Port-based flow limiting is safer for the patient during vitreoretinal surgery because it is so instantaneous, does not occur at the console, and helps prevent retinal breaks.
The goal in port-based flow-limiting vitreoretinal surgery is to avoid causing retinal breaks, although this problem seldom can be eliminated completely.
Two scenarios exist in which retinal breaks can be created by pulsatile vitreoretinal traction. In one, an already detached retina moves to the port.
In another scenario, similar to an occlusion break during phacoemulsification, iatrogenic retinal breaks can occur after fluid surge through the port after epiretinal membrane or dense fibrous tissue deforms through the port, Dr. Charles said.
Port-based flow limiting is a term meaning that the flow is limited at the port and not at the console, he said. There is a difference between the two. With flow limiting at the console, there is a time lapse between the surgeon recognition of the retina moving toward the port and his or her response to that and an additional delay as the fluid pulse twice traverses 7 feet of tubing.
"Reaction time is about 400 milliseconds," he said, during which time fluid continues to flow through the port. The pulse flow rate, cut rates, vacuum, duty cycle, and lumen size all affect the port-based flow limiting as well as the properties of the vitreous. He explained the effect of adjusting the parameters during vitreoretinal surgery.
"Smaller lumen diameters limit flow and therefore … pulse flow, which is the amount that goes through each time the port opens and closes," he said. When the pulse flow is decreased there is less pulsatile vitreoretinal traction, which in turn decreases the motion of the detached retina and the pulsatile forces on the attached retina.
"Low pulse flow limits effects to the area around the port," he said.
Higher cut rates produce greater port-based flow limiting and thereby reduce the pulsatile vitreoretinal traction and less retinal movement.