An adaptor comprises comprising a base element and a nozzle element which are operatively coupled to each other. The base element defines a throat and at least one entrainment port facilitating a path of fluid communication between the throat and ambient air. The nozzle element includes a jet nozzle, and a connector which is adapted to facilitate the fluid coupling of the nozzle element to a bi-lumen tube of the patient circuit. The connector includes both a delivery port and a sensing port. The jet nozzle and the delivery port collectively define a delivery line or lumen which fluidly communicates with the throat of the base element, and is placeable into fluid communication with the delivery lumen of the bi-lumen tube.