Automatic determination of passive elastic and resistive properties of the respiratory system during assisted mechanical ventilation

• US 5,884,622 A
• Filed: 12/20/1996
• Issued: 03/23/1999
• Est. Priority Date: 12/20/1996
• Status: Expired due to Term

• Current Assignee: University of Manitoba
• Original Assignee: University of Manitoba
• Inventors: Younes, Magdy
• Primary Examiner(s): Lewis, Aaron J.
• Application Number:
  US08/771,216
• Time in Patent Office: 823 Days
• Field of Search: 128/204.18, 128/204.21, 128/204.23
• US Class Current: 128/204.21
• CPC Class Codes: A61M 16/00 Devices for influencing the respiratory system of patients by gas treatment, e.g. mouth-to-mouth respiration; Tracheal tubes stimulating the respiratory movement by mechanical, pneumatic or electrical means, iron lungs combined with gas breathing means A61H31/00
• CPC Class Codes: A61M 16/0006 with means for creating vibrations in patients' airways
• CPC Class Codes: A61M 16/024 including calculation means, e.g. using a processor
• CPC Class Codes: A61M 2016/0036 in the breathing tube and used in both inspiratory and expiratory phase
• CPC Class Codes: A61M 2230/46 Resistance or compliance of the lungs

Abstract

Passive elastic and resistance values for a particular patient are estimated during proportional assist ventilation and other forms of assisted ventilation to permit reliable adjustment for volume-related and flow-related assist gains to meet continuously varying patient muscle pressure. Independent but simultaneous procedures are effected to determine the respective values. The pressure-volume relationship is determined by a modification of the inspiratory hold technique while the pressure-flow relationship is determined by deliberately introducing brief perturbations in pressure, flow and volume and observing the effects obtained.

Claims

1. A method for estimating a passive pressure-volume relationship of a respiratory system in a patient on mechanical ventilatory support and producing spontaneous respiratory effort (assist mode of ventilation) to guide selection of a volume-assist component of a proportional assist ventilation mode (PAV), which comprises:

a) placing a ventilator in a proportional assist mode of ventilation, using empiric values of elastance and resistance, or values of elastance and resistance determined by other conventional methods, for initial adjustment of volume-related and flow-related assist components of the proportional assist ventilation mode,

b) monitoring airway pressure (P_aw) and flow (V) and volume (V) to the patient,

c) holding flow at or near zero in selected breaths for a period beyond termination of a patient'"'"'s inspiratory phase,

d) measuring P_aw at a point as far away as possible from the onset of an inspiratory hold but sooner than latency for a behavioral respiratory response to the inspiratory hold to provide P_hold,

e) measuring the tidal volume (V_T) of the breaths selected for the inspiratory hold step, and

f) establishing a relationship between P_hold and V_T in said selected breaths to provide a pressure-volume relationship over the V_T range encountered during proportional assist ventilation to permit subsequent adjustment of a volume-related assist component for proportional assist ventilation.

2. The method of claim 1, including measuring said P_hold at a time no greater than about 250 msec from the onset of an inspiratory hold.

3. The method of claim 1, including measuring said P_hold at about 200 to about 250 msec from the onset of an inspiratory hold.

4. The method of claim 1, including using linear functions to fit the relationship between said V_T and said P_hold.

5. The method of claim 1, including using non-linear functions to fit the relation between said V_T and said P_hold.

6. The method of claim 1, including determining said P_hold by non-linear extrapolation to an asymptote of multiple P_aw values measured in the interval between onset of inspiratory hold and latency for a behavioral respiratory response to the inspiratory hold.

7. The method of any one of claims 1 to 6, including minimizing scatter around a fit between said V_T and said P_hold by including factors that can theoretically alter this relation in multiple regression analysis of said relation.

8. The method of claim 7 including selecting said factors from ventilation, V_T, expiratory time, expiratory flow and expiratory P_aw in the interval preceding the breaths selected for inspiratory hold and/or inspiratory time of the selected breaths.

9. The method claimed in claim 1, including transiently increasing or decreasing the gain of volume-assist in PAV in said selected breaths to cause transient increase or decrease in V_T to expand the range of V_T over which the relation between V_T and P_hold is determined.

10. The method claimed in claim 1, including preferentially targeting breaths in which V_T is predicted to be substantially larger or smaller than average V_T for the inspiratory hold procedure in order to expand the range of V_T over which the relation between V_T and P_hold is determined.

26. The method of claim 1 or 11, including eliminating faulty data points from the analysis of the relation between V_T and P_hold or between pressure and flow, such faulty nature being established according to specified criteria including patterns of P_aw, V and/or V during breaths used in analysis, or during breaths preceding said breaths, that are outside the usual range of variability established from the majority of breaths in the patient being ventilated.

27. The method of claim 1 or 11, including extrapolating the pressure-volume and pressure-flow relationships beyond the range of actual observations.

28. The method of claim 1 or 11, including discarding old data points as new ones are collected in order to provide a continuous update of the pressure-volume and pressure-flow relationships.

29. The method of claim 1 or 11, including storing the pressure-volume and pressure-flow relationships computed at different times in the course of mechanical ventilation of a given patient for later display to provide time-related trends in such relationships.

30. The method of claim 1 or 11, including automating the procedures.

31. The method of claim 30, including automatically adjusting results of automated procedures to automatically adjust the volume-assist and/or flow-assist gains of the proportional assist ventilation.

11. A method for estimating a passive pressure-flow relationship of the respiratory system in a patient on mechanical ventilatory support and producing spontaneous respiratory efforts (assist mode of ventilation) to guide selection of a flow-assist component in a proportional assist ventilation (PAV) mode, which comprises:

a) placing a ventilator in a proportional assist ventilation mode using empiric values of elastance and resistance or values of elastance and resistance determined by other conventional methods, for initial adjustment of volume-related and flow-related assist components of the proportional assist ventilation mode,

b) monitoring airway pressure (P_aw) and flow (V) and volume (V) to the patient,

c) applying brief perturbations in pressure, flow and volume of at least two different forms to selected breaths, such perturbations occurring at a predetermined time during the inspiratory phase of said selected breaths,

d) determining the P_aw, flow (V) and volume (V) at predetermined times during the perturbation less than latency for a behavioural respiratory response to the perturbation.

e) determining the P_aw, V and V at similar times to those used in step (d) in unperturbed breaths,

f) averaging results of a number of each form of perturbed and of unperturbed breaths, and

g) utilizing the average values of P_aw, V and V obtained from the different forms of perturbed and from unperturbed breaths to provide a pressure-flow relationship to permit subsequent adjustment of the flow-related assist component for proportional assist ventilation.

12. The method of claim 11 including selecting said perturbation of at least two different forms from positive perturbations of two or more amplitudes, negative perturbations of two or more amplitudes, biphasic perturbations, and a mix of positive and negative perturbations.

13. The method of claim 11 wherein including determining said P_aw, V and V during the perturbation in less than about 200 msec.

14. The method of claim 11, including effecting step (g) by mathematically fitting the values determined in step (f) to Rohrer'"'"'s equation:

space="preserve" listing-type="equation">P.sub.res =V·

K.sub.1 +V.sup.2 ·

K.sub.2

where P_res is resistive pressure and K₁ and K₂ are constants reflecting laminar and turbulent components of resistance, respectively.

15. The method of claim 11, including replacing observations in unperturbed breaths by one additional form of perturbation.

16. The method of claim 11, including applying perturbations when a specific flow is reached rather than a specific time into inspiration.

17. The method of claim 11, including basing determination of the pressure-flow relationship on differences between individual breath values within different forms of perturbations rather than values obtained from averages of individual breath values of perturbed and unperturbed breaths.

18. The method of claim 11, including using average values obtained from unperturbed breaths as reference for calculation of change in P_aw, V and V produced by the different forms of perturbation to result in average ▴

P_aw, ▴

V and V for each form of perturbation with the overall pressure-flow relationship being determined from said average differences.

19. The method of claim 18, including using said differences in ▴

P_aw, V and V to calculate two or more incremental resistance values.

20. The method of claim 11, including applying perturbations at more than one point in time into inspiration.

21. The method of claim 11, including producing the transient perturbations by deliberately increasing or decreasing the command signal that controls ventilator output in the PAV mode of ventilation by specified amounts, for a specified time, without changing the values of the flow-related and volume related assist of the PAV mode of ventilation.

22. The method of claim 11, including producing the transient perturbations by a transient change in the flow-related and/or volume-related assist gain of the PAV mode of ventilation.

23. The method of claim 11, including producing transient perturbations by a mechanical system, independent of the ventilator itself, and incorporated in the external tubing.

24. The method of claim 11, including applying the method during volume-cycled ventilation, rather than during PAV;

by adding and/or subtracting specified amounts of flow for specified transients, to the ongoing programmed flow command routine.

25. The method of claim 11 including applying the method during modes of ventilation other than PAV and volume cycled assist, including CPAP mode, pressure support ventilation (PSV) or airway pressure release ventilation (APRV), whereby perturbations are produced by superimposing positive and/or negative transients to the usual control signal of the relevant mode.