Everything you ever wanted to know about Phaco Fluidics
Fluidics play an important part in phacoemulsification and should be understood clearly by the beginner surgeon. Flow and vacuum both attract pieces towards the probe whereas phaco power repels pieces.
FLUID INFLOW: Fluid enters the anterior chamber (AC) from the infusion bottle. This flow may be regulated by adjusting the bottle height when it is a purely gravity dependent mechanism (approximately 11 mmHg above ambient atmospheric pressure for every 15cm bottle height above the patient's eye) or it may be in the form of pressurized infusion (either as an air pump connected externally to the infusion bottle or as inbuilt gas forced infusion).
FLUID OUTFLOW: Fluid exits the AC through the aspiration tubing as well as via leakage from incisions. This outflow is determined by the Aspiration flow rate (AFR), vacuum, incision size and geometry.
Aspiration flow rate: This refers to the amount of fluid that leaves the eye through the aspiration tubing per unit time. It is measured in cc/minute. A high AFR results in more rapid events. It can therefore result in more rapid removal of nuclear fragments but also result in less time for the surgeon to react, which may result in complications. Inflow needs to be increased when using high AFR to maintain a stable anterior chamber. This is better achieved using pressurized air infusion; the other option being to elevate bottle height. The AFR can act as a third hand within the AC by producing fluid currents that can be used to direct nuclear material into the aspiration port. These fluid currents flow from the irrigation ports in the sleeve towards the aspiration port. Faster AFR produces stronger currents. Some degree of turbulence occurs because of incisional leakage and other variables. The currents are stronger closer to the aspiration port and therefore, followability of material can be increased either by increasing AFR or by taking the aspiration port closer to the fragments
Vacuum: This is generated by the phaco machine pump and is the vacuum effective in the AC. It is measured in mmHg. Vacuum may be created by either peristaltic or venturi pumps. Vacuum is produced in Peristaltic pumps (flow based pump) on occlusion of the port. However, it may also be produced without occlusion at high flow rates and thus can emulate a Venturi pump at higher flow rate settings. Flow and vacuum may be adjusted independently. Vacuum is created even without phaco tip occlusion in Venturi pump (vacuum based pump), however vacuum and flow rate cannot be adjusted independently. Rapid flow rates and rapid rise times are seen with Venturi pumps. Alcon uses a peristaltic pump; the AMO pump can switch between peristaltic and venturi mechanisms in the same surgery whereas B&L has both peristaltic and venturi but these cannot be switched in the same surgery. Pre-set or Maximum vacuum refers to the maximum vacuum level set by the surgeon. Actual vacuum depends on foot pedal position (with linear setting), maximum pre-set level, AFR, port size and degree of occlusion. Vacuum rise time is the speed with which the maximum pre-set vacuum is attained following complete occlusion. Low rise times make surgery more rapid but give lesser time to react. This is dependent on AFR, tubing compliance and venting mechanism. Higher the AFR, faster the rise time; lower the compliance of the tubings, faster the rise time. Fluid vented machines also have a faster rise time than air vented machines.
SURGE: Surge refers to a sudden shallowing or collapse of the anterior chamber in response to an excess of outflow as compared to inflow. It occurs when occlusion is broken. This can result in an anterior movement of the posterior capsule and a posterior movement of the cornea both of which can result in complications such as a posterior capsular rent or endothelial loss. Various phaco machines deal with surge by in-built mechanisms. This includes change in pump speed, vacuum rise time or AFR during occlusion; increased inflow on break of occlusion; low compliance tubings, venting etc. Alcon has the Intrepid Fluid Management System, AMO has Fusion Fluidics and B&L has EQ Fluidics.
GAS FORCED INFUSION (Air pump): Surge causes an unstable AC. A simple remedy suggested by Sunita Agarwal was the air pump. A simple fish tank aquarium is connected by a 20 G needle and tubing to a non-expandable infusion bottle. This gas forced into the infusion bottle causes a pressure rise that increases the amount of fluid entering the eye thus preventing surge even at higher vacuum levels. A millipore filter is used to prevent infection and to ensure particulate free air. The advantage with in-built gas forced infusion is the ability to actively and digitally control the parameters during surgery according to the conditions or the surgical steps of the individual case. This makes even difficult cases like mature white and hard brown cataracts, small pupils etc easier. It increases the fluid inflow thus resulting in a well formed, deep and stable AC. This decreases chances of damage to endothelium and capsule. As it allows for higher AFR, it makes surgery more rapid and brings nuclear material towards the probe. The fluid acts as a third hand. The air pump can make bimanual phaco easier and allows use of smaller bore instruments, making even sub-
1mm surgery possible.
THERMAL BURNS: Overheating of the phaco tip can produce thermal burns at the clear corneal incision. Coaxial phaco depends on adequate needle cooling by fluid flow around the needle through the infusion sleeve whereas in biaxial phaco, needle is cooled by incisional leakage, smaller needle diameter and higher flow parameters. Most burns occur during tip occlusion. Tip heating can occur within 1-3 sec with inadequate irrigation and aspiration flow. Inadequate irrigation can occur from an unnoticed empty infusion bottle, inadequate bottle height, a crimped infusion sleeve, kinked tubings or a tight incision. AFR can be decreased by tip occlusion, low vacuum, crimped aspiration line or improperly loaded cassette/ tubings.
~ Dr. Soosan Jacob is a Senior Consultant Ophthalmologist at Dr. Agarwal's Eye Hospital, Chennai,
India and can be reached at email@example.com
Wednesday, January 10, 2018