Ever since the introduction of Phacoemulsification in 1967, till today, the fundamentals have all remained the same. The three basics principles of phacoemulsification which needs to be satisfied for an uneventful surgery are –
1. Inflow equal to outflow. Irrigation flow to the eye should not be any way less than the aspirated fluid flow from the eye.
2. Less turbulence in the eye during the course of the phaco surgery.
3. Minimal heat buildup near the incision site, that is, consistent thermal protection near the incision site particularly.
These are the cardinal principles of phaco, and both surgical skills and sophistication of the Phacoemulsification machine need to go hand in hand toward its fulfilment.
Now, we would take a deep dive into the basics of phaco to help new surgical sales force to help have at least an elementary understanding of the phaco process, and the seniors who may be happy to have a quick brush up.
By phaco fluidics we mean largely the interplay and management of the following – Irrigation to the eye (fluid flow), Aspiration from the eye , Vacuum generated, and Phaco Power or energy. However, before we deep dive into the fluidics, it is important to have a basic understanding of the different types of pump systems that run the phaco machines like the engine that drives a car.
The two broad types of pumps used to run such machines are the Peristaltic and the Venturi based pump systems. While the former is often termed as a passive vacuum system, the Venturi pumps are based on active vacuum. Let us first have an understanding of the pumps first.
Venturi Pumps – As vacuum and flow (aspiration) are essential elements in phaco, the basic difference between the two pump systems are how they are created. In a Venturi machine, vacuum is generated due to compressed gas/air
flowing through the pipe, creating a pressure differential in the cassette chamber, which starts the flow from the eye to fill the vacuum. Thus vacuum is always active when the surgeon presses the footswitch to position 2, while the flow starts due to generation of vacuum. This is a distinguishing factor of a Venturi machine. There can be no flow without vacuum generated, and vacuum can be generated only when the surgeon is in ftsw position 2.
Since vacuum is always live in Venturi pump systems ( once the surgeon is in footswitch position 2) phaco machines based on these pump systems are fast. However, with an inexperienced or a beginner surgeon, the Venturi based systems can prove to be very unsafe. Figure 1 explains how Venturi vased pump systems generate a flow.
Peristaltic Pump – The word peristaltic comes from peristaltis. If you think of a snake eating its prey, you would understand how such pumps work. The snake squeezes and releases its prey down its body to its stomach. A peristaltic pump has a hub roller and several pinch rollers. The hub roller rotates on its axis, and so does the pinch rollers rotate. Drawing an analogy, the sun (hub roller) rotates on its axis. The planets like Earth, Mars etc (pinch rollers) revolve around the Sun and also rotate on their own axis. The tubing, through which the aspiration or flow is to be generated is housed on the pinch rollers. Thus as the hub roller rotates, so does the pinch rollers revolve and rotate around the hub roller. This leads to a pinch and release action on the tubing, eventually generating the flow from the eye. Like the snake eating its prey.
In a Peristaltic pump ( Figure 2) , the flow is generated only when the surgeon is in ftsw position 2 ( remember in Venturi the vacuum is generated in position 2 as a result of which flow is also generated). The vacuum and the flow are independent of each other in Peristaltic pump. The fact that there is flow generated due to the activation of pump does not mean that their will be vacuum also. Then how does the vacuum gets generated in a Peristaltic pump. In a peristaltic pump, vacuum can only be generated when there is a occlusion ( or block ) on the phaco tip. In such a case the pump still rotates clearing the fluid from the tubing and as a result the vacuum starts building up. We will look into this aspect in more depth later.
So a peristaltic pump has historically been slow, but safe. Over a period of time peristaltic pumps underwent several design improvements, as a result they have today been more safer, but at the same time been moderately fast. Such design improvements are beyond the subject of this WeekEnd Reading, but would be covered in subsequent editions.
Before we go deeper into the science of phaco fluidics subsequently, let us take a pause and understand how does the surgeon control the phaco machine during the surgical process with the help of the footswitch.