Basics of Phaco - 2
In Part 1 we discussed about two basic pump systems and the footswitch positions. Against the backdrop of these information let us now understand the basics of phaco fluidics with respect to Peristaltic pump systems.
We know the phaco fluidics is all about the irrigation running into the eye, the aspiration that runs out of the eye, the vacuum generated subsequently during the phaco process, and the interplay of all these with the energy or power applied. The surgeon would get the irrigation pressure into the eye as he steps into footswitch (ftsw) position 1. In the Infiniti/Laureate you would have the pinch valve open with a sound thereby letting the irrigation pressure into the eye. I would say pressure and not flow, because in a (relatively) close system you will only have a flow to the eye (irrigation) as fluid escapes (aspiration) from the eye. This would only happen as the surgeon moves down to ftsw position 2. As the pump rotates drawing fluid out from the eye, the irrigation flow from the bottle/bag would replace the fluid escaped from the eye. For example, if you hold a bottle of water upside down and squeeze the bottle, pressure is generated, but the flow would only start when you rotate the cap of the bottle (in this case pump).
We came to know that the first principle of
phaco is to equalize inflow to outflow. Hence the fluid escaping from the eye must equal to the fluid moving into the eye from the bag. The fluid escaping from the eye or aspiration, is determined by the rotation of the pump, also called the Aspiration Flow Rate (AFR), measured in cc/min.
This is the primary determinant factor of the amount of fluid escaping from the eye, the other being leakage from main incision and sideports (secondary incision).
The AFR determines the flow of phaco materials towards the phaco probe. The faster the pump rotates, the better is the followability of the phaco pieces towards you. Hence a higher flow rate is required to help generate sufficient followability. Refer Figure 1. In Figure 1 the surgeon is in ftsw position 2 where the pump is rotating and aspiration generated thereby attracting the phaco pieces toward the tip. At this point there is no vacuum generated as the pieces though close to phaco tip, are still not occluding the tip.
That being said, we have to be also conscious of the Phaco first principle. Hence to avoid a situation where the outflow exceeds the inflow (irrigation), you may have to keep a balance between the two. You cannot trade in one for the other. Because to gain something more, you may risk compromising on something else. No free lunch J
To balance the inflow to the outflow, you may reduce the aspiration. But that may impact the followability of the phaco materials to the probe. The other option is to have a sufficient bottle height, or if not, increase the bottle height. Higher the bottle height, higher the irrigation flow into the eye. The manufacturers all along have depended in increasing the bore size of the irrigation tubing and reducing the aspiration tubing bore, to maintain such balance. In the OR, you may compare an irrigation tubing with the bore size of the aspiration tubing. You will find it yourself. But be careful of sterility in the OR, before you do so.
Having a sufficient bottle height to compensate for the aspiration is important to balance inflow to outflow. You may find competition machines having a very high bottle height so that outflow never goes beyond inflow. That is a good strategy. Over the years, untill the introduction of Infiniti, and specially OZIL, we too depended on such strategy. In subsequent series we will look into how Ozil changed the concept, and where we stand out from the competition today. But for now, let us still be focused on our basics.
A challenge with keeping a very high bottle height is that as irrigation exceeds aspiration, in coaxial phaco, followability is often compromised. This is because, in coaxial phaco, irrigation and aspiration happens through the same incision site. As irrigation pressure is more than AFR, the flow towards the phaco piece is obstructed, like rain water flowing down the drain from two opposite directions create whirl/spin (turbulence) before finally moving into the pit.
Hence if you have an unusually higher bottle height, with low AFR, followability may be compromised. You may then consider of either lowering the bottle height, but this would affect the chamber maintainence (inflow = outflow). You may also consider to ramp up the aspiration by increasing the AFR. This may increase followability, but be careful of not compromising chamber stability (inflow = outflow). Ah, that is a dilemma !!
That dilemma may be solved as we introduce the third & fourth factor in phaco fluidics – vacuum and energy (power).
Remember our school days, and if you were as naughty as me you would have had landed with a number of fights with your friends. I did. But I was not as clever as my friends. I used to try to punch them, but they would avoid the punch by keeping their body away from my fist. When my little friend wanted to tease me, he would hold me by my collar and then hit me. That gave him the punching power. An unholy example, but nevertheless effective.
Vacuum in phaco is like my friend holding me before landing the punch (power). In phaco the vacuum is the hold that the phaco tip would have before the surgeon applies power to break it. In conventional phaco, or longitudinal phaco, this is crucial as application of phaco power to break the pieces would be more effective with the strong adhesion of nucleus pieces on to the phaco tip at the time of applying power. Hence vacuum can be correlated with holding force. In phaco world this is often called as “holdability”. Vacuum is to holdability of the phaco pieces, as AFR or aspiration flow rate is to followability or attractability of phaco pieces to the tip. Followability and holdability are crucial in phaco surgery, and goes a long way in determining the effectiveness of the phacoemulsification machine.
This brings us to an important question – when or how does the vacuum generate in a phaco machine based on Peristaltic pump. At this point you should have a look at last WeekEnd Reading (16) to gain an understanding of the pump system. In a Peristaltic pump, vacuum is generated only when there is an occlusion or block of the phaco tip by nucleus pieces. As the pieces are attracted towards the phaco tip, due to the flow or aspiration (because of rotation of the pump), these pieces ultimately starts occluding the phaco tip. With the phaco tip occluded or blocked, no BSS can flow out from the eye through the tip. With the pump still rotating, whatever fluid is in the aspiration tubing would be removed. With no fluid left in the phaco tip and the connected aspiration tubing, and the pump still rotating, vacuum rises to the preset vacuum that the surgeon has asked for in the Graphic User Interface (screen). As vacuum reaches to the preset level, the pump stops. The information that the vacuum has reached the preset level is fed back by the Vacuum Pressure Sensor, also called the Aspiration Pressure Sensor, thereby commanding the pump to stop.
The phaco tip has now a good hold on the nucleus picece, making the surgeon ready to apply the phaco power to break the pieces.
To be continued…….