Phacoemusification machines can be overwhelming at first. Here i will try to simplify them as best i can to help those that are just getting started using them. I think it is important to have a good understanding of how they operate and not simply rely on past settings, equipment representatives, and/or the circulating nurses to run these devices critical to your success as a cataract surgeon.
These machines have four main components and software that ties the components together. First a system of irrigation which is typically just a bottle that is hung at variable heights above the surgical eye. Second the foot pedal which allows the surgeon to control the machine. Third an ultrasound hand piece which typically has crystals which vibrate the phaco needle with various power waveforms controlled by the phaco machine. And finally and most importantly, the pump, which is classically either a flow based or vacuum based device.
These machines have four main components and software that ties the components together. First a system of irrigation which is typically just a bottle that is hung at variable heights above the surgical eye. Second the foot pedal which allows the surgeon to control the machine. Third an ultrasound hand piece which typically has crystals which vibrate the phaco needle with various power waveforms controlled by the phaco machine. And finally and most importantly, the pump, which is classically either a flow based or vacuum based device.
The irrigation system on phaco machines is typically is just an adjustable bottle held higher than eye to allow infusion of fluid. The machine can adjust the bottle height for various phases of the surgery. For example when the vacuum goes up during segment removal the bottle height typically will need to be higher to maintain the anterior chamber. similarly when you are doing anterior vitrectomy the bottle height should go way down. The machine can also turn the fluid on and off. When the foot pedal goes from position 0 to 1 the fluid is typically turned on by the phaco machine. Some phaco machines can detect when the irrigating fluid bottle is getting near empty but most cannot.
The foot pedal is typically controlled with the dominant foot (w/o shoes w/socks). although some such as the famous surgeon Jim Davidson (marshaltown iowa) suggests that the phaco foot pedal is simpler than the microscope pedal and uses his non-dominant foot for the phaco machine saving the dominant foot for the microscope. The most basic function of the accelerator like portion of the foot pedal is common across all brands with 4 positions: Position 0 – everything is off; Position 1 – irrigation is on, no pump, no U/S; Position 2 – irrigation is on, pump is on, no U/S; Position 3 – irrigation is on, pump is on, U/S is on. some of the foot pedals are wireless now such (eg. Stellaris) but most have a cable that connects to the phaco machine
The ultrasound (U/S) hand piece vibrates the phaco needle at a set rate in the 20,000 to 40,000 HZ range. The vibration is typically delivered by electrically stimulating crystals with a resonate frequency and the crystals are connected to the phaconeedle. many of the modern hand pieces have as many a 4 crystals to allow the needle to handle more load from a hard cataract. Increasing the U/S power typically increases the excursion of the needle but not the frequency which usually remains stable. With increasing load, such as a very hard cataract, the frequency and excursion may not keep up. when setting up the pahco machine for the case one must "tune" the handpiece. the machine sends pulse to the handpiece and sees how much power must be delivered to move the needle. this "tuning" helps to adjust for subtle variations from different needles and handpieces.
The phaco machine can set up a variety of ultrasound modes. In continuous mode the ultrasound energy is on on when the pedal is in position 3 and increases in excursion or power the deeper the surgeon is into position 3 up to a set maximum. In pulse mode the ultrasound contains pulses of ultrasound where it is on (set %) and then off (set %) for a set frequency. the deeper the pedal is into position 3 the more power each pulse will have up to a set maximum. The typical pulse will have a 50% duty cycle with an on time equal to the off time. the classic pulse setting is the howard fine "choo choo chop" setting with a low frequency of about 4 Hz and 50% duty cycle that is useful to acquire a piece for chopping and kind of sounds like a choo choo train. another common setting is to increase the frquency to about 100 Hz and have an on time which is less than off time creating hyperpulses which seem to run more cool to protect against wound burn. finally most machines feature a burst mode where stepping further into position 3 decreases the time between bursts of phaco such that when the pedal is fully engaged the power is continuous.
Recently some machine handpieces have featured an oscillatory component in addition to the classic longitudinal ultrasound. AMO features a figure 8 motion of the phaco needle in it latest phaco machine. Alcon in the infinity Ozil machine has a rotational feature in addition to the longitudinal ultrasound. as this rotational energy does not directly push away the nucleus pieces like the longitudinal ultrasound does the nuclear bits seem to come more readily to the tip in these modes.
The phaco pump is the most important and complex part of the phaco machine. The pump comes in two basic varieties: vacuum based (eg venturi) and flow based (eg peristaltic). A vacuum based pump creates more vacuum (mmHg) when the pump works harder. A flow based pump creates more flow (cc of fluid/min) when the pump works harder. In a real world it is hard to separate flow from vacuum as the resisitance in the tubing keeps the two related. the parameters of the pump will depend on the phase of the surgery. you will want very little fluid flow during sculpting and you will want alot of vacuum when removing the segments or when holding onto the nucleus during chopping. You might want to look over the classic definitive text in this area by Barry S. Seibel, Phacodynamics.
In pumps the vacuum, flow rate and resistance to flow in the tubing are related. I find it useful (maybe because i used to be an electrical engineer) to compare the fluid relationship to Ohm's law (E=IR) where the relationship between current I (analgous to flow rate) , voltage E(analogous to vacuum) and resistance R (analogous to resistance to flow in tubing) are related. just like it is impossible to have a pure current source or voltage source it impossible to create a pure vacuum or flow based pump. but these pumps do behave differently in practice and so it is important to understand how to operate them to your specifications.
Vacuum Pumps. The most common of these are Venturi pumps (Stellaris, Millennium, Accurus) where compressed air passes over a column of air creating a vacuum proportional to the flow of air over the column (this is similar to the way a wing creates lift). Increasing pump power increases vacuum directly; flow rate indirectly based on the amount of resistance to flow. Typically a Venturi pump requires an external source of compressed air (Millennium) or an internal compressor (Accuris or Stelaris) which has limited acceptance of this pump. The compressed gas flows over the open top of a rigid column or cassette attached to tubing creating vacuum. Flow rate for a particular amount of vacuum is then dependant on the resistance of flow to the fluid. This is roughly analogous to electric current voltage relationship (Ohm’s. This law) i=e/r where e = voltage (analogous to vacuum); i = current (analogous to flow rate); r = resistance (analogous to tubing and occlusion). As such with a given vacuum setting of the pump when you have less resistance in the tubing the flow rate will increase and conversely when you have more resistance you will get less flow through the tubing.
Vacuum Pumps. The most common of these are Venturi pumps (Stellaris, Millennium, Accurus) where compressed air passes over a column of air creating a vacuum proportional to the flow of air over the column (this is similar to the way a wing creates lift). Increasing pump power increases vacuum directly; flow rate indirectly based on the amount of resistance to flow. Typically a Venturi pump requires an external source of compressed air (Millennium) or an internal compressor (Accuris or Stelaris) which has limited acceptance of this pump. The compressed gas flows over the open top of a rigid column or cassette attached to tubing creating vacuum. Flow rate for a particular amount of vacuum is then dependant on the resistance of flow to the fluid. This is roughly analogous to electric current voltage relationship (Ohm’s. This law) i=e/r where e = voltage (analogous to vacuum); i = current (analogous to flow rate); r = resistance (analogous to tubing and occlusion). As such with a given vacuum setting of the pump when you have less resistance in the tubing the flow rate will increase and conversely when you have more resistance you will get less flow through the tubing.
Controlling the vacuum based pump is very simple as you only have to set the vacuum and have no setting for flow rate. Typically you would use a fixed vacuum (ie. no matter how deep you are into position 2 or 3 the vacuum stays the same) for sculpting and to hold while chopping. You would typically use a variable vacuum (ie. the deeper into position 2 the more vacuum and faster the pump) to remove epinuclear material and for I/A of the cortex.
Flow based pumps. The most common flow based pump is the peristaltic pump (Infinity, Sovereign, and Legacy). With peristaltic pumps the faster the pump goes the more cc/min of fluid passes through the tubing or the more flow. This is often refered to as the aspiratiion flow rate (AFR). Increasing the power of the pump increases the flow rate directly and vacuum indirectly through the resistance of the tubing. So the vacuum is just dependant on the amount of fluid flow and this relationship is 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 resistance). As such you will only get some vacuum if there is some resistance to flow (or some occlusion).
Flow based pumps. The most common flow based pump is the peristaltic pump (Infinity, Sovereign, and Legacy). With peristaltic pumps the faster the pump goes the more cc/min of fluid passes through the tubing or the more flow. This is often refered to as the aspiratiion flow rate (AFR). Increasing the power of the pump increases the flow rate directly and vacuum indirectly through the resistance of the tubing. So the vacuum is just dependant on the amount of fluid flow and this relationship is 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 resistance). As such you will only get some vacuum if there is some resistance to flow (or some occlusion).
These flow based machines typically have a setting for the flow rate but also have a vacuum cut off. the vacuum cut off is the point at which the pump will stop if a certain vacuum is reached. so you set the flow rate and pump humms along until the vacuum rises (due to increased resistance) to the vacuum cut off point and then the pump simply stops. so even though with peristaltic machines you have a vacuum and a flow rate setting you can only make the pump work harder by increasing the flow rate. setting the vacuum higher only sets the point higher at which the pump stops when this vacuum is reached.
With modern peristaltic pumps (eg. Infiniti) for each foot position 2 you can have fixed or variable flow; fixed or variable vacuum cut off. if you want the pump to be responsive to pressing harder on the pedal (eg for I/A) you would use a variable setting such that the aspiration flow rate or at least the vacuum cut off increases as you step down into position 2. when you set both the flow rate and the vacuum cut off to be variable so that it increases as you step into position 2 the peristaltic pump begines to feel more like a venturi pump. you typically would set the pump low and fixed for sculpting (80 mmHg vacuum cut off: 20 cc/min flow rate); higher but still fixed for chopping and segment removal (300 mm Hg and 30 cc/min); and high and vairable for I/A where you need more control (500 mmHg and 50 cc/min flow rate). i usually use roughly a 10:1 ratio of vacuum cut off:flow rate with the infinity and legacy for segment removal, chopping and I/A.
Which pump is better?. There is no clear favorite for every situation. For certain parts of the procedure the flow based pumps seem better like sculpting the groove as you can set the vacuum low with a reasonble flow rate. For other parts of the procedure like I/A and anterior vitrectomy vacuum based pumps are better as the vacuum is not related as much to occlusion (resistance to flow). for years the flow based pumps were most popular in part at least because the early vacuum based pumps required an external compressed gas line and as the peristalitic were cleverly marketed as "safer" for divide and conquer. recently, as the phaco procedure has moved more toward higher vacuum for chopping and away from scupting the vacuum based pumps are getting more popular.
Which pump is better?. There is no clear favorite for every situation. For certain parts of the procedure the flow based pumps seem better like sculpting the groove as you can set the vacuum low with a reasonble flow rate. For other parts of the procedure like I/A and anterior vitrectomy vacuum based pumps are better as the vacuum is not related as much to occlusion (resistance to flow). for years the flow based pumps were most popular in part at least because the early vacuum based pumps required an external compressed gas line and as the peristalitic were cleverly marketed as "safer" for divide and conquer. recently, as the phaco procedure has moved more toward higher vacuum for chopping and away from scupting the vacuum based pumps are getting more popular.
Vacuum based pumps seem to have less post occlusion surge during segment removal and material seems to come to the tip better for irrigation aspiration. vacuum based pumps are clearly better for vitrectomy as the vitreous comes to the tip even without occlusion from the guillotine which can be frustrating with flow based pumps. The disadvantage of the venturi pump which is the most common vacuum based pump is the need for compressed gas and the need for a rigid cassette.
Flow based pumps seem to be better for low vacuum jobs like sculpting. With modifications such as setting the vacuum and the flow to increase with increasing position 2 on the foot pedal the flow based pumps can be more responsive for I/A like the vacum based pumps. The flow based pumps do not require compressed gas.
No comments:
Post a Comment