Power Modulations Come of Age

After a decade of fluidic advancements such as enhanced pump function, anterior chamber pressure sensing, less compliant tubing, and microprocessor controls, new digitally driven, rapidly cycling, and programmable microbursts of phacoemulsification have heralded the age of the power modulations. First introduced by Advanced Medical Optics, Inc. (Santa Ana, CA), on its Sovereign cataract extraction system with Whitestar Technology in 2001, advanced power modulations such as hyperpulse technology have recently been adapted by both Alcon Laboratories, Inc. (Fort Worth, TX), in its new Infiniti system and by Bausch & Lomb (Rochester, NY) for the Millennium microsurgical system's Custom Control Software (CCS) upgrade.

By rapidly cycling micropulses of phaco energy with variable duty cycles, the surgeon gains three key advantages: (1) better phaco efficiency, the ability to achieve the same cutting power with less total phaco energy; (2) a decreased heat buildup and risk of thermal injury; and (3) enhanced followability with less chatter due to the better balance between phaco repelling forces and the flow/vacuum attractive forces. These three factors allow for a better, safer cataract surgery, and they are responsible for surgeons' recent movement toward bimanual microincisional phacoemulsification.

A byproduct of the novel power modes is new, sometimes confusing nomenclature. This article attempts to clarify what the newest power modulations mean, what they actually do, and how those of different systems compare.

THE BASICS

Continuous phacoemulsification preceded traditional pulse, which was followed by burst mode and then hyperpulse with a variable duty cycle. The way to understand these modes is to break each one down to its basics. Aside from continuous phacoemulsification, all of the power modes are essentially combinations of phaco-on periods followed by phaco-off periods. The percentage of phaco-on time to total time is called the duty cycle.

PULSE

With traditional pulse (which in the original software of all three platforms was limited to a maximum of 10 to 20 pulses per second), the duty cycle was fixed at 50%. A setting of four pulses per second therefore meant that each pulse interval was 250 milliseconds long, comprising a 125-millisecond phaco-on and 125-millisecond phaco-off period. Increasing the pulse rate to 10 would decrease the pulse interval to 100 milliseconds (50 milliseconds phaco-on and 50 milliseconds phaco-off). No matter the pulse setting (from two to the previous limit of 20), the phaco power was always on 50% of the time (a 50% duty cycle). With pulse mode chosen, foot position three on the footswitch controlled linear power with a fixed phaco-on and phaco-off interval.

BURST MODE

Burst mode was the first power modulation to indirectly allow variation of the duty cycle. In traditional burst mode, the phaco-on time is preset, and the phaco-off time was linearly controlled or decreased with foot position three. Phaco power remained constant in all positions of foot position three. For example, the Millennium's CCS software allows the surgeon to set a phaco-on period of between 4 and 600 milliseconds. The phaco-off time is variably controlled with foot position three. Starting at the beginning of foot position three, the cycle interval becomes progressively shorter than 1,200 milliseconds as the pedal is depressed. At full pedal excursion, the phaco-off period goes to zero, and the equivalent of continuous phacoemulsification is achieved. Burst mode fixes the phaco power and phaco-on time but effectively allows the surgeon to vary the duty cycle. If, for example, the burst-mode phaco-on time is preset to 300 milliseconds, then the duty cycle goes from 25% (300 milliseconds phaco-on with 900 milliseconds phaco-off) at the top of foot position three all the way to 100% at full foot pedal depression.

PREPROGRAMMED POWER MODES

On all three companies' systems, the newest hyperpulse power modulations allow surgeons to choose variable duty cycles as well as newer pulse settings of 100 pulses per second and beyond. By preprogramming different combinations of settings, ophthalmologists can employ unique power modulations for different lens densities or disassembly techniques. For example, one may approach soft nuclei with lower overall phaco energy settings such as a power of 30%, a frequency of 60 pulses per second, and a low duty cycle of 10% to 40%. Hard nuclei can be approached with 40% power, 30 pulses per second, and a duty cycle of 40% to 50%.

Similarly, different power modulations are advantageous for different stages of the procedure. Continuous mode works best for sculpting, in which low vacuum settings and a stationary, in-the-bag lens are present. By contrast, short, higher-powered microbursts of energy coupled with longer phaco-off periods avoid repulsion and achieve good impaling ability during quick chopping, which employs high vacuum and requires rapid, strong occlusion (Figure 1). For quadrant removal or other supracapsular techniques, followability is critical and can be enhanced by using a hyperpulse setting in the 50- to 100-pulses-per-second range, with a 25% to 40% duty cycle. Minimizing the percentage of ultrasound and allowing more rapid cycling between the phaco-on and phaco-off periods decreases the repulsive forces and allows the forces of fluidic attraction to predominate. This type of setting creates so-called magnetic followability, which minimizes chatter and wasted phaco energy. Many surgeons feel that hyperpulse mode enhances the efficiency of cavitation by preventing the wasteful buildup of cavitation bubbles on top of each other as occurs in continuous mode.

OCCLUSION MODE

Another power modulation, again first introduced by Advanced Medical Optics, Inc., is occlusion mode, which is now also available on the Infiniti (although Advanced Medical Optics, Inc., is seeking an injunction against the phaco power occlusion mode offered by Alcon Laboratories, Inc.). This technology, currently available only with the peristaltic pump (not the Millennium's venturi pump), allows the application of different phaco settings before and after tip occlusion.

On the Sovereign, an occlusion threshold vacuum setting can be programmed that will automatically switch phaco parameters. For example, if the vacuum level is 325mmHg, the surgeon can set a threshold vacuum level of 250mmHg, which when reached would automatically change to new settings. Typically, postocclusion settings should have a higher Whitestar duty cycle, a higher maximum power, and a lower aspiration flow rate. In Figure 2, power increases from 35% to 45%, the Whitestar duty cycle increases from CL (20%) to CD (43%) (the two-letter nomenclature is explained later), and aspiration flow is decreased from 30 to 24mL/min. Once a nuclear fragment occludes the port, thus causing the vacuum to rise and exceed the predetermined threshold, the machine switches to more phaco power for greater cutting efficacy and decreases the flow rate to avoid a postocclusion surge. After the occlusion is cleared, the vacuum level drops, and the settings return to their original lower level.

Care must be taken not to increase the power and duty cycle too high in postocclusion mode. Because the tip is occluded and flow is minimal or zero, the tip can heat up very rapidly, potentially causing tissue damage.

The Infiniti system's occlusion mode differs somewhat. Instead of just relying on a threshold vacuum level, it uses a sophisticated algorithm based on irrigation pressure and aspiration vacuum to determine occlusion onset and full occlusion. The Infiniti's postocclusion settings, however, are limited—only allowing for a reduction of power, not the more logical increase in power or duty cycle.

COMPARING THE THREE SYSTEMS
Infiniti

Alcon Laboratories, Inc., increased the maximum pulse rate from 15 pulses per second on its Legacy machine to 100 pulses per second on the Infiniti (a software upgrade to the Legacy called Everest allows similar settings). Furthermore, the surgeon can now set the pulse duty cycle (formerly fixed at 50%) from 5% to 95%. This change gives the surgeon predictable and complete control over all phaco-on and phaco-off parameters.

The company also enhanced traditional burst mode (named fixed burst on the Infiniti) with linear burst. In this mode, the phaco-on interval is fixed (from 5 to 500 milliseconds), and the phaco-off time—as well as the phaco power—is linearly controlled with the foot pedal (Figure 1). This change allows a dual step-up of duty cycle and phaco power as the surgeon depresses foot position three.
Moreover, the Infiniti system offers the company's proprietary Neosonix technology, which allows for additional sonic oscillatory energy to be added to the ultrasound. Neosonix can now be applied in hyperpulse or any of the burst modes.

Sovereign With Whitestar Unlike the Infiniti, which allows for numerous combinations of pulse frequency and duty cycle, Whitestar Technology has 10 preset hyperpulse/duty cycle settings from which to choose. The different settings are defined by the fixed period of phaco-on and phaco-off intervals. Advanced Medical Optics, Inc., has clinically validated and chosen the 10 settings to represent a manageable range of options.

Trying to compare the Whitestar settings with the hyperpulse settings of the Infiniti or Millennium can be confusing, due to the different nomenclature used by Advanced Medical Optics, Inc. For example, the lowest duty cycle setting on Whitestar alternates 4 milliseconds of phaco-on followed by 24 milliseconds of phaco-off. The highest duty cycle phaco setting has 8 milliseconds of phaco-on with 4 milliseconds of phaco off. The company uses a two-letter nomenclature for each setting; the letter's order in the alphabet represents a 2-millisecond multiple in time. The first letter represents the phaco-on period, and the second letter the phaco-off. In the first example just given, the setting is dubbed BL. B is the second letter of the alphabet, and multiplying two by 2 milliseconds yields 4 milliseconds of on time. L is the 12th letter, and multiplying 12 by 2 milliseconds represents the 24 milliseconds of off time. The second example is named DB (D as 4th letter times 2 milliseconds equals 8 milliseconds of on time, and B signifies 4 milliseconds of off time).

Some simple calculations are necessary to express the Whitestar BL mode in terms of pulses per second and duty cycle—the adjustable variables on the Infiniti and the Millennium's CCS. With 4 milliseconds on and 24 milliseconds off, the duty cycle is 14% (four divided by 28 equals 0.142), and the pulse frequency is 36 pulses per second (1,000 milliseconds divided by an interval length of 28 milliseconds). Likewise, the DB setting can be mimicked on the other machines with settings of 83 pulses per second and a 67% duty cycle.

The Sovereign allows surgeons to layer one of the 10 preset Whitestar hyperpulse modes on top of a traditional pulse mode called long pulse. In other words, a slow, fixed, 75% duty cycle long pulse of five pulses per second (with 150 milliseconds phaco-on and 50 milliseconds phaco-off) is further modulated by having the phaco-on portion comprise hyperpulses instead of continuous phacoemulsification. Combining a Whitestar setting with a long-pulse setting effectively reduces the Whitestar duty cycle by 25%, thus further decreasing the buildup of heat.

The recent 6.0 software upgrade includes the latest power modulation, called variable Whitestar. In this mode, the surgeon can customize a set of four different Whitestar settings to be cycled through with depression of the foot pedal in position three (Figure 3). For example, the beginning of foot position three can be set with a low duty cycle mode such as CL (20% duty cycle), and progressive depression can shift to modes CF (33%), CD (43%), and DB (67%). This setup allows the surgeon a bimodal linear increase in phaco power and duty cycle with foot pedal depression so that he may enhance cutting efficiency and total phaco energy when needed. During microincisional phacoemulsification, a set of lower duty cycle settings may be chosen, as seen in set 1 of Figure 3. Keeping the duty cycle below 33% greatly limits the buildup of heat.

Millennium With CCS

The CCS upgrade allows the surgeon to preprogram hyperpulse settings in three modes: hyperpulse; fixed burst; and multiple burst. Multiple burst mode is similar to traditional burst mode (described earlier); the phaco-on time and phaco power are fixed, and the phaco-off time is controlled with the foot pedal. One little modification is the ability to set the maximum (full foot pedal depression) duty cycle at between 50% and 100%, as opposed to traditional burst mode, which is fixed at 100% continuous phacoemulsification at maximal excursion. This capability is useful during bimanual phacoemulsification, where the duty cycle is best kept below 50% to lessen heat.

The new hyperpulse mode is set similarly to the Infiniti's, with a pulse rate of up to 120 pulses per second and an adjustable duty cycle of between 10% and 90% (Figure 4). What is termed fixed burst mode on the Millennium's CCS is analogous to the Whitestar method of describing the hyperpulse setting. In this mode, the surgeon sets fixed phaco-on and phaco-off intervals (each is independently set at between 4 and 600 milliseconds). The pulse intervals stay fixed, with linear power control in foot position three. Unlike the 10 preprogrammed combination settings of Whitestar, the Millennium's CCS offers thousands of potential combinations. Again, it is important to realize that, on the Millennium, hyperpulse mode and fixed burst mode can achieve identical phaco-on/off settings—just expressed in different terms.

The Millennium's CCS has personalized “submodes” that allow surgeons to toggle between different preset duty cycle and hyperpulse settings. While in foot position two, the surgeon can kick left to change to a higher duty cycle and pulse rate for a particularly dense piece of nucleus. Up to three different submodes can be programmed per memory setting. The system's proprietary dual-linear foot pedal already allows the surgeon separate linear control of both phaco energy and vacuum. The new submodes are another means of adjusting the duty cycle with the foot pedal.

CONCLUSION

Overall, the newest power modulations add more complexity and sophistication to current phaco systems. Understanding the terminology and properly setting up the new modes are critical to maximizing phaco success. With more efficient energy use, enhanced followability, and better thermal safety, power modulations enhance the surgical experience and outcomes for physicians and patients alike.

Tal Raviv, MD, FACS, is Assistant Professor of Ophthalmology, New York Medical College, Valhalla, New York; is an attending cornea and refractive surgeon at the New York Eye and Ear Infirmary; and is in private practice in New York City. He states that he does not hold any financial interest in the products or companies mentioned herein. Dr. Raviv may be reached at (212) 448-1005; tal.raviv@nylasereye.com.