Minimizing imposed expiratory resistance of mechanical ventilator by optimizing exhalation valve control

1. A method for controlling exhalation during ventilation of a patient on a ventilator, the method comprising:

determining a control command for an exhalation valve, wherein the control command targets a pressure at the exhalation valve between a minimum pressure and a steady-state pressure for a period of time;

controlling the exhalation valve based on the control command during one or more exhalation cycles;

monitoring an end exhalation pressure and a flow undershoot during the one or more exhalation cycles;

comparing the end exhalation pressure to a predetermined pressure range;

comparing the flow undershoot to a predetermined flow threshold; and

based on the comparing, updating the control command in order to maintain a positive-end expiratory pressure (PEEP) at the end of the one or more exhalation cycles.

2. The method of claim 1 , wherein the pressure is lower than the PEEP.

3. The method of claim 1 wherein the period of time is two breath cycles.

4. The method of claim 1 , wherein the control command is determined by an equation:

u ( t k )=( u ss −u 0 )*(1−exp(− t k /tau ))+ u 0 ,

wherein tau=sum*(the predetermined number of breath cycles)/(u ss −u 0 ),

wherein sum=sum+u ss −u 0 ,

wherein t k is an exhalation time,

wherein u 0 is the minimum pressure during the period of time, and

wherein u ss is the steady-state pressure during the period of time.

5. The method of claim 1 wherein comparing the end exhalation pressure to the predetermined pressure range further comprises:

determining an average end exhalation pressure based on the end exhalation pressures of a prior number of exhalation cycles; and

determining whether the average end exhalation pressure falls outside of the predetermined pressure range.

6. The method of claim 5 wherein the prior number of exhalation cycles is 10.

7. The method of claim 5 wherein determining whether the average end exhalation pressure falls outside of the predetermined pressure range further comprises:

calculating a difference between the average end exhalation pressure and the PEEP; and

determining whether the difference falls outside of the predetermined pressure range.

8. The method of claim 1 wherein comparing the flow undershoot to the predetermined flow threshold further comprises:

measuring an expiratory flow of the exhalation valve during the one or more exhalation cycles;

determining the flow undershoot of the expiratory flow; and

determining whether the flow undershoot falls under the predetermined flow threshold.

9. The method of claim 1 wherein the predetermined flow threshold is 0.2 L/min.

10. The method of claim 1 further comprising terminating control of the exhalation valve upon detection of an end condition.

11. A ventilator system, comprising:

means for determining a control command for an exhalation valve, wherein the control command targets a pressure at the exhalation valve between a minimum pressure and a steady-state pressure for a period of time;

means for controlling the exhalation valve based on the control command during one or more exhalation cycles;

means for monitoring an end exhalation pressure and a flow undershoot during the one or more exhalation cycles;

means for comparing the end exhalation pressure to a predetermined pressure range;

means for comparing the flow undershoot to a predetermined flow threshold; and

based on the comparing, means for updating the control command in order to maintain a positive-end expiratory pressure (PEEP) at the end of the one or more exhalation cycles.

12. A computer-readable medium having computer-executable instructions for performing a method controlling exhalation during ventilation of a patient on a ventilator, the method comprising:

determining a control command for an exhalation valve, wherein the control command targets a pressure at the exhalation valve between a minimum pressure and a steady-state pressure for a period of time;

repeatedly controlling the exhalation valve based on the control command during one or more exhalation cycles;

repeatedly monitoring an end exhalation pressure and a flow undershoot during the one or more exhalation cycles;

repeatedly comparing the end exhalation pressure to a predetermined pressure range;

repeatedly comparing the flow undershoot to a predetermined flow threshold; and

based on the comparing, repeatedly updating the control command in order to maintain a positive-end expiratory pressure (PEEP) at the end of the one or more exhalation cycles.

13. The method of claim 12 , wherein the pressure is lower than the PEEP.

14. The method of claim 12 wherein repeatedly comparing the end exhalation pressure to the predetermined pressure range further comprises:

repeatedly determining an average end exhalation pressure based on the end exhalation pressures of a prior number of exhalation cycles; and

repeatedly determining whether the average end exhalation pressure falls outside of the predetermined pressure range.

15. The method of claim 14 wherein repeatedly determining whether the average end exhalation pressure falls outside of the predetermined pressure range further comprises:

repeatedly calculating a difference between the average end exhalation pressure and the PEEP; and

repeatedly determining whether the difference falls outside of the predetermined pressure range.

16. The method of claim 12 wherein repeatedly comparing the flow undershoot to the predetermined flow threshold further comprises:

repeatedly measuring an expiratory flow of the exhalation valve during the one or more exhalation cycles;

repeatedly determining the flow undershoot of the expiratory flow; and

repeatedly determining whether the flow undershoot falls under the predetermined flow threshold.