A vent typically does not function if it is inserted too deeply and tissues get sucked into its holes. This is a potentially a dangerous condition that can cause LV perforation if the vent is intra-ventricular. It is necessary to always disconnect the vent suction source and then reposition the vent to avoid ventricular injury.
Typically excessive suction is indicated by collapse of the vent tubing and also "line chatter"
This can be avoided by avoiding active suction and using gravity drainage but this may not be very effective in open chambers and may limit the amount of suction. A needle to "vent the vent" placed in the vent line can prevent excessive negative pressure and allow the vent to function well.
Use of a sump type of vent can also obviate this problem.Stopping a vent, disconnecting the vent from the suction source and withdrawing the vent into the middle of a chamber usually allows it to start functioning.
Excessive need to vent could be due to inadequate drainage or due to excessive collaterals.
A vent that drains greater than 10 % of the pump flows should prompt for the search of a major aorto-pulmonary connection like a patent ductus arteriosus.