|Decreasing Phaco Energy
Optimizing surgical parameters in the age of refractive cataract surgery.
By Uday Devgan, MD, FACS
|During the past 2 decades, ophthalmologists have become increasingly convinced that the amount of phaco energy delivered in an eye directly affects corneal endothelial cells and, in some cases, causes corneal decompensation and compromised vision. Technological advances and surgical techniques aimed at minimizing the iatrogenic effects of phaco energy have evolved rapidly, but surgeons today are faced with an even bigger challenge: high patient expectations in the age of refractive cataract surgery.
Currently, patients routinely elect to undergo the removal of early cataract, before the significant compromise of their preoperative vision. These patients expect not only the removal of their cataracts, but also the elimination of their refractive error, a high degree of safety, and immediate recovery of their sharp vision. Although the accuracy of IOL calculations has increased and the incidence of intraoperative complications has decreased, achieving a clear cornea and sharp vision immediately postoperatively continues to pose a challenge.
The single most important predictor of good vision and a clear cornea postoperatively is the total amount of phaco energy delivered into the eye.1 With advances in phaco power modulation, surgeons can dramatically reduce the phaco time and power used in every cataract case.
To achieve these objectives, I upgraded to the new Millennium Custom Control Software (Millennium CCS; Bausch & Lomb, Rochester, NY) and have seen my phaco energy cut in half, which results in clear corneas and sharp vision for nearly all of my patients immediately after surgery.
MEASURING PHACO ENERGY
At present, no phaco platform offers surgeons a direct or standard measure of the energy that is placed into the eye during cataract surgery in units such as Joules. However, all modern machines record the total time spent in foot pedal position 3 (phaco time) and the application of average phaco power. The product of these two variables is often referred to as the absolute phaco time (APT) or the effective phaco time, which is the equivalent phaco time at 100% power (APT = phaco time X average phaco power).
In order to maximally decrease the APT, the phaco time and average phaco power must be decreased. The lower the APT, the clearer the cornea, and the sharper the vision on postoperative day 1.
BASIC POWER MODULATIONS
The most basic form of modulating phaco power is having linear control of power in continuous mode (eg, the farther the pedal is depressed, the higher the phaco power is). Maximum phaco power is set and then delivered between 0% and the maximum preset power, depending on how much the foot pedal is depressed.
In the pulse method of modifying phaco power, the same linear control of phaco power via the foot pedal exists, but the energy is delivered in pulses instead of continuously. With each pulse programmed “on” and “off” for an equal amount of time, the total energy delivered is cut in half as compared with the continuous mode.
In this mode, a burst of phaco energy is delivered, with a time interval between identical bursts that varies and depends on the depression of the foot pedal. The farther the foot pedal is pressed, the shorter the “off” period between bursts is.
With maximum foot pedal depression, this “off” period is infinitely small, and the energy is continuous. Linear control of phaco power is stopped in burst mode, and instead variable control of the burst interval is achievable.
These basic forms of power modulation are helpful in decreasing phaco energy, but they are limited in their level of programmability and efficiency (Figure 1).
ADVANCED POWER MODULATIONS
Hyperpulse and hyper burst refer to an extended range of programmability of conventional pulse and burst modes. The traditional pulse mode is limited to approximately 20 pulses per second, whereas hyperpulse can be increased to as many as 120 pulses per second.
In hyperpulse mode, increasing the pulses per second does not change the total energy delivered, but it does reduce heat because each pulse is immediately followed by a brief “off” period. This mode also increases the effectiveness of cutting, particularly of making grooves with phaco techniques such as divide-and-conquer, when a very high rate (eg, 120 pulses per second) is used. If a surgeon who traditionally performs divide-and-conquer surgery switches from continuous phacoemulsification to hyperpulse, the total phaco energy delivered will be cut by 50%.
Similarly, in traditional burst mode, the previous minimum microburst of energy the surgeon could choose was 80 milliseconds, whereas, with hyper burst, the minimum microburst of energy can be programmed as low as 4 milliseconds. The ability to program the microburst of energy as low as 4 milliseconds affords delivery of smaller bursts of phaco energy during nuclear removal and thereby minimizes the build-up of heat and the total phaco energy used.
In pulse mode, the default duty cycle is 50%, which means that each pulse of energy is “on” for the same duration as the “off” period that follows it. A “cycle” consists of one “on” phase followed by its “off” phase. Twenty pulses per second is the same as 20 cycles per second (20Hz). If the duration of the “on” phase is decreased and the duration of the “off” increased, the same number of pulses per second can still be delivered with the benefit of less thermal energy (Figure 2).
In addition, with the Millennium CCS, a variable duty cycle such as 50% can be programmed as an end point in burst mode, a setup that prevents any continuous phaco energy delivery to the eye, even if maximum foot pedal depression is applied.
Variable Rise Time
When using phaco power in pulse or burst mode, total phaco energy decreases, and the efficiency of nuclear removal increases. Traditional pulses or bursts are delivered in square waves, but, with recent advances in software, surgeons now have the option of gradually ramping up pulses and bursts as well as of delivering waveform-modulated packets of energy. This gradual ramping up of power achieves a “pulsed pulses” effect. The waveform modulation reaches the same peak power as the conventional square wave pulse, but with less total energy expended (Figures 3 and 4).
Another major advantage of variable rise time is it increases the followability of nuclear fragments during surgery. Phaco power is a repulsive force like a jackhammer; attacking a nuclear fragment with high-powered phacoemulsification pushes the fragment away from and causes chatter at the phaco tip. By ramping up the power via the variable rise time, a surgeon can grasp nuclear pieces with lower power. Once he gains purchase, delivering higher power will emulsify the nucleus. This process results in an almost magnetic followability of the nuclear pieces and increases the surgical efficiency.
Prior to upgrading to the new Millennium CCS, I used pulse mode (which was limited to 20 pulses per second) and kept my phaco energy low at a maximum of 10% for routine cases. With these settings and my flip-and-chop technique, I was able to achieve an APT of 2 to 4 seconds for routine, 3+ nuclear cataracts.
I have switched to using hyper burst mode with a pulse duration of 20 to 25 milliseconds, an end point of 50% duty cycle, and 5% power. With these settings and a flip-and-chop technique, I cut my APT in half and am now averaging only 1 to 2 seconds of APT (often, the APT is even less than 1 second) (Figures 5 and 6). As an added benefit, nuclear removal is now more efficient, and my total case time has dropped by 10% to 20%.
This dramatic reduction in APT has resulted in the majority of my patients’ having clear corneas and sharp visual acuity postoperatively. My ability to meet and even exceed patients’ surgical expectations has led to increased word-of-mouth referrals to my practice.
CHOOSING APPROPRIATE SETTINGS
These new advances in power modulation offer a nearly endless number of programming possibilities to ensure a customized fit to every surgeon for every technique. I suggest starting with settings that do not cause excessive heat build-up to ensure maximum safety, followed by variations that will reduce APT (Table 1).
Phaco Pinch Test
A very simple yet effective method for measuring heat production at the phaco tip is the phaco pinch test. It requires the surgeon to remove the silicone sleeve from the phaco tip, disconnect or block the irrigation line, pinch the phaco tip with his fingers, and then depress the phaco pedal. If the power modulation settings cause significant heat to buildup, the surgeon will quickly feel the heat and possibly burn his fingers.
Using program settings with advanced power modulation techniques such as hyperpulse, hyper burst, reduced duty cycles, and variable rise times reduces heat production at the phaco tip and either minimizes or eliminates the likelihood of an iatrogenic corneal burn. These advantages are particularly important in bimanual microincisional cataract surgery in which the phaco needle is used without the silicone irrigating sleeve and is in direct contact with the corneal stroma (Figures 7 and 8).
Maximum Phaco Power
Many surgeons set their maximum phaco power at a very high level and will then use more total phaco energy than is absolutely necessary to remove the cataract. I suggest that surgeons perform a typical case with their usual settings and then record the average phaco power used at the end of the case. Doubling the latter figure will result in the maximum phaco power that they should use for their next case. Surgeons will likely not notice the difference intraoperatively, but, at the end of the case, they will see that their average phaco power and the APT will be significantly less.
In the age of refractive cataract surgery, optimizing phaco parameters is paramount to success. With the advanced power modulations possible with CCS, I have been able to reduce my total phaco energy by 50%, thereby minimizing endothelial trauma and consistently delivering clear corneas and same-day vision to my patients. n
Uday Devgan, MD, FACS, is Assistant Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine, and he is in private practice in Sun Valley, California. He states that he does not hold financial interest in the company or product mentioned herein, nor does he accept funds or honoraria for ophthalmic industry consulting. Dr. Devgan may be reached at (818) 768-3000; firstname.lastname@example.org.
1. Fine IH, Packer M, Hoffman RS. Power modulations in new phacoemulsification technology: Improved outcomes. J Cataract Refract Surg. 2004;30:1014-1019.
|For a downloadable pdf of this article, including Tables and Figures, click here.|