Heat generation studies test concept of cold phaco

San Francisco-Heat generation during phacoemulsification while varying pulse rate and duty cycle was investigated in two experimental settings, reported Jaime Zacharias, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

Studies designed to investigate the influence of changing pulse rate and duty cycle on heat generation at the wound site during phacoemulsification provide evidence to refute the idea that surgery using hyperpulse settings (>15 pps) has an advantage for minimizing temperature increase relative to procedures performed with slightly lower pulse frequencies that can be achieved with standard phaco equipment, said Dr. Zacharias, of the Pasteur Clinic, Santiago, Chile.

Dr. Zacharias measured heat at the incision site in two experimental settings-one involved operating in porcine eyes and the other used an artificial wound-like setup that was built around a phaco needle equipped with a silicone sleeve.

Wound temperature assessed

When the duty cycle was held at 50%, wound temperature increased as the pulse rate was raised progressively from 1 to 6 pps in the simulated wound model and from 1 to 3 pps in the animal eyes. However, in both experiments, as the pulse rate was raised above those thresholds, there were no further changes in wound temperature. For the variations in duty cycle, the temperature at the wound was seen to increase proportionally as duty cycle increased.

"There appears to be confusion regarding the effects of various power modulation schemes that permit higher pulse rates and adjustable duty cycles on heat generation and risk of wound burn," said Dr. Zacharias, explaining his motivation for undertaking this study. "In particular, I believe the statement that higher ultrasonic pulse rates produce less heat in an equivalent power basis is controversial and not supported by good scientific evidence.

"The results of the present studies demonstrate that there is a risk of wound burn with the use of very low pulse rates that are the equivalent of ultrasound power being continuously on and then continuously off for relatively long periods," Dr. Zacharias said. "On the contrary, we found there is no difference in heat generation at the incision site during surgery using higher pulse frequencies in the range of 10 to 15 pps that can be achieved with standard phaco machines compared with even higher settings attainable only with hyperpulse-capable equipment. In other words, the term 'cold phaco' is misleading because increasing the pulse rate into the hyperpulse range does not result in any reduction of the temperature at the wound site."

In the experiment performed with animal eyes, the temperature-sensitive film was implanted midstroma and a typical cataract incision was constructed approximately 200 μm deeper than the position of the temperature-sensitive film. Then, the phaco probe with a silicone sleeve was inserted into the wound. The temperature-sensitive film was compressed over the silicone sleeve in the artificial wound setup.

Dr. Zacharias contrasted his method for measuring heat generation with other studies that have used infrared thermal imaging.

"The cornea is a poor conductor of infrared energy and so the infrared camera measures temperature near the surface but not deeper in the incision," he concluded. "This temperature-sensitive thin film implanted at the midstroma captures more relevant data."