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Two sources of fluid improve small incision cataract surgery, surgeon says
An anterior chamber maintainer allows for reduction of bottle height and increases fluid availability at the same time.
Michael Blumenthal |
NAPLES – Multiple small-incision surgery with two sources of fluid creates a stable anterior chamber with less turbulence and fluctuation, making phacoemulsification easier and safer, according to Michael Blumenthal, MD.
During the Joint Meeting of Ocular Surgery News, the Italian Association of Cataract and Refractive Surgery and the International Society of Refractive Surgeons, Prof. Blumenthal noted that the new developments in phaco surgery, such as microincision cataract surgery (MICS) and cold phaco, require increased fluid availability. Although this can easily be obtained by elevating the irrigation bottle, a high irrigation supply inflow rate creates unstable conditions in the anterior chamber, strong jet fluid currents and unfavorable fluctuations of the iris and capsule.
“We need to reduce bottle height and increase fluid availability at the same time,” he said.
Bottle height and IOP
Prof. Blumenthal said there are three factors that control fluidics in the eye: suction, bottle height and outflow resistance. He showed in a video presentation how fluidics management, as well as anterior chamber IOP stability, depends on the balancing of these variables.
The IOP, which equals the hydrostatic pressure in the zero outflow condition, is determined by the height of the irrigation bottle, he said. For example, in a zero outflow situation with no suction, when the irrigation bottle is located at a 130-cm height, the IOP equals almost 100 mm Hg, he explained.
The interrelation between different bottle heights and IOP is summarized in the Table.
Outflow and IOP
The overall dynamic of the fluidics changes if the outflow variable is introduced, Prof. Blumenthal said. He pointed out that there are two ways in which the irrigating solution leaves the eye: aspiration and leakage. The IOP drops accordingly, depending on the suction rate.
“Aspiration is a surgeon-controlled parameter. In absence of leakage, when suction decreases the IOP raises until it reaches its original hydrostatic pressure,” he said.
Leakage is a far less controllable parameter, but it can be significant in phaco surgery, he added.
“You cannot measure the amount of liquid leaving the eye, although you can to some extent reduce it or prevent it by small incisions. Leakage rate can reach 20 to 30 mL per minute, but varies from patient to patient and from surgeon to surgeon, and is of course closely connected with bottle height,” Prof. Blumenthal said.
Leakage, which has the same effect on IOP as suction, drops the IOP in proportion to the amount of saline solution that leaves the eye, he said.
A difficult balance
According to Prof. Blumenthal, outflow-induced changes in anterior chamber pressure should be immediately detected and corrected by overflow with the irrigation tip. However, this maneuver is potentially unsafe for endothelial integrity, he said.
“With a bottle placed at a height of 1 m and a total outflow of 40 to 50 mL per minute, due to leakage and suction together, the shearing rate of the jet stream can be higher than 1,500 units, which is the upper limit a healthy endothelium can tolerate during surgery,” Prof. Blumenthal said.
By reducing bottle height to 60 cm, shear stress decreases to 200 units and IOP falls to a safe range. However, he added, the lower the irrigation bottle is located, the faster IOP drops during suction, eventually reaching the point of anterior chamber collapse.
One way of maintaining IOP within a safe range without increasing the shear stress is by reducing suction rate, Prof. Blumenthal said. But low suction does not protect against sudden IOP drops, and the surgeon is therefore forced to elevate the irrigation bottle once again, to not less than 1 m, he said.
“But again, even with the bottle at 1 m, if suction increases suddenly during surge, the shear rate increases and, more important, the IOP drops to a critical level,” Prof. Blumenthal said.
In some cases it is necessary to increase bottle height to even more than 1 m in order to increase fluid availability and to prevent the surge collapsing effect, he said.
Another possible solution is to reduce inflow resistance and increase fluid availability by using instruments with a larger diameter, the surgeon said. However, this would be an unacceptable step backwards from small-incision surgery and “microincision would become macroincision,” Prof. Blumenthal pointed out.
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Images: Blumenthal M | ||
Two sources of fluid
After trying several different approaches to balancing the factors of fluidics, Prof. Blumenthal concluded that the safe limit for the bottle height is 60 cm, and found the lower the bottle is placed, the better control is during surgery. He addressed the drawbacks of lowering the irrigation bottle by introducing a second source of fluid.
“To overcome this dilemma, I recommend the use of two separate inflow irrigation sets introduced into the anterior chamber at the same time through separate incisions,” Prof. Blumenthal said.
The first instrument is a self-retaining anterior chamber maintainer (ACM). Inserted in the lower part of the cornea, the instrument provides continuous irrigation, maintaining hydrostatic pressure throughout the operation, he said.
The second instrument can either be an irrigating chopper or a phaco handpiece, he said.
By simultaneously activating two bottles of irrigating solution placed at 60 cm, “the total amount of flow in the eye is reduced rather than increased and a friendlier fluid current equilibrium is created,” Prof. Blumenthal said. “Positive IOP is maintained during surgery, stabilizing the anterior chamber depth. Adverse events such as surge, high-shearing-rate jet streams and anterior chamber collapse are avoided. Offering this kind of safety is just a matter of an extra moment. We deserve it, and our patients deserve it too.”
For Your Information:
- Michael Blumenthal, MD, can be reached at Ein Tal Eye Center, 17 Brandeis Street, Tel Aviv 62001, Israel; 972-3-5433213; fax: 972-3-5441222; e-mail: blumenthal@doctor.com or secretary@eintal.com.
- Naples ’04, the 9th Annual Joint Meeting of Ocular Surgery News, the Italian Association of Cataract and Refractive Surgery and the International Society of Refractive Surgeons, was held May 20-22. For information on future meetings, e-mail meetingregistration@slackinc.com.
- Michela Cimberle is an OSN Correspondent based in Asolo, Italy.