The aspiration system of the phaco machine is a critical element in the performance of various maneuvers. Thus, an understanding of this system can greatly improve the efficiency of the surgeon’s phacoemulsification technique. Ideally, the surgeon is able to utilize fluidics to maximize phacoemulsification efficiency and to grasp nuclear fragments without inadvertently damaging the iris, capsule, or other intraocular tissues. For example, adjusting the aspiration flow rate can help attract nuclear or cortical material into the aspiration port of the phaco tip or irrigation/aspiration handpiece. Adjusting the vacuum determines how tightly particulate material that has occluded the aspiration port is grasped.
Two main types of aspiration pumps are used in phaco machines: flow pumps (of which peristaltic pumps are the most common example) and vacuum pumps (of which Venturi pumps are the most common example) (Table 8-1). In general, both peristaltic and Venturi pumps are effective, although the vacuum rise time varies between the different pump designs. The latest generation of machines feature continuous feedback sensors that measure flow and vacuum and make immediate adjustments.
Table 8-1 Flow Pump Versus Vacuum Pump: Machine Parameter Effects With and Without Aspiration Port Occlusion
Peristaltic (or Flow) Pumps
A peristaltic pump (Fig 8-4) directly creates flow by moving a set of rollers along flexible tubing, pushing fluid through it. The pressure differential between the lower-pressure aspiration tubing and the higher-pressure anterior chamber creates a relative vacuum at the aspiration port of the phaco tip. Direct linear control of the aspiration flow rate can be achieved by depressing the foot pedal farther down into position 2, thereby increasing the speed of the pump rollers. Higher aspiration flow rates will cause the nuclear fragments to more quickly approach the phaco tip. If the anterior chamber collapses while there is steady flow, the irrigation bottle height can be increased, or the aspiration flow rate can be decreased.
Figure 8-4 Illustration of the peristaltic pump.
(Redrawn with permission from Practical Phacoemulsification: Proceedings of the Third Annual Workshop. Medicopea International; 1991:43–48.)
Although a vacuum limit is set on the machine, a peristaltic pump does not directly produce this level of vacuum. Rather, the peristaltic pump directly controls the aspiration flow rate, which indirectly controls the vacuum level produced. The vacuum level depends on the resistance in the fluidic circuit. Vacuum (grip) builds upon occlusion of the aspiration port. When the aspiration port is not occluded, a higher aspiration flow rate will attract material to the aspiration port more quickly. With complete occlusion, the vacuum level will build up to (but not exceed) the preset vacuum limit and will determine how tightly a nuclear fragment is held onto the aspiration port. The vacuum rise time (the amount of time required to reach a given level of vacuum) is related to the aspiration flow rate: during occlusion, a higher aspiration flow rate will cause a faster vacuum rise time.
During complete occlusion, modern peristaltic phaco machines do allow for linear control of the vacuum because there is no flow. For example, if the vacuum limit has been reached during full occlusion, lifting the foot pedal up while still remaining in position 2 will decrease the amount of applied vacuum (by a slight reversal of the pump rollers), even though the aspiration port is still completely occluded.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.