Optimizing phaco fluidics creates more stable environment for cataract surgery
Inflow that is balanced with outflow results in a safer, more efficient procedure.
With milder cataracts, techniques of phaco chop and femtosecond laser lens softening, phaco fluidics play more of a role in cataract removal than delivery of ultrasonic energy. A more accurate term for modern-day cataract surgery may be phaco-assisted aspiration of the cataract because the primary modality for removal of nuclear pieces is vacuum aspiration. Optimizing the phaco fluidic parameters can make cataract surgery safer and more efficient.
Understanding inflow, outflow balance
The key parameter in the fluidic settings is the balance of the inflow and outflow of balanced salt solution in all phaco platforms, regardless if they have peristaltic or venturi vacuum pumps. During phaco, we are working in a very small space comprised of the anterior and posterior chambers, which constitute approximately 0.5 cc of fluid. When aspirating cataract material and fluid from this space, we must instantly replace it with an infusion of balanced salt solution to prevent collapse of the eye. In addition, this flow of fluid creates the currents that draw the cataract pieces to the phaco tip and allow for followability.
We need to balance the inflow of fluid with the outflow of fluid. If the outflow exceeds the inflow, for even a second, we see anterior chamber instability and a higher risk of complications such as posterior capsule rupture. There are two sources of fluid outflow from the eye: that which we aspirate directly through the phaco tip and leakage from our incisions. In contrast, there is only one source of fluid inflow: the balanced salt solution that flows from the hanging bottle and is delivered via gravity. If using a peristaltic pump, we can directly set the rate of fluid aspiration through the phaco tip, typically 20 to 40 cc/minute depending on the technique, and we can estimate the fluid leakage from our incisions, usually about 5 to 10 cc/minute. This needs to be balanced by an inflow level that is the sum of these two outflow rates.
Image: Devgan U
Addressing intraoperative situations
In some situations, there is a need to make adjustments in real time as we are operating. During surgery, if the anterior chamber is noted to be unstable with anterior movement of the iris and capsule, particularly after occlusion break, we must increase the inflow and decrease the outflow. The inflow is increased by raising the bottle height, and the outflow is decreased by altering the phaco machine parameters. In the case of an excessively leaky incision, a suture can be placed to tighten the wound to decrease fluidic loss.
If we have a patient with a medical condition, such as congestive heart failure, that requires the patient to be in a semi-reclined position instead of completely supine, the phaco bottle height must be increased if we are relying on a simple gravity-propelled system. The difficulty in these situations is being able to quickly react to any change in conditions because the changes are made manually after the physician calls out new settings to his operating room team.
Using a method to force fluid into the eye during surgery has been done for more than a decade in cataract surgery and for an even longer period in vitrectomy surgery. The idea is to make the incisions more watertight with less leakage and then to use a gas pump or a mechanical means to actively pressurize the infusion instead of relying solely on gravity.
Among the major phaco platforms available in the U.S., two offer a method of forced infusion: the Bausch + Lomb Stellaris, which uses an air pump to pressurize a rigid bottle of balanced salt solution, and the Alcon Centurion, which uses mechanical pressure on a compressible bag of balanced salt solution.
The Stellaris phaco system has been available with a gas-forced infusion system for many years. The system utilizes an air pump to pressurize the hanging bottle of balanced salt solution, thereby allowing a greater inflow rate than if gravity alone were used. The advantage comes when we move toward microincision cataract surgery. As we decrease the size of the phaco incisions, the size of the phaco tip also decreases, and this can result in a reduction in fluid flow into the eye. We can increase the outflow of fluid by increasing the vacuum setting, with levels of 500 mm Hg and higher being commonplace with sub-2 mm phaco tips. To increase the inflow level, relying on simple gravity-based infusion is a challenge because most operating rooms have a limited ceiling height. The gas-forced infusion is a more elegant solution because the pressurized balanced salt solution bottle allows for a higher and more stable inflow of fluid during surgery.
Retina surgeons have known for many years that the key to balancing fluidics in vitrectomy surgery is to have a system of forced infusion in an eye with incisions that are not leaky. We first saw this technology brought over to the cataract side by Bausch + Lomb more than a decade ago, and now with the newer platforms, it will gain an even wider adoption. By balancing the inflow with the outflow, we can create a more stable environment for surgery, which will lead to more efficiency and a higher margin of safety.