tips or flair tips, change fluid flow rates. There is an increased resistance to the flow due to the reduced internal lumen of the needle.

The Alcon ABS (aspiration bypass system) tip modification is available with a 0° tip, a Kelman tip, or a flare tip. The flare is a modification of power intensity and the ABS a flow modification. In the ABS system, a 0.175-mm hole in the shaft permits a variable flow of fluid into the needle, even during occlusion. Therefore occlusion is never allowed to occur (Fig. 3.6). This flow adjustment serves to minimize surge.

3.Flow can be modified by utilizing smaller diameter tips. These tips have a thin flexible outer sleeve to seal the phaco incision. Coaxial-micro incisional phaco makes use of these tips to accomplish a tight seal and permit a low-flow phaco without the danger of wound stretching and wound burns.

Phaco power energy emulsifies the lens nucleus. The phaco tip must operate in a cool environment and with adequate space to isolate its actions from delicate intraocular structures. This portion of action of the machine is dependent on its fluidics.

3.4 Fluidics

The fluidics of all machines depend upon a balance of fluid inflow and fluid outflow.

Inflow is determined by the bottle height above the eye of the patient, and irrigation tubing diameter. Potentially new improvements will certainly include pressurized infusion systems with anterior chamber pressure sensors in the form of a computer generated virtual anterior chamber to regulate the inflow volume contingent on the outflow volume. One type of pressurized infusion consists of an irrigation bag with two compartments. One compartment contains BSS. The other is pressurized by a pressure pump in the machine to compress the BSS and therefore increases inflow.

Outflow has two components.

1.Unregulated Fluid Outflow: This is determined by the sleeve-incision relationship, as well as the paracentesis size. The incision length selected should create a snug fit with the phaco tip selected. This will result in minimal uncontrolled incisional outflow with resultant increased anterior chamber

stability. The paracentesis should not be unnecessarily large, thereby leading to unnecessary fluid outflow.

2.Regulated Fluid Outflow: This is defined as the fluid, which passes through the phaco needle. The volume of the fluid exiting through the needle is restricted by the needle diameter, length, and configuration. If the diameter of the needle is smaller, less fluid can egress per unit time. Similarly, the tubing diameter and compliance act in an analogous manner. Tubing modifications such as “cruise control” (manufactured by Staar Surgical) can have a potent effect on fluid egress and preserve anterior chamber depth in the face of major potential episodes of surge.

Aspiration rate, (or flow), is defined as the flow of the fluid, measured in cc/min, through the tubing. With a peristaltic pump, it is determined by the speed of the pump. Flow determines how well particulate mater is attracted to the phaco tip. Aspiration level (or vacuum) is defined as the magnitude of negative pressure created in the tubing. It is measured in mmHg. Vacuum is the determinant of how well, once occluded on the phaco tip, particulate material will be held to the tip.

3.5 Vacuum Sources

There are three categories of vacuum sources or pumps. These are flow pumps, vacuum pumps, and hybrid pumps.

1.The primary example of the flow pump type is the peristaltic pump. These pumps allow for independent control of both aspiration rate (flow) and aspiration level (vacuum) (Fig. 3.7).

2.The primary example of the vacuum pump is the venturi pump. This pump type allows direct control of only vacuum level. Flow is dependent upon vacuum level setting. Examples of additional types are the rotary vane and the diaphragmatic pumps (Fig. 3.8).

3.The primary example of the hybrid pump is the AMO Signature peristaltic pump or the Bausch & Lomb Stellaris Peristaltic Pump. These pumps are interesting as they are able to act either like a vacuum or like a flow pump depending on programming.

They are generally controlled by digital inputs creating incredible flexibility and responsiveness.

The challenge to the surgeon is to balance the effect of phaco power intensity, which tends to push nuclear fragments away from the phaco tip, with the effect of

Fig. 3.7 Hybrid Pump. The AMO digital pump. A peristaltic pump with digital controls allows forward, backward, or to and fro movement to precisely control flow. This pump is capable of performing like a venturi or peristaltic pump