Jagdish Dukre

FLUIDICS The fluidics of the machine refers to the integrated functions

performed by infusion and aspiration systems by which a

stable AC is maintained.

It consists of :

1. Infusion system

2. Aspiration System

Infusion System The infusion system consists of a

bottle, the height of which

provides the gradient for flow.

The tubing from the bottle is run

through a pinch valve which is

controlled by the foot pedal.

A bottle height of 3 ± 1 ft

maintains a safe IOP with

sufficient fluid entering the eye.

In addition, when the machine flow rate is increased,

increased fluid evacuation from the anterior chamber requires

increased inflow to maintain the steady-state system.

Therefore, when the machine flow rate is increased, the bottle

height should also be increased.

Raising the bottle height too much can have undesirable

effects due to repeated iris prolapse, especially if the pupil is

small and wound is large.

Aspiration System

The two functions of the aspiration system are lavage of the

anterior chamber and creation of a hold for emulsification or

crushing of the nucleus.

Lavage is governed by the flow rate.

The hold is a function of the vacuum generated by the system.

Flow rate (FR) is the quantity of the fluid pulled from the eye

per minute.

Flow rate therefore helps in bringing the material towards the

tip and is measured in cc/min.

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.

The aspiration systems consist of a pump that is either flow

based or vacuum based.

The common type of flow pump is the Peristaltic pump.

Venturi is the prototype of a vacuum-based machine.

Peristaltic System

Advantages of Peristaltic Pump

In this system, vacuum will be built up only after the tip is occluded.

There is a large safety margin in this pump as it is slower in building up vacuum.

There is no inadvertent pull on the ocular structure since vacuum builds up only on occlusion

Flow rate and vacuum can be set independently in a peristaltic system.

Disadvantages of Peristaltic Pump

The vacuum build-up is in a stair-stepped pattern causing

more pulsations in the anterior chamber.

The vacuum build-up is directly related to the density of

occlusion which in turn would depend upon the bevel angle

of the titanium tip.

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.

Venturi System

Advantages of Venturi Pump 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 Venturi Pump

This pump has the least safety margin.

The incidence of iris trauma and posterior capsular rents have been reported to be much higher with this pump.

In the venturi system only the level of vacuum can be controlled and not the flow rate.

The flow rate is a fixed fraction of the vacuum.

However, the change in vacuum level doesn’t always lead to a proportionate change in the flow rate since port size and resistance in the passage also modify flow rate.

Rise Time (RT) The rise time is the time taken by a

machine to reach maximum preset

vacuum after occlusion has been

achieved.

In a Venturi system, the RT is fast,

linear and dependent upon the

highest preset vacuum.

In a peristaltic pump, RT depends

on the FR of the machine.

The higher the FR, the lesser the

RT though the relationship is not

absolutely linear.

Followability Followability refers to the tendency of the nuclear

fragments/cortical matter to come into the tip.

The positive pressure due to the infusion and the negative pressure created by the aspiration pump are responsible for the creation of a pressure gradient at the tip.

This in turn leads to eddy currents from the infusion orifice to the phaco tip.

The area encompassed by these eddy currents is known as the zone of followability.

The area just in front of the tip is the

area of highest followability.

There are some areas of no

followability.

Here the positive pressure from the

infusion pushes the pieces out of the

eye.

Surge A principal limiting factor in the selection of high levels of

vacuum or flow is the development of surge.

Sudden withdrawal of fluid from AC after occlusion breaks

is called surge.

There are various methods of controlling the surge.

Some are incorporated into the newer machines and there

are some measures that the surgeon can apply.

Surge prevention by the machine Property of the tubing to collapse (deform under pressure) is

the compliance of the tubing.

This extent of collapse of the tubings will depend on the

thickness of the tube.

The collapse is less if the walls are thicker (less compliant

tubing).

In a peristaltic machine, these high vacuum tubings and

cassettes are used to decrease the compliance of the tubings

thus preventing surge.

Venting The machine has a sensor which detects occlusion break and

releases fluid into the system to fill the volume of the re-

expanding tubing.

This prevents fluid being drawn out of the AC.

ABS tip The tip has a 0.175 mm hole drilled

in the shaft of the phaco needle.

When occlusion occurs at the tip,

fluid flows into this hole.

The amount of flow depends on the

vacuum and flow settings.

Because some flow always exists, in

reality there is never complete

occlusion.

This modification must be used with

the high vacuum tubing or it does

not function properly.

Partial-Occlusion Phacoemulsification In partial-occlusion phaco, micro-pulse phaco is used to avoid

total occlusion to prevent surge.

The nuclear fragment is brought close to the phaco tip with a

4-millisecond period of aspiration until the fragment partially

occludes it.

With the onset of a 4-millisecond burst of phaco energy, the

fragment is emulsified before it can totally occlude the phaco

tip.

Therefore, flow never falls to zero and vacuum never builds

to maximum, thus preventing surge.

Surgeon’s Control of Surge

Increasing the infusion by raising the bottle height may be

useful in some cases.

The use of an ACM is useful for decreasing surge.

Increased viscosity of the AC contents: The flow rate

settings are for clear fluids like BSS/Ringers.

A thicker fluid increases the resistance and does not flow out

easily.

The use of visco-elastic is particularly useful in hard

cataracts where the settings are usually high.

This will maintain occlusion and prevent fluid from the AC

being aspirated.

Good foot pedal control is importance in controlling surge

and utilizing it to surgeon’s own advantage.

As soon as the occlusion is about to break (i.e. the piece is

about to be aspirated into the tip) is the surgeon lifts the FP to

IAo, the piece will go in on its own momentum and without

any of surge as the FR will decrease.

Thus fluid withdrawn from the AC will be very little to

overcome the compliance of the system.

However, if the FP is withdrawn too early and there is not

enough momentum then it will take more time to build up

vacuum again.