgery proceeds more slowly is that the vacuum often cannot build to its maximum level. Maximum vacuum requires occlusion of the tip. Since the pulse rate delivers specified pulses of ultrasound energy to the section at the tip, these pulses often push the piece off the tip. The advantage of this is that the piece keeps moving on the tip. As the piece bounces at the tip, there is less likelihood of vacuum surge as the last bit of nucleus is removed. The disadvantage is that occlusion is disrupted and vacuum fails to build. For the beginning surgeon, pulse mode is often the preferred method for removing nuclear sections since overall efficiency is still quite high, and yet, the removal of tissue progresses in a slightly slower, more controlled fashion. In addition, there are fewer variables with pulse mode that need to be directly managed. For the more experienced surgeon, the increased speed and efficiency of burst becomes highly desirable.
4. Specific uses of burst and pulse
Burst is usually the preferred method of nuclear section removal when conditions are optimal. “Optimal conditions” imply that the nucleus is of average density and that all steps in the surgery prior to section removal have proceeded smoothly. If any of these steps have been compromised, pulse is recommended. Specific examples of instances in which pulse might be preferred to burst are the following:
- Small capsulorrhexis
- Anterior capsular tear
- Very dense nucleus
- Sharp section edges
- Shelved sections
The surgeon may elect to switch to burst once the initial sections have been removed and there is more room to maneuver the remaining sections. If there is any compromise of the capsule or zonules, then the surgeon will likely use pulse exclusively especially once any vitreous has been properly addressed.
D. Epinucleus Removal
If there is an epinuclear plate, it can be efficiently and safely removed using settings that use ultrasound, aspiration, and vacuum all at low to moderate levels. Table 12-1 shows typical settings for epinucleus removal. The goal during epinucleus removal is to bring the epinuclear tissue to the tip and evacuate it, but to leave the posterior capsule intact. For this reason, more aspiration and vacuum are needed than in the sculpting phase, but less than in section removal or chop. Often, removal of epinucleus is aided by viscodissection so that aspiration and vacuum can work even more efficiently.
Using basic principles, even the beginning surgeon can manipulate settings to his or her advantage in phacoemulsification. If these principles are used at the start of the learning curve, the surgeon will quickly progress to removing the cataract safely and with maximum efficiency.
*Dikutip dari Buku Essentials Of Cataract Surgery 2nd Ed, halaman 117-122