Small incision cataract surgery requires the flow of fluid through the eye in order to dissipate the heat created by the phaco needle, to remove the emulsified nucleus, to strip the residual cortical material, and to remove the viscoelastics. The flow must be controlled well enough to maintain the anterior chamber and avoid damage to the delicate structures of the eye. In this closed system, the movement of the fluid is modulated by varying height of the infusion bottle, the aspiration flow rate, and the vacuum settings of the pump (Figure 11-1). Modern phacoemulsification machines allow for fixed or linear control of most of these parameters as well as other features such as surge control, automatic attenuation of phaco power under low flow conditions, and variable rise times. Other factors that affect the movement of material through this system include the nature of the material in the anterior chamber and the lines and the amount of occlusion at the phaco or irrigation/ aspiration (I/A) tip. The use of burst and pulsed modes of phaco power also can influence flow.
Infusion of fluid into the eye can be created by a programmable pump, but it is often a gravity feed system in which the pressure is created by a difference between the bottle height and the patient’s eye (not the bottle height above the machine although this ideally should be the same). Infusion flow needs only to be high enough to match current and anticipated outflow.
Bottle height and inflow must be adjusted as outflow is altered by changes in the machine settings or by surgical maneuvers, otherwise an unstable anterior chamber will result. The bottle height will usually need to be higher with higher aspiration rates and with incisions that are less competent (leaky). Air pump or forced infusion systems, founded on systems such as the Alcon ACCURUS, utilize air generated by a pump to actively push fluid into the eye independent of bottle height. The advantages are a more constant intraoperative anterior chamber volume and minimal surge.
Inflow tubing is usually wider in diameter and softer than outflow tubing as compliance is less of an issue on the inflow side and the softer tubing improves ergonomics. On the outflow side tubing needs to be more rigid (less compliant) to minimize surge (see following text).
Control of outflow is multifactorial and influenced by several factors at the same time. These include incision architecture, phacoemulsification and I/A tip and port size, pump type, tubing size and compliance, vacuum, venting, and aspiration settings.
Incision width should be standardized for the phaco tip and I/A instrument used. The incision should not be watertight. Consequently some flow around the instrument is inevitable, and for the most part is useful in helping to avoid wound burns and prevent heating of the phaco needle. If the incision is too large, it may result in damage to the eye tissues from excessive flow and difficulty in maintaining a stable anterior chamber. If it is too narrow (tight) and/or too long, it may lead to crimping of the infusion sleeve as the instrument is manipulated, impairing inflow and causing an unstable anterior chamber or thermal damage.
Most of the fluid exiting the eye is removed by the phaco machine pump. Two major types of pump are in common use in phaco systems: flow pumps (peristaltic) and vacuum pumps (Venturi). Hybrid pumps with features of both systems could be considered to be a third type.
Peristaltic pumps (Figure 11-2), a type of flow pump, control fluid movement through the outflow line by the action of a series of rollers that move along the flexible outflow tubing, forcing fluid through the system. This also creates a relative vacuum at the aspiration port when it is partially or completely occluded. The flow rate is controlled by the speed of the rollers, as is the rise time (the interval between occlusion and reaching maximum vacuum). It should be noted that flow and aspiration with the pump system are essentially the same. The vacuum level is determined by how long the rollers run when the line is occluded. The advantage of this system is that vacuum and flow (aspiration) can be independently controlled. In this type of pump, vacuum is used primarily to hold the material at the tip of the hand piece (like glue). Flow (aspiration rate) is adjusted to modify the speed at which material is swept toward the tip (like a magnet). Partial occlusion reduces flow through the system.
A common form of vacuum pump (Figure 11-3) is based on the Venturi principle: the flow of gas or liquid across a port creates a vacuum proportional to the rate of flow. The vacuum is then utilized in these pumps to create a fluid flow. Decreasing the vacuum decreases the pump flow. Vacuum level and aspiration flow rates are not independently controlled. The rise time depends on vacuum level and varies inversely with it. Vacuum pumps characteristically have a rapid rise time.
In a Venturi pump, the direct linear control of vacuum allows indirect linear control of aspiration rate as long as the aspiration port is not occluded. Unlike a flow pump, in vacuum pumps, bottle height and inflow will influence outflow unless the aspiration port is occluded.
Hybrid pump systems such as Allergan’s SOVEREIGN and Bausch and Lomb’s CONCENTRIX pumps can be digitally programmed to behave like a peristaltic or a vacuum pump with the goal of adding flexibility for the surgeon.
*Dikutip dari Buku Essentials of Cataract Surgery 2nd Ed, halaman 107-110