lated and rectified. Verification of a clog is achieved by placing a test chamber over the visibly unobstructed phaco needle and engaging pedal position 2 while observing inadequate or absent activity in the irrigating bottle’s drip chamber. The accumulation of emulsate can sometimes be visualized in the aspiration line, often at one end; in these cases, digital massage of the tubing at this area often breaks up the obstruction. Sometimes, very high commanded flow and vacuum along with high ultrasound will free a nonvisualized obstruction; remember to perform this maneuver extraocularly with a test chamber over the phaco needle.

The greatest danger of an aspiration line obstruction is the surgeon’s failure to recognize and rectify the situation. The most benign outcome of such a failure is the impairment of the machine’s effectiveness in producing desired intraoperative flow and vacuum. However, the greater danger occurs if, as a result of the subsequently impaired followability and grip, the surgeon chases after nuclear fragments into the periphery of the anterior chamber rather than maintaining the phaco needle in a safer, more central position and having machine fluidics attract fragments to and into the aspiration port. With the needle in a peripheral position, an aspiration line obstruction might spontaneously clear, inducing a surge that can incarcerate and damage the juxtaposed iris or capsule. Furthermore, if the obstruction does not clear spontaneously, the probability of a corneal wound burn becomes progressively greater as more ultrasound energy is engaged without sufficient cooling flow.

The appropriate setting of the machine’s vacuum parameter (measured in mm Hg) is another key element in avoiding complications. As discussed previously, adjusting the commanded vacuum on a vacuum priority pump (e.g., Venturi, rotary vane, or Concentrix pump in vacuum mode) proportionately adjusts the flow rate when the phaco tip’s aspiration port is not occluded. When the phaco tip is occluded, then adjusting the commanded vacuum (vacuum priority pump) or the vacuum limit preset (flow priority pump; either Concentrix pump in flow mode or peristaltic pump) proportionately adjusts the grip and deformational force that is applied to the material that is occluding the aspiration port. The amount of grip for a given amount of pump vacuum is proportional to the surface area of the phaco needle’s aspiration port; the surgeon should therefore anticipate the need for increasing vacuum from the usual levels when changing to either a smaller gauge or less beveled phaco needle. As with any parameter, the vacuum should be adjusted appropriately for a given surgical function; a higher adjustment would needlessly compromise the operation’s safety margin.

For example, a high vacuum level of 250 mm Hg might be required during a modified chop maneuver

(as described by Paul Koch and Ron Stasiuk) to grip and pull the engaged heminucleus centrally so as to facilitate the peripheral placement of the chopping instrument. However, once the heminucleus is mechanically fixated between the chopper and the phaco tip, high vacuum is no longer required. Indeed, it can subsequently become a liability if maintained beyond the completion of the chop with breaking of the vacuum seal between the phaco tip and the nucleus. When using a vacuum pump, the high induced flow from the high vacuum level with an unoccluded tip can produce shallowing or collapse of the anterior chamber.

POSTOCCLUSION SURGE

When using either a flow or vacuum pump, the high vacuum level can further compromise anterior chamber stability because of a postocclusion surge; this phenomenon results from the sudden equilibration of compliance that was pulled out of the system during occlusion of the aspiration port with high vacuum.

IMMEDIATELY PREOCCLUSION

When the phaco tip is occluded with a nuclear fragment, and aspiration is engaged (foot pedal position 2), the pump continues to create vacuum. Because the tip is occluded, there is no flow (Fig. 25–1). The vacuum will increase to the preset maximum. The aspiration tubing may collapse.

OCCLUSION

The fragment is held firmly to the phaco tip. There is no flow. Vacuum is at maximum. The aspiration tubing is collapsed. The combination of high vacuum and collapsed aspiration tubing creates a system with a great deal of potential energy. The chamber is deep (Fig. 25–2).

POSTOCCLUSION

Once emulsification is activated (foot pedal position 3) the fragment is emulsified. The potential energy of the system is transformed into kinetic energy. Thus, an instantaneous rush of fluid from the anterior chamber into the phaco tip ensues. Augmenting this flow is the additional vacuum created by the now expanding aspiration line tubing. Fluid leaves the anterior chamber faster than it can be replaced by infusion. The sudden evacuation of fluid causes the anterior chamber to shallow. The posterior capsule moves anteriorally and the cornea collapses. The posterior capsule may forcefully “snap” forward,

initiating a tear. Alternatively, the posterior capsule may be suddenly stretched over a nuclear fragment and in this way causing it to tear (Fig. 25–3).

LATE POSTOCCLUSION

If the event is sufficiently violent, before inflow and outflow can equilibrate, the tear may become quite large, and the vitreous face may rupture. Vitreous will create resistance and as it is aspirated, the vacuum will rise slightly1 (Fig. 25–4).

More frequently, the surge results in sudden shallowing of the anterior chamber but results in nothing

FIGURE 25–1 Immediately preocclusion. The nuclear fragment is almost occluded on the phaco tip. There is no flow. Vacuum will rise to the preset limit. The aspiration tubing begins to collapse.

more than surgeon discomfort. Eventually, the vacuum decreases and inflow equilibrates with outflow. The anterior chamber deepens to the preocclusion depth.

FLUIDIC COMPLICATIONS—VACUUM

MANAGEMENT

The vacuum should be dynamically titrated to a lower level as a chop is completed so as to avoid these problems with anterior chamber maintenance; this dynamic titration requires linear pedal control of vacuum in phaco mode. Furthermore, the surgeon

should compensate accordingly when observing early signs of surge, such as intermittent dimpling of the cornea and constriction of the pupil during carouseling emulsification of a nuclear fragment. Appropriate steps would include raising the bottle height, lowering the vacuum level, or changing to a phaco needle with a smaller internal diameter and subsequently higher fluidic resistance.

In addition to producing anterior chamber instability, unnecessarily high vacuum levels can disrupt inadvertently incarcerated iris or capsule. Furthermore, such a high vacuum level can be counterproductive with regard to a desired surgical goal. For

FIGURE 25–3 Postocclusion. After the application of phaco power the fragment is emulsified and vacuum immediately falls to 0. Flow immediately rises to the preset maximum. This allows the high vacuum and expanding aspiration tubing to pull fluid out of the anterior chamber more rapidly than it can be replaced by infusion. The rapid loss of anterior chamber volume causes the posterior capsule to move anteriorally and the cornea to collapse. The posterior capsule is literally “snapped,” or alternatively stretched, around the nuclear fragment, resulting in a tear.

example, an efficient technique for removing the epinucleus involves using just enough vacuum to grip and manipulate it so as to facilitate flipping as described by Howard Fine. Once the engaged epinucleus is safely and optimally positioned away from the capsule, it can then be aspirated by engaging mild ultrasound. If the surgeon uses an inappropriate and unnecessarily high vacuum level for epinucleus removal, the adherent tissue will be abruptly aspirated upon contact with the phaco tip, precluding the possibility of controlled manipulation as described (Fig. 25–5). The surgeon is subsequently forced to reengage the epinucleus repeatedly at its

FIGURE 25–5 (Top) The epinucleus is aspirated with too high a vacuum setting, resulting in a chunk of epinucleus breaking off the plate. (Bottom) The epinucleus must then be engaged further out in the periphery, where aspiration of the capsule is more likely.

more precarious in situ position adjacent to the capsule, thereby greatly increasing the chances of inadvertent capsule incarceration and rupture (Fig. 25–5 bottom). This same logic is applicable for cortical removal, in which an appropriately titrated vacuum level enables the engaged cortex to be stripped away from the capsule in large sheets prior to safe, controlled aspiration via increased vacuum.

As mentioned previously, an appropriately titrated vacuum level enables the surgeon to firmly grip an engaged heminucleus so as to displace it centrally prior to chopping; this maneuver enables the chopper to be more easily engaged at the heminuclear periphery while minimizing danger to the anterior and peripheral capsule. If the phaco tip pulls out of the heminucleus instead of pulling it centrally, the surgeon may perhaps suspect that the vacuum parameter was insufficient. However, this ineffective grasp might have been caused by a vacuum level that was, in fact, too high for the given nuclear density, such that the nuclear material just around the tip was abruptly aspirated (Fig. 25–6). Without a relatively snug fit around the phaco tip to give a good vacuum seal, it is impossible to effectively transmit the machine’s gripping force, regardless of whether a flow pump or a vacuum pump is used. A similar disruption of material just around the tip might have been caused by the excessive use of ultrasound when embedding the tip. Only mild ultrasound should be used at this point to ensure a tight vacuum seal, and it should always be discontinued as soon as

FIGURE 25–6 Excessive vacuum in relation to nucleus density leads to removal of excess nuclear material contiguous with the tip. This disrupts the vacuum seal. There is resultant poor holding power.

the tip is buried to an adequate depth of 1 to 1.5 mm. Maintaining ultrasound beyond this point not only can preclude a good vacuum seal, as just described, but also will further preclude an effective grip of a heminucleus by allowing the needle to vibrate free when pulled rather than pulling the heminucleus with it.

With regard to the importance of a good vacuum seal as discussed above, the concept of the phaco needle bevel must be examined relative to its ease of occludibility. Although it has been suggested in the past that a 0-degree tip occludes more easily than a more beveled tip, it can be seen that for practical clinical purposes the bevel angle is irrelevant to the effectiveness and ease of occludibility. A good occlusion is obtained when the aspiration port is buried a sufficient amount over its entire surface. If part of the aspiration port is only shallowly embedded in the nucleus, the vacuum seal can easily be broken with only modest physical manipulation of the phaco (Fig. 25–7). The surgeon might mistakenly think that more vacuum parameter is needed when in fact the needle bevel and the surface to be occluded simply needed to be adjusted so that they are parallel to each other to achieve an effective vacuum seal. It can be seen that both a 0-degree tip and a 45degree tip can achieve either a good or a poor occlusion based on this relationship (Fig. 25–8 top and inset). It is only necessary to turn the beveled tip bevel down to maximize the capability for occlusion (Fig. 25–8 bottom). If bevel-down sculpting is to be