Introduction

Phacoemulsification surgery can be described based on its two fundamental components. First, ultrasound energy is used to emulsify the nucleus. Second, a fluidic circuit is used to remove the emulsate through a small incision while maintaining the anterior chamber (Slide 1). This circuit is supplied by an elevated irrigation bottle that supplies both the fluid volume and pressure to maintain the chamber hydrodynamically and hydrostatically, respectively; anterior chamber pressure is directly proportional to the height of the bottle. The fluid circuit is regulated by a pump that not only clears the chamber of emulsate, but also provides significant clinical utility. When the phaco tip is unoccluded, the pump produces currents in the anterior chamber (measured in cubic centimeters [cc], or milliliters [mL], per minute), which attract nuclear fragments. When a fragment completely occludes the tip, the pump provides holding power or vacuum (measured in millimeters of Mercury [mm Hg]), which grips the fragment. To fully exploit the potential of a phaco machine, a surgeon must understand the logic behind setting the parameters of ultrasound power, vacuum, flow, and bottle height. Whereas the phacodynamics of ultrasound will be discussed by William Fishkind, MD, in another tutorial, this tutorial discusses fluidic phacodynamics.

Slide 1

Slide 1

The key to understanding fluidics in phaco surgery begins with a categorization of the various pumps that are used. For clinical purposes, I believe that all current and future phaco pumps may be classified as either a flow pump or a vacuum pump.

Flow pumps

With a flow pump, a surgeon commands a given flow rate while vacuum varies up to a point that is also surgeon selected; the vacuum varies according to the fluidic resistance at the aspiration port of the phaco tip. For example, a surgeon may command a flow rate of 26 cc per minute, which may result in a vacuum level of 100 mm Hg when sculpting and aspirating viscous nuclear emulsate or viscoelastic, or both. However, this commanded flow rate will result in a lower vacuum level of approximately 30 mm Hg as the thick emulsate and viscoelastic are cleared from the aspiration line and replaced by less viscous balanced salt solution.

More specifically, the flow pump, also known as a positive displacement pump, differs from a vacuum pump in the following characteristics:

Slide 2

Slide 2
Slide 3

Slide 3

An important characteristic of a flow pump is its ability to independently control flow and vacuum. Flow rate, also known as aspiration flow rate, is directly proportional to the rotational speed of the pump head, measured in revolutions per minute (rpm); this relationship applies when the aspiration port is not occluded (Slide 3). Because the pump head physically interdigitates with the fluidic circuit via the aspiration line tubing, it regulates the flow rate independently of the amount of pressure in the line via the elevated irrigating bottle. Therefore, flow rate is independent of bottle height when a flow pump is used.

Slide 4

Slide 4

However, regardless of pump type, actual aspiration flow rate depends on the degree of phaco tip occlusion. Flow rate decreases with increasing tip occlusion (i.e., decreased effective aspiration port surface area with subsequent increased fluidic resistance) until flow ceases upon complete tip occlusion (Slide 4); the irrigation bottle’s drip chamber mirrors the activity in the anterior chamber. Aspiration flow control on the phaco machine is important with complete tip occlusion because it controls the rotational speed of the pump head. Although no actual flow exists with complete occlusion, a surgeon can control the speed of vacuum buildup via pump speed control. The amount of time required to reach a given vacuum preset, assuming complete tip occlusion, is defined as rise time.

Slide 5

Slide 5

Rise Time
Rise time is inversely proportional to the rotational speed of the pump head (Slide 5). All graphs represent the same phaco machine, with time 0 beginning with full aspiration port occlusion and full engagement of pedal position 2. However, when the flow rate is cut in half (from 40 cc to 20 cc per minute), the rise time is doubled (from 1 second to 2 seconds). Rise time is doubled again to 4 seconds when flow rate is halved again to 10 cc per minute. A longer rise time gives the surgeon more time to react in cases of inadvertent incarceration of the iris, capsule, or other material. Both residents in training and experienced surgeons appreciate the enhanced safety margin afforded by a longer rise time.

Rise time is adjusted via manipulation of the phaco machine’s flow rate control. However, as discussed previously, no actual flow exists when the tip is completely occluded, which is necessary to efficiently build vacuum at the phaco tip. Adjusting the machine’s flow parameter directly affects the rotational speed of the pump head. Vacuum builds more quickly as the rollers more rapidly traverse the aspiration line tubing in the pump head, even though no additional fluid is removed from the anterior chamber through the occluded phaco tip.

Although no fluid flows from the eye when the phaco tip is occluded, a minute amount of fluid is pumped from the aspiration line tubing as vacuum is built up, thus accounting for the relationship of pump speed to rise time. Because fluid is noncompressible and nonexpansile, theoretically, no change in aspiration line fluid volume would occur as the pump head exerts pressure on the fluid. However, two factors account for this not being true with peristaltic pumps. First, the use of the aspiration line tubing as a conduit for transmitting the pump rollers’ force results in slippage between the pump rollers and the tubing and between the opposed internal surfaces of the aspiration line tubing. Secondly, a peristaltic pump’s mechanism of action requires enough aspiration line tubing compliance to allow for collapse by the pump rollers. This compliance must be overcome during rise time in the form of some tubing constriction as minute amounts of fluid are removed from the line (not the eye) by the pump even with complete tip occlusion. Modern peristaltic pumps minimize the system’s compliance to the minimum level compatible with the functioning of the pump, thereby attaining fairly rapid potential rise times. By placing the pump element directly in the aspiration fluid path, a scroll pump further reduces the need for aspiration line compliance to the minimum amount required for ergonomic handpiece control. This type of pump can, therefore, achieve the tightest possible control of rise time with the most rapid vacuum buildup attainable.

Slide 6

Slide 6

Finally, a maximum attainable vacuum can be preset on the machine. A variety of methods can be used to prevent vacuum buildup past this level. For example, the pump head can be stopped when the preset value is reached. Additionally, vacuum can be regulated with a moving pump head by venting air or fluid into the aspiration line if the preset value is exceeded. Venting is also used when a surgeon must release material that is held to the phaco tip with vacuum. Air venting has the disadvantage of increasing the fluidic circuit’s compliance relative to fluid venting. Higher compliance increases rise time and decreases the machine’s responsiveness to foot pedal vacuum control. Slide 6 illustrates this principle, whereby an air bubble that was vented into the circuit to decrease vacuum must be first stretched out by the pump before vacuum power can begin to build again in the aspiration line. By using either air or fluid venting to regulate vacuum buildup, a flow pump directly controls flow but also allows indirect control of vacuum.

Vacuum Pumps

In contradistinction, a vacuum pump directly controls vacuum, and it can indirectly control flow. The rotary vane, diaphragm, and venturi pumps are examples of vacuum pumps.

Following are characteristics of a vacuum pump:

Slide 7

Slide 7
Slide 8

Slide 8

The applied vacuum in a vacuum pump’s drainage cassette proportionately produces flow when the aspiration port is unoccluded (Slide 7). When the tip is occluded, flow ceases and vacuum is transferred from the cassette down the aspiration line to the occluded tip (Slide 8). In contrast to a using a flow pump, a surgeon who uses a vacuum pump will command a given vacuum while flow varies according to fluidic resistance at the aspiration port of the phaco tip (a flow pump produces a commanded flow rate whereas vacuum varies according to fluidic resistance at the tip). For example, if a surgeon commands a vacuum level of 140 mm Hg, the pump will produce this level of vacuum in the machine. If the material entering the tip is viscous (dense nuclear emulsate and/or viscoelastic, both with high fluidic resistance), the subsequent flow will be relatively low. However, once this viscous material passes through, the same commanded vacuum level will result in a faster flow rate as less viscous balanced salt solution traverses the fluidic circuit.

For any type of pump, the level of vacuum just inside the incompletely occluded phaco tip is always lower than the level at the machine’s pump (Slide 7); the differential in these vacuum levels (pressures) is what produces a flow. With complete occlusion of the phaco tip and an active pump (i.e., foot pedal position 2), the vacuum power throughout the aspiration line equilibrates as flow ceases and the pressure becomes hydrostatic (Slide 8). If a surgeon commands a vacuum level of 250 mm Hg on a vacuum pump, the level of vacuum is produced at the level of the machine. Upon occlusion of the tip, the vacuum level rapidly transfers to the occluding material at the tip. If a surgeon who uses a flow pump had set a vacuum limit of 250 mm Hg, the surgeon may wait longer for vacuum to build up in the line, depending on the pump speed (recall discussion on the relationship of commanded flow rate to rise time with flow pumps). Because no rollers are required to collapse the tubing as with peristaltic pumps, vacuum pumps can use more rigid tubing with less compliance. This lower compliance and the short times needed for vacuum transfer from the cassette to the phaco tip or irrigation and aspiration (I&A) tip result in typically lower rise times with vacuum pumps as compared with flow pumps.

Slide 9

Slide 9

Low rise times can be a potential liability when using high vacuum techniques; if unwanted material is inadvertently incarcerated in the aspiration port, the surgeon has little time to react before potentially permanent damage occurs. When using a flow pump with a high vacuum preset, a low flow rate can be set to produce longer rise times which give the surgeon more time to react to unwanted occlusions. Most vacuum pumps do not allow attenuation of rapid rise times, although the Storz Millennium and Premiere machines are exceptions. These pumps allow the surgeon to set a time delay for full commanded vacuum buildup which starts when the surgeon enters foot pedal position 2. However, once this delay has elapsed, any subsequent engagement of material will be exposed to a typically rapid vacuum pump rise time. An even better solution to this issue is the Dual Linear foot control of the Millennium (Slide 9); this separates simultaneous linear control of vacuum and ultrasound into two planes of pedal movement (pitch and yaw). With linear control of vacuum in phaco mode, the surgeon can approach material with safer lower vacuum levels and increase it only after desired material is positively engaged.

Slide 10

Slide 10

Direct linear control of vacuum has another advantage with vacuum pumps in that it allows subsequently indirect linear control of aspiration flow rate when the tip's aspiration port is unoccluded (Slide 7). Increased resistance from partial or complete occlusion affects actual flow just as with a flow pump (Slide 10). However, because flow is indirectly controlled by these pumps, it is more sensitive to resistive variances in the fluidic circuit. For example (Slide 11), a vacuum pump will produce a certain flow rate at a particular vacuum when using a phaco tip; this same flow rate can also be produced on a flow pump.
Slide 11

Slide 11
Slide 12

Slide 12
However, changing to an I&A tip (with its smaller surface area aspiration port and subsequently higher fluidic resistance) will decrease actual flow in both systems, but to a greater degree in the vacuum pump. This indirect control of flow by a vacuum pump has another important clinical corollary with regard to bottle height. Unlike a flow pump, a vacuum pump's flow rate is affected by bottle height as a result of a higher pressure head from a higher bottle height pushing fluid through the open circuit more rapidly (compare the fluidic schematics in Slide 2 and Slide 7, noting again the interdigitation of the flow pump rollers and the aspiration line tubing).

Phaco Technique

To appropriately adjust the machine parameters for various stages of surgery, it is necessary to analyze the function of those parameters for a given stage and for a given pump type. For example, sculpting requires both titration of ultrasound power as well as enough flow to clear the anterior chamber of the emulsate produced by ultrasound. Furthermore, sufficient flow is required to cool the phaco tip; a modest flow of 20 cc/min is usually adequate for all of these functions. This flow rate may be directly commanded on a flow pump machine or indirectly achieved on a vacuum pump machine by a low commanded vacuum (e.g., 30 mm Hg to 50 mm Hg depending on the diameters/fluidic resistance of the particular phaco needle and aspiration line used).

There is little need for vacuum during sculpting with regard to its role in gripping material; there are not yet any fragments that need to be occluded and gripped. Vacuum is not needed to counteract the repulsive action of ultrasound because the nucleus is held stationary by the capsule, zonules, and its intact structure at this point. Therefore, a low vacuum is adequate for sculpting. Zero vacuum (vacuum limit preset of 0 mm Hg) sculpting was once advocated by some surgeons to maximize safety. However, a vacuum setting of 0 mm Hg is not practical in that it would result in cessation of pump activity (flow) once any nuclear emulsate is produced which increases viscosity and therefore increases vacuum even slightly past the preset limit of 0. Therefore, a slightly higher level of 30 mm Hg (either with a flow pump or a vacuum pump) still provides significant safety (in case of contraincisional peripheral epinuclear or capsule incarceration) while decreasing the likelihood of a clogged aspiration line.

The actual number for the flow or vacuum parameter is not nearly so important as a surgeon’s visual assessment of the machine’s efficacy through the operating microscope. During sculpting, adequate power should be used so that the phaco needle gently carves through the nucleus without pushing it and stressing zonules. Adequate flow should be ascertained by keeping the anterior chamber clear of nuclear emulsate (lens milk). Accumulation of lens milk should prompt the surgeon to increase flow rate. Prompt attention should be paid to the occlusion indicator bell (on flow pumps), especially when the aspiration port seems to be clear; this is another sign of inadequate flow and can result in an incisional burn if ultrasound power is maintained without first increasing aspiration outflow either by increasing the proper parameter (flow on a flow pump and vacuum on a vacuum pump) or by inspecting the aspiration line for any signs of an obstruction such as a kink or a clog of nuclear emulsate.

Once the nucleus is debulked or grooved, rotation or cracking should be performed. These maneuvers should be performed in foot pedal position 1 so that the chamber will be pressurized without any pump action which might inadvertently aspirate unwanted material. Once the nucleus is debulked or cracked into fragments, machine parameters must adapt to the needs of emulsifying these fragments. Ultrasound power requirements are lower at this stage relative to sculpting because of the increased efficiency of phacoaspiration with complete or almost complete tip occlusion. Even with only moderate ultrasound levels, though, flow rate and vacuum usually must be increased from the sculpting levels to overcome the repulsive action of ultrasound at the axially vibrating needle tip. Although a 26-cc/min flow rate (flow pump) and 120 mm Hg vacuum power (vacuum pump or flow pump) are reasonable baseline values at this stage, these parameters should ideally be linearly titrated intraoperatively to a given ultrasound level and nuclear density. This level of control has only recently been available to the surgeon with the advent of the Dual Linear pedal. As with sculpting, the surgeon must assess intraoperative efficacy; nuclear chatter at the phaco tip should prompt the surgeon to increase attractive fluidic parameters (flow and vacuum on a flow pump or vacuum on a vacuum pump) or to decrease the repulsive ultrasound parameter.

Chopping maneuvers often require further manipulation of parameters. When discussing parameter needs for chopping, it is first helpful to classify chopping methods into either horizontal or vertical, a concept popularized by David Chang, MD. Horizontal techniques include Nagahara’s original method, as well as the later variants, such as stop and chop by Paul Koch, MD, and mini-chop by Ron Stasiuk, MD. In these methods, the chopper is initially placed at the nuclear periphery and then drawn toward the centrally imbedded phaco tip to create the chop; some lateral separation of the instruments is subsequently required to complete the chop. The actual chop may require only moderate vacuum because the nucleus is mechanically fixated between the phaco tip and the chopper. However, higher vacuum levels of 200 mm Hg to 250 mm Hg can be used advantageously to grip and manipulate the nucleus in preparation for chopping. For example, the gripped nucleus can be displaced so that the chopper is more centrally located when engaging the nuclear periphery. This maneuver is especially effective if the nucleus was previously grooved and hemisected as has been described by Koch and Stasiuk (Slide 12).

Slide 13

Slide 13

The preceding technique must be changed slightly when a surgeon uses a vertical chopping technique, such as phaco crack (Vladimir Pfiefer, MD), quick chop (Dave Dillman, MD), or snap and split (Hideru Fukasaku, MD). With these methods, the chopping instrument is placed centrally in front of the centrally imbedded phaco tip and then pressed downward to impale the nucleus. Because this downward force is at a right angle to the phaco tip, it produces a destabilizing shear force that typically requires a higher vacuum setting relative to the somewhat lower setting that is adequate in a horizontal chop when the chopper is drawn from the periphery directly toward the phaco tip, mechanically fixating the nucleus. As always, proper technique must precede adjustments in technology, and the destabilizing force can be minimized in vertical chopping by placing the chopper closer to the phaco tip to minimize the induced torque (Slide 13).

If a flow pump is used for chopping, 28 cc/min is a useful compromise between a reasonably rapid rise time and a reasonable safety margin against surge. If a vacuum pump is used at 200 mm Hg to 250 mm Hg, the induced flow rate and surge potential are especially high. When the chop is completed and the occlusion breaks, the subsequent induced flow with a standard needle would be more than 60 cc/min. A MicroFlow or similar needle with a reduced inner diameter (and therefore increased fluidic resistance) reduces this flow by about 40% to a safer level. The safest technique, though, would be to use the high vacuum level during the actual manipulation and chop when gripping the nucleus (i.e., tip occluded and therefore no flow) and then to dynamically decrease the vacuum with pedal control just as the chop is completed to minimize the surge potential. Once the chops are completed, the surgeon may return to the lower and safer levels that were previously discussed for quadrant removal; this type of frequent adjustment is best accomplished by linear pedal control of fluidics in addition to phaco power.

Slide 14

Slide 14

Surge occurs when an occluded fragment is held by high vacuum and is then abruptly aspirated (i.e., with a burst of ultrasound); fluid tends to rush into the tip to equilibrate the build up of vacuum in the aspiration line with potentially consequential shallowing or collapse of the anterior chamber (Slide 14). In addition to preventive measures, phaco machines use a variety of methods to combat surge. Fluidic circuits are engineered with minimal compliance which will still allow adequate ergonomic manipulation of the tubing as well as functioning of the pump mechanism, the latter being primarily important for peristaltic pumps. The Surgical Designs machine incorporates a second, higher irrigating bottle in which fluidic circuit is engaged upon detection of a surge. A number of newer machines use a vacuum sensing feedback loop via microprocessor control to assess and hopefully blunt potential surges in the anterior chamber. Small bore aspiration line tubing, utilized by Allergan and Alcon, provides increased fluidic resistance which obtunds surges in a manner similar to that of the MicroFlow needle. However, placing increased fluidic resistance in the aspiration line as opposed to the vibrating phaco needle can be more likely to lead to potential clogging of the aspiration line from nuclear emulsate and consequential interruption of flow.

Although all of these designs are helpful, it is ultimately a surgeon’s responsibility to set parameters that optimize surge prevention of a given machine for a given patient. Vacuum level and bottle height are the most relevant parameters and should be decreased and increased, respectively, when a surgeon first notices modest chamber shallowing immediately following occlusion breaks. If a surgeon is unable to produce adequate surge control (i.e., due to the inadvertent use of standard tubing instead of smaller bore high vacuum tubing), then he or she must change the technique from chopping (which requires higher vacuum levels) to a method that works with lower vacuum levels, such as Four Quadrant Divide and Conquer (Joel Shepherd, MD).

The bottle height setting has a constant function during all phases of surgery — to keep the chamber safely formed without overpressurization which may stress zonules, misdirect aqueous into the vitreous, or cause excessive incisional leakage. Approximately 10 mm Hg of hydrostatic pressure is produced intraocularly for every 15 cm that the bottle is above the eye. However, it is vital that the appropriate bottle height be set hydrodynamically with the pump operating (foot pedal position 2 or 3) and the tip unoccluded so that an adequate pressure head will be established to keep up with the induced aspiration outflow from the eye. The surgeon will typically want to raise the bottle slightly higher (e.g., 15%) to allow a safety margin against potential transient increases over this baseline flow level (i.e., surges).

Conclusion

This discussion has stressed the importance of appropriate machine parameter settings. However, surgical technique is not only just as important, but also integrally related. For example, if a surgeon wishes to grip and pull a heminucleus in preparation for chopping, yet finds that the tip pulls away from the lens material instead, the tendency would be to increase the vacuum parameter to give a stronger grip. However, the full preset vacuum can be produced at the phaco tip only with complete tip occlusion. Therefore, if an adequate vacuum seal is not obtained, the preset value will often not be reached; this is true for both flow pumps and vacuum pumps. Increasing the vacuum preset will not affect the clinical performance in the absence of a good vacuum seal, which is obtained by imbedding the phaco tip at least 1 mm to 1.5 mm with light ultrasound energy so as to avoid excessive cavitation (Slide 15). The tip is also imbedded in the central densest nucleus as opposed to more peripheral, softer material which might irregularly aspirate, again causing a loss of the vacuum seal (Slide 16). Another example of the importance of technique would be chamber instability; a surgeon should ensure that he or she is not excessively distorting the incision and causing extraneous leakage before starting to adjust bottle height and flow. This subtle attention to technique pays off with the machine being used to its most effective potential by minimizing the need for excessively high parameters and therefore maximizing safety and efficiency.

Modern phaco machines offer unprecedented levels of control and safety. To fully capitalize on these values, a thorough understanding of the principles by which the machines and surgical maneuvers operate is essential. Particularly, a surgeon must appropriately adjust flow rate, vacuum, ultrasound power, and bottle height as necessary for a given patient and for a given stage in the operation. This vigilance and attention, coupled with meticulous technique designed to optimize the machine's performance, will result in the safest, most efficient phacoemulsification surgery.

Slide 15

Slide 15
Slide 16

Slide 16