EyeWorld Asia-Pacific September 2013 Issue

32 EWAP CATARACT/IOL September 2013 Go with the flow Bob Avery, MD Associate professor of ophthalmology, Medical director, UNMH Eye Clinic, University of New Mexico Medical School,Albuquerque, NM, USA I t is helpful to think about cataract surgery in terms of the forces that are used. These include fluid dynamics, mechanical forces, and ultrasound power. In many ways, the most interesting (but least intuitive) of these are the fluid dynamics. Many of us speak of the “fluid circuit” because of its analogy to the flow of electricity. Here are three tips to help surgeons understand and optimize fluid dynamics: 1. Know the functions of the fluid circuit. The fluid circuit performs three vital functions during cataract surgery. Most importantly, it stabilizes the volume of the eye so that the contents (especially in the anterior chamber) do not shift. The fluid circuit also provides attractive force that brings mobile objects to the phaco tip. Finally, fluid flow prevents heat from building up and damaging the eye—especially the cornea. 2. Appreciate the importance of maintaining equilibrium in the fluid flow. The fluid circuit works best when it is in equilibrium: when the outflow is matched by the inflow. One must consider all the ways that fluid can enter and exit the eye at any one time. The only meaningful way that fluid enters the eye is via irrigation— either through the phaco sleeve or a separate irrigation port. The obvious way that fluid leaves the eye is through the aspirating tip of the handpiece, but it is important to remember that fluid can (and often does) exit through all of the surgical wounds. If the inflow cannot immediately replace all fluid losses, the chamber will not maintain stability. 3. Think of the aspiration rate setting as a “flow rate.” The “aspiration rate” is the rate at which fluid enters the phaco tip. It should not be considered the “sucking force” of the tip. Aspiration is a flow rate, measured in units of volume per time, not units of force. Fluid is drawn to the tip so that objects can be removed from the eye. Think of the fluid flow within the eye as a river current that starts at the irrigation sleeve port, flows through the eye, then exits via the phaco tip opening. Increasing the “aspiration flow rate” has two important effects on attracting mobile structures. A higher flow rate will draw objects to the tip faster (the river flows faster). A higher flow rate will also apply an attractive force to mobile objects farther away from the tip (the current pulls more “stuff” into it). Like most things in medicine, the more a surgeon understands the fluid circuit, the more interesting and useful it becomes. Uday Devgan, MD Private practice, Devgan Eye Surgery, Los Angeles, Calif., USA Chief of ophthalmology, Olive View UCLA Medical Center Associate clinical professor, Jules Stein Eye Institute, UCLA School of Medicine One of the primary challenges of phacoemulsification is working within the very small space of the anterior segment of the eye. Certainly, every drop of aspirated fluid must be replaced in order to prevent instability and collapse of the anterior chamber. It follows that the most important principle of phaco fluidics is balancing the inflow and outflow of fluid within the eye. The basic concepts of fluidics are explained by Poiseuille’s law, which states that the factors influencing flow are length of the tubing, viscosity of the fluid, change in pressure, and, most importantly, radius of the tubing. This is easily remembered by a simple analogy: If you use a small straw to drink a milkshake, you will need high vacuum in your mouth to get a low amount of flow. If you use a larger bore straw, then you’ll require less vacuum and achieve more flow. For phacoemulsification, there is only one source of fluid inflow: the hanging bottle of balanced salt solution. The bottle height determines the change in pressure due to gravity to drive the fluid into the eye. There are, however, two sources of fluid outflow: the fluid that is aspirated through the phaco needle and the leakage of fluid around the incisions, which Cataract tips from the teachers continued on page 38 keeps them cool. We must balance the total outflow of fluid with a sufficient amount of inflow fluid in order to prevent surge, which is anterior chamber instability that can lead to complications such as posterior capsule rupture. To choose fluidic parameters, the first step is to determine the fluid inflow that you can achieve with the phaco tip you have chosen. Use a measuring device such as an empty 60 cc syringe and with the bottle height at your standard setting and the phaco probe at the patient eye level, see how much fluid flows in 60 seconds. This is your maximum inflow rate, measured in cc/minute, and it should be about 50 cc/min, but it could vary from 40-60 cc/ min depending on the size of your phaco tip. For outflow, expect about 10 cc/min of fluid loss from your incisions. This leaves a maximum aspiration flow rate setting of 40 cc/ min (50 cc/min inflow minus 10 cc/ min outflow from leakage) with a peristaltic phaco machine. If you notice bounce or instability of the anterior chamber during surgery, you can increase the inflow rate by raising the bottle height, lower the outflow rate by dropping the aspiration flow rate, or both. The aspiration flow rate also controls the speed at which cataract material will be attracted to your phaco tip. The vacuum setting determines holding power during maneuvers like phaco chop as well as ability to suction cataract pieces through the phaco needle. For a peristaltic phaco system, remember that the pre-set maximum vacuum level is only achieved once the phaco tip is occluded with cataract material. By adjusting these three basic parameters (bottle height, aspiration flow rate, and vacuum level), we can optimize the fluidics of phacoemulsification to provide a higher level of safety and better visual outcomes for our patients.