Saturday, October 01, 2005

Chapter 3 - Phaco Machine Settings Primer


Four main components and software to tie them together

  • Pump - most important variable


    • Parameters are dependant on tubing diameter and compliance

    • Parameters are dependant on phaco needle diameter

    • Allows removal of the emulsified lens material

    • Set low during sculpting and higher during quadrant removal and chopping


  • Irrigation System


    • Typically is just an adjustable bottle held high to allow infusion of fluid

    • Machine can adjust bottle height

    • Machine can turn fluid on and off


  • Ultrasound (U/S) hand piece


    • Vibrates needle at a set rate in the 20,000 to 40,000 HZ range

    • Increasing the ultrasound power increases the excursion of the needle

    • With increasing load the frequency and excursion lessens

    • Modern multiple crystal hand pieces can better handle load


  • Footswitch


    • Typically controlled with dominant foot (without shoes)

    • Accelerator like pedal is common across all brands

    • Position 0 - everything is off

    • Position 1 - irrigation is on, no pump, no ultrasound

    • Position 2 - irrigation is on, pump is on, no ultrasound

    • Position 3 - irrigation is on, pump is on, ultrasound is on



Phaco Pumps

  • Please read the classic definitive text for more details: Barry S. Seibel, Phacodynamics, Slack

  • Flow rate: amount of fluid passing through the tubing (cc/min) also aspiration flow rate

  • Vacuum: difference in fluid pressure among two points, e.g. tip of needle and AC (mm Hg)
  1. Vacuum based Pumps – e.g. Venturi pump ( Millennium, Accurus ), diaphragm


    • Increasing pump power increases vacuum directly and flow rate indirectly

    • Venturi pump requires external source of compressed air


      • This has limited acceptance of this pump (make sure your ASC has the proper air lines)

      • Compressed gas flows over open top of rigid cassette attached to tubing

      • Flow of gas creates vacuum much as flow over wing creates lift


    • Flow rate is dependant on resistance of flow

    • Roughly analogous to electric current voltage relationship (Ohm's law)


      • i=e/r where e = voltage (analogous to vacuum)


        • i = current (analogous to flow rate)

        • r = resistance (analogous to tubing and occlusion)


      • more flow rate with less resistance (fixed vacuum)

      • more flow rate with more vacuum (fixed resistance)


    • Pump settings -- No settings for flow rate only vacuum


      • Fixed: no matter how deep the into position 2 or 3 vacuum is fixed


        • Great for chopping and quadrant removal


      • Variable: vacuum increases from 0 to max when deeper into pos 2 or 3


        • Great for I/A can slowly increase vacuum to just what you need




  2. Flow based pumps – e.g. peristaltic pump ( Infinity, Sovereign, and Legacy )


    • Increasing pump power increases flow rate directly and vacuum indirectly

    • Vacuum is dependant on resistance of flow


      • Roughly analogous to electric current voltage relationship (Ohm's law)


        • e=ir where e = voltage (analogous to vacuum)


          • i = current (analogous to flow rate)

          • r = resistance (analogous to tubing and occlusion)



      • more vacuum with more resistance (fixed vacuum)

      • more vacuum with more flow rate (fixed resistance)


    • Pump Settings


      • Set vacuum cutoff and flow rate


        • Vacuum cut off


          • Seems like you are setting the vacuum

          • Really setting the vacuum at which the pump stops

          • Increasing the vacuum does not increase pump speed

          • Flow rate or Aspiration FR rate (AFR) sets pump speed (cc/min)



      • with modern peristaltic pumps (eg. Infiniti ) for each foot position you can have:


        • Fixed or variable flow

        • Fixed or variable vacuum cut off
Variables affecting Phacoemulsification

Flow rate

Vacuum
Cut off

Comment/Application

Fixed

Fixed

Independent of depth in foot position
Low numbers good for sculpting

Fixed

Variable

More depth higher vacuum cut off
Limited control

Typical I/A setting on Alcon 20,000

Variable

Fixed

More depth faster pump
More control pump speed changes
Bimodal setting on Alcon 20,000

Variable

Variable

Both change with depth in foot position
Feels like a venturi pump


Phaco Pump Comparison


Pump

Pros

Cons

Vacuum
e.g. Venturi

Less posterior occlusion surge
Better for vitreous removal
Material comes to tip easily

Need source of compressed gas
Need rigid cassette

Flow
e.g. Peristaltic

Better for sculpting
No need for compressed air

Post occlusion surge
Need occlusion for vacuum to build


Ultrasound Control

  • Four ultrasound modes: continuous, pulse, burst, and hyperpulse
  • Continuous

    • Phaco is on in position three
    • Usually increasing ultrasound power with depth into foot position

  • Pulse

    • Phaco pulses with duty cycle on and off
    • Usually with equal on and off time or 50% duty cycle (time on/cycle time)
    • Usually the rate (or inverse of duty cycle) is fixed (Hz)
    • Usually increasing ultrasound power with depth into foot position

  • Burst
    • Bursts of power come with off time that decreases with depth into foot position
    • Usually when floored in position 3 -- ultrasound power becomes continuous
    • ultrasound power is fixed

  • Hyperpulse
    • Uses short on time pulses e.g. 25% on; 75% off
    • Fixed duty cycle; fixed pulse rate
    • Usually increasing ultrasound power with depth into foot position

Advantages & Disadvantages of Various Modes

Mode

Advantages

Disadvantages

Applications

Continuous

Simple

Repels nuclear material
Hot

Sculpting

Pulse

Less hot

Can repel nuclear material

Choo choo chop
Segment removal

Burst

Less hot
Holds material well

Chopping

Hyperpulse

Followability with Long off cycle
Cool with long off cycle

Sculpting
Bimanual small incision


My Typical Settings

[click on table for larger view]


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