9

The Fluidics and Physics of Phaco

Barry S Seibel

Introduction

Phacoemulsification is comprised of two basic elements: (i) ultrasound energy is used to emulsify the nucleus, and (ii) a fluidic circuit is employed to remove the emulsate through a small incision while maintaining the anterior chamber (Fig. 9.1). This circuit is supplied by an elevated irrigating bottle which 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 which not only clears the chamber of the emulsate, but also

FIGURE 9.1

provides significant clinical utility. When the phaco tip is unoccluded, the pump produces currents in the anterior chamber, measured in cc per minute, which attract nuclear fragments. When a fragment completely occludes the tip, the pump provides holding power, measured in mm Hg vacuum, which grips the fragment. In order to fully exploit the potential of a phaco machine the surgeon must understand the logic behind setting the parameters of ultrasound power, vacuum, and flow.

Categorization of Pumps

A discussion of flow and vacuum in phaco surgery must begin with a categorization of the various pumps which are utilized. There are two basic types of pumps in phaco: (i) the flow pump, and (ii) the vacuum pump.

Flow Pump

The flow pump also known as a positive displacement pump, physically regulates the fluid in the aspiration line via direct contact between the fluid and the pump mechanism. Although the scroll pump is the newest example of a flow pump, the peristaltic pump is the most commonly employed in current phaco machines and serves as a good schematic example of the flow pump’s principles (Fig. 9.2).

One important characteristic of a flow pump is its ability to independently control flow and vacuum. Flow rate, also known as aspiration flow rate, is measured in cc per minute and is directly proportional to the rotational speed of the pump head, measured in revolutions per minute (rpm). Note that 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 using flow pumps.

However, actual fluid flow rate is very dependent on the degree of phaco tip occlusion. Flow rate decreases with increasing tip occlusion (i.e. decreased effective aspiration port surface area)

FIGURE 9.2

Phacoemulsification 134

FIGURE 9.3

until flow ceases completely with complete tip occlusion. Note that in Figure 9.3 the irrigation bottle’s drip chamber mirrors the activity in the

FIGURE 9.4

anterior chamber. Aspiration flow control on the phaco machine is still important with complete tip occlusion in that it controls the rotational speed of the pump head, and even though no actual flow exists with complete occlusion, the surgeon can control the speed of vacuum build-up via pump speed control; the amount of time required to reach a given vacuum preset, assuming complete tip occlusion, is defined as rise time.

Rise time is inversely proportional to the rotational speed of the pump head (Fig. 9.4). All graphs represent the same machine, but note that when the flow rate is cut in half (from 40 to 20 cc per minute), the rise time is doubled (from 1 second to two seconds). Rise time is doubled again to four 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 iris, capsule, or other unwanted material, although a useful setting for training residents, even experienced surgeons appreciate the enhanced safety margin afforded by a longer rise time.

Several points should be made about the preceding discussion on rise time. First, rise time was adjusted via manipulation of the machine’s flow rate control. However, as discussed previously, no actual flow exists with complete occlusion, which is necessary to efficiently build vacuum at the phaco tip. Adjusting the machine’s flow parameter, measured in cc per minute, actually 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.

The second point regarding the rise time discussion concerns the fact that although no fluid flows from the eye with tip occlusion, a minute amount of fluid is pumped from the aspiration line tubing as vacuum is built up, thus, accounting for the relation 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 exerted pressure on the fluid. However, two factors account for this not being true with peristaltic pumps: (i) the use of the aspiration line tubing as a conduit for transmitting the pump rollers’ force results in some inefficiency in the form of slippage both between the pump rollers and the tubing as well as in between the opposed internal surfaces of the aspiration line tubing, and (ii) the mechanism of action of a peristaltic pump 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 some fluid is removed from the line (not the eye) by the pump even with complete tip occlusion (Fig. 9.5). The most 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 into 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 potential control of rise time with the most rapid vacuum build-up attainable.

The final point concerning rise time and flow pumps is the fact that a maximum attainable vacuum can be preset on the machine. In order to prevent vacuum build-up past this level, a variety of methods are employed. For example, the pump head can be stopped when the preset value is reached. Alternatively, 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 employed if the surgeon wishes to release material which 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. Figure 9.6 illustrates this principle, whereby an air-bubble which was vented into the circuit to

decrease vacuum must be first stretched out by the pump before vacuum can begin to build in the aspiration line again. By employing either air or fluid venting to regulate vacuum build-up, a flow pump therefore not only directly controls flow but also allows indirect control of vacuum.

Vacuum Pump

In contrast, a vacuum pump directly controls vacuum although it can indirectly control flow. Vacuum pumps represent the second main category of phaco pump, with examples being the rotary vane pump, the diaphragm pump, and the Venturi pump. Vacuum pumps have in common a rigid drainage cassette attached to the aspiration line tubing. The various pumps are linked to the cassette and produce vacuum in it which in turn proportionately produces flow when the aspiration port is unoccluded (Fig. 9.7). When the tip is occluded, flow ceases and vacuum is transferred

FIGURE 9.7

from the cassette down the aspiration line to the occluded tip (Fig. 9.8). Because no rollers are required to collapse the tubing as with peristaltic pumps, vacuum pumps can employ more rigid tubing with less compliance. This lower compliance coupled with the short times needed for vacuum transfer from the cassette to the phaco (or IA) tip result in low rise times with vacuum pumps.

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

Phacoemulsification 138

little time to react before potentially permanent damage occurs. Recall that when using a flow pump with a high vacuum preset, 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 donot allow attenuation of rapid rise times, although the Storz Millennium and Premiere

FIGURE 9.8

machines are exceptions. These pumps allow the surgeon to set at time delay for full commanded vacuum build-up which starts when the surgeon enters 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, if not elegant, solution to this issue is the dual linear foot control on the millennium (Fig. 9.9), this separates simultaneous linear control of vacuum and ultrasound in 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.

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 unocclu-

FIGURE 9.9

ded (Fig. 9.7). However, because flow is thus indirectly controlled by these pumps, it is more sensitive to resistive variances in the fluidic circuit. For example (Fig. 9.10), a vacuum pump will

FIGURE 9.10

produce a certain flow rate at a particular vacuum when using a phaco tip; this same flow rate could also be produced on a flow pump. However, changing to an IA tip (with its smaller surface are 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

Phacoemulsification 140

fluid through the open circuit more rapidly (compare the fluidic schematics in Figures 9.2 and 9.7, noting again the interdigitation of the flow pump in its fluidic circuit).

Application of Ultrasound Power

Besides setting fluidic parameters, the surgeon must also decide on the application of ultrasound power, which is produced most often by a piezoelectric crystal oscillating between approximately 20,000 and 60,000 times a second for most machines. This frequency is fixed on a given machine. Ultrasound power is varied by changing the amplification voltage of the handpiece. Increased voltage translates to increased stroke length at the phaco needle tip, up to a maximum of about five microns on most machines (Fig. 9.11). Usually, a maximum ultrasound limit is preset on the machine’s front panel, and the surgeon then titrates with linear pedal control the percentage of this preset maxi-

FIGURE 9.11

mum which is appropriate to a given intraoperative instant.

The actual mechanism of action of ultrasonic phacoemulsification is somewhat controversial. One school of thought centers around the acoustic breakdown of lenticular material as a result of sonic wave propagation through the fluid medium. Another theory concerns the microcavitation bubbles produced at the distal phaco tip, the implosion of these bubbles produces brief instances of intense heat and pressure which is thought to emulsify adjacent lens material. Yet another potential mechanism of action is via the tips axial oscillations through its stroke length, this resultant jackhammer affect is thought to mechanically break down lens material. This last mechanism also explains the clinical phenomenon of repulsion of free floating lens material with high ultrasound power levels, these levels need appropriate fluidic titration of the attractive parameters of flow and vacuum to counteract this repulsion.

Ultrasonic phaco needles are available in a variety of configurations. One basic design parameter is the distal bevel angle, which is most commonly 0°, 15°, 30°, or 45° as shown in Figure 9.12. The sharper 45° angle is thought to carve dense

FIGURE 9.12

FIGURE 9.13

nuclei more efficiently to the extent that the jack-hammer mechanism of action is valid, whereas the 0° tip would be more efficient to the extent that the microcavitation theory is valid (the 0° tip has more frontal surface area perpendicular to the axis of oscillation, thereby, producing more cavitation bubbles). In practice, it is difficult to quantitatively compare these efficiencies on a standard density nucleus. Another traditional teaching regarding tip angulation is that a 0° tip occludes more readily than 45° tip, this

Phacoemulsification 142

observation is correct only in that the smaller surface area and perimeter of the 0° tip does seal more readily than does the tip with a larger bevel. However, this axiom is less relevant intraoperatively. A tip occludes readily when the surface to be occluded is parallel to the needle bevel, the surface can and should be manipulated as necessary to achieve this configuration (Fig. 9.13, which illustrates the attempted gripping of a heminucleus during a stop and chop maneuver).

FIGURE 9.14

When titrating ultrasound power, the surgeon must be aware of interrelated clinical variables affecting the resistance to emulsification, especially sculpting. This resistance is directly proportional to both the linear speed of sculpting as well as the amount of the tip engaged. In Figure 9.14, it can be noted that for the increased resistive load caused by the increased tip engagement, the surgeon must compensate by either increasing phaco power or decreasing in linear speed of sculpting. Either solution is satisfactory, as long as the interrelationship among the above variables is respected, so as to facilitate the needle carving through the nucleus instead of pushing it and stressing the zonules or capsule.

Adjustment of Machine Parameters

In order 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. For example, sculpting requires titration of ultrasound power as described in the previous paragraph. Furthermore, it requires enough flow to clear the anterior chamber of the emulsate produced by ultrasound as well as sufficient flow to cool the phaco tip, a modest flow of 18 cc/min is usually adequate for these functions. There is little need for vacuum during sculpting, as there are not yet any fragments which need to be occluded and gripped. Furthermore, vacuum is not needed to counteract the repulsive action of ultrasound since the nucleus is held stationary by the capsule, zonules, and its intact structure at this point.

Therefore, a low vacuum is adequate for sculpting. Although 0 mm Hg is advocated by some surgeons, a slightly higher level of 15 to 30 mmHg still provides significant safety (in case of contraincisional peripheral epinuclear or capsule incarceration) while decreasing the likelihood of a clogged aspiration line.

Once the nucleus is debulked or grooved, it then needs manipulation such as rotation or cracking. These maneuvers should be performed in 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 need to adapt to the needs to 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 their sculpting levels in order to overcome the repulsive action of ultrasound at the axially vibrating needle tip. Although 26 cc/min flow rate and 120 mmHg vacuum 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 previously described.

Chopping maneuvers often require further manipulation of parameters. 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 to 250 mmHg can be used advantageously to grip

FIGURE 9.15

and manipulate the nucleus. 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 Drs. Paul Koch and Ron Stasiuk (Fig. 9.15). If a flow pump is used, 26 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 to 250 mmHg, the surge potential is especially high. When the chop is completed and the occlusion breaks, the subsequent induced flow with a standard needle would be over 60 cc/min. A Microflow or similar needle with a reduced inner diameter (therefore increased fluidic resistance)

Phacoemulsification 144

reduces this flow by about 40 percent 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 and then to dynamically decrease the vacuum with pedal control just as the chop is completed to minimize the surge potential.

Surge, as has been discussed, 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 built-up vacuum in the aspiration line with potentially consequent shallowing or collapse of the anterior chamber (Fig. 9.16). In addition to the preventive measures mentioned in

FIGURE 9.16

the previous paragraph, phaco machines employ 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. Small bore aspiration line tubing, utilized by Allergan and Alcon, provide increased fluidic resistance which obtunds surges in a manner similar to that of the Microflow needle previously discussed. The Surgical Designs machine incorporates a second, higher irrigating bottle whose fluidic circuit is engaged upon detection of a surge. While all of these designs are helpful, it is ultimately up to the surgeon to set parameters which optimize a given machine for a given patient with regard to surge prevention.

The parameter of bottle height has a constant function during all phases of surgery—to keep the chamber safely formed without overpressurization which might stress zonules, misdirect aqueous into the vitreous, or cause excessive incisional leakage. Approximately, 10 mm Hg hydrostatic pressure is produced intraocularly for every 15 cm bottle height above the eye.

However, it is vital that the appropriate bottle height be set hydrodynamically with the pump operating (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.

Phacoemulsification 146

nucleus as opposed to more peripheral, softer material which might irregularly aspirate, again causing a loss of the vacuum seal (Fig. 9.18). This subtle attention to technique pays off with the machine being used to its most effective potential.

Summary

Modern phaco machines offer unprecedented levels of control and safety. In order to fully exploit these values, a thorough understanding of the principles by which the machines operate is essential. In particular, the 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.