Phaco Fundamentals Part 8: Variable Duty Cycle

The range of programmability of the pulse and burst phaco settings has expanded considerably. While previous generations of phaco platforms had pulse rates of up to 20 pulses per second, the newer generation machines have the ability to deliver up to 200 pulses per second. Similarly, the older machines had burst widths as narrow as 30 milliseconds, while the new platforms are able to deliver burst widths as fine as just 1 or 2 milliseconds.

The advantage of this upgraded range of programmability is the smoothness and precision of power delivery. With the standard settings in pulse mode, where each pulse is as long as each rest period, the pulse mode can deliver good cutting power with half the energy of continuous phaco energy. The more pulses per second we can give, the smoother the power delivery will be – very similar to serrations on a knife.

If we want to harness the sculpting and cutting ability of the phaco hand-piece for grooving of the cataract nucleus, it makes sense that a smooth blade knife would cut well. A coarsely serrated knife with large widely spaced serrations would not cut as smoothly. However, if we use a very finely serrated knife, it would likely cut the best of all. Using a very high pulse rate of 100 or more pulses per second results in the cutting ability of a very finely serrated knife while delivering half of the energy of continuous phaco power.

Hyper settings in burst mode allow finer and more precise delivery of bursts of phaco power. If we use continuous phaco energy mode and try to use our foot to deliver small bursts of phaco power, the best we can do is about a half-second of energy per pulse, which is 500 milliseconds. Using the newer hyper settings we can set a burst mode as small as 4 milliseconds, which is 100 times finer and more precise than using manual control by the surgeon.

Modern surgery is primarily phaco-assisted aspiration of the nucleus. The majority of the forces that are used to remove the nucleus from the eye are fluidic forces – the flow, aspiration, and vacuum forces. The ultrasonic power delivery is there to assist the fluidics once a denser piece of nucleus is encountered. If we are using burst mode, with a very fine burst width, then as the pedal is depressed further in foot-position 3, the rest interval between bursts decreases until the burst width and rest interval are equal, resulting in a 50% duty cycle. The effective number of bursts per second increases as the rest interval decreases and using a burst width of 5 milliseconds and allowing 5 milliseconds of rest between each burst, the maximum number of bursts per second is 100. (Math: 1 second / 10 millisecond cycle = 100 bursts per second). This results in being able to effectively control the duty cycle and burst rate per second at the same time via the foot-pedal.

For surgeons who wish to continue to perform their standard technique of phaco-emulsification, simply changing from continuous phaco power to a hyper pulse rate of 100 pulses per second will allow them to cut the energy delivery in half. This halving of the ultrasound energy will result in less endothelial cell damage, less heat production, and clearer corneas and sharper vision immediately post-op. For surgeons who perform the divide-and-conquer method of nucleus phaco, make the switch to a hyper pulse mode and you will immediately perform better surgery without a change in your technique.

Changing the number of pulses per second does NOT change the amount of power delivered into the eye. Whether we give 2 pulses per second or 8 pulses per second, note that the total energy, as represented by the green blocks is the same. The same applies when we compare 10 pulses per second to 100 pulses per second. The reduction in the amount of energy delivered is due to the ratio of the on:off pulses, which is known as the duty cycle.

To program in a change in this ratio, there are two distinct methods: entering a new duty cycle or direct pulse programming. For example, if I am using 10 pulses per second and I’d like to slightly reduce the ultrasound energy, I can decrease it from a 50% duty cycle to a 40% duty cycle. This can be done by dropping the duty cycle ratio as seen on the control panel of the phaco platform. Alternatively, I can delineate the specific on and off periods for each cycle, with an on time of 40 milliseconds on followed by 60 milliseconds off, I will achieve the same result – a total cycle time of 100 milliseconds, which corresponds to 10 pulses per second, with a ratio of the on:off time resulting in a 40% duty cycle.

In the pulse mode, the default duty cycle is 50%. For instance, the pulse is on the “on” position 250 msec and “off” for 250 msec. The benefit of the new power modulation software is that the duty cycle can be changed for example to 20% which results in 10 msec on, 400 msec off giving a ratio of 20:80. We can then harness the benefits of a lower duty cycle which results in longer cooling time for the phaco needle, thus decreasing the amount of phaco energy delivered to the eye. In addition, during the extended “off” time, no energy is delivered and nuclear fragments can be easily aspirated.

When do we want higher or lower duty cycles? The answer depends on the phase of surgery. For sculpting the nucleus, such as with the technique of divide-and-conquer, we need to deliver sufficient energy to be able to cut the grooves. This requires a duty cycle of about 40 to 60%. Once we have the grooves placed in the nucleus and we have achieved cracking resulting in quadrants, we can use a lower duty cycle during the phaco-assisted aspiration of the quadrants. For this quadrant removal, a lower duty cycle of 20 to 40% can be used since the principal force for suction is the fluidics and not the ultrasound.

Using the variable duty cycle programming allows the surgeon to deliver just the right amount of ultrasound energy during each phase of surgery. The concept to remember is that a higher duty cycle results in better cutting power but increased heat generation and more energy-related damage to the corneal endothelium. Using the lower duty cycle allows more fluidic aspiration of nuclear fragments while minimizing heat and phaco power, resulting in clear corneas immediately after surgery. And we all know that clear corneas on post-op day one make for good visual acuity and very satisfied patients.

Some phaco machines give the choice between longitudinal ultrasound power (which is in a back and forth linear motion) and rotational ultrasound power (which is in a circular, elliptical, or torsional pattern). These two modes can also be mixed together to give different ratios.