Rethinking Intraocular Pressure Requirements
Panelists describe the clinical impact of IOP compensation.
Dr. Packo: Retina surgeons have been conditioned to use intraocular pressures (IOPs) that are higher than what we would think is healthy for the eye. According to the 1999 Preferences and Trends (PAT) survey by the American Society of Retina Specialists, the most commonly chosen vented gas/forced infusion (VGFI) setting for vitreous surgery is in the mid-30s.
Dr. Ho, tell us about your experience with the new IOP compensation function.
ACTUAL VS ESTIMATED PRESSURES
Dr. Ho: I have come to realize that infusion pressure of 35 mmHg to 40 mmHg is the pressure in the air above the balanced salt solution that drives the fluid down to the eye. It does not reflect the pressure drop across the 84 inches of infusion line or the pressure in the eye at a specific moment. The IOP compensation function allows us to determine real pressures in the eye, which means we need to rethink where we should set our pressure. The system has a detection mechanism and a servo-control that allows IOP to remain as constant as possible. This technology reacts faster than we can on the fly. I think leaving the VGFI setting at 15 mmHg or 20 mmHg, pressures that we thought were too low with the Accurus, would be appropriate with the Constellation.
Dr. Packo: We believe 15 mmHg is a perfect pressure for the human eye naturally. Why is 15 mmHg too low during vitrectomy with the older technology?
Dr. Ho: The pressure of 15 mmHg is too low because it represents the pressure in the bottle and does not take into account the pressure through the tubing, with all of its resistance, and the distance to the eye.
Dr. Packo: Additionally, the IOP can drop to very low levels when we are removing fluid from the eye because the infusion cannot keep up with the probe removal. In fact, the IOP can drop almost 50% at flow rates used for typical core vitrectomies (Huculak J, Alcon Surgical Inc., Irvine, Calif., oral communication, 2005). I have seen the eye begin to collapse during fluid aspiration even with higher than "normal" infusion settings. Gravity-fed or conventional gas-driven infusion cannot always keep up with the demands of the aspiration. The VGFI setting at the top of the bottle is actually matched in the eye, but only when nothing is moving. As soon as we start removing fluid from the eye, the pressure can drop significantly. When we see striae develop on the cornea, it is because we are overcoming the limits of technology with conventional VGFI.
Dr. Dugel, what is your typical VGFI setting with the Accurus? Are you doing anything differently with the Constellation?
Dr. Dugel: My settings on the previous generation system are basically the same as those reported in the PAT survey, usually in the 30s. The new system, however, measures real pressure as opposed to estimated pressure. It has internal, noninvasive sensors that measure the flow through the infusion line and cannula. If you look at the mathematical equation — pressure is equal to the flow rate divided by the resistance — essentially, resistance is measured during the IOP priming. The flow is being measured constantly through the infusion setting, so we are solving the Ohm's Law equation by measuring two variables directly. We have not had that capability before.
|"Infusion pressure of 35 mmHg to 40 mmHg is the pressure in the air above the balanced salt solution that drives the fluid down to the eye. It does not reflect the pressure drop across the 84 inches of infusion line or the pressure in the eye at a specific moment."|
—Allen C. Ho, MD
|Added Safety in Air-Fluid Exchange|
|Dr. Packo: How is doing an air-fluid exchange with the Constellation different than with the Accurus?|
Dr. Ho: With the Constellation, the surgeon has absolute control over going to different modes from fluid to air. You do not have to rely on your scrub nurse, who may not be right next to you, to switch a stopcock. It is in your control, done in combination with IOP control.
Dr. Packo: It is also foot pedal-controlled. I have found it interesting to toggle back from air to fluid. If you want to rinse some fluorocarbon off the retinal surface, for example, you can easily go back, throw a little fluid in the eye and then very quickly come back to air. You are not commanding an assistant or a scrub nurse to do anything.
Dr. Ho: Think about how much safer that will be. When switching between modes, you will not have that sense of the globe collapsing where you might have an airlock in the line. It is not only a convenience issue but a safety issue.
Dr. Pollack: I believe the greatest clinical value of IOP control technology is in underperfused eyes, such as those frequently found in diabetes and glaucoma patients. We are likely causing ocular injury when we operate on diabetes patients with ocular ischemia or patients with endstage glaucoma using infusion pressures of 30 mmHg to 40 mmHg.
When operating on hypoperfused eyes with the older technology and infusion pressure of 30 mmHg, I find that it is common for the optic nerve vessels to either pulsate or lack perfusion at all. This creates the need for the surgeon to drop the infusion pressure to 25 mmHg or even 21 mmHg before reperfusion occurs. The problem with low infusion pressures is that it is difficult for lower infusion to keep up with typical vacuum settings, resulting in the need to significantly slow the surgery. Intraocular pressure control technology was designed to address this issue.
Dr. Rizzo: Intraocular pressure control is also important when we operate on highly myopic eyes, to avoid choroid problems due to sudden pressure changes, or, for example, to avoid secondary endothelial damage at the collapse of the anterior chamber as we unexpectedly relax during aspiration of the eye indentation. When using contact panoramic viewing systems like I do, it is also important to avoid corneal folds and, therefore, poor visibility when aspiration is done excessively.
Dr. Packo: Potentially, we could be saving corneal epithelium intraoperatively. On rare occasions with high myopia, we may see a sudden occurrence of either serous or hemorrhagic choroidal detachments, and that may be due to the pressure swings in surgery.
The eye felt different to me when using the IOP control during the scleral depression maneuver. As Dr. Rizzo pointed out, when we do scleral depression, we worry about losing corneal epithelium with the collapse of the globe when you let go and the eye tries to reform. With this technology, however, the cornea does not show ill effects of the pressure swings.
Although rare, we all have seen a surgery scenario wherein everything went well and yet, the patient has a poor visual outcome. We are left with an idiopathic vision loss from vitreous surgery and wonder why. Maybe we now have an answer.
|"I believe the greatest clinical value of IOP control technology is in underperfused eyes, such as those frequently found in diabetes and glaucoma patients."|
—John S. Pollack, MD
ADVANTAGE FOR SURGEON AND PATIENT
Dr. Murray: Like everyone else, I have been using relatively high IOP settings to protect the integrity of the globe during some of the surgical maneuvers that are necessary during vitrectomy. To be able to go from IOPs of 35 mmHg to 40 mmHg down to 20 mmHg and to maintain control of the IOP through that closed loop mechanism, I think is advantageous to the surgeon and, more importantly, to our patients. RP
|1. Packo, KH, Pollack, JS, American Society of Retina Specialists Annual Preferences and Trends Survey, 1999; access for members only at http://www.asrs.org.|