- Email: [email protected]
Phacoemulsifier occlusion break surge volume reduction
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LABORATORY SCIENCE
Phacoemulsifier occlusion break surge volume reduction Andrew Thorne, BA, David W. Dyk, BS, Douglas Fanney, BS, MBA, Kevin M. Miller, MD
Purpose: To compare the volumetric occlusion break surge responses of phacoemulsification units from 1 company over 3 generations under varying vacuum limits and target intraocular pressure (IOP) settings. Setting: Alcon Research, Ltd., Lake Forest, California, USA. Design: Experimental study. Methods: Three generations of phacoemulsification units (Infiniti Vision System, Centurion Vision System, and Centurion Vision System with Active Sentry upgrades) were tested. Volumetric surge responses were measured after occlusion breaks at vacuum limits of 200 mm Hg, 300 mm Hg, 400 mm Hg, 500 mm Hg, and 600 mm Hg and target IOPs of 30 mm Hg, 55 mm Hg, and 80 mm Hg. An
O
ne of the great remaining challenges to safe and efficient cataract surgery is the occlusion break surge phenomenon.1 Vacuum builds to a preset limit within the aspiration tubing and cassette when the aspiration port of a phacoemulsification system is occluded by lens fragments or other tissues. As vacuum equilibrates between the cassette and the occluded aspiration port, compliant components within the system collapse and trapped air expands.2 At the moment of an occlusion break, these elements recoil to their original dimensions, releasing stored energy and pulling fluid suddenly from the anterior chamber to fill the recoiled volume. A partial or complete collapse of the anterior chamber can ensue, which is termed the occlusion break surge response. Because surges occur on a millisecond timescale, they are too quick for human intervention. This sudden reduction in aqueous volume can traumatize the cornea, iris, posterior capsule, or zonular fibers.3 In extreme cases, the anterior chamber might collapse entirely. Historically, surge responses have been characterized by associated pressure changes within a rigid test chamber.
acrylic test chamber with a piston attached to 3 springs modeled the human eye in this study. The springs were calibrated to mimic volumetric changes in the eye over a wide range of IOPs.
Results: Occlusion break surge volumes varied from 17.4 mL to 153 mL, corresponding to 7% and 61%, respectively, of the aqueous volume in the average phakic eye and to 4% and 33% of the aqueous volume in the average aphakic eye. Conclusion: Occlusion break surge volumes decreased with increasing target IOP, decreasing vacuum limit, and each generational increment in the phacoemulsification system. J Cataract Refract Surg 2018; 44:1491–1496 Q 2018 ASCRS and ESCRS
Supplemental material available at www.jcrsjournal.org.
The measured transient drop in chamber pressure is used as a proxy for the severity of the occlusion break surge response.4,5 More important clinically, however, is the volume change associated with an occlusion break. It would be particularly useful to characterize volume changes as a percentage of total aqueous volume because this has clinical implications. In addition, because volumetric surge depends on ocular compliance, surge responses should be modeled with a test chamber that has a compliance similar to that of the human eye. The spring eye model relies on 3 springs that engage in succession to mimic the compliance of a typical human eye.6 In this model, the springs resist piston movement in a manner that mimics human eye-wall rigidity. A laser sensor monitors displacement of the piston, permitting changes in volume to be measured accurately. The objective of this laboratory evaluation was to measure the volumes after occlusion break surge produced by 3 different phacoemulsification units from the same company, representing 3 generations of technology, under a variety of vacuum limit and target intraocular pressure (IOP) paradigms.
Submitted: September 15, 2017 | Final revision submitted: December 4, 2017 | Accepted: January 2, 2018 From the Stein Eye Institute and Department of Ophthalmology (Thorne, Miller), David Geffen School of Medicine at University of California Los Angeles, Los Angeles, and Alcon Research Ltd. (Dyk, Fanney), Lake Forest, California, USA. Supported in part by Alcon Research Ltd., the Viola Hyde Surgical Research Award at UCLA, and the Albert Sarnoff Endowed Cataract Fund at UCLA. Corresponding author: Kevin M. Miller, MD, Stein Eye Institute, Edie and Lew Wasserman Building, 2nd Floor, 300 Stein Plaza, UCLA, Los Angeles, California, 900957000, USA. E-mail: [email protected]. Q 2018 ASCRS and ESCRS Published by Elsevier Inc.
0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2018.01.032
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LABORATORY SCIENCE: OCCLUSION BREAK SURGE VOLUME REDUCTION
MATERIALS AND METHODS Phacoemulsification Systems and Fluidics Configurations The 3 phacoemulsification units tested were the Infiniti Vision System (first-generation device), the Centurion Vision System in the active-fluidics mode (current-generation device), and the Centurion Vision System with Active Sentry upgrades (next-generation device). All instruments use a peristaltic pump to control aspiration flow. In each experiment, the first-generation device was outfitted with an Infiniti Ozil handpiece, Infiniti Intrepid Plus fluidics management system, and balanced salt solution (BSS, Alcon Laboratories, Inc.) bottle. A bubble suppression insert was not used. The current-generation device was outfitted with a Centurion Ozil handpiece, Centurion active fluidics management system, and Centurion balanced salt solution bag. The next-generation device (upgraded system) was outfitted with an Active Sentry handpiece, an active fluidics management system, and a Centurion balanced salt solution bag. To minimize variations across experiments and achieve reliable and complete occlusion, an Ultrasleeve and a straight 0.9 mm miniflared 0 degree round "blank" tip without an aspiration system bypass hole were used for all experiments. Test Configuration The anterior chamber of the human eye was modeled using a spring eye model.6 The device was calibrated per Alcon protocol at the beginning of each experiment to ensure that its compliance behavior would accurately mimic that of an average human eye. Volumetric changes within the spring eye model were measured as a function of piston displacement using a laser sensor (model LT-9030M, Keyence Corp.). Target IOPs and aspiration pressures were measured using a custom assembly of pressure transducers (model 26PCCFG6G, Honeywell Corp.), and programmable strain gauge amplifiers (model 1169-01-50-200-A, Raetech Corp.). The accuracy of the pressure transducers was checked using a separate factory-calibrated digital pressure meter (model DPM4, Fluke Biomedical). Pressure and volume changes were recorded as voltages on a digital oscilloscope (Waverunner 606Zi, Teledyne LeCroy). Each ultrasound (US) sleeve and tip combination was inserted into an opening in a handpiece test chamber matching the diameter of the proximal end of the sleeve to create a watertight seal. No
leakage was allowed in any experiment. Figure 1 shows a schematic diagram of the test setup. The handpiece and test chamber were positioned at patient eye level on the current-generation device system and the nextgeneration device system. Because the 0 setting for patient eye level on the first-generation device console is 5 cm below that of the 2 Centurion configurations, an additional 5 cm water (H2O) was added to each first-generation device bottle height setting to avoid having to reposition the handpiece and test chamber to achieve consistent test IOPs. Occlusion was initiated using an actuation control box that controlled a pneumatic cylinder (Airpel, Airpot Corp.) connected to a lever arm. When activated, the lever brought a segment of super-soft latex rubber tubing (McMaster-Carr) into contact and flush with the tip of a handpiece under test, thereby creating a watertight seal. The pneumatic cylinder was actuated through a pair of solenoid valves with air regulated at 3.5 pounds per square inch gage, sufficient to ensure full occlusion at the lever’s end. An electronic timer relay connected to the solenoid valves ensured full occlusion for 3.5 seconds before automatically signaling the solenoids to reverse the air supply and break occlusion. The current-generation device and first-generation device systems were run in the “cortex” panel setting so that US would not activate and cut the rubber tubing and compromise the occlusion. The next-generation device system was run in the “pre phaco” setting with power set to 0% to take advantage of the automatic patient eye level and occlusion surge mitigation features while avoiding damage to the rubber tubing. Experiments Each phacoemulsification configuration was primed before each experiment according to the manufacturer’s instructions. The test chamber was primed with every change of fluidics management system or phacoemulsification unit to ensure the evacuation of air bubbles, which are highly compliant. Aspiration flow was kept constant at 30 cc a minute across all trials for consistency. The pump stops turning when vacuum limit is reached in a peristaltic system. Thus, the aspiration flow rate does not appreciably alter surge volume measurements. The first-generation system with gravity fluidics was tested at bottle heights yielding infusion pressures of 41 cm H2O, 75 cm H2O, and 109 cm H2O. The actively controlled fluidics
Figure 1. Schematic of electrical and pneumatic components of the experimental setup (IOP Z intraocular pressure).
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LABORATORY SCIENCE: OCCLUSION BREAK SURGE VOLUME REDUCTION
configurations of the current-generation device and nextgeneration upgraded systems were tested using target IOP settings of 30 mm Hg, 55 mm Hg, and 80 mm Hg. These gravity and active fluidics settings (respectively: 41 cm H2O and 30 mm Hg; 75 cm H2O and 55 mm Hg; 109 cm H2O and 80 mm Hg) yield equivalent infusion pressures at an infusion flow rate of 0 cc/min (full occlusion).7 The conversion from pressure at a given bottle height to target IOP was 1.0 cm H2O Z 0.74 mm Hg. The vacuum limits at which occlusion was broken were 200 mm Hg, 300 mm Hg, 400 mm Hg, 500 mm Hg, and 600 mm Hg. Three fluidics management system cassettes per phacoemulsification system were tested across all fluidics settings. Surge volumes from the 3 fluidics management system tests were averaged for each setting. During the experimental runs, a target IOP and vacuum limit were set for each test. Aspiration flow was then initiated. After steady-state conditions were achieved as determined by oscilloscope readings, occlusion was initiated and the predetermined vacuum limit was achieved. Finally, occlusion was broken suddenly based on the timer relay signal. Piston displacement was captured on the oscilloscope (Figure 2).
Data Analysis Oscilloscope voltage measurements were converted to pressures using the measured sensitivities of the pressure transducers. The pressure measurements were used to confirm that proper IOP and vacuum were achieved. Volume measurements were obtained from the laser output using the following conversion: volume Z (p/4) D2/A, where D is the diameter of the piston (1.9 cm) and A is the laser sensitivity (100 V/cm). The mean surge volume from each of the 3 fluidics management systems for each
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phacoemulsification system under each test condition was then calculated. The aqueous volume of the average phakic eye was chosen to be 0.25 mL (250 mL), the midpoint of the range of volumes (0.15 to 0.35 mL) in the model human eye.8 The aqueous volume of the average aphakic eye was chosen to be 0.465 mL (465 mL). This is the approximate midpoint volume of the lens in 60- to 80-yearold eyes (0.215 mL; range 0.213 to 0.218 mL) added to the aqueous volume of the average phakic eye (0.25 mL).8
RESULTS Average absolute surge volumes were converted to percentage aqueous volume losses for average phakic eyes (Figure 3) and aphakic eyes (Figure 4). Because in general the spread of data between the 3 separate runs for each fluidics management system was less than 1.0%, error bars were not added to the graphs. The spread in surge volumes increased when the target IOP was 30 mm Hg and the vacuum limit at occlusion break was 500 or 600 mm Hg. The largest surge volume that can be measured by the spring eye model is approximately 0.17 cc, or 170 mL. The surge volume from the 1 first-generation device fluidics management system at the 500 mm Hg vacuum limit and 30 mm Hg IOP set point could not be measured accurately because it hit the displacement limit of the piston in the eye model. The subsequent test on this sample at the 600 mm Hg vacuum limit and 30 mm Hg IOP was also not run for the same reason. The remaining 2 first-generation
Figure 2. Fluctuation of volume within the spring eye model during an experimental trial simulating occlusion break surge.
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Figure 3. Percentage aqueous volume lost during an occlusion break surge in the average phakic eye (0.25 mL) by the phacoemulsification system, vacuum limit, and target IOP. Each bar represents the mean of 3 experimental runs (IOP Z intraocular pressure).
device fluidics management system’s surge volumes at the 500 mm Hg vacuum limit and 30 mm Hg IOP did not bottom out the piston. However, the following 600 mm Hg vacuum limit tests on these 2 fluidics management systems caused the piston displacement to reach its limit. As a result, the surge volumes for the first-generation system at these vacuum limits and 30 mm Hg IOP are likely larger than reported. DISCUSSION This experimental study analyzed the volumetric occlusion break surge responses of 3 generations of phacoemulsification instruments from the same company. This study is the first to use the spring eye model to measure the transient changes in anterior chamber volume after occlusion break. With 1 exception,9 previous studies have characterized the occlusion break phenomenon by transient pressure changes in a rigid test chamber, a measurement that lacks clinical relevance.3,10 The Zacharias model9 was unique and
relevant in 2 respects. It used a compliant test chamber rather than a rigid test chamber to model the human eye, and it measured displaced volume using an open pipette. The transient changes in volume within the pipette had to be observed visually or recorded on video. Our study expands on the Zacharias approach by using a compliant eye model that mimics an experimentally determined human eye’s average compliance and adds a laser sensor to precisely measure volume changes. Our experiments showed a wide range of occlusion break surge volumes in the phakic eye and aphakic eye states. A few observations are readily gleaned from inspection of the bar graphs in Figures 3 and 4. First, occlusion break surge volumes increased as vacuum limit increased. This is a well-known phenomenon. Dialing down the vacuum limit is one of the principal ways cataract surgeons manage surge. Second, occlusion break surge volumes decreased with an increasing target IOP. This effect was seen in both the gravity fed and active fluidics configurations.
Figure 4. Percentage aqueous volume lost during an occlusion break surge in the average aphakic eye (0.465 mL) by the phacoemulsification system, vacuum limit, and target IOP. Each bar represents the mean of 3 experimental runs (IOP Z intraocular pressure).
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This is not as intuitive as the vacuum limit effect but is a consequence of the compliance of a human eye. At low IOPs, the eye is relatively soft. Occlusion break surge results in large volume changes with relatively small pressure changes. At high IOPs, the eye is more rigid. Third, occlusion break surge volume losses have been reduced with the introduction of each successive generation of phacoemulsification equipment from this manufacturer. The Centurion system improves upon the Infiniti system through optimized cassette and aspiration tubing design. The primary modification introduced in the transition from the Centurion system to the Centurion Active Sentry system was the addition of a surge mitigation feature. This dampens the surge volume demand once the onset of an occlusion break is detected by the Active Sentry irrigation line pressure sensor. A final observation is that surge volumes as a percentage of total aqueous volume were much greater in the phakic eye than in the aphakic eye. The largest surge volume, measuring 154 mL, occurred on the first-generation system at a target IOP of 30 mm Hg and a vacuum limit of 600 mm Hg. As stated, the actual surge was likely higher; however, our measurement was limited by the constraints of piston displacement. This measurement corresponds to 62% of the aqueous volume of an average phakic eye. Had we modeled a hyperopic eye with a small aqueous volume of 150 mL, the surge loss would have collapsed the anterior chamber. The lowest surge volume of 17.4 mL corresponds to 7% of the volume of the average phakic eye. It occurred using the currentgeneration system at a target IOP of 80 mm Hg and a vacuum limit of 200 mm Hg. The next-generation and first-generation systems were very similar in their responses under these conditions. Generational improvements in fluidics technology are impressive. The surge volumes under the most strenuous target IOP of 30 mm Hg with the Centurion system with Active Sentry upgrades were comparable to those of the base Centurion system under the most favorable target IOP of 80 mm Hg, regardless of the vacuum limit. The decline in occlusion break surge volume with each generation has implications in the operating room. Negative effects of increased intraoperative IOP during surgery have been reported. A study of porcine eyes11 found that a lower IOP during simulated surgery led to less damage to the corneal endothelium. Highly myopic eyes are more comfortable when IOP is low. The Centurion Active Sentry system keeps the surge volume under 25% in the normal phakic state, even with a target IOP of 30 mm Hg. Thus, the surgeon can work at IOPs that are comfortable for the patient and safe for the cornea and optic nerve without risking sudden chamber collapse. Given various ethical considerations, it would not be possible to perform this study in living human eyes. When such experiments are performed on human and porcine cadaver eyes, reproducibility is usually poor. The spring eye model was patterned after compliance measurements of fresh human cadaver eyes. Because the spring eye fixture is a mechanical model rather than a tissue model, it
can be subjected to repeated testing without measureable fatigue effects. In addition, the spring eye model eliminates the effect of incision leakage and intereye variability. Our experiments should be highly reproducible by anyone with access to the model. Occlusion break surge is one of a few safety hazards during cataract surgery. Until the Zacharias model,9 surge had yet to be measured precisely in terms of volume loss. The spring eye model adds convenience and accuracy to the Zacharias approach. This study found a stepwise decrease in occlusion break surge volume with decreasing vacuum limit, increasing target IOP, and each new generation of phacoemulsification equipment brought to market. In this study, we compared the influence of phacoemulsification technology advances over time from the same manufacturer. An obvious next step would be to compare the occlusion break surge response of latest technology phacoemulsification instruments from different manufacturers. This we have done.12
WHAT WAS KNOWN The occlusion break surge response of phacoemulsification systems was characterized previously by transient changes in pressure within a rigid test chamber.
WHAT THIS PAPER ADDS Occlusion break surge volumes can now be measured using a spring eye model and those volumes reported as percentage of aqueous volume loss. Occlusion break surge volumes decreased with decreasing vacuum limit and increasing target IOP. Each new generation of phacoemulsification equipment from the same manufacturer reduced surge volumes at equivalent vacuum limits and target IOPs.
REFERENCES 1. Han YK. Fluidics of phacoemulsification systems. In: BissenMiyajima H, Koch DD, Weikert MP, eds. Cataract Surgery: Maximizing Outcomes Through Research. Tokyo, Japan, Springer Japan, 2014; 113–126 2. Nejad M, Injev VP, Miller KM. Laboratory analysis of phacoemulsifier compliance and capacity. J Cataract Refract Surg 2012; 38:2019–2028 3. Sharif-Kashani P, Fanney D, Injev V. Comparison of occlusion break responses and vacuum rise times of phacoemulsification systems. BMC Ophthalmol 2014; 14:96. Available at: http://www.biomedcentral.com /content/pdf/1471-2415-14-96.pdf. Accessed March 13, 2018 4. Kageyama T, Yagushi S. [In vitro evaluation of pressure fluctuations with differing height of the infusion bottle in phacoemulsification] [Japanese]. Nippon Ganka Gakkai Zasshi 2000; 104:312–316; (English abstract of article published in Jpn J Ophthalmol 2000; 44:690–691) 5. Georgescu D, Payne M, Olson RJ. Objective measurement of postocclusion surge during phacoemulsification in human eye-bank eyes. Am J Ophthalmol 2007; 143:437–440 6. Dyk DW, Miller KM. Mechanical model of human eye compliance for volumetric occlusion break surge measurements. J Cataract Refract Surg 2018; 44:231–236 7. Nicoli CM, Dimalanta R, Miller KM. Experimental anterior chamber maintenance in active versus passive phacoemulsification fluidics systems. J Cataract Refract Surg 2016; 42:157–162 8. Report of the Task Group on Reference Man. A report prepared by a task group of Committee 2 of the International Commission on Radiological
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9. 10.
11.
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Protection adopted by the commission in October 1974. International Commission on Radiological Protection no. 23. Ann ICRP 1975; issue 1; errata iii. Available at: http://journals.sagepub.com/doi/pdf/10.1016/S007 4-2740%2875%2980015-8. Errata available at: http://journals.sagepub .com/doi/pdf/10.1016/0146-6453(79)90123-4. Accessed March 13, 2018 Zacharias J, Zacharias S. Volume-based characterization of postocculsion surge. J Cataract Refract Surg 2005; 31:1976–1982 Wilbrandt HR. Comparative analysis of the fluidics of the AMO Prestige, Alcon Legacy, and Storz Premiere phacoemulsification systems. J Cataract Refract Surg 1997; 23:766–780 Suzuki H, Oki K, Shiwa T, Oharazawa H, Takahashi H. Effect of bottle height on the corneal endothelium during phacoemulsification. J Cataract Refract Surg 2009; 35:2014–2017 Aravena C, Dyk DW, Thorne A, Fanney D, Miller KM. Aqueous volume loss associated with occlusion break surge in phacoemulsifiers from 4 different manufacturers. J Cataract Refract Surg 2018; 44:884–888
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Disclosures: Mr. Dyk and Mr. Fanney are employees of Alcon Research Ltd. Dr. Miller is an investigator in and consultant to Alcon Laboratories, Inc. Mr. Thorne has no financial or proprietary interest in any material or method mentioned.
First author: Andrew Thorne, BA Stein Eye Institute and Department of Ophthalmology, David Geffen School of Medicine at University of California Los Angeles, Los Angeles, USA
LABORATORY SCIENCE
Phacoemulsifier occlusion break surge volume reduction Andrew Thorne, BA, David W. Dyk, BS, Douglas Fanney, BS, MBA, Kevin M. Miller, MD
Purpose: To compare the volumetric occlusion break surge responses of phacoemulsification units from 1 company over 3 generations under varying vacuum limits and target intraocular pressure (IOP) settings. Setting: Alcon Research, Ltd., Lake Forest, California, USA. Design: Experimental study. Methods: Three generations of phacoemulsification units (Infiniti Vision System, Centurion Vision System, and Centurion Vision System with Active Sentry upgrades) were tested. Volumetric surge responses were measured after occlusion breaks at vacuum limits of 200 mm Hg, 300 mm Hg, 400 mm Hg, 500 mm Hg, and 600 mm Hg and target IOPs of 30 mm Hg, 55 mm Hg, and 80 mm Hg. An
O
ne of the great remaining challenges to safe and efficient cataract surgery is the occlusion break surge phenomenon.1 Vacuum builds to a preset limit within the aspiration tubing and cassette when the aspiration port of a phacoemulsification system is occluded by lens fragments or other tissues. As vacuum equilibrates between the cassette and the occluded aspiration port, compliant components within the system collapse and trapped air expands.2 At the moment of an occlusion break, these elements recoil to their original dimensions, releasing stored energy and pulling fluid suddenly from the anterior chamber to fill the recoiled volume. A partial or complete collapse of the anterior chamber can ensue, which is termed the occlusion break surge response. Because surges occur on a millisecond timescale, they are too quick for human intervention. This sudden reduction in aqueous volume can traumatize the cornea, iris, posterior capsule, or zonular fibers.3 In extreme cases, the anterior chamber might collapse entirely. Historically, surge responses have been characterized by associated pressure changes within a rigid test chamber.
acrylic test chamber with a piston attached to 3 springs modeled the human eye in this study. The springs were calibrated to mimic volumetric changes in the eye over a wide range of IOPs.
Results: Occlusion break surge volumes varied from 17.4 mL to 153 mL, corresponding to 7% and 61%, respectively, of the aqueous volume in the average phakic eye and to 4% and 33% of the aqueous volume in the average aphakic eye. Conclusion: Occlusion break surge volumes decreased with increasing target IOP, decreasing vacuum limit, and each generational increment in the phacoemulsification system. J Cataract Refract Surg 2018; 44:1491–1496 Q 2018 ASCRS and ESCRS
Supplemental material available at www.jcrsjournal.org.
The measured transient drop in chamber pressure is used as a proxy for the severity of the occlusion break surge response.4,5 More important clinically, however, is the volume change associated with an occlusion break. It would be particularly useful to characterize volume changes as a percentage of total aqueous volume because this has clinical implications. In addition, because volumetric surge depends on ocular compliance, surge responses should be modeled with a test chamber that has a compliance similar to that of the human eye. The spring eye model relies on 3 springs that engage in succession to mimic the compliance of a typical human eye.6 In this model, the springs resist piston movement in a manner that mimics human eye-wall rigidity. A laser sensor monitors displacement of the piston, permitting changes in volume to be measured accurately. The objective of this laboratory evaluation was to measure the volumes after occlusion break surge produced by 3 different phacoemulsification units from the same company, representing 3 generations of technology, under a variety of vacuum limit and target intraocular pressure (IOP) paradigms.
Submitted: September 15, 2017 | Final revision submitted: December 4, 2017 | Accepted: January 2, 2018 From the Stein Eye Institute and Department of Ophthalmology (Thorne, Miller), David Geffen School of Medicine at University of California Los Angeles, Los Angeles, and Alcon Research Ltd. (Dyk, Fanney), Lake Forest, California, USA. Supported in part by Alcon Research Ltd., the Viola Hyde Surgical Research Award at UCLA, and the Albert Sarnoff Endowed Cataract Fund at UCLA. Corresponding author: Kevin M. Miller, MD, Stein Eye Institute, Edie and Lew Wasserman Building, 2nd Floor, 300 Stein Plaza, UCLA, Los Angeles, California, 900957000, USA. E-mail: [email protected]. Q 2018 ASCRS and ESCRS Published by Elsevier Inc.
0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2018.01.032
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LABORATORY SCIENCE: OCCLUSION BREAK SURGE VOLUME REDUCTION
MATERIALS AND METHODS Phacoemulsification Systems and Fluidics Configurations The 3 phacoemulsification units tested were the Infiniti Vision System (first-generation device), the Centurion Vision System in the active-fluidics mode (current-generation device), and the Centurion Vision System with Active Sentry upgrades (next-generation device). All instruments use a peristaltic pump to control aspiration flow. In each experiment, the first-generation device was outfitted with an Infiniti Ozil handpiece, Infiniti Intrepid Plus fluidics management system, and balanced salt solution (BSS, Alcon Laboratories, Inc.) bottle. A bubble suppression insert was not used. The current-generation device was outfitted with a Centurion Ozil handpiece, Centurion active fluidics management system, and Centurion balanced salt solution bag. The next-generation device (upgraded system) was outfitted with an Active Sentry handpiece, an active fluidics management system, and a Centurion balanced salt solution bag. To minimize variations across experiments and achieve reliable and complete occlusion, an Ultrasleeve and a straight 0.9 mm miniflared 0 degree round "blank" tip without an aspiration system bypass hole were used for all experiments. Test Configuration The anterior chamber of the human eye was modeled using a spring eye model.6 The device was calibrated per Alcon protocol at the beginning of each experiment to ensure that its compliance behavior would accurately mimic that of an average human eye. Volumetric changes within the spring eye model were measured as a function of piston displacement using a laser sensor (model LT-9030M, Keyence Corp.). Target IOPs and aspiration pressures were measured using a custom assembly of pressure transducers (model 26PCCFG6G, Honeywell Corp.), and programmable strain gauge amplifiers (model 1169-01-50-200-A, Raetech Corp.). The accuracy of the pressure transducers was checked using a separate factory-calibrated digital pressure meter (model DPM4, Fluke Biomedical). Pressure and volume changes were recorded as voltages on a digital oscilloscope (Waverunner 606Zi, Teledyne LeCroy). Each ultrasound (US) sleeve and tip combination was inserted into an opening in a handpiece test chamber matching the diameter of the proximal end of the sleeve to create a watertight seal. No
leakage was allowed in any experiment. Figure 1 shows a schematic diagram of the test setup. The handpiece and test chamber were positioned at patient eye level on the current-generation device system and the nextgeneration device system. Because the 0 setting for patient eye level on the first-generation device console is 5 cm below that of the 2 Centurion configurations, an additional 5 cm water (H2O) was added to each first-generation device bottle height setting to avoid having to reposition the handpiece and test chamber to achieve consistent test IOPs. Occlusion was initiated using an actuation control box that controlled a pneumatic cylinder (Airpel, Airpot Corp.) connected to a lever arm. When activated, the lever brought a segment of super-soft latex rubber tubing (McMaster-Carr) into contact and flush with the tip of a handpiece under test, thereby creating a watertight seal. The pneumatic cylinder was actuated through a pair of solenoid valves with air regulated at 3.5 pounds per square inch gage, sufficient to ensure full occlusion at the lever’s end. An electronic timer relay connected to the solenoid valves ensured full occlusion for 3.5 seconds before automatically signaling the solenoids to reverse the air supply and break occlusion. The current-generation device and first-generation device systems were run in the “cortex” panel setting so that US would not activate and cut the rubber tubing and compromise the occlusion. The next-generation device system was run in the “pre phaco” setting with power set to 0% to take advantage of the automatic patient eye level and occlusion surge mitigation features while avoiding damage to the rubber tubing. Experiments Each phacoemulsification configuration was primed before each experiment according to the manufacturer’s instructions. The test chamber was primed with every change of fluidics management system or phacoemulsification unit to ensure the evacuation of air bubbles, which are highly compliant. Aspiration flow was kept constant at 30 cc a minute across all trials for consistency. The pump stops turning when vacuum limit is reached in a peristaltic system. Thus, the aspiration flow rate does not appreciably alter surge volume measurements. The first-generation system with gravity fluidics was tested at bottle heights yielding infusion pressures of 41 cm H2O, 75 cm H2O, and 109 cm H2O. The actively controlled fluidics
Figure 1. Schematic of electrical and pneumatic components of the experimental setup (IOP Z intraocular pressure).
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LABORATORY SCIENCE: OCCLUSION BREAK SURGE VOLUME REDUCTION
configurations of the current-generation device and nextgeneration upgraded systems were tested using target IOP settings of 30 mm Hg, 55 mm Hg, and 80 mm Hg. These gravity and active fluidics settings (respectively: 41 cm H2O and 30 mm Hg; 75 cm H2O and 55 mm Hg; 109 cm H2O and 80 mm Hg) yield equivalent infusion pressures at an infusion flow rate of 0 cc/min (full occlusion).7 The conversion from pressure at a given bottle height to target IOP was 1.0 cm H2O Z 0.74 mm Hg. The vacuum limits at which occlusion was broken were 200 mm Hg, 300 mm Hg, 400 mm Hg, 500 mm Hg, and 600 mm Hg. Three fluidics management system cassettes per phacoemulsification system were tested across all fluidics settings. Surge volumes from the 3 fluidics management system tests were averaged for each setting. During the experimental runs, a target IOP and vacuum limit were set for each test. Aspiration flow was then initiated. After steady-state conditions were achieved as determined by oscilloscope readings, occlusion was initiated and the predetermined vacuum limit was achieved. Finally, occlusion was broken suddenly based on the timer relay signal. Piston displacement was captured on the oscilloscope (Figure 2).
Data Analysis Oscilloscope voltage measurements were converted to pressures using the measured sensitivities of the pressure transducers. The pressure measurements were used to confirm that proper IOP and vacuum were achieved. Volume measurements were obtained from the laser output using the following conversion: volume Z (p/4) D2/A, where D is the diameter of the piston (1.9 cm) and A is the laser sensitivity (100 V/cm). The mean surge volume from each of the 3 fluidics management systems for each
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phacoemulsification system under each test condition was then calculated. The aqueous volume of the average phakic eye was chosen to be 0.25 mL (250 mL), the midpoint of the range of volumes (0.15 to 0.35 mL) in the model human eye.8 The aqueous volume of the average aphakic eye was chosen to be 0.465 mL (465 mL). This is the approximate midpoint volume of the lens in 60- to 80-yearold eyes (0.215 mL; range 0.213 to 0.218 mL) added to the aqueous volume of the average phakic eye (0.25 mL).8
RESULTS Average absolute surge volumes were converted to percentage aqueous volume losses for average phakic eyes (Figure 3) and aphakic eyes (Figure 4). Because in general the spread of data between the 3 separate runs for each fluidics management system was less than 1.0%, error bars were not added to the graphs. The spread in surge volumes increased when the target IOP was 30 mm Hg and the vacuum limit at occlusion break was 500 or 600 mm Hg. The largest surge volume that can be measured by the spring eye model is approximately 0.17 cc, or 170 mL. The surge volume from the 1 first-generation device fluidics management system at the 500 mm Hg vacuum limit and 30 mm Hg IOP set point could not be measured accurately because it hit the displacement limit of the piston in the eye model. The subsequent test on this sample at the 600 mm Hg vacuum limit and 30 mm Hg IOP was also not run for the same reason. The remaining 2 first-generation
Figure 2. Fluctuation of volume within the spring eye model during an experimental trial simulating occlusion break surge.
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LABORATORY SCIENCE: OCCLUSION BREAK SURGE VOLUME REDUCTION
Figure 3. Percentage aqueous volume lost during an occlusion break surge in the average phakic eye (0.25 mL) by the phacoemulsification system, vacuum limit, and target IOP. Each bar represents the mean of 3 experimental runs (IOP Z intraocular pressure).
device fluidics management system’s surge volumes at the 500 mm Hg vacuum limit and 30 mm Hg IOP did not bottom out the piston. However, the following 600 mm Hg vacuum limit tests on these 2 fluidics management systems caused the piston displacement to reach its limit. As a result, the surge volumes for the first-generation system at these vacuum limits and 30 mm Hg IOP are likely larger than reported. DISCUSSION This experimental study analyzed the volumetric occlusion break surge responses of 3 generations of phacoemulsification instruments from the same company. This study is the first to use the spring eye model to measure the transient changes in anterior chamber volume after occlusion break. With 1 exception,9 previous studies have characterized the occlusion break phenomenon by transient pressure changes in a rigid test chamber, a measurement that lacks clinical relevance.3,10 The Zacharias model9 was unique and
relevant in 2 respects. It used a compliant test chamber rather than a rigid test chamber to model the human eye, and it measured displaced volume using an open pipette. The transient changes in volume within the pipette had to be observed visually or recorded on video. Our study expands on the Zacharias approach by using a compliant eye model that mimics an experimentally determined human eye’s average compliance and adds a laser sensor to precisely measure volume changes. Our experiments showed a wide range of occlusion break surge volumes in the phakic eye and aphakic eye states. A few observations are readily gleaned from inspection of the bar graphs in Figures 3 and 4. First, occlusion break surge volumes increased as vacuum limit increased. This is a well-known phenomenon. Dialing down the vacuum limit is one of the principal ways cataract surgeons manage surge. Second, occlusion break surge volumes decreased with an increasing target IOP. This effect was seen in both the gravity fed and active fluidics configurations.
Figure 4. Percentage aqueous volume lost during an occlusion break surge in the average aphakic eye (0.465 mL) by the phacoemulsification system, vacuum limit, and target IOP. Each bar represents the mean of 3 experimental runs (IOP Z intraocular pressure).
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This is not as intuitive as the vacuum limit effect but is a consequence of the compliance of a human eye. At low IOPs, the eye is relatively soft. Occlusion break surge results in large volume changes with relatively small pressure changes. At high IOPs, the eye is more rigid. Third, occlusion break surge volume losses have been reduced with the introduction of each successive generation of phacoemulsification equipment from this manufacturer. The Centurion system improves upon the Infiniti system through optimized cassette and aspiration tubing design. The primary modification introduced in the transition from the Centurion system to the Centurion Active Sentry system was the addition of a surge mitigation feature. This dampens the surge volume demand once the onset of an occlusion break is detected by the Active Sentry irrigation line pressure sensor. A final observation is that surge volumes as a percentage of total aqueous volume were much greater in the phakic eye than in the aphakic eye. The largest surge volume, measuring 154 mL, occurred on the first-generation system at a target IOP of 30 mm Hg and a vacuum limit of 600 mm Hg. As stated, the actual surge was likely higher; however, our measurement was limited by the constraints of piston displacement. This measurement corresponds to 62% of the aqueous volume of an average phakic eye. Had we modeled a hyperopic eye with a small aqueous volume of 150 mL, the surge loss would have collapsed the anterior chamber. The lowest surge volume of 17.4 mL corresponds to 7% of the volume of the average phakic eye. It occurred using the currentgeneration system at a target IOP of 80 mm Hg and a vacuum limit of 200 mm Hg. The next-generation and first-generation systems were very similar in their responses under these conditions. Generational improvements in fluidics technology are impressive. The surge volumes under the most strenuous target IOP of 30 mm Hg with the Centurion system with Active Sentry upgrades were comparable to those of the base Centurion system under the most favorable target IOP of 80 mm Hg, regardless of the vacuum limit. The decline in occlusion break surge volume with each generation has implications in the operating room. Negative effects of increased intraoperative IOP during surgery have been reported. A study of porcine eyes11 found that a lower IOP during simulated surgery led to less damage to the corneal endothelium. Highly myopic eyes are more comfortable when IOP is low. The Centurion Active Sentry system keeps the surge volume under 25% in the normal phakic state, even with a target IOP of 30 mm Hg. Thus, the surgeon can work at IOPs that are comfortable for the patient and safe for the cornea and optic nerve without risking sudden chamber collapse. Given various ethical considerations, it would not be possible to perform this study in living human eyes. When such experiments are performed on human and porcine cadaver eyes, reproducibility is usually poor. The spring eye model was patterned after compliance measurements of fresh human cadaver eyes. Because the spring eye fixture is a mechanical model rather than a tissue model, it
can be subjected to repeated testing without measureable fatigue effects. In addition, the spring eye model eliminates the effect of incision leakage and intereye variability. Our experiments should be highly reproducible by anyone with access to the model. Occlusion break surge is one of a few safety hazards during cataract surgery. Until the Zacharias model,9 surge had yet to be measured precisely in terms of volume loss. The spring eye model adds convenience and accuracy to the Zacharias approach. This study found a stepwise decrease in occlusion break surge volume with decreasing vacuum limit, increasing target IOP, and each new generation of phacoemulsification equipment brought to market. In this study, we compared the influence of phacoemulsification technology advances over time from the same manufacturer. An obvious next step would be to compare the occlusion break surge response of latest technology phacoemulsification instruments from different manufacturers. This we have done.12
WHAT WAS KNOWN The occlusion break surge response of phacoemulsification systems was characterized previously by transient changes in pressure within a rigid test chamber.
WHAT THIS PAPER ADDS Occlusion break surge volumes can now be measured using a spring eye model and those volumes reported as percentage of aqueous volume loss. Occlusion break surge volumes decreased with decreasing vacuum limit and increasing target IOP. Each new generation of phacoemulsification equipment from the same manufacturer reduced surge volumes at equivalent vacuum limits and target IOPs.
REFERENCES 1. Han YK. Fluidics of phacoemulsification systems. In: BissenMiyajima H, Koch DD, Weikert MP, eds. Cataract Surgery: Maximizing Outcomes Through Research. Tokyo, Japan, Springer Japan, 2014; 113–126 2. Nejad M, Injev VP, Miller KM. Laboratory analysis of phacoemulsifier compliance and capacity. J Cataract Refract Surg 2012; 38:2019–2028 3. Sharif-Kashani P, Fanney D, Injev V. Comparison of occlusion break responses and vacuum rise times of phacoemulsification systems. BMC Ophthalmol 2014; 14:96. Available at: http://www.biomedcentral.com /content/pdf/1471-2415-14-96.pdf. Accessed March 13, 2018 4. Kageyama T, Yagushi S. [In vitro evaluation of pressure fluctuations with differing height of the infusion bottle in phacoemulsification] [Japanese]. Nippon Ganka Gakkai Zasshi 2000; 104:312–316; (English abstract of article published in Jpn J Ophthalmol 2000; 44:690–691) 5. Georgescu D, Payne M, Olson RJ. Objective measurement of postocclusion surge during phacoemulsification in human eye-bank eyes. Am J Ophthalmol 2007; 143:437–440 6. Dyk DW, Miller KM. Mechanical model of human eye compliance for volumetric occlusion break surge measurements. J Cataract Refract Surg 2018; 44:231–236 7. Nicoli CM, Dimalanta R, Miller KM. Experimental anterior chamber maintenance in active versus passive phacoemulsification fluidics systems. J Cataract Refract Surg 2016; 42:157–162 8. Report of the Task Group on Reference Man. A report prepared by a task group of Committee 2 of the International Commission on Radiological
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Protection adopted by the commission in October 1974. International Commission on Radiological Protection no. 23. Ann ICRP 1975; issue 1; errata iii. Available at: http://journals.sagepub.com/doi/pdf/10.1016/S007 4-2740%2875%2980015-8. Errata available at: http://journals.sagepub .com/doi/pdf/10.1016/0146-6453(79)90123-4. Accessed March 13, 2018 Zacharias J, Zacharias S. Volume-based characterization of postocculsion surge. J Cataract Refract Surg 2005; 31:1976–1982 Wilbrandt HR. Comparative analysis of the fluidics of the AMO Prestige, Alcon Legacy, and Storz Premiere phacoemulsification systems. J Cataract Refract Surg 1997; 23:766–780 Suzuki H, Oki K, Shiwa T, Oharazawa H, Takahashi H. Effect of bottle height on the corneal endothelium during phacoemulsification. J Cataract Refract Surg 2009; 35:2014–2017 Aravena C, Dyk DW, Thorne A, Fanney D, Miller KM. Aqueous volume loss associated with occlusion break surge in phacoemulsifiers from 4 different manufacturers. J Cataract Refract Surg 2018; 44:884–888
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Disclosures: Mr. Dyk and Mr. Fanney are employees of Alcon Research Ltd. Dr. Miller is an investigator in and consultant to Alcon Laboratories, Inc. Mr. Thorne has no financial or proprietary interest in any material or method mentioned.
First author: Andrew Thorne, BA Stein Eye Institute and Department of Ophthalmology, David Geffen School of Medicine at University of California Los Angeles, Los Angeles, USA