The fluidic systems for phaco units are designed to be effective and safe with the use of hand pieces and tubing of various configurations, and the companies who manufacture them have recommended settings of their machines. Nonetheless, surgeons have the opportunity and responsibility to adjust these settings in order to maximize the efficiency and safety of the procedure for various surgical circumstances. Understanding the nuances of this subject is gained chiefly through surgical experience. However, some principles are basic.
Inflow should be adequate to maintain the anterior chamber even as the outflow through the incisions and the pump varies. The bottle height will usually need to be higher with higher aspiration flow rates and with larger, leakier incisions. However, if the incision is very large, as in converting a phaco case to an extracapsular extraction for example, the bottle height will need to be lowered in order to reduce flow and reduce the risk of iris prolapse and other complications. The high inflow pressures used in contemporary phaco surgery are only acceptable with a controlled outflow system.
Some phaco experts have advocated very low flow rates, inflow and outflow, in order to slow the pace of events in the anterior chamber, especially for beginning phaco surgeons. This makes sense as higher flow rates speed up movement of nuclear material, and potentially of iris and capsule; however, when reducing flow it is important to maintain sufficient flow to cool the phaco tip and prevent incision burns.
Bottle height will have to be raised (elevated) with the higher flow rates associated with quadrant removal or phaco chop techniques. Bottle height should be evaluated hydrodynamically with the pump running and the port unoccluded so that adequate inflow will match outflow. Consideration should also be given for the effects of surge (see below).
Sculpting of the nucleus (low aspiration, low vacuum in a peristaltic system) can be done with very little vacuum as the nuclear material is stabilized by the zonules and capsule. Vacuum is not needed to hold the nuclear material at the phaco tip. The aspiration rate needs only be adequate for removing the nuclear material and cooling (approximately 20 cc per minute is usually adequate).
When manipulating nuclear fragments or quadrants, vacuum and aspiration (flow) usually have to be higher, especially with dense cataracts, in order to counter the tendency of the longitudinal ultrasound energy to push material away from the tip. Baseline settings of 30 cc per minute flow and 200 mm Hg vacuum are a reasonable starting point. Torsional phaco tips, such as Alcon’s OZiL, decrease tip occlusion time and repulsion associated with traditional longitudinal ultrasound. This, in turn, decreases the need for higher flow and vacuum settings, and therefore reduces the risk of surge.
For chopping, a similar aspiration flow rate with a vacuum of 300 mm Hg is a compromise between an acceptable rapid rise time and a safety margin against surge. The higher vacuum level may be needed in order to grasp and manipulate the nucleus. Surge, or postocclusion surge, occurs when a fragment that is occluding the port is suddenly aspirated at high vacuum levels usually as the fragment is phacoemulsified. Surge is due in large measure to the compliance of the plastic tubing within the pump. As the vacuum pressure increases, the tubing partially collapses. The tubing then snaps open when the vacuum is released, causing a sudden surge of fluid into the line. This causes a partial collapse of the anterior chamber and a sudden forward movement of the posterior capsule. Surge effects can be minimized by using lower vacuum, lower aspiration rates, and higher infusion bottle height. Some phaco needles also have an aspiration bypass port to allow slight flow when the tip is occluded. Such bypass systems, such as that found on the Alcon phaco needles, incorporate small holes in the shaft of the phaco tip to provide this alternate fluid pathway during tip occlusion, thereby decreasing surge. Manufacturers also utilize more rigid (less compliant) tubing in aspiration lines. The inherent resistance to flow of aspirated material in the line also probably modulates surge, especially with smaller hose tubing. Microprocessors are also now available to modify surge by sensing the restoration of flow after occlusion and immediately lowering vacuum by slowing or reversing the pump. When utilizing equipment with such surge-control features, vacuum levels in excess of 500 mm Hg may be safely used.
Scroll pumps put the pump directly in the aspiration flow path and there- fore can utilize tubing with the minimum compliance needed for hand piece control, hence decreasing surge potential.
Vacuum pumps can also utilize relatively noncompliant tubing, but un- less the machine has a preset available to alter rise time, and several do, the rise time is usually very rapid and with occlusion break the flow is high, and therefore so is the risk of surge.
I/A of residual cortex can be accomplished with modest aspiration flow rates as the instrument tip can generally be placed near the cortical material to be aspirated. This, in addition to whatever surge control has been built into the machine by the manufacturer, reduces surge risk. Linear vacuum with a high limit is used so that the cortex can be stripped into the center of the anterior chamber where vacuum is utilized to pull it through the 0.2- or 0.3-mm aspiration port.
Rise time is the interval between occlusion of the phaco or I/A tip and at- tainment of maximum preset vacuum. In a peristaltic or flow system, this is governed by the flow rate setting. Even though there is no flow when the tip is occluded, the aspiration flow rate setting governs the speed of the pump. Doubling the flow rate halves the rise time. In some systems, for example the Alcon’s Infinity, the rise time can be altered by a preset in which the machine senses an increase in pressure in the inflow line and increases vacuum (indicating occlusion) and then changes the speed of the pump, up or down as pre- decided by the surgeon. Increasing the pump speed increases the risk of surge.
V. FOOT PEDAL CONTROL OF FLUIDICS
Most foot pedals (Figure 11-4) can be set up for fixed and/or linear control of surgical functions—aspiration, vacuum, and power. There is usually slight resistance, when moving from one position to the next. Commonly in phaco mode there are 4 positions. Position 0, when the pedal is not depressed, is set for no irrigation, no aspiration, and no power. Position 1, initial depression, is set for irrigation (at a rate determined by the bottle height and incision leakage) without aspiration or phaco. When manipulating nuclear material, as rotation of the nucleus in “divide and conquer” techniques, it is important to be in position 1 so that the inflow maintains the anterior chamber and capsular bag architecture. Position 2, further foot pedal depression, is set for irrigation plus aspiration at a fixed rate and no phaco. The rate of aspiration will be influenced by the amount of occlusion of the aspiration port. Position 3, still further foot pedal depression, adds phaco power at either a fixed level or with linear control by further depression of the foot pedal. Additional phaco variations include pulsed phaco or burst mode. Pulse mode delivers phaco power in regular “on” and “off” time intervals, linearly controlled by foot pedal position. In burst mode, stable power strokes are delivered with varying time intervals. The frequency of bursts increases proportional to foot pedal depression, such that with maximal depression, continuous phaco is delivered. Also available is the addition of sonic vibrations such as with Alcon Ozil.
Phaco power indirectly affects fluidics. Phaco power breaks up nuclear material and allows it to be aspirated. It also tends to push nuclear material away from the tip. Therefore phaco, in general, tends to increase flow through the system. Lowering phaco power tends to allow aspiration flow to pull material toward the tip partially or completely occluding it and thereby reducing or stopping flow. Pulsed phaco appears to even out flow rates as the case proceeds. Vacuum is ordinarily a preset in phaco mode and varies with the amount of occlusion and rise time.
In I/A mode, gravity feed irrigation is on in positions 1, 2, and 3 and the flow rate is determined by the bottle height. Both vacuum and aspiration are activated in positions 2 and 3 and both can be fixed or linear. If fixed vacuum is chosen, occlusion will result in a rise in vacuum to that set level. If linear vacuum is selected, the vacuum can be raised to whatever maximum is chosen by further depressing the foot pedal. Fixed aspiration provides for a fixed aspiration flow rate in pedal positions 2 and 3. Linear aspiration control provides for linear control of aspiration flow rate proportional to the foot pedal position. Combining linear aspiration and linear vacuum creates a Venturi- like system.
Occasionally during a procedure unwanted material will be aspirated, occluding the tip of the phaco or I/A hand piece. Phaco machines are designed with a venting system so that returning to position 0 will break the vacuum. In the Alcon Infinity, there is a reflux preset, that limits reflux pressure to the current infusion pressure plus a specified additional amount. This can be used to release capsule, iris, or lens material. Reflux is usually a factory set default and is foot pedal controlled.
VI. AUDIBLE SIGNALS
- Vacuum tone: Pitch varies relative to the amount of vacuum. The volume can be reduced but cannot be turned off.
- Occlusion tones: Intermittent beeping indicates that vacuum is at or near its preset limit and aspiration flow is reduced or stopped to avoid exceeding this limit. Continued phaco with no aspiration flow may result in thermal injury, especially if the corneal incision is tight.
- Phaco occlusion tone: High-pitched double beep that cannot be turned off.
- I/A occlusion tone: Lower single beep that can be turned off. Voice messages indicate mode changes.
VII. TROUBLE SHOOTING
A. Problem: Inadequate Infusion With Unstable Anterior Chamber
Possible causes include the following:
(a) Bottle was not vented.
(b) Bottle is empty.
(c) Height of bottle relative to patient’s eye is too low.
(d) Infusion line is kinked.
(e) The fitting to the hand piece is loose.
(f) The incision is too tight.
(g) Manipulation of the hand piece is compressing the infusion sleeve.
(h) There is too much viscoelastic in the anterior chamber.
(i) Outf low has increased.
B. Problem: Cannot Attract and Hold Nuclear Material at the Phaco Tip
Possible causes include the following:
(a) Too much phaco power is pushing the material away, or creating toolarge a space in the nuclear fragment to allow occlusion. Therefore, use less phaco power.
(b) The vacuum setting is too low—unlikely unless it is very low. Except in Venturi-type systems, raising vacuum has little effect on aspiration flow rate. Enough vacuum is necessary to maintain occlusion.
(c) Aspiration flow rate is set too low. Possible but consider option (a) first.
C. Problem: Inadequate Aspiration
Possible causes include the following:
(a) There is a leak in the aspiration line.
(b) There is air in the aspiration line.
(c) The hand piece is clogged.
(d) The flow rate or vacuum setting is too low—consider options (a), (b),and (c) before changing settings that usually work for you.
Finally, quoting from the manual that comes with Alcon’s Infinity phaco machine, “Good clinical practice dictates testing for adequate irrigation, aspiration flow, reflux and operation as applicable for each hand piece prior to entering the eye.”
*Dikutip dari Buku Essentials Of Cataract Surgery 2nd Ed, halaman 110-115