Optimizing phaco fluidics settings helps to achieve efficient, safe cataract surgery

Bottle height, flow rate, maximum vacuum level and phaco needle size are the primary adjustable parameters on a peristaltic system.

Issue: June 1, 2006

ByUday Devgan, MD

The challenge of cataract surgery is in large part due to the small confines of the working space. The anterior and posterior chambers combined typically comprise less than 1 cm³ of space and provide little room for error. The function of phaco fluidics is to balance the inflow and outflow of fluid to maintain the working space, bring cataract material to the phaco tip and prevent collapse of the eye. Optimizing the phaco fluidics settings is instrumental to the efficiency and safety of phacoemulsification surgery.

With a typical peristaltic phaco machine platform — the most common type in the U.S. market — only a few primary parameters are adjustable: bottle height, flow rate, maximum vacuum level and phaco needle size.

Uday Devgan

Perhaps the most important parameter is phaco needle size. From previous columns, we recall that the difference in flow between a larger-bore needle and a smaller-bore needle varies exponentially due to Poiseuille’s equation (Ocular Surgery News Feb. 15, 2006). To summarize briefly, smaller-bore phaco needles are suited for high-vacuum, low-flow fluidics, while larger-bore needles are better suited for high-flow, low-vacuum fluidics. The analogy of drinking a milkshake through a small-bore cocktail straw vs. a larger-bore drinking straw illustrates this point.

The flow rate of a peristaltic machine is typically given in cubic centimeters of fluid per minute. This is determined by the rate at which the peristaltic rollers milk the fluid along the outflow tubing. With the phaco needle unobstructed, the maximum flow rate is achieved, and this in large part determines the speed at which things happen in the eye. Upon occlusion of the phaco needle with cataractous material, the flow rate declines and approaches zero.

The bottle height determines the inflow rate of fluid into the eye. Much like a water tower in a small town, the height of the fluid above the eye creates a forceful infusion of fluid via gravity: The higher the infusion bottle, the greater the inflow pressure and inflow rate.

With an unobstructed phaco needle, the flow rate is at the maximum, but the vacuum level is low – far from the maximum vacuum level that the surgeon has selected. The vacuum level in a peristaltic-based system is achieved only upon occlusion of the phaco tip (See Figure). The higher the vacuum, the greater the holding power – and the holding power is used to fixate the cataract while we mechanically chop it. The effect of the vacuum level varies with the bore of the phaco needle due to the surface area. The larger the cross-sectional surface area of the phaco needle, the greater the holding power given the same amount of vacuum. The vacuum level determines the “holding power” or “grip” of the phaco tip onto nuclear pieces.

Occlusion required to achieve maximum vacuum

Occlusion of the phaco tip is required to achieve the preset maximum vacuum level with a peristaltic system.

Image: Devgan U

Optimizing settings

In order to optimize the phaco fluidics settings, it is important to match the parameters to the technique and the surgeon’s preference.

The first decision is the selection of phaco needle size, with the most common sizes being the smaller-bore 0.9-mm needle and the larger-bore 1.1-mm needle. If your preference is a faster procedure with rapid nucleus removal, the 1.1-mm needle size is preferred because it will give a significantly greater flow rate. If your preference is a slower but more controlled procedure, then the 0.9-mm needle is more suited to your technique.

The bottle height determines the inflow of fluid into the eye. In order to help prevent surge, it is important to keep the inflow of fluid greater than the outflow of fluid at all times. The inflow of fluid comes from only one source, the bottle of balanced salt solution, while the outflow of fluid comes from two sources, the suction via the phaco needle and the leakage from the incisions. If at any time the outflow outstrips in the inflow, the eye will collapse, and there is a high likelihood of posterior capsule rupture. It is often advantageous to start with a high bottle height to ensure a sufficient inflow of fluid and then to taper it downward to minimize the posterior displacement of the lens-iris diaphragm due to the infusion pressure. If you sometimes notice corneal striae and anterior chamber instability during surgery, you may benefit from increasing the bottle height.

For phaco chop, holding power is important in order to securely fixate the nucleus while using the chopper to mechanically disassemble it. This requires a relatively high vacuum, such as 200 to 250 mm Hg with the 1.1-mm needle or 300 to 400 mm Hg with the 0.9-mm needle. Once the nucleus has been broken into smaller fragments, the speed at which the fragments are attracted to the phaco tip is determined by the peristaltic flow rate, with 20 cc/min being slow and 50 cc/min being fast. The same vacuum and flow-rate settings can be used for the entire nucleus removal procedure during phaco chop.

For divide-and-conquer, there are two distinct parts of nucleus removal: sculpting of the nucleus and quadrant removal. Different fluidics settings are required for each. For grooving and sculpting of the nucleus, the work is done by ultrasonic energy, and thus the flow and vacuum settings are low – just enough to aspirate the nuclear material removed from each forward stroke of the phaco probe. A vacuum level of less than 100 mm Hg and a flow rate of less than 30 cc/min is sufficient for this purpose. For quadrant removal, a moderate amount of holding power is required to bring each quadrant into the phaco tip. Using a higher vacuum level of 200 to 300 mm Hg and a flow rate of 30 to 50 cc/min, depending on the needle size, is typically sufficient for this purpose.

With knowledge of the concepts behind the variables, it is easy to tailor the fluidics settings to the surgeon and technique. Understanding the concepts behind the phaco fluidic settings is instrumental in optimizing the parameters for increasing the efficiency and safety of your phaco technique.

For more information:

Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.