Compensation for unmeasurable inspiratory flow in a critical care ventilator

Abstract

A system and method for operating a ventilator to compensate for external gas flow reaching a patient from an external device, such as a nebulizer. A control unit of the ventilator monitors the gas flow rate from the ventilator and compares the gas flow rate from the ventilator to an expired gas flow rate from the patient. The difference between the inspired flow rate and the expired flow rate is due to the external device. The control unit modifies the operation of the ventilator to compensate for the external gas flow rate such that the flow of gas reaching the patient is maintained at a desired level.

1 . A method of operating a ventilator to provide an inspiratory gas flow to a patient including an external gas flow from a medical device separate from the ventilator, the method comprising the steps of:
 monitoring a flow of gas from the ventilator in a control unit of the ventilator;   monitoring an expiratory flow of gas from the patient in the control unit;   determining an external flow rate in the control unit based on the difference between the expiratory flow rate and the ventilator flow rate; and   operating the control unit to adjust the ventilator flow rate based upon the determined external flow rate.   
     
     
         2 . The method of  claim 1  wherein the ventilator flow rate is adjusted such that the combination of the ventilator flow rate and the external flow rate provides a desired inspiratory flow rate to the patient. 
     
     
         3 . The method of  claim 1  wherein the flow of gas from the ventilator and the expiratory flow of gas are determined by sensors associated with the ventilator. 
     
     
         4 . The method of  claim 3  wherein the external gas is injected into the flow of gas from the ventilator downstream from the sensor contained within the ventilator. 
     
     
         5 . The method of  claim 1  wherein the external gas is provided by a nebulizer that provides a nebulized agent in a nebulizer gas flow. 
     
     
         6 . The method of  claim 1  wherein the flow of gas from the ventilator and the expiratory flow rate are determined by sensors external to the ventilator and coupled to the control unit of the ventilator. 
     
     
         7 . The method of  claim 2  wherein the desired inspiratory flow rate for the patient is determined in the control unit of the ventilator based upon user input. 
     
     
         8 . The method of  claim 7  wherein the desired inspiratory flow rate is determined through the user input entered into a user input device of the ventilator, wherein the user input device is coupled to the control unit. 
     
     
         9 . The method of  claim 5  wherein the nebulizer operates independently from the operation of the ventilator and the control unit. 
     
     
         10 . A method of adjusting a flow rate from a ventilator to compensate for a gas flow from a nebulizer to provide a desired inspiratory flow rate to a patient, comprising the steps of:
 monitoring the flow of gas from the ventilator in a control unit of the ventilator;   monitoring an expiratory flow of gas from the patient;   determining a nebulizer flow rate in the control unit based upon the difference between the expiratory flow rate and the ventilator flow rate; and   operating the control unit to adjust the ventilator flow rate such that the combination of the nebulizer flow rate and the ventilator flow rate reaches the desired inspiratory flow rate.   
     
     
         11 . The method of  claim 10  wherein the flow of gas from the ventilator and the expiratory flow are determined by sensors contained within the ventilator. 
     
     
         12 . The method of  claim 11  wherein the nebulized agent is injected into the flow of gas from the ventilator downstream from the sensor contained within the ventilator. 
     
     
         13 . The method of  claim 10  wherein the desired inspiratory flow rate for the patient is determined in the control unit of the ventilator based upon user input. 
     
     
         14 . The method of  claim 13  wherein the desired inspiratory flow rate is determined based on a desired breath volume entered into a user input device of the ventilator, wherein the user input device is coupled to the control unit. 
     
     
         15 . The method of  claim 10  wherein the nebulizer operates independently from the operation of the ventilator and the control unit. 
     
     
         16 . The method of  claim 10  further comprising the steps of:
 assigning a reference flow rate to the nebulizer flow rate prior to determination of the nebulizer flow rate; 
 reducing the flow rate from the ventilator by the reference flow rate; and 
 providing the reduced ventilator flow rate to the patient prior to determining the nebulizer flow rate. 
 
     
     
         17 . A system for providing an inspiratory gas to a patient at a desired inspiratory gas flow rate:
 a ventilator that provides an output flow of gas to an inspiratory limb of a patient circuit;   an inspiratory sensor that monitors the output flow of gas into the inspiratory limb;   an external device that injects an external gas flow into the inspiratory limb downstream of the inspiratory sensor;   an expiratory sensor that monitors an expiratory gas flow from the patient in an expiratory limb of the patient circuit; and   a control unit contained within the ventilator and in communication with both the inspiratory sensor and the expiratory sensor to calculate a net gas flow rate as the difference between the output gas flow from the ventilator and the expiratory gas flow from the patient,   wherein the control unit reduces the output gas flow from the ventilator by the net gas flow such that the desired inspiratory gas flow rate is delivered to the patient.   
     
     
         18 . The system of  claim 17  wherein the external device is a nebulizer. 
     
     
         19 . The system of  claim 17  wherein the inspiratory sensor and the expiratory sensor are each associated with the ventilator. 
     
     
         20 . The system of  claim 18  wherein the nebulizer injects a medical agent into the inspiratory limb including the ventilator output gas flow.