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Back to Basics column provides a concise review
This new column is set to serve as a guide to the basic concepts in ophthalmology.
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While reading the ophthalmic journals, I’ve often wished for a column that would explain some of the basic concepts that form the building blocks for modern day ocular surgery. In this light, Ocular Surgery News has started the “Back to Basics” column to provide practicing ophthalmologists a concise review of the basic concepts behind the current techniques and technologies used in ocular surgery, primarily cataract and anterior segment surgery.
This column is aimed at the practicing ophthalmologist who has already acquired surgical knowledge and skills, but wishes to stay current with the latest trends. This is a surgical column in a publication that serves the surgical as well as the medical needs of the ophthalmic community, and as such, we’ll be focusing on surgical concepts, surgical techniques and surgical technologies.
For example, it is great to read about the new trends in “cold phaco” and “ultra-pulse with variable duty cycles,” but if I do not fully understand the difference between simple “pulse” and “burst” modes, then an explanation of these more esoteric concepts may escape me. Without an understanding of these basics, I am more likely to stick with “continuous” phaco mode, which is not as beneficial to my patients and is not applicable to newer sleeveless phaco techniques.
This is easily remedied as these concepts are easily explained from one surgeon to another, in a manner entirely different from acquiring the knowledge from nonsurgeons, such as company-sponsored representatives.
I would like to start by reviewing the concepts of phaco power modulation – software that can minimize the phaco energy used and maximize the efficiency of cataract surgery. I am always open to suggestions, corrections and questions and can be reached at the e-mail address listed below.
Power modulation: pulse mode
The single most important factor in achieving clear corneas and sharp vision immediately after cataract surgery is the amount of phaco energy delivered into the eye during the procedure. We can help protect the corneal endothelium from the ultrasound energy via the use of viscoelastics, but it is often more effective to decrease the total phaco energy that we use during surgery.
We can effectively decrease phaco energy via use of mechanical disassembly of the nucleus, such as performing quick chop. However, this requires a change in technique for surgeons who are not well-versed in this maneuver, and they may be hesitant to switch.
We can also decrease phaco energy via the use of phaco power modulations: using software within the phaco machine to change the delivery of the ultrasonic power in a manner that minimizes its iatrogenic effects while increasing its efficacy. If appropriate settings are used, this software can effect these improvements without requiring a change in the surgeon’s surgical technique.
The traditional phaco setting is continuous mode, which varies the phaco power in a linear manner as the foot pedal is depressed (Figure 1). The energy level can vary from zero to the surgeon-set maximum with foot pedal position. This mode results in a lot of energy delivery into the eye, and it does not provide any time for cooling of the phaco tip, as the energy is continuous.
In pulse mode, the phaco energy can be cycled on and off at a surgeon-set frequency. The energy is still delivered in a linear manner, depending on foot pedal position, however the energy is delivered in pulses, at a rate determined by the surgeon (Figure 2). The pulse rate is given in hertz (cycles per second), often noted as pps (pulses per second). In addition to decreasing the phaco energy placed into the eye, pulse mode allows for cooling of the phaco tip during the brief off periods of each cycle.
With earlier generation phaco machines, it was not possible to use pulse rate for all techniques primarily because the maximum pulse rate was insufficient. If we think of phaco power delivery as a knife blade, then continuous is like a straight blade, and pulse mode is like a serrated knife, where the higher the pulse rate, the finer the serrations (Figure 3).
Rates differ
If we are performing the grooving aspect of divide-and-conquer, continuous phaco power would work well, but a low pulse rate would be too coarse and insufficient for smooth cutting. However, once we use a high pulse rate, it’s like using a finely serrated knife to cut a tomato – it works better that a straight blade. This illustrates the importance of choosing the correct pulse rate: lower rates tend to work better for segment removal, while higher rates tend to work better for grooving and sculpting.
It is important to note that simply changing the pulse rate does not change the total energy delivered into the eye, rather it divides up the energy into smaller pulses. Whether the pulse rate is 2, 8 or 100, the total energy delivered is the same (Figure 4). In pulse mode, the energy is decreased as compared to continuous mode by virtue of the ratio of the on:off segments in each cycle. The default cycle for pulse mode is to have the phaco energy on and then off for equal periods, in a 50:50 ratio, thus giving a “duty cycle” of 50%.
In my next column, we will examine the effects of varying the duty cycle and identify the situations where it would be beneficial. In the meantime, I encourage you to try pulse mode with a pulse rate that is suitable for your personal technique of cataract surgery.
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.