FIELD OF THE INVENTION

The present invention relates to a method for estimating Lung Compliance and Lung Resistance, preferably during the inhalation phase of a VCV or PCV breath during passive ventilation.

BACKGROUND OF THE INVENTION

During ventilation of ICU patients or patients undergoing surgical procedures, it is desirable and certainly very helpful to have information about the patient's respiratory system, among other things. In particular, it is common practice to estimate the patient's lung compliance and resistance and observe their evolvement as it may indicate worsening or improving condition of the patient. To assess the patient's lung Compliance and Resistance, the prior art performs a Respiratory Mechanics maneuver called "Static Mechanics

Maneuver" or "Inspiratory Hold". This technique uses a breath in Volume Control

Ventilation, with a Square waveform and keeps the exhalation valve closed at the end of the inhalation phase, to estimate the Lung Compliance and Resistance of the patient, see for instance the paper titled "Respiratory Mechanics Derived From Signals in the Ventilator Circuit" by Umberto Lucangelo MD, Francesca Bernabe ^' MD, and Llui ^' s Blanch MD PhD, published by aarc.org.

The inventor of the present invention has appreciated that an improved method and system is of benefit, and has in consequence devised the present invention.

SUMMARY OF THE INVENTION

It would be advantageous to achieve a method and system for estimating Lung Compliance and Lung Resistance without the need for execution of occlusion maneuvers or introduction of pulses (positive or negative) while ventilation is carried out. It would also be desirable to enable health care persons to gain access to information on Lung Compliance and Lung Resistance at any time during ventilation of patients. In general, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination. In particular, it may be seen as an object of the present invention to provide a method that solves the above mentioned problems, or other problems, of the prior art.

To better address one or more of these concerns, in a first aspect of the invention an method of estimating physical parameters on a patient's lungs when the patient is connected to a ventilation system is presented wherein the ventilation system comprises a gas supply source and a patient circuit arranged for delivering the gas to the patient's lungs, a pressure sensor arranged for detecting pressure at patient airways opening and a flow meter for measuring the flow of gas in or out of the patient's lungs via the patient's airway. The method may comprise the steps of detecting pressure measured by the pressure sensor, detecting flow measured by the flow meter, estimating, when flow of gas from the gas supply becomes zero in magnitude, the patients compliance using the equation: CLung

=VLung/Delta_Py, where CLung is Patient's lung compliance, VLung is Gas volume accumulated in the patient's lungs during the inhalation phase of a breath, and Delta Py is Pyeoi - Pyboi, where Pyboi is Py estimate at the beginning of a physiological inhalation Pyeoi is Py estimate at end of a physiological inhalation.

The pressure at the patient's airway opening and the flow in and out of the patient lungs may be directly measured or estimated using methods familiar to those experienced in the art.

The present invention is related to the process of estimation of Lung

Compliance and Lung Resistance, when a patient is passive, i.e. in assisted breathing. The method has the advantage that there is no need for performing partial occlusion or any other maneuver. The method utilizes that the pressure in the patient's lung is the same as the pressure in the tubing circuit when the Lung flow magnitude is zero. So at the beginning of the physiologic inhalation (i.e. when the lung Flow crosses zero and becomes positive) and the beginning of the physiological exhalation (i.e. when the lung Flow crosses zero and becomes negative), it is possible to calculate the pressure difference that occurred in the lung during the inhalation phase and thus estimation of the lung compliance can be carried out via a the simple equation mentioned above.

By not having to perform partial occlusion or any other invasive maneuver the patient need not be disturbed. As the patient is passive, i.e. not breathing on his or her own, the system is the only supply of gas. By not having to disrupt the patient's breathing the values may be continuously available to an operator, e.g. health care person.

The present invention is usable without the need for execution of occlusion maneuvers or introduction of pulses (positive or negative) while ventilation is carried out. Therefore the method may be used on a continuous basis (i.e. every breath), since there is no disturbance of the breathing pattern or the patient's comfort and provide up to date data to the clinician at any desired time. Since the prior art requires disturbance of the breathing pattern and the user must program or command the ventilator to perform these maneuvers, the up to date data is only available upon execution of the maneuvers.

Furthermore, the invention is not restricted to Volume Control Ventilation as is the prior art, but may be used without loss of accuracy in Pressure based Controlled Ventilation. Note that just as the prior art, the requirement for the patient being passive still remains.

The present invention relates to technique for estimating Lung Compliance and Resistance, while the method does not make use of the inspiratory hold technique but instead utilizes that at zero lung flow, the pressures at the tubing circuit wye is the same as that of the lung since the pressure drop across that patient airways is zero. Also, once the Compliance is determined, the value of the lung Resistance may be determined at any point during the inhalation phase.

Advantageously the Pyboi is the Py estimate at the beginning of a physiological inhalation is determined at the point (boi) where the flow QLung = 0 before it becomes positive. Using these points for determining the Lung Compliance and Lung Resistance a relatively precise estimate may be obtained.

Advantageously the Pyeoi is the Py estimate at end of a physiological inhalation at the point (eoi) where the flow QLung = 0 before it becomes negative.

Advantageously detection is performed using samples, and determination of the crossing points is carried out by using linear interpolation of pressures and flow samples. This could include comparison to zero of the magnitude and sign for adjacent or close samples used for the linear interpolation of the flow QLung signal. By sampling a digital processor may be used. A signal processor may be used for obtaining the samples.

Advantageously the Lung Resistance estimation may be performed by reconstruction of the lung pressure starting at either end of the inhalation phase and by using the equation: RLung = (Py o - PLung o) / QLung o, where RLung is the airway resistance, Py_o is the magnitude of the pressure measured at the distal end of the patient's airway at time = To, PLung o is the magnitude of the pressure estimated at the Lung at time = To, and QLung o is the magnitude of the flow into the patient's lungs at time = To.

Advantageously the steps of the method may be repeated in a loop for continuous, periodic or selective monitoring of Patient's airway resistance. This allow the method to be utilizes for monitoring the condition of a patient, e.g. during an operation or when a patient is in the ICU. Once the CLung estimate is found, estimates for the airway resistance (RLung) can be obtained at different points in time (say time =To) throughout the just completed inhalation phase.

Advantageously the method may be used in a Volume Control Ventilation system or in a Pressure based Controlled Ventilation system. The method is useful for both kinds of systems.

A second aspect of the present invention relates to a ventilating system for ventilating a patient's lungs when the patient is connected to the ventilation system, the ventilation system comprising a gas supply source, a patient circuit arranged for delivering the gas to the patient's lungs, a pressure sensor arranged for detecting pressure at patient airways opening and a flow meter for measuring the flow of gas in or out of the patient's lungs via the patient's airway, a processor operatively coupled to the sensor and flow meter for obtaining signals therefrom, the processor configured for detecting when flow of gas from the gas supply becomes zero in magnitude and subsequently estimating the patients compliance using the equation: CLung =VLung/Delta_Py, where CLung is Patient's lung compliance, VLung is Gas volume accumulated in the patient's lungs during the inhalation phase of a breath, and Delta Py is Pyeoi - Pyboi, where Pyboi is Py estimate at the beginning of a physiological inhalation Pyeoi is Py estimate at end of a physiological inhalation.

The system may be or be part of a respirator, where the ventilating part comprises the mentioned parts. Further an output device, e.g. a screen and/or a sound producing unit, may be used for informing an operator of the patient's current compliance.

Either or both of the pressure sensor and/or flow meter may be replaced by a device for estimating pressure at patient airways opening and/or the flow of gas in or out of the patient's lungs via the patient's airway. Two devices may be supplied for estimation of the values.

Advantageously the processor may be further configured for determining Lung resistance by the equation: RLung = (Py o - PLung o) / QLung o, where RLung is the airway resistance, Py o is the magnitude of the pressure measured at the distal end of the patient's airway at time = To, PLung o is the magnitude of the pressure estimated at the

Lung at time = To, and QLung o is the magnitude of the flow into the patient's lungs at time = To. Advantageously the system may be arranged for continuously determining an estimate of Patient's lung compliance. As before the continuous operation of the system may be useful for monitoring the condition of the patient, and/or the progress of the patient.

A third aspect of the present invention relates to a computer implemented method of estimating physical parameters on a patient's lungs when the patient is connected to a ventilation system, the ventilation system comprising a gas supply source and a patient circuit arranged for delivering the gas to the patient's lungs, a pressure sensor arranged for detecting pressure at patient airways opening and a flow meter for measuring the flow of gas in or out of the patient's lungs via the patient's airway, the ventilation system further comprising a processor adapted for carrying out the computer implantation of the method, the method comprising the steps: detecting pressure measured by the pressure sensor, detecting flow measured by the flow meter, estimating, when flow of gas from the gas supply becomes zero in magnitude, the patients compliance using the equation: CLung =VLung/Delta_Py, where CLung is Patient's lung compliance, VLung is Gas volume accumulated in the patient's lungs during the inhalation phase of a breath, and Delta Py is Pyeoi - Pyboi, where Pyboi is Py estimate at the beginning of a physiological inhalation Pyeoi is Py estimate at end of a physiological inhalation.

The computer implemented method may advantageously be used for operating a system according to the second aspect of the present invention, e.g. by being configured to operate using a processor in the system. As mentioned above, the steps of detecting pressure measured by the pressure sensor and detecting flow measured by the flow meter may be replaced by estimating the values, e.g. using an estimation device, or e.g. computer implemented methods.

The computer implemented method may advantageously comprise any features mentioned in relation to the first and/or second aspect.

In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 schematically illustrates a graph relating Py, QL and PLung, Fig. 2 schematically illustrates a graph depicting the signals in the area where the lung flow crosses zero at the onset of the biological exhalation,

Figs. 3 and 4 schematically illustrate experimental results obtained by ventilation of a glass bottle (Clung = 18.8 ml/cmH20, measured using the syringe method) through a linear resistor.

DESCRIPTION OF EMBODIMENTS

A patient's Lung Compliance can be estimated, without the aid of special manoeuvres and independent of the flow waveform, during ventilation of passive patients in Volume Control Ventilation (VCV) and Pressure Control Ventilation (PCV) modes. The main principle applied for this purpose is that when the flow into or out of the lung goes through a sign change (i.e. becomes zero in magnitude), the pressures in the Lung and the airway opening are the same. Thus, by detecting these crossing points and estimating the corresponding pressures at these points, one can estimate the patient's compliance via the equation below.

CLU N G = Vhung/ Delta_Py Equation 1

Where:

- C mg = Patient's lung compliance = inverse of lung elastance (E _Lu n _g )

V uig = QLung * dtj = Gas volume accumulated in the patient's lungs

during the inhalation phase of a breath.

QLung = Patient Flow = Gas flow in or out of the patient's lungs via the patient's airway. This variable may be either measured or estimated.

P _y = Pressure at the patient's Airways opening.

Delta Py Pyeoi Pyboi

Pyboi = Py estimate at the beginning of a physiological inhalation. That is at the point (boi) where Qmng = 0 before it becomes positive.

Pyeoi = Py estimate at end of a physiological inhalation. That is at the point (eoi) where Qmng = 0 before it becomes negative.

Detecting the boi and eoi crossing points in a sampled based system is carried out by simple comparison to zero of the magnitude and sign for the current and previous samples of the Qmng signal. Once these points are found, three possibilities exist: a) the last sample of the Q ng signal is zero, in which case the magnitude of the last sample for the P _y signal is the value we are looking for; b) the current sample of the Qmng signal is zero, in which case the magnitude of the current sample for the P _y signal is the value we are looking for and lastly, the magnitude for neither the current or the last sample of the Qmng signal is zero in which case we estimate P _y t, ₀ i or P _yeo i (as appropriate) using linear interpolation via the equation below.

P _y @o = ( Py2*QLun _g i - Pyi *QLun _g 2 )/(Q 1 - Q2) Equation 2 Where:

P _y @o = Value of P _y at the point of zero crossing of Qmng P _y i = Value of P _y prior to the point of zero crossing of Qmng - P _y 2 = Value of P _y after the point of zero crossing of Qmng

Qmngi = Value of Qmng prior to the point of zero crossing Qmng2 = Value of Qmng after the point of zero crossing

Once the Cmng estimate is found, estimates for the airway resistance (Rmng) can be obtained at different points in time (say time =T ₀ ) throughout the just completed inhalation phase. This is accomplished by using Lung flow, airway pressure and Lung volume data at one or more points in the inhalation, and by using the equation below at each of the points.

Rmng ⁼ (Py o - Pmng o) / QLung o Equation 3

Where:

- T ₀ is any time in the inhalation phase during which Cm _ng was estimated.

P _{y o} is the magnitude of the pressure measured at the distal end of the patient's airway at time = T ₀ ;

QLung _o is the magnitude of the flow into the patient's lungs at time = T ₀ . PLung o is the magnitude of the pressure estimated at the Lung at time = T ₀ , itself dependent of the Cmng estimate as seen in the equation below:

PLung o Pyboi + (V _Lu ng_o)/ C _Lu ng Equation 4a

PLung o Pyeoi " (V _Lu ng_o)/ Cmng Equation 4b

Where:

VLung o ⁼ if equation 4a is used I _TO ° ¹ QLung * dtj if equation 4b is used

The example below illustrates the estimation process.

A lung simulator system configured with a compliance of 60ml/cm and a 20 cmH20/L/sec linear resistance was used to obtain the data required to calculate Cmng and P Lung. Pressure Control with a pressure target of 20 cmH20, a PEEP of 0 cmH20, an inspiratory time of 1 sec and a rise time of 500 msec were used to generate the data. The flow, Pressure and Volume signal traces for the breath in which the estimations took place appear in the figure below.

Fig. 1 schematically illustrates a graph relating Py, QL and PLung.

Fig. 2 schematically illustrates a graphs for illustrating the principle upon which this invention is based, Fig. 2 depicts the signals in the area where the lung flow crosses zero at the onset of the biological exhalation.

Note that the PLung and the Py signals have the same magnitude at the points where QLung crosses or is equal to zero (see also the beginning of the biological inhalation in the first figure).

From the data generated, the value for Pyboi was found to be equal to 0 (zero) and the value of Pyeoi = 10.654 cmH20 was found using equation 2 with Pyl = 10.95, Py2 = 10.53, QLungl = 1.548, QLung2 = -0.976. Then Delta Py = 10.654 and using equation 2 with Pyl and Py2 substituted by Vlungl= 639.2 ml and Vlung2 = 639.3 ml then VLungo = ( VLung2*QLungl - VLungl *QLung2 )/(Ql- Q2) = 639.2703 ml. Thus CLung = 60.005 ml/cmH20 is found using equation 1. Note that the error is less than 0.5% which is quite acceptable and while in this particular case the sample interval was 0.005 sec, if the sample interval is made much smaller, the use of equation 2 is probably not required and either Pyl or Py2 could be used in the CLung calculation and the same goes for VLungl and VLung2. Py, QLung and VLung data at several points within the inhalation phase were used in conjunction with the Clung estimation found above to be able to estimate PLung, using equation 4a or 4b, and using equation 3 above to calculate RLung at these different points.

The following table 1 illustrates the results.

Table 1 :

Clearly, not only RLung was estimated correctly but also PLung was as well.

Experimental results obtained by ventilation of a glass bottle (Clung = 18.8 ml/cmH20, measured using the syringe method) through a linear resistor (nominal value = 20 cm/L/sec @ 60 1pm), are shown in Figures 3 and 4 and table 2 below.

The results of the compliance estimation are:

- CLung = (VLung = 340.65 ml) / [(Pyeoi = 18.133 cmH20) - (Pyeoi = 0.268 cmH20)] = 19.07 ml/cmH20

This represents an error less than 1.45% which is very good when compared to the specifications for the Static Mechanics Maneuver for the V200 or the PB-840 ventilators, namely ± (1 ml/cmH20 + 20% for true value).

The results for the resistance estimation were calculated using equations 3 and

4a and appear in the table below, at points in time where the flow magnitude is large so that the signal to noise ratio is maximized. Plung es1 imated start ing at Pyboi Acceptabli ϊ range

Time Flow Riung meassured Riung estimate Lower Higher

(sec) (1pm) (cmH20/L/sec) (cmH20/L/sec) Limit Limit

0.10 23.52 20.19 20.35 13.1558 27.23369

0.20 31.12 21.81 22.33 14.45185 29.17778

0.30 30.25 21.81 23.13 14.44858 29.17287

0.40 26.82 20.74 23.06 13.59594 27.89391

0.50 21.70 21.39 24.66 14.10937 28.66405

0.60 16.72 21.12 26.13 13.89811 28.34717

Again, these results are very good. The acceptable range that appears in the table is based on the specifications for the Static Mechanics Maneuver for the V200 or the PB-840 ventilators, namely ± (3 cmH20/L/sec + 20% for true value), which does not apply for the PB-840 when CLung < 5 ml/cmH20 or when the Peak flow is < 20 1pm.

Note that the experimental results for Riung measured were obtained by measuring the pressure delta that existed between the Py and the PLung pressure sensing sites and dividing the delta by the flow.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.