The key point in fluid dynamics during phacoemulsification is that the amount of fluid going into the eye should exceed the amount of fluid going out of the eye at all times. When it does not, the anterior chamber will shallow and the posterior capsule will move upward and potentially come into contact with the phaco tip, which can result in posterior capsular rupture.
Inflow and Outflow, Pump systems
The inflow of fluid is determined solely by the irrigation flow. The hydrostatic pressure of the water column in the line from the irrigation fluid bottle to the level of the drip chamber underneath the bottle is expressed in centimeters (bottle height). The resistance in the entire irrigation line, including the narrow space between the sleeve and the phaco tip, determines the final irrigation flow (ie, irrigation pressure/irrigation resistance).
When the phaco tip is completely occluded and no leak flow occurs through any of the incisions, one must be aware of the actual intraocular pressure (IOP). In this state, the entire fluid (water) column of the irrigation line is applying pressure in the eye. For instance, a bottle height of 75 cm H2O translates to 750 mm H2O. Divided by the relative weight of mercury (13.6 g/mL), this results in an IOP of approximately 55 mm Hg. Likewise, an extreme bottle height of 150 cm results in an IOP of 110 mm Hg when the tip is occluded. One should be extremely cautious about utilizing bottle heights that exceed 100 cm, as this level can result in pressure spikes of more than 73 mm Hg.
Outflow from the eye during phaco surgery is affected by several components, including aspiration flow, leak flow, and surge flow. Before discussing outflow, it is important to understand the two types of pump systems available in existing phaco machines: peristaltic and venturi pumps.
Peristaltic pumps consist of rollers that push fluid through flexible aspiration tubing. The aspiration flow increases with increasing speed of the rollers (Figure 1A). On the other hand, venturi pumps create vacuum in a rigid cassette by forcing gas through a pipe connected to the cassette (Figure 1B). With more gas force blown through the pipe, higher vacuum is created in the cassette, attracting more fluid from the aspiration line.
The main difference between the two systems is that, in venturi pump systems, the vacuum and aspiration flow are directly linked to each other and with a peristaltic pump, vacuum and aspiration flow can be controlled independently. With a venturi pump, one cannot set a high vacuum and a low flow. Therefore, I prefer peristaltic pumps, as the ability to control vacuum and flow separately is essential for managing challenging cases.
Leak and aspiration flows
Leak flow occurs through either the main incision, when the wound is too large relative to the size of the phaco sleeve, or the sideport. Leak flow should be limited as much as possible, and all incisions should be adequately sized, meaning they are only large enough to allow easy entry of the phaco sleeve and sideport instrument.
In a peristaltic system, the speed of the rollers in the cassette determines the aspiration flow and can be set on the machine in mL/min. Aspiration flow can occur only when the tip is not fully obstructed. When the phaco tip is fully occluded, there is no flow. The flow passing through the aspiration line is dependent on the force of the phaco pump pulling the fluid and on the total resistance in the aspiration line. Pump capacities and aspiration line lumen sizes vary among the available phaco machines.
The preset values displayed on the machines do not necessarily correspond with what is happening in real time. For example, with an aspiration flow setting of 50 mL/min, the actual aspiration flow can be close to that value with a large bore phaco tip of 0.7 mm and normally large lumen aspiration tubing. In contrast, the preset value of 50 mL/min on the same machine will not be reached through the small 0.3-mm opening of the I/A port. The actual flow can be easily less than half of that value, depending on the system.
The vacuum level displayed on the machine is the preset maximum level. When the tip is occluded, the pump rollers will continue to spin until the preset maximum vacuum level is reached. The time needed to reach this maximum vacuum level (vacuum rise time) is dependent on the speed of the rollers (aspiration flow setting). The same high vacuum level can be reached either at a high speed/high flow setting or at a slower pace/low flow setting. The maximum vacuum level is lost when the tip occlusion breaks, which normally happens when ultrasound is activated in footpedal position 3. Vacuum is only built up in footpedal position 2 when both irrigation and aspiration occur.
Vacuum is the holding force of the machine that keeps lens material at the tip to be emulsified and pulls the lens material through the aspiration line, but there is an ongoing debate about the required level of vacuum in phaco surgery. In essence, the required level can be stated as only high enough to do the job; surge flow must be avoided.
Another physics phenomenon is often overlooked by surgeons. The vacuum setting displayed on machine will be reached if the phaco tip is fully occluded; however, if the tip is not occluded and aspiration flow runs through the tubing, there is vacuum drop along the aspiration line. The vacuum drop increases with higher aspiration flow. There is also a higher vacuum drop when narrower-lumen aspiration tubing is used. This can result in a halt of the pump at lower vacuum settings with relatively high flow settings. When the aspiration tubing is filled with ophthalmic viscosurgical device and/or lens material, the vacuum drop can be more significant.
The phenomenon of surge flow occurs only at the moment of occlusion break and vacuum loss. It lasts for a fraction of a second, when the contracted aspiration line under vacuum suddenly springs back to its original shape and volume when vacuum is lost (Figure 2).
The severity of the surge flow is determined by the following factors:
- Vacuum level: surge increases with higher vacuum;
- Phaco tip lumen: a smaller lumen will restrict the amount of fluid during the surge;
- Sleeve size: a larger sleeve will allow more fluid into the eye during surge;
- Infusion pressure: a higher bottle will push more fluid into the eye during surge; and
- Compliance of tubing: softer tubing material will contract more, resulting in higher surge.
Figure 3 depicts the IOP cycle during the brief moment of an occulsion break. The sharp downward spike right after occlusion break must not result in a negative IOP; in that case, the anterior chamber shallows immediately, and the posterior capsule comes upward.
Beware of Air
When a significant amount of air is inadvertently aspirated, the postocclusion surge response can be dramatically higher because air is more compliant than fluid. The air in the aspiration line will enlarge significantly under vacuum. On occlusion break, the air will return to its original volume, markedly adding to the force of the surge. Air can easily be inadvertently aspirated when the phaco tip is retracted from the eye while in footpedal position 2. This happens often, and many surgeons are unaware of the danger.
If air is aspirated inadvertently, one should place the phaco tip in a fluid container and aspirate fluid until the air has emptied completely from the aspiration line.
The basics of fluid dynamics are important to understand the benefits and limitations of fluidics settings. This knowledge will help the cataract surgeon to improve efficiency and maintain safety at all times during the phaco procedure.
Khiun F. Tjia, MD, is an Anterior Segment Specialist at the Isala Clinics, Zwolle, Netherlands. Dr. Tjia states that he is a consultant to Alcon Laboratories, Inc. He is a Chief Medical Editor of CRST Europe and may be reached at e-mail: firstname.lastname@example.org.