Phaco Fundamentals Part 2: Foot Pedal & Inflow/Outflow

Part 2 in our series focuses on the Foot Pedal Position and then balancing Inflow versus Outflow to achieve stable fluidics during surgery. New installments will be posted to every Wednesday.

All phaco platforms share the same basic structure and concepts. The phaco machine aims to balance fluidics within the eye, while delivering ultrasonic energy and vacuum in order to emulsify and aspirate the cataract through a small incision.

The three main functions of the phaco machine are: (1) to provide irrigation into the eye, (2) to create vacuum/aspiration to remove the cataract, and (3) to deliver ultrasound energy in order to emulsify the nucleus. These three functions correspond to the three phaco foot-pedal positions.

The phaco foot pedal is the primary instrument used to control the phaco machine during cataract surgery. This foot pedal traditionally works by depressing it towards the floor with the dominant foot (the right foot for most surgeons). Each foot pedal position is additive to the previous positions, so that while the pedal is in position 2 (vacuum/aspiration) it is also providing the full function of position 1 (irrigation). Similarly, once the pedal is in foot position 3 (ultrasound energy), it is also providing the function of position 2 (vacuum/aspiration), as well as position 1 (irrigation).

Foot Position 1: Irrigation

It’s important to realize that during phacoemulsification, we are working in the very small space of the anterior and posterior chambers, compromising well under one cubic centimeter of space together. During the surgery, we must always maintain the stability and structure within the eye, particularly to prevent collapse of the anterior and posterior chambers which can lead to severe complications.

The irrigation function of the phaco machine is meant to provide a source of fluid infusion into the eye during the surgery. By depressing the foot pedal to position 1, the infusion is turned on. There is no linear control of the infusion – the infusion is either turned on or turned off.

The height of the infusion bottle determines the relative infusion pressure and flow rate during the surgery. To keep the eye inflated during surgery, we need to make sure that the fluid inflow rate is greater than the fluid outflow rate.

Foot Position 2: Vacuum / Aspiration of fluid

Phaco foot position 2 is the control of the relative aspiration and vacuum level of the fluid from the eye. There is a linear control of vacuum and flow, so that the top of foot position 2 provides less vacuum or flow than the middle or bottom range of the same foot position 2. This is similar to the gas pedal in a car, where the car’s throttle is opened more as the gas pedal is further depressed.

To create the vacuum and the aspiration flow of fluid, the phaco machine must have a fluid pump. The most common types of fluid pumps are peristaltic and venturi. (These will be explained in detail in the upcoming parts of the Phaco Fundamentals series.)

The vacuum and aspiration levels that are created draw the fluid out of the eye and into a waste fluid collection via the outflow tubing. The regulation of vacuum and aspiration is controlled by the foot pedal, with more depression of the pedal resulting in higher levels. There are two primary sources of fluid outflow during phacoemulsification: the outflow from the phaco probe created by the fluid pump, and the leakage of fluid from the incisions.

Foot Position 3: Ultrasound Energy

The bottom-most position of the foot pedal is position 3, which controls the delivery of ultrasound energy into the cataract. There is linear control of the ultrasound energy level so that further pedal depression results in more ultrasound energy, such as would be needed for a denser cataract.

Note that if the pedal is in position 3, we are already engaging the full function of both positions 1 and 2. The irrigation is on, and the vacuum and aspiration level is at its highest preset level. Ultrasound energy should only be applied once the tip of the phaco probe is in contact with part of the cataract.

When we look at the phaco probe closely, we see that there are three lines attached: (1) the infusion tubing carrying fluid into the eye, (2) the outflow tubing that removes the fluid via flow that is created by the phaco machine’s fluid pump, and (3) the line that carries the electrical signals to control the ultrasound energy at the tip of the phaco probe. These three lines correspond to the three phaco foot pedal positions.

Changing Pedal Functions for Anterior Vitrectomy

There are two distinct modes under the anterior vitrectomy setting of your phaco machine and they change the function of foot pedal position 2 and 3.

  • I/A Cut: This means that your foot pedal does: position 1 irrigation, position 2 aspiration, position 3 the mechanical cutting action. It is the same order as the name “I/A Cut” – 1. Irrigation, 2. Aspiration, 3. Cut. This is great for aspirating cortex material or removing viscoleastic at the end of the case. Do not use this mode for removing vitreous since it will put traction on the vitreous and that could damage the retina. If you hear the ding sound that the machine makes when it is occluded, you likely have vitreous blocking the port and you should not be using this mode.
  • Anterior Vitrectomy: This means that your foot pedal does: position 1 irrigation, position 2 mechanical cutting action, position 3 aspiration. This is great for removing prolapsed vitreous, but it does not work well for stripping away cortex material. This mode will help minimize the traction that the vitreous can place on the retina, which helps lower the risk of a retinal break or detachment. Also, be careful as this mode can easily damage the remaining capsular support or even the iris.

Measuring Actual Inflow

The vast majority of cataract surgeons have never measured the actual inflow of fluid during surgery and phaco machines do not track this value either. With your normal bottle height (or infusion pressure), hold the phaco probe at approximately patient eye level. This is important because the infusion pressure and flow will be determined by this height difference, much like a water tower in a residential neighborhood. Now step down to foot pedal position 1 (irrigation) for 60 seconds and collect the balanced salt solution. In the picture here, I am using an empty 60 cc syringe as a measuring cup or cylinder. The total amount of fluid during the 60 seconds will be the actual inflow rate measured in cc/minute. This value changes based on the phaco needle, sleeve, and tubing being used as well as the bottle height and patient eye level.

Always keep Inflow balanced with Outflow

Due to the small volume of the anterior and posterior chambers, the control of fluidics during phacoemulsification surgery is important to ensure efficient removal of the cataract while preventing complications due to tissue collapse. Balanced fluidics means that the infusion pressure is sufficient to keep up with the outflow of fluid and that gives a stable anterior chamber and prevents the posterior capsule from flapping uncontrollably in the fluid currents.

Remember that there is just one source of inflow: the balanced salt solution from the infusion bottle or bag. There are two sources of fluid outflow: most of it is aspirated by the phaco handpiece (or I/A handpiece for cortex removal), but there is also fluid leakage from the incisions. We want a small amount of fluid leakage from the main incision since this cools the phaco probe which would otherwise get very hot from friction due to the ultrasonic motion.

In the example above, if we measured the actual inflow to be 60 cc/min, then we should expect up to 10 cc/min of fluid leakage from the incisions. This leaves us with maximum advisable outflow rate of 50 cc/minute which we determine from our fluidic pump settings.

The basic concept of fluidics is that the inflow of fluid must be greater than the outflow of fluid. By keep a constant infusion pressure and limiting the outflow, we can ensure that the eye stays inflated and stable during surgery. If we allow the outflow to exceed the fluid inflow, even for just a fraction of a second, we experience surge within the eye and this can cause chamber instability, collapse of the eye, and aspiration of the posterior capsule. The primary rule for phaco fluidics is to keep the inflow greater than the outflow.

Click below to watch the video explaining Part 2 of Phaco Fundamentals:

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