Ординатура / Офтальмология / Английские материалы / The Art of Phacoemulsification_Mehta, Alpar_2001

22 THE ART OF PHACOEMULSIFICATION

Phacoemulsification Terminology

Phaco power Phaco power is the ability of the phaco handpiece to cut or emulsify a cataract. Phaco power is directly related to stroke length, frequency and efficiency of handpiece.

Stroke length Stroke length is the distance by which the titanium phaco tip moves to and fro. It is the most important factor in deciding the phaco power. The stroke length can be altered by changing the phaco power setting of the machine.

Frequency Frequency is the number of times the tip oscillates and is fixed for a particular phaco handpiece. It is measured in kHz.

Preset levels Each surgeon sets his level which he does not wish to exceed during the surgery, both for minimum levels as well as maximum levels. This is done so that the safe levels are not exceeded inadvertently during the stress of surgery.

Linear v/s panel In linear control, pressing the foot-pedal leads to gradual rise of the parameters from zero to preset maximum with a linear relation to the footpedal control. In panel mode, the parameter reaches the preset panel maximum on pressing the foot switch without any linear foot pedal control. Essentially it panel is simply on or with no variables in between, panel mode is normally utilized for diathermy, flow rate or for vitrectomy settings, never for power settings or aspiration settings on phacoemulsification.

Constant v/s pulse phaco power In constant mode, power is delivered continuously and it can be linear or panel controlled. Pulse mode allows phaco power to be delivered at preset intervals which can be varied. The pulse mode gives relative intervals where there is absence of tIP movement. This improves the flow characteristics and helps in evacuating small nucleus particles towards the end of the surgery. The pulse mode is also relatively safer for the epinucleus because a more consistent and predictable cutting power will provide greater stability in the posterior chamber.

Maximum phaco power Maximum phaco power is preset by the surgeon. It determines the maximum obtainable ultrasonic energy when the foot pedal is fully depressed.

Actual phaco power Actual phaco power in a machine with a linear foot-pedal control is proportional to foot pedal position and denotes the power actually being delivered at a given time.

Effective phaco time (EPT) Effective phaco time is the total phaco time at 100 percent phaco power. It can be less than the total foot-pedal time. EPT is very significant as less EPT indicates proportionately less energy delivered to the eye thereby reducing the side effects of phaco power. When one compares the energy used between different types of procedures, or even between different instruments, one has to compare the EPT which is a much more accurate index.

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How does phacoemulsificatlon work?

There are various steps involved in the actual phacoemulsification process.

•Mechanical contact of the tip with the lens

•Acoustical wave transmitted through fluid in front of the tip

•Cavitation At the cessation of the forward stroke, the tip has imparted forward momentum to the fluid and the lens particles in front of it. On the tip being

retreated, the fluid cannot follow thereby creating a void in front of the tip. The void is collapsed by the implosion (cavitation) of the tip thereby creating

additional shock waves.

•There is an impact of fluid and lens particles being pushed forward in front of the tip.

Considering the mechanics of phaco it is clear that there is attenuation of energy

on phacoemulsifying within nuclear material. This reduces the deleterious effects on the corneal endothelium. Therefore, posterior chamber phaco helps to maintain the safety of the procedure by increasing the working distance from the endothelium. Further, if phaco power is used only when the tip is in the nucleus, the safety margin is significantly enhanced.

The ultrasonic handle has three functions, namely, irrigation, aspiration and fragmentation. These can be operated separately or simultaneously. The dynamics of irrigation and aspiration are now considered in detail.

Irrigation System

In most phacomachines, irrigation during phacoemulsification is provided by gravity feed through the space between the titanium phaco tip and the sleeve. The amount of irrigation is determined by the bottle height relative to the patient’s eye, by the sleeve diameter, and most importantly by the loss of fluid from the eye.

Stable anterior chamber dynamics:

Irrigation = aspiration + leakage from the wound.

Rigid sleeves may be preferred over flexible sleeves because the irrigation does not get compromised while manipulating the handpiece in the incision. The height of the irrigation bottle during phaco is usually placed between 65 cmand 75 cm above the eye level. The eye should be at the same level above the floor as the pump (cassette) of the phacoemulsifier.

Aspiration System

Aspiration is defined as the evacuation of fluid through a closed system. Two important concepts concerning aspiration are flow rate and vacuum level.

Flow rate Flow rate is the quantity of the fluid pulled from the eye per minute through the instrument tip and irrigation tubing. Flow rate therefore helps in bringing the material towards the tip. Flow rate is measured in cc/min and is dependent on the level of vacuum created in the aspiration tubing by the aspiration pump

24 THE ART OF PHACOEMULSIFICATION

and surface area of the port of aspirating tip. Flow rate determines the rate of rise of the aspiration vacuum when the aspiration port is occluded.

Vacuum Vacuum level is the difference in pressure between atmospheric pressure and the pressure inside the aspiration tubing. This is a negative suction pressure that is created by the pump.

Port vacuum (mm Hg/min) = the vacuum created (mm Hg)

port area (mrn)2

The vacuum level created at the port therefore varies inversely with the diameter of the tip. The vacuum or negative suction force created helps in holding the material to the tip and its final aspiration.

Aspiration Pumps

Depending on the machine, three kinds of pumps are used to control aspiration and produce the negative suction pressure, i.e. the vacuum (Fig. 2.7). They are

•Peristaltic pump

•Venturi pump

•Diaphragmatic pump.

Fig. 2.7: The peristaltic pump of the opticon P4000 machine

The peristaltic pump is also known as a “constant flow” pump while the Venturi and the diaphragmatic pumps are of the “constant vacuum” variety.

Peristaltic Pump

Peristaltic pump (Figs 2.8 and 9) was popularized by the heart-lung machine. In these pumps, a pressure differential is created by compression of the aspiration tubing in a rotatory motion. When the rotational speed is low, vacuum develops only when the aspiration port is occluded. On occlusion, vacuum builds up to preset

26 THE ART OF PHACOEMULSIFICATION

•Because of the stair-stepped pattern of the vacuum build-up, there could be more pulsations in the anterior chamber

•True linear aspiration is not seen, however newer pumps do simulate a linear build-up of vacuum

•One has to mechanically approach the nuclear or cortical matter to first achieve occlusion for vacuum to build up in order to aspirate the tissue. However, the rapid rotation mode has significantly improved the followability of the tissue,

even in the peristaltic pump.

Venturi Pump

A Venturi pump uses compressed gas to create inverse pressure. Vacuum generated is related to gas flow which in turn is regulated by a valve (vacuum build-up occurs linearly in a consistent manner from zero to a preset value. The build-up is almost instantaneous on pressing the foot-pedal. Due to this there is an increased risk of iris trauma and posterior capsular rents which make these pumps unsafe, particularly so for beginners.

Advantages of a Venturi Pump

•There is a good follow ability of the tissue

•The vacuum build-up is linear

•There is a consistent increase in the vacuum from zero to the preset level on depressing the foot switch

•Nuclear and cortical material can be attracted towards the probe on depressing the foot pedal.

Disadvantages of a Venturi Pump

•This pump has the least safety margin and is not forgiving to the surgeon

•The rise time is too fast

•There is an immediate rise in the vacuum on pressing the foot switch to position 3 without any linear foot pedal control

•The incidence of iris chaffing and posterior capsular rents have been reported to be much higher with this pump as compared to’ the peristaltic pump

•Venturi pump does not allow either zero to high . vacuum phaco.

Diaphragm Pump

A diaphragm pump uses a flexible membrane within a cassette to generate vacuum. Build up of vacuum is more linear and reaches the preset level even without occlusion. This makes it unsafe. However, lens material can be aspirated without having to mechanically approach it.

Advantages of Diaphragm Pump

•There is an improved linearity of vacuum build-up

•The flow rate and aspiration are faster

•Tissue can be pulled towards the center as vacuum builds up to preset even without occlusion

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•There is a greater control with the diaphragm pump during posterior segment surgery.

DIsadvantages of a DIaphragm Pump

•This being a faster pump it offers lesser safety margin

•Foot pedal depression does not have a very good graded control over vacuum

build-up

•Rise of vacuum depends on the fluid in the chamber

•Vacuum build-up reaches preset level even without occlusion. This leads to inadvertent pull on ocular tissue resulting in a higher complication rate

•A Venturi is not a forgiving pump and has to be handled by newcomers with caution though in the hands of an expert it can give excellent results.

Physics of Phaco: Certain Aspects

Aspiration pressure It is modified depending on the stage of surgery and is inversely proportional to the diameter of the aspirating port. The ultrasonic tip has a port diameter of 1.00 to 1.20 mm with which the maximum vacuum achievable is 70 to 100 mm of Hg. However, in new machines (Alcon’s Legacy, and the Allergan Sovereign series, etc.) the vacuum can be raised to 500 mm of Hg in the phaco mode. The I-A tip has a diameter of 0.3 mm and the aspiration pressure may be increased to 500 mm of Hg.

Rise Time and Pump Flow

Rise time Rise time is a measure of how rapidly vacuum builds up once the aspiration port is occluded.

Pump flow Pump flow is a measure of the rotational speed of the peristaltic pump head (which in turn determines flow rate and aspiration). This changes from machine to machine.

RelationshIp of Illse TIme and Pump Flow

As the pump flow increases, vacuum builds rapidly as the tip is occluded and therefore the rise time decreases.

Pump flow is usually preset by the surgeon and is measured as a percentage. Normally 100 percent is equal to flow rate of 35 cc/minute. It is an overall measurement of fluid turnover in the eye. Pump flow determines rise time and event time.

Vacuum Settings

Maximum vacuum Determine the maximum obtainable vacuum when the aspiration port is fully occluded. Maximum vacuum is preset by the surgeon and is measured in mm of Hg. Typical settings are 65 to 75 mm Hg for phaco and 400+ mm Hg

28 THE ART OF PHACOEMULSIFICATION

for irrigation aspiration. The speed at which this vacuum is achieved is determined by the pump flow setting and the bore of the aspirating tube.

Actual vacuum It indicates pressure at the aspirating port at a given time. This depends on the maximum preset pump flow, degree of tip occlusion and position of the foot pedal when linear control is used.

RelationshIp Between Pump Flow, Irrigation and Aspiration

With an increase in the pump head rotational speed, the pump flow increases. Due to this, both aspiration rate and irrigation flow also increase.

Relationship between pump flow, rise time and vacuum To reach a preset vacuum, as pump flow increases the rise time decreases, e.g. if pump flow is doubled the rise time gets halved.

Fluidic Balance

Fluidic balance is the balance between inflow of fluid into the eye and the outflow of fluid out of the eye, which helps in maintenance of the IOP.

An adequate fluidic balance provides

•Constant lOP

•Stable anterior chamber

•Protects corneal endothelium and posterior capsule.

The amount of irrigation is determined by the bottle height relative to the patient’s

eye, by the sleeve diameter, and, most importantly, by the loss of fluid from the eye. The following situations may exist:

•Balanced anterior chamber dynamics: Irrigation = Aspiration + Leakage

•Tight Wound (Irrigation decreases): <Irrigation <Aspiration + Leakage

•Large Wound (Leakage increases): >Irrigation »Aspiration + Leakage

A tight wound can limit irrigation if the space between the flexible sleeve and

the phaco tip is compromised when manipulating the handpiece. This problem is not present with a rigid sleeve, although such a sleeve is not generally used because ‘contact with the rapidly vibrating phaco tip can cause serious overheating problems and tip damage if the sleeve is metallic.

Excessive leakage between the rigid sleeve and the wound can also occur. Most surgeons have a shelved incision to reduce iris prolapse and create an efficient wound, although some fluid will, and should, always leak through the wound entering the anterior chamber. With an appropriate size keratome, i.e. 3.00 mm to 3.20 mm, proper irrigation is facilitated.

Irrigation Bottle Height

The irrigation bottle during phaco is usually placed between 65 em and 75 em above the eye. The eye should be at the same level above the floor as the pump (cassette) of the phacoemulsifier. It must be however understood that merely raising the bottle will not cure the problem of anterior chamber collapse caused by an

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excessively large wound, since irrigation and leakage are increased concomitantly by the increased force created by the higher bottle.

Height of the Tubes Running from the Handpiece to the Machine

Often forgotten and a sadly neglected fact that water always runs down hill. If the tubes are not supported but allowed to sag, when the phaco is turned off, the

chamber will continue to empty which will lead to a chamber collapse. In addition it leads to air being sucked in the tubes which will lead to a compromised suction

when the handpiece is used again. Always support the tubes at the same level as the patient’s eye. Thus a table which can be raised (A Mayo trolley) is an integral part of the set-up for good phaco.

Enhancing Fluidics in a Phaco emulsification Machine

The various ways to achieve good fluidics in a machine are

1.Narrower bore aspiration tubes Different size of irrigation tubing and aspiration tubing, irrigation being larger than aspiration. By making the aspiration tube with a narrower bore yet with a thicker wall decreases the compliance of the tubes and enhances response time with a more accurate aspiration pressure. It is the use of these high compliance tubing’S, (also called HiVac tubing’s) coupled with computerized controls which have made the new third-generation machines use higher and higher vacuum (and lower and lower ultrasound timings) with great safety.

2.By using software-driven pump systems where aspiration rate slows as the vacuum rises. Aspiration is zero at maximum vacuum, i.e. on total occlusion. As the occlusion clears aspiration rate slowly starts to rise from zero to maximum. A number of pumps at the time of release will actually reverse the pump a bit to make sure that the release is accurate.

Foot Pedal

The mode of operation in which the instrument is functioning on depressing the foot pedal in a linear manner is shown by the position indicator.

Position 0 : Instrument turned on, no fluid flows, no ultrasonic vibration. Position 1 : Only irrigation solution is flowing.

Position 2 : Irrigation and aspiration occur simultaneously.

Position 3 : Irrigation, aspiration and fragmentation take place simultaneously. There is no position 2 or 3 in irrigation mode. Similarly there is no position

3 in I-A mode.

A useful option in some machines is a foot pedal reflux control wherein fluid is pushed from the aspiration line to the eye in order to disengage tissue that has been inadvertently engaged in the tip.

Programmability of foot switch In most of the machines these positions are prefixed. But in new advanced machines the positions are programmable, e.g. in Alcon Legacy, and the Allergan Sovereign.

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•Full function remote is extremely valuable. It can be easily kept in the sterile area to facilitate change of parameters by the surgeon during the procedure.

•Multiple programmability facility must be available. This allows individual

surgeons to feed in their own parameters for surgeries on various grades of cataract as also for different stages of the surgery in a given case.

•Facility for Reusable tubing’s and cassettes The machine should be cost-effective. In Indian circumstances, it is imperative that one be able to reuse the tubing’s, sleeves, etc.

Fig. 2.10: P4000 machine (Opticon)

The ophthalmologists need to carefully look at the various options available to them before purchasing a phacoemulsifier. Though several manufacturers are selling their products, one needs to remember that phaco surgery is significantly machine dependent. A good service back-up is therefore essential. It would always be prudent to take the opinion of a contemporary who is more experienced in the art of phaco surgery regarding which equipment to buy and the reasons for the same before taking a final decision.

FURTHER READING

1.Arensten IT et al: Corneal opacification occurring after phacoemulsification and phaco fragmentation. Am J Ophthalmol 73: 794-804, 1977.

2.Barbell A: Health devices: Phacoemulsification systems. ECRI 18: 392, 1989.

3.Benolken RM, Emery JM, Landis OJ: Temperature profiles in the anterior chamber during phacoemulsification. Invest Ophthalmol 13: 71-74, 1974.

4.Binder P: Corneal endothelial damage associated with phacoemulsification. Am J Ophthalmol 82: 4854, 1976.

5.Ito K: Experimental studies on clinical and pathological changes of neighboring tissues of lens by ultrasonic vibrating tip for phacoemulsification. Japanica 74: 725, 1970.

6.Kelman CD: Physics of ultrasound in cataract removal. Int Ophthalmol Clin 9: 739-44, 1969.

7.Maloney WF, Maloney K: Two handed method. Video Journal Ophthalmology 3(4): 1987.

8.Pollack FM, Sugar A: The phacoemulsification procedure. II Corneal endothelial changes. Invest Ophthal

15:458-69, 1976.