1. Field of the Invention
This invention relates generally to breathing ventilators, and more particularly to a pnuematically driven, electronically controlled ventilator system for mixing gas proportions and delivering breathing gas for either controlled or spontaneous breaths.
2. Prior Art
Conventional breathing ventilators generally provide a positive over pressure of breathing gas to a patient, and may provide respiratory gas under pressure in a predetermined manner responsive to pressure variations in the breathing pattern of the patient, or may operate to supply a predetermined volume of respiratory gas to a patient in a controlled manner for each breath. The breathing gas is generally provided with an elevated concentration of oxygen. An oxygen concentrator having a reservoir for receiving the oxygenated product gas, including a sensor for monitoring withdrawal of the product gas from the reservior, and a microprocessor for determining the minimum time for charging of the gas to provide a product gas with a selected oxygen concentration at the sensed rate of withdrawal is described in U. S. Pat. No. 4,561,287 (Rowland). A plurality of canisters with molecular sieve beds for absorbing nitrogen provide the oxygenated gas to the reservoir, and a valve mechanism is provided for directing air from a compressor to alternate sieve beds. Another system for mixing the two gases in predetermined proportions involves the introduction of two gases via separate inlets into a pressure vessel receiving the first gas up to a first pressure, and then receiving the second gas up to a second pressure, to provide the proper proportioning of the mixed gases. This system is described in U.S. Pat. Nos. 4,022,234 and 4,023,587 (Dobritz). The mixed gases are withdrawn until the initial pressure within the receiving vessel is reached, at which time the withdrawal of gas is interrupted, and the mixing cycle is again commenced. The feedback control of the rate of flow and pressure of breathing gas to a patient by an inspiration servounit is also described in U.S. Pat. No. 3,741,208 (Jonsson, et al.)
It would be desirable to provide a ventilator system which can mix the constituent gases in a respiratory gas in a manner responsive to pressure within the respiratory gas contained in the receiving vessel, and responsive to the various modes of breathing for which the respiratory gas is being provided.