PHACODYNAMIC LINKS TO
COMPLICATIONS

Barry Seibel

Complications related to the phaco machine can generally be grouped into two categories. First, the inappropriate use of machine parameters may result in direct tissue damage through such mechanisms as chamber collapse, corneal wound burn, iris incarceration, and capsule rupture. Second, failure to optimally adjust machine parameters so as to facilitate efficient surgery will result in the surgeon having to compensate by more frequent and exaggerated manual maneuvers that can increase the possibility of tissue damage. Although some surgical complications are inevitable by-products of preexisting anatomy, many can be avoided by judiciously examining the relationship of the machine technology and the fundamentals of the microsurgical techniques to modern phaco surgical methods. Many aspects of the machine technology’s contribution to surgical complications can be understood by first simplifying the role that the machine plays in a routine case. That role is to create and maintain a fluidic circuit that starts at the elevated irrigating bottle, passes through the eye and then the phaco pump, before draining into a collection chamber.

If a cataract has a soft, gel-like density, then the nuclear material can propagate along the fluidic circuit as the phaco needle’s aspiration port is placed against the nucleus, aspirating it. As the nuclear density increases, the pump must create more vacuum to deform the nuclear material sufficiently for aspiration. If the nucleus has still greater, crystalline-type density, then an appropriate amount of ultrasound must be titrated so that the material can be emulsified sufficiently to allow pump vacuum and flow to deform it into and through the phaco needle and aspiration line. The ultrasonic vibrating phaco needle tends to push material away from it. During sculpting, the nucleus is held in place by capsule and zonules, but during carouseling emulsification of cracked or chopped fragments, fluidic parameters of vacuum, flow, and bottle height must be adjusted to counteract the ultrasonic repulsion so as to aspirate the nuclear material into the phaco needle.

The anterior chamber must be maintained during all phases of cataract surgery; its fluid pressure must therefore be greater than ambient atmospheric pressure. Any vacuum created in the fluidic circuit by the pump is optimally located between the aspiration port and the pump; any vacuum (pressure lower than atmospheric pressure) in the eye portion of the fluidic circuit would result in a chamber collapse, leading in turn to potential damage to the capsule, zonules, and corneal endothelium. One of the main determinants of intraocular pressure (IOP) is the height of the irrigating bottle, which yields an IOP of 11 mm Hg (above ambient atmospheric pressure) for every 15 cm (6 inches) of elevation above the eye. This relationship is accurate for hydrostatic pressures in pedal position 1. In pedal positions 2 and 3 IOP decreases (but remains greater than atmospheric pressure) with induced flow in proportion to the commanded pump strength, as well as the degree of aspiration port occlusion.

PUMP TYPES

To create vacuum, a pump of some type is necessary. Classically, pumps have fallen into two categories. The flow pump, the best example being a peristaltic pump, allows the surgeon to control both flow and vacuum parameters. The vacuum pump, for example the Venturi pump, allows surgeon control over vacuum only. The amount of flow is dependent on the vacuum setting and cannot be set by the surgeon. Recently, pumps have evolved such that flow pumps are so responsive they can be programmed to respond as if they were vacuum pumps. Additionally vacuum pumps can be manipulated to act as if they were flow pumps. These modern pumps are therefore named “hybrid pumps.”

Irrespective of pump selection, flow, measured in cubic centimeters per minute, (cc/min) is the force that brings material toward the phaco tip. In general, the higher the flow, the faster events will occur within the anterior chamber. Vacuum setting, measured in millimeters of mercury (mm Hg), will hold material on the phaco tip, once occlusion has occurred.

FLUIDIC COMPLICATIONS—FLOW MANAGEMENT

As discussed above, flow rate (cc/min) is an important factor in determining IOP. Flow rate can be increased directly on a flow pump by increasing the commanded flow rate or indirectly on a vacuum pump by increasing the commanded vacuum; in both cases, the actual flow rate is dependent on the degree of aspiration port occlusion. The actual flow rate is also affected by the fluidic circuit’s resistance, which is in turn determined by the internal diameters of the phaco needle as well as the (especially aspiration line) fluidic tubing. In addition, flow rate is affected proportionately by bottle height, but only when using a vacuum-priority pump. One potential arrangement, which may lead to complications, is to use a vacuum-priority pump for a high-vacuum technique. A high commanded vacuum setting produces a high flow rate with the unoccluded phaco tip. The surgeon may compensate for this with an elevated bottle height to maintain adequate IOP in the face of the high flow rate. However, the higher bottle height produces an even higher flow rate that not only diminishes the effectiveness of increasing the IOP, but also produces potentially dangerously fast intraocular currents that can uncontrollably attract and incarcerate unwanted material such as iris and capsule.

The induced flow rate from a high commanded vacuum can be lowered to a safer level by the use of a high-resistance phaco needle with a small internal diameter, such as a MicroFlow or MicroSeal needle. In addition, the surgeon should titrate the amount of commanded vacuum during phaco with linear pedal control according to the clinical application and the status of the aspiration port. Appropriately high vacuum may be used safely when the aspiration port is occluded, for example, when gripping a heminucleus in preparation for chopping. However, when anticipating an occlusion break at the end of a chop or during carouseling ultrasonic emulsification, the surgeon should linearly titrate vacuum to a lower, safer, and more appropriate level.

ASPIRATION LINE OBSTRUCTION

Another potential area of complication related to IOP maintenance and flow concerns is the presence of an obstruction between the aspiration port and the pump. Although kinked aspiration line tubing can produce this effect, it is most often caused by the localized accumulation of nuclear emulsate, clogging the aspiration line, especially when sculpting dense, mature cataracts. This type of obstruction can significantly impair the effectiveness of the phaco machine by limiting the pump’s ability to transfer its force (either via flow or vacuum) past the obstruction to the phaco tip where it is needed. The surgeon can recognize this problem when free (e.g., chopped) nuclear fragments fail to be effectively drawn to the phaco tip when in pedal position 2 and using a flow setting that usually is effective in this situation. Similarly, an aspiration line obstruction is suspected when faced with insufficient grip of an occluding nuclear fragment when using a vacuum setting that usually achieves a good grip. An aspiration line clog is also a strong possibility when observing intraocular flocculence (“lens milk” or “phaco dust”) during sculpting, indicating the inability for pump-induced flow to effectively clear the anterior chamber of the ultrasonically induced emulsate.

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.