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Ramping up phaco energy improves followability, reduces chatter at tip
A variable rise time produces a ramped wave that results in improved efficiency and better outcomes.
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Keep in mind that ultrasonic phaco power is a repulsive force – like a jackhammer, the phaco needle moves back and forth into the cataract at a fixed frequency (between 28,500 times/second and 40,000 times/second depending on the platform). We have all seen it during surgery, and we often call it “chatter” – when the ultrasonic power mechanically pushes the nucleus off the phaco tip.
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![Uday Devgan, MD, FACS [photo]](/images/commonart/devgan.jpg)
To reduce the repulsive force of phaco we can decrease the phaco power, but this is not always the best answer, particularly when a nucleus is dense and requires more power for emulsification. Instead, if we initially attack the nucleus with lower power, then hold on to it with the vacuum fluidics of the phaco machine, we can ramp up the power to a higher level. The new phaco power modulation software on most platforms allows this automatically, with millisecond precision.
Variable rise times
Burst and pulse modes deliver square-wave energy by default, which means the power goes from zero to the preset level immediately and the resulting waveform on the oscilloscope looks like a square. With a variable rise time, we can have the phaco energy ramp up over the course of each individual pulse or burst, resulting in a ramped wave (Figure 1).
Images: Devgan U |
This ramping up of the energy allows better followability of the nuclear pieces and less chatter at the phaco tip, and it results in less energy and less heat delivered into the eye (Figure 2). There are situations where it is difficult to use a variable rise time, such as when the pulse width or burst width is so short that there is insufficient time to fully ramp up each packet of phaco energy. For example, if it takes 40 milliseconds to ramp up the power from zero to the preset level, but the defined burst width is just 25 milliseconds, the desired phaco power level will not be achieved.
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Suggested settings
First, remember to keep your phaco needle and all vacuum and flow levels the same as what you are used to. Also, no change in your surgical technique is needed. The only thing that we will be changing is the way that the phaco power will be delivered.
If you are accustomed to continuous phaco mode, you will likely have an easy time starting with a hyper-pulse mode at 60 to 120 pulses/second, initially at a 50% duty cycle, and using the same maximum phaco power that you’re used to. This one simple change will likely cut your total phaco time and energy in half with virtually no effect on your technique (Figure 3).
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If you are accustomed to a pulsed phaco mode, you will have an easy time staying with about the same number of pulses per second and keeping your maximum phaco power the same, while decreasing your duty cycle to 25% to 45%. You can then implement a variable rise time in order to further decrease the total phaco time and energy, and enhance purchasing power and followability.
Phaco chop surgeons will have an easier time adapting to hyper-burst mode. Keeping in mind that you will be controlling the interval between identical bursts via the third position, you should keep the maximum phaco power level relatively low. You will be unable to vary the percentage power level with your foot pedal, so setting a maximum level of 10% to 30% is suggested. Keep the burst width short, between 20 and 80 milliseconds, and make sure that you use an “endpoint duty cycle” of 50%. Depending on your machine, you may have to enter this as a “minimum burst interval,” which should be set equal to your burst width in milliseconds to achieve the effective endpoint duty cycle of 50% (Figure 4).
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You can further tailor your settings to better suit your technique and your patient population, without changing your surgical technique. Transitioning to the new phaco power modulation software is an easy way to improve your surgical outcomes and efficiency while decreasing the heat and energy placed into the eye.
For Your Information:
- Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.