"True" IOP Control: A CONSTELLATION® Vision System Advanced Technology
A first in vitrectomy systems, integrated pressurized infusion with IOP control compensates and provides control of infusion pressure, producing more stable pressures within +/− 2 mmHg, resulting in more predictable, more efficient surgery.
By Christopher D. Riemann, MD, and Daniel M. Miller, MD, PhD
The CONSTELLATION® Vision System has generated much interest among retina specialists. For those of us who were happy with our ACCURUS® Surgical Systems, however, it was something of a hard sell. We asked: "Why invest a lot of money on a brand new vitrectomy system when there's nothing wrong with the vitrectomy system we have? The ACCURUS® Surgical System is a beautiful machine. It's easy to use. It does everything it's supposed to do."
Then we had the opportunity to road-test the CONSTELLATION® Vision System and find out firsthand if its theoretical performance advantages, such as the High Performance 5000 cpm ULTRAVIT® Probe, the integrated pressurized infusion with IOP control and the V-LOCITY® Efficiency Components, made a significant positive difference in practice. What we learned convinced us to make the investment.
We found that because setup is simpler and more straightforward for our surgical assistants, turnover is faster with the CONSTELLATION® Vision System than with the ACCURUS® Surgical System. The new efficiency features help our facility expedite our day. We also found that the CONSTELLATION® system improves intraoperative and intraocular surgical precision and safety. Two notable features are the ultra-high cut rate ULTRAVIT® Probe (5000 cuts per minute) and the surgeon-selected duty cycle control.
Another major advantage of the new system is the IOP control feature. For the first time, we have a machine that allows us to control IOP to within +/− 2 mmHg.
Evolution of infusion pressure control technology
Intraocular pressure is a critical factor in retinal surgery. We raise IOP to control bleeding. We lower it in cases where there is retinal ischemia or advanced glaucoma. It can fluctuate significantly during surgery. Yet, until now, we have not had a vitrectomy machine that can regulate IOP in "real time."
It used to be, and it still is with some vitrectomy systems, that infusion pressure was created by controlling the height at which the bottle was hung over the patient. We called this the gravity system, and it worked. You could measure the distance from the bottle to the patient and adjust the pressure that way. The higher the bottle, the higher the pressure. The problem is that you cannot adjust the pressure quickly, and there can be huge changes, lag times and pressure surges when changing bottle height and when beginning to aspirate or cut.
The ACCURUS® system improved upon the gravity system with vented gas forced infusion (VGFI). With this system, the bottle is set at a fixed height and then pressurized by placing an air pressure head against it. One click of the foot pedal instantly drives a higher air pressure head into the bottle to raise fluid pressure in the eye. The limiting factor with the VGFI tubing set is that you need to change the gas and fluid pressure manually. If you are doing something that aspirates more, then you must manually increase the gas pressure, driving fluid out of the infusion bottle into the eye. Conversely, if you are doing something where you are not aspirating a lot — peeling a membrane, for example — then you must manually lower the eye pressure, adding more variables to the many we must consider when performing vitrectomy surgery. To simplify this aspect of the surgery, many surgeons use slightly higher than needed pressures for low and medium aspiration scenarios and increase the pressure when aspirating a lot or when there is a need for a higher pressure.
The CONSTELLATION® Vision System incorporates an IOP compensation feature — called integrated pressurized infusion with IOP control — which uses noninvasive flow sensors to measure what we are infusing into the eye, and it directly compensates for that in real time to maintain a desired pressure. This is a tremendous advantage in small-gauge pars plana vitrectomy. It enables us to operate at lower IOP and prevents "outrunning your infusion" when performing maximal cut and/or aspiration. So if you step on the gas and are aspirating a lot, the IOP control will automatically move more volume through the infusion cannula to compensate for increased aspiration, resulting in far greater stability of the posterior segment. We have found it is easier to control bleeding in difficult diabetic cases, for example, because there is less fluctuation when we raise IOP.
CONSTELLATION® noninvasive flow sensor.
A Closer Look at How the CONSTELLATION® Vision System Controls IOPThe CONSTELLATION® Vision System can set and maintain intraocular pressure in real time to +/− 2 mmHg. The system uses a sensor embedded in the cassette to measure infusion flow. Because the geometry of the infusion tubing, stopcock and cannula are precisely known, hydraulic resistance to flow through the system is also precisely defined. Fluid dynamics are actively calculated for the full range of possible infusion flow rates, and bottle pressure is appropriately compensated to give truly precise control of the intraocular pressure at the tip of the infusion cannula within the eye in real time. As infusion fluid flows into the globe, the system automatically anticipates the resulting pressure drop and automatically increases the infusion pressure to match and compensate for the pressure drop across the infusion cannula. For the first time, we know actual, as opposed to estimated, IOPs within +/− 2 mmHg and can be confident of these during the surgical case.
Precise control of globe stability
Many diseases of the retina are plagued by vasculopathy. Half of the patients in our retina clinic are diabetics, and diabetes is associated with poor ocular blood flow. Of the nondiabetic patients we care for in the retina clinic and the OR, many are known vasculopaths, and of those who are not, many are old and, therefore, may have undiagnosed vasculopathies. To maintain a stable globe intraoperatively with the ACCURUS® machine, we would routinely run a pressure at 30 mmHg or 35 mmHg. These pressures are generally considered unhealthy for a healthy eye, and they could be outright dangerous for a vasculopathic eye or an eye with advanced glaucoma.
With the CONSTELLATION® IOP control feature, we routinely run a pressure at 15 mmHg to 20 mmHg, and sometimes as low as 5 mmHg to 10 mmHg in select high-risk patients. The IOP control feature allows us to do this with consistent and usually improved globe stability when compared with the ACCURUS® vitrectomy system. This is remarkable, considering the higher efficiency of the CONSTELLATION® cutter, which moves vitreous out of the eye more efficiently than the ACCURUS® system does. The CONSTELLATION® cutter has an adjustable optimized duty cycle, an 8% higher aspiration vacuum (650 mmHg vs 600 mmHg), and a larger vitreous cutter port size.
Transducers ultrasonically measure the flow through the IOP Control Flow Sensor upstream and downstream. If there is no movement, then it knows the eye is stable. If there is fluid movement, then it will adjust the IOP to +/− 2 mmHg.
The CONSTELLATION® Vision System's new vitreous probe provides a tremendous advantage. Its large port and enhanced fluidics result in improved vitreous cutting efficiency and increased aspiration. This meshes well with the ability to more precisely control IOP.
Although we are moving vitreous out of the eye more efficiently with the CONSTELLATION® system, the IOP compensation module maintains globe stability and a satisfactory IOP while utilizing pressures that, in some cases, are half of what we used to use with the ACCURUS® Surgical System. That is a beautiful thing to reflect on when doing a vitrectomy on an ischemic diabetic patient or a patient with advanced glaucoma, or when doing a vitrectomy for an epiretinal membrane or any other disease on an elderly patient who may have some underlying carotid vascular disease and, therefore, perhaps an undiagnosed ocular ischemic syndrome. There is no doubt that running lower pressures without any compromise in the stability of the intraocular environment is desirable.
Safety and efficiency
With the improved efficiency of the cutter, especially in 25-gauge, it is important to improve the functionality of the infusion, and Alcon has done that beautifully with the integrated pressurized infusion with IOP control of the CONSTELLATION® Vision System. Not only is the system able to keep up with the increased demands on the infusion cannula caused by the more efficient vitreous cutter, but it allows us to operate at a lower infusion pressure. That translates to more efficient and safer surgery in patients who have vascular compromise.
Christopher D. Riemann, MD, and Daniel M. Miller, MD, PhD, practice at the Cincinnati Eye Institute, one of the largest ophthalmology practices in the country. Both are voluntary assistant professors at the University of Cincinnati Department of Ophthalmology. Dr. Miller has received lecture fees and research support from Alcon.