Patent Description:
First, a brief introduction about how inspiration occurs and how a ventilatory assist affects lung expansion will be provided.

Inspired lung volume or inflation of lungs is determined by the pressure distending the lungs, which is called the transpulmonary pressure PTR, and the mechanical properties of the lung, such as elastance and resistance of the lung. PTR is generated by the respiratory muscles, which through an outward action acts to expand the lungs. In respiratory failure, increased load or inspiratory muscle weakness results in an inability to adequately ventilate the lungs such that ventilation will become inefficient. In spontaneously breathing patients, addition of mechanical ventilation (artificial respiration) is used to aid the (presumably weak) respiratory muscles to overcome the increased inspiratory load. The level of ventilatory assist is currently determined rather arbitrarily with a major focus to restore adequate blood gases. In spontaneously breathing patients, the ventilatory assist level should be high enough to ensure that adequate ventilation can take place, however, one should avoid too high levels of ventilatory assist since this may result in disuse atrophy of the inspiratory muscles. There are currently no methods available to monitor and ensure that ventilatory assist levels are adequate.

Using a neurally controlled ventilator, that is a ventilator that responds to patients' neural effort in both time (triggering and termination of assist) and space (magnitude of assist) (as disclosed in <CIT>, granted to <NPL>"), ventilatory assist is uniquely synchronized to patient effort and the mechanical ventilator could be considered as an additional artificial inspiratory breathing muscle under the influence of the brain's respiratory centers and neural respiratory feedback systems. Given the neural integration of such a system, it is not possible to set the assist or ventilation to too high values. Consequently, the system can unload muscles, improve ventilation to levels that are preferred by the patient's respiratory centers. However, a neurally controlled ventilator system resists "over assist" of the patient. Therefore, muscle unloading only takes place by overcoming inertia, elastic and resistive loads. Unlike the conventional systems ("not neurally controlled in time and space"), it is not possible to hyperventilate to very low breathing frequencies or apnea, such that the respiratory drive and respiratory muscle activity, due to chemo receptor influence, will always persist (by <NPL>).

In unhealthy lungs, some air sacs may collapse, meaning that in those collapsed sacs, gas cannot enter or leave them, thus preventing gas exchange through the collapsed air sacs; in this case, a ventilator will supply a higher concentration of oxygen in order to provide proper blood oxygenation. Also, a ventilator can supply positive end-expirtory pressure (PEEP) to recruit or maintain airways open.

During the inflation process of the lungs, by increasing the transpulmonary pressure PTR, the collapsed air sacs will start to open up. When the collapsed air sacs start to open up, they are said to be recruited and the pressure at which the recruitment happens is called the critical opening pressure. However, continuing to increase the transpulmonary pressure PTR will lead to overinflation, which can be dangerous for the patient since it may cause lesions in the lung tissues, which will lead to air leakage out of the lung.

Furthermore, underinflation may also cause problems, such as atelectasis, when the recruited air sacs are de-recruited at a pressure threshold referred to as the critical closing pressure. Therefore, proper pressure provided by the mechanical ventilator should fall inside the thresholds of overinflation and underinflation pressures. In <CIT>, granted to <NPL>", a method and apparatus for controlling the ventilation pressure are disclosed. By increasing incrementally the pressure, the lung volume is measured and then compared to a previous volume measure. If the increase in the lung volume is higher than <NUM>% when compared to the past value, then the critical opening pressure has been reached. Therefore, the ventilatory apparatus will stop increasing the pressure. To measure the critical closing pressure, the pressure in the lungs is decrementally decreased and, at each decremental decrease, the lung volume is measured and then compared to the previous value. If a change in the volume of more than <NUM>% is observed, then it means that the critical closing pressure has been reached. And the mechanical ventilatory assist machine stops decreasing the pressure. This method presents the drawback of depending on very slow inflations to measure a static pressure.

In <CIT>, and entitled "A Method for Examining Pulmonary Mechanics and a Breathing Apparatus System", a method and apparatus for examining the pulmonary mechanics in a respiratory system is disclosed. More specifically, the apparatus determines a flow, volume and pressure of the gas streaming through the respiratory system. Furthermore, the apparatus compares the measured/determined flow, volume and pressure with reference values set by an operator and then produces an error signal for adjusting accordingly the apparatus. This method depends on oscillations in patients who are not breathing spontaneously.

In <CIT>, and entitled "Target Drive Ventilation Gain Controller and Method", a device for adjusting the degree of inspiratory assist, in relation to the patient's respiratory drive, representing a real need of the patient, is disclosed. This device first detects a signal representative of a respiratory drive, then compares this signal to a target drive and finally adjusts the gain of a controller of a lung ventilator in order to control the lung ventilator in relation to the respiratory drive. However, such a method of controlling inspiratory proportional pressure assist ventilation requires no knowledge of the mechanics of the lung, such as its elastance and resistance. <CIT> discloses a device for controlling positive pressure assist during expiration.

Therefore, until now, no dynamic measurements of the mechanics of the lungs have been proposed, using a respiratory neural drive for controlling a ventilator assist.

An object of the present invention is therefore to provide a device for determining dynamically respiratory features in spontaneously breathing patients receiving mechanical ventilatory assist.

The present invention is concerned with a device for determining dynamically a respiratory feature in a spontaneously breathing patient receiving mechanical ventilatory assist. The device comprises: a ventilator for applying mechanical ventilatory assist to the patient, a controller of the ventilator for modifying a level of mechanical ventilatory assist to the patient, an airway pressure detector for measuring an airway pressure and detecting a change of gradient of the measured airway pressure, and a calculator, connected to the airway pressure detector, for determining the respiratory feature based on the airway pressure measured upon detecting the change of gradient of the measured airway pressure.

The present invention still further relates to a device for determining a respiratory feature in a spontaneously breathing patient receiving mechanical ventilatory assist. The device comprises: a ventilator for applying mechanical ventilatory assist to the patient, a controller of the ventilator for modifying a level of mechanical ventilatory assist to the patient, a respiratory neural drive detector for measuring a respiratory neural drive and for detecting a lowest level of the measured respiratory neural drive, and a calculator for determining the respiratory feature based on the detected lowest level of respiratory neural drive.

The foregoing and other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of an illustrative embodiment thereof, given by way of example only with reference to the accompanying drawings.

Generally stated, the following description is concerned with a non-restrictive illustrative embodiment of the present invention for:.

in spontaneously breathing patients who receive mechanical ventilatory assist.

Furthermore, this non-restrictive illustrative embodiment according to the present invention also pertains to the measurements of:.

in a patient who is breathing, for example, on a neurally controlled ventilator, which delivers pressure in proportion to the inspiratory activity or effort. This can be performed by, for example, using a mechanical ventilator controlled by neural drive (diaphragm electrical activity EAdi) as outlined in <CIT>, or using a mechanical ventilator that delivers ventilatory assist between the beginning and end of the diaphragm electrical activity EAdi as described in <CIT>.

As stated hereinabove, the transpulmonary pressure PTR, which represents the pressure required for distending the lungs, is given by the difference between the airway pressure Paw and pleural pressures, the latter being usually measured through an esophageal balloon (Pes). Thus, the transpulmonary pressure PTR equals the airway pressure Paw minus the pleural pressure, which is typically estimated by Pes, so that PTR= Paw - Pes. During a non-assisted inspiration (i.e. without a mechanical positive pressure ventilator for example), the lung distending pressure (i.e. the gradient of transpulmonary pressure acting to expand the lungs) is similar to Pes - atmospheric pressure, i.e. change in PTR = change in Pes. If a positive pressure ventilator is applied to the airways, then the transpulmonary pressure PTR can be calculated from the difference Paw - Pes.

Generally stated, the non-restrictive illustrative embodiment of the present invention is based on a progressive increase of ventilatory assist starting from a zero level (or small level) to a high level. The increase of assist is preferably linear but can also be arbitrary or follow a non-linear function. When ventilatory assist from a mechanical ventilator is controlled using diaphragm electrical activity EAdi as outlined in <CIT>, the ventilatory assist, which corresponds to the pressure Paw delivered to the patient, is obtained by multiplying the diaphragm electrical activity EAdi with a proportionality constant γ such that Paw=EAdi ● γ. Hence, an increase of the proportionality constant γ when/if EAdi remains constant, increases the ventilatory assist (i.e. increases the pressure Paw in the respiratory circuit). If EAdi is decreasing when the proportionality constant γ is increased, then the rate at which Paw increases will decrease.

More specifically, <FIG> illustrates a system <NUM> according to the non-restrictive illustrative embodiment of the present invention. The system <NUM> comprises a mechanical ventilator <NUM>, a controller <NUM>, an EAdi level detector <NUM>, a Paw detector <NUM> and an elastance and resistance calculator <NUM>.

The ventilator <NUM> can be the mechanical ventilator disclosed in <CIT>, whose ventilatory assist is controlled as a function of a respiratory neural drive such as EAdi (electrical activity of the diaphragm). Furthermore, the ventilator <NUM> is connected to the controller <NUM> for adjusting the degree of assist from the ventilator <NUM>, through the above described proportionality constant γ. By increasing the proportionality constant γ, the degree of ventilatory assist is increased and vice versa if the proportionality constant γ is decreased.

When ventilatory assist is applied to a patient (not shown), the EAdi level detector <NUM>, which is connected both to the patient and the ventilator <NUM>, detects and records the EAdi level in response to ventilatory assist. The EAdi level detector <NUM> is further designed to detect a lowest level of EAdi, meaning that it can determine the point where the EAdi level reaches a plateau and the value of this plateau. From this level on, EAdi becomes insensitive to an increase of ventilatory assist, meaning that EAdi is no longer decreasing even though the level of assist may still be increasing. At this level of lowest EAdi, Pes is close to zero, therefore Paw = PTR.

The Paw detector <NUM> is connected to the EAdi level detector <NUM>. The Paw detector <NUM> detects a change in the gradient of the airway pressure Paw supplied by the ventilator <NUM>. When a change in the gradient of Paw is detected, then it means that respiratory unloading has satisfied respiratory centers.

Once the lowest level of EAdi and a change in the gradient of Paw are determined, the mechanics of the lungs such as elastance and resistance can be calculated by the calculator <NUM>.

Now, turning to <FIG>, a method <NUM> for physiological determination of respiratory unloading is described, using the system <NUM> of <FIG>.

At the beginning, the proportionality constant γ is set to a small value, in operation <NUM>. It can be set to zero, for example.

In operation <NUM>, the proportionality constant γ is then increased by a certain increment by the controller <NUM> of <FIG>, or in a linear manner. The value and nature of the increment can depend on many factors such as the health of the patient and the category of the patient (infant, child, adult, etc.). As a consequence of the increase of the proportionality constant γ, the ventilatory assist also increases to a higher level. Therefore, there is an increase in Paw associated with increasing the proportionality constant γ.

At some point in time, corresponding to a certain level of increase of the proportionality constant γ, the rate of increase in Paw will slow down and/or reach a new gradient or plateau. Therefore, in operation <NUM>, a change in the gradient of Paw is checked, through for example the Paw detector <NUM> of <FIG>.

If no change in the gradient of Paw is detected, then the method <NUM> goes back to operation <NUM> for continuing to increase the proportionality constant γ through the controller <NUM> of <FIG>.

If a change in the gradient of Paw is detected in operation <NUM>, then Paw is measured and recorded in operation <NUM>.

When the gradient of Paw changes, this indicates that the rate of decrease of EAdi has increased so as to reduce the increase of Paw. This also suggests that the level of ventilatory assist satisfies the muscular receptors sensitive to muscle load and pulmonary receptors responsive to lung recruitment/stretch. The point where the rate of increase of Paw (gradient) changes or reaches a plateau can be determined, for example, by visual inspection, or by algorithms for calculating the rate of increase of Paw or by applying trigonometric functions. Such functions can be implemented in the Paw detector <NUM>.

Hence, the value of Paw and the proportionality constant γ observed at the point where the rate of increase of Paw (gradient) changes or reaches a plateau indicate a level of ventilatory assist that is likely to satisfy the patient's need for respiratory muscle unloading, as determined by the patient respiratory centers. It should be noted that repeated titrations would increase the reliability of the measurements of Paw. Also, if the patient improves his/her respiratory function or capability of breathing by himself/herself, one would expect a lower level of Paw at the point where the rate of increase of Paw (gradient) changes or reaches a plateau and vice versa if the patient's respiratory function is deteriorated.

According to an alternative implementation of the non-restrictive embodiment of the present invention, the point where the rate of increase of Paw (gradient) changes or reaches a plateau can also be determined by starting with a high assist/high proportionality constant γ and then reducing the assist/proportionality constant γ until a change of gradient of Paw is observed. This alternative method <NUM> is illustrated in <FIG>.

At the beginning, in operation <NUM>, the proportionality constant γ is set to a high value. This value can be easily determined by a person of ordinary skill in the art and will depend on parameters such as the health of the patient and the patient's category (infant, child, adult, etc.).

Then, in operation <NUM>, the proportionality constant γ is decreased, for example in a linear manner.

In operation <NUM>, a change in the gradient of Paw is checked. If there is no change in the gradient of Paw, then the method <NUM> goes back to operation <NUM> for continuing to decrease the proportionality constant γ.

If a change in the gradient of Paw occurs, then in operation <NUM>, the current value of Paw is recorded and represents the level of pressure required for respiratory muscle unloading. Also, the current value of the proportionality constant γ is stored and represents the level of the proportionality constant γ that satisfies muscular receptory sensitive to muscle load (muscles no longer need to work).

Now, turning to <FIG>, a method <NUM> for determining the respiratory lung mechanics is described.

In accordance with the non-restrictive illustrative embodiment of the present invention, in operation <NUM>, the method <NUM> carried out by the device of the invention, starts with a low proportionality constant γ, for example, it can be set to zero (γ=<NUM>). In the case where γ=<NUM>, it means that no ventilatory assist is delivered to the patient. If the patient's response is normal, then an absence of assist will result in an increased/high respiratory drive and high level of diaphragm electrical activity (i.e. high level of EAdi).

The proportionality constant γ is then increased in operation <NUM>, for example by a given increment or in a linear manner. In consequence, EAdi decreases as illustrated in the upper curve of <FIG>. Furthermore, <FIG> shows an example of EAdi and Paw during titration with a linearly increasing proportionality constant y starting from zero (<NUM>).

More specifically, as illustrated in <FIG>, as the proportionality constant γ is increased, the airway pressure Paw will increase at a rate determined by the rate of increase of the proportionality constant γ and the EAdi response. Typically, the dynamics between these different variables are as follows: EAdi will first remain at a high level or slowly decrease, and Paw will increase at a relatively fast rate. At some point, EAdi will decrease at a more rapid rate, thus slowing the rate of increase in Paw.

In operation <NUM>, the EAdi level detector <NUM> (see <FIG>) determines if the level of EAdi has reached a plateau or not.

If the level of EAdi has not reached a plateau yet, the method <NUM> goes back to operation <NUM> in order to continue to increase the proportionality constant γ. Indeed, continuing to increase the proportionality constant γ will at some point cause the neural drive EAdi to reach a lowest level of neural drive where it will plateau.

If the level of EAdi has reached a plateau, then in operation <NUM>, a value for Paw is obtained. The point of lowest level of neural drive EAdi most likely represents the point where the respiratory work load is compensated as indicated by the abolished inspiratory Pes in <FIG>. In other words, the pressure required to inflate the lungs is provided via the mechanical ventilator.

The lowest level of the neural drive EAdi is determined at the point where EAdi stops to decrease, which is referred to as the lowest EAdi level in the present specification. This lowest EAdi level represents the case where the mechanical ventilatory assist replaces the inspiratory muscles' work to expand the lungs.

Hence, when the lowest EAdi level has been reached, in operation <NUM>, the pressure delivered by the ventilator (Paw) has eliminated the patient's own inspiratory pressure generation (i.e. the deflections in Pes are close to zero) such that Paw represents the transpulmonary pressure PTR (i.e. the pressure for distending the lungs), so that Paw = PTR.

<FIG> shows an example of curves corresponding to EAdi flow, volume, Paw and Pes at the lowest EAdi level.

Given that inspired lung volume is determined by the transpulmonary pressure PTR and the mechanical properties of the lungs, it is possible to calculate the resistive and elastic mechanical properties of the lungs. Therefore, in operation <NUM>, using Paw, which is equal to the transpulmonary pressure PTR at the level of lowest EAdi, dynamic elastance and resistance of the lungs are calculated, through the calculator <NUM> of <FIG>, for example.

As a non-limitative example, to determine dynamic elastance of the lungs, volume and transpulmonary pressure PTR can be used. An example of curves of the volume in function of Paw is illustrated in <FIG> and <FIG>. It should be noted that the same curves as those in <FIG> can also be obtained for only expiration to determine expiratory mechanics.

To determine dynamic resistance of the lungs, flow and transpulmonary pressure PTR can be used. An example of curves of the flow in function of Paw is illustrated in <FIG> and <FIG>. It should be noted that the same curves as those in <FIG> can also be obtained for only expiration to determine expiratory mechanics.

Several methods can be used to calculate lung elastance (or compliance) and resistance. They can be implemented in the calculator <NUM> of <FIG>.

For example, elastance can be estimated by measuring the inspiratory transpulmonary pressure swing and the corresponding lung volume during an inspiration and then calculate the pressure to volume ratio. Inspiratory resistance can be obtained by calculating a ratio between the inspiratory transpulmonary pressure swing and the flow rate during, for example, mid-inspiratory volume of an inspiration.

Another example for calculating lung elastance consists of applying a multiple linear regression analysis using the transpulmonary pressure PTR as the dependent variable and flow and volume as the independent variables; regression coefficients for flow and volume can then be determined. Hence, it is possible to calculate the amount of pressure necessary to generate a given volume and obtain a value representative of elastic properties of the lungs or calculate how much volume is obtained for a given transpulmonary pressure PTR (i.e. the compliance of the lungs). From the same regression analysis, it is also possible to calculate the amount of transpulmonary pressure PTR necessary to generate a given flow (i.e. resistive components of the lungs).

Since the pleural/esophageal pressures are negligible when the assist level has been increased to the level where the lowest EAdi level is reached, Paw is then similar to the transpulmonary pressure PTR at the lowest EAdi level. Therefore, Paw can be used to calculate the lung mechanics in operation <NUM>.

It should be noted that once it is confirmed that the level of EAdi has reached its lowest level, the titration should be discontinued in order to avoid over-assist.

Based on the results obtained by using the methods <NUM>, <NUM> and <NUM> as described in <FIG>, <FIG> or <FIG>, variable determination, such as elastance and resistance of the lungs can then be determined for different levels of PEEP, as will be described hereinbelow. Indeed, the lowest level of EAdi and the value of Paw at the point when its gradient changes allow for dynamic measurements.

Now, referring to <FIG>, a method <NUM> for determining a level of PEEP associated to the least impaired level of respiratory mechanics is described.

At the level of ventilatory assist representative of the lowest EAdi level, different levels of positive end-expiratory pressure (PEEP) can be applied through an expiratory valve (not shown) for example. Increasing PEEP acts such as to distend the lungs and is clinically used to keep the airways open (i.e. to avoid lung collapse/atelectasis). If the lungs collapse, then more transpulmonary pressure PTR is required during each inspiration since the collapsed airways need to open up. This will, for example, increase the dynamic elastance of the lungs, and thus the lungs are less compliant.

If PEEP is applied to a level that prevents collapse of the airways, then less transpulmonary pressure PTR will be needed to generate a given inspiratory volume i.e. the elastance of the lungs is increased (the lungs are more compliant).

If the PEEP level is further increased, the further expansion of the lungs will make the lungs stiffer such that more pressure will be needed to generate a given inspiratory volume, and thus the lungs are less compliant.

By applying PEEP at various levels, at the level of ventilatory assist that represents the lowest EAdi level, the elastic and resistive properties of the lungs can be dynamically determined as described above for each level of PEEP applied.

Therefore, the method <NUM> of <FIG> starts by applying a PEEP of lower level, for example, through an expiratory valve (not shown), in operation <NUM>, at the level of ventilatory assist (pressure Paw) representative of the lowest EAdi level found in operation <NUM> of <FIG>, by using the corresponding value of the proportionality constant γ.

Then the level of PEEP is increased, for example, linearly, in operation <NUM>.

In operation <NUM>, for the level of PEEP as determined in operation <NUM>, the elastic properties of the lungs are calculated, using methods well known to those of ordinary skill in the art and using the calculator <NUM> of <FIG>, for example. This value of the PEEP is recorded.

In operation <NUM>, for the same level of applied PEEP, the dynamic resistive properties of the lungs are calculated, using methods well known to those of ordinary skill in the art and using the calculator <NUM> of <FIG>, for example. This value of the PEEP is recorded.

Then, in operation <NUM>, it is checked to see if the level of applied PEEP has reached a higher level, which can be determined by a clinician or a person of ordinary skill in the art, according to the needs of each patient.

If the level of applied PEEP has not reached yet the higher level, then the method <NUM> goes back to operation <NUM> in order to continue to increase, for example, linearly, the level of PEEP to an increased level and then to calculate the dynamic elastic and resistive properties of the lungs (respectively operations <NUM> and <NUM>).

If the level of PEEP has already reached the higher level of operation <NUM>, then, in operation <NUM>, it is possible to determine which level of PEEP, among the different increased levels of PEEP, is associated with the least impaired respiratory mechanics by comparing the different calculated values of elastic and resistive properties. In other words, by comparing elastance and resistance values at various levels of PEEP at the level of ventilatory assist that represents the lowest EAdi level, one can determine which PEEP level is associated with the lowest level of elastic and resistive loads. Generally, the level of PEEP associated with the lowest elastic and resistive loads is the one that is most likely related to ideal lung recruitment.

Referring now to the flow chart in <FIG>, a method <NUM> for quantifying a respiratory drive is described.

As stated before, since the lowest EAdi level that can be reached with increasing levels of assist/proportionality constant γ corresponds to a level where the lung distending pressure generated by the patient is eliminated, the pressure delivered by the ventilator (Paw) then represents the transpulmonary pressure PTR (i.e. the pressure distending the lungs). Hence, the mechanical load necessary to inflate the lungs is abolished and one can assume that the respiratory drive at this level of ventilatory assist is little affected by respiratory load or respiratory muscle weakness but mainly influenced by metabolism, blood gases, and patients comfort level and similar variables.

Moreover, it is assumed that the point where the rate of increase in Paw (gradient) changes or reaches a plateau represents the level of assist/proportionality constant γ that satisfies muscular receptors sensitive to muscle load and pulmonary receptors responsive to lung recruitment/stretch.

Hence, by calculating a difference or ratio in EAdi between the point of lowest level of EAdi and the point where the rate of increase in Paw (gradient) changes or reaches a plateau, the amount of respectively absolute or relative increase in EAdi contributed by the respiratory mechanical load is determined.

It should be noted that breathing frequency and tidal volume do normally not change between these points.

More specifically, the method <NUM> is based on the lowest EAdi level determined in operation <NUM>, in the method <NUM> of <FIG>. The method <NUM> also uses the value of Paw determined in operation <NUM> of the method <NUM> of <FIG> or in operation <NUM> of the method <NUM> of <FIG> when its gradient changes.

In operation <NUM>, a level of EAdi corresponding to the point when the gradient of Paw changes is determined, as illustrated in <FIG>.

In operation <NUM>, a difference or ratio is calculated between the lowest level of EAdi and the EAdi level corresponding to a change in gradient of Paw. As mentioned hereinabove, this difference or ratio allows to express the amount of absolute or relative increase in EAdi contributed by the respiratory mechanical load. The difference or ratio in EAdi can be calculated through a calculator (not shown), for example.

Therefore, the amount of absolute or relative increase in EAdi allows for quantification of respiratory drive and partition of the respiratory drive into chemical/habitual drive and load related drive.

Claim 1:
A device (<NUM>) for determining dynamically a respiratory feature in a spontaneously breathing patient receiving mechanical ventilatory assist, comprising:
a ventilator (<NUM>) for applying mechanical ventilatory assist to the spontaneously breathing patient;
a controller (<NUM>) of the ventilator (<NUM>) for modifying a level of mechanical ventilatory assist to the spontaneously breathing patient; and
a respiratory neural drive detector (<NUM>) for measuring a respiratory neural drive (EAdi) of the spontaneously breathing patient and for detecting a lowest level of the measured respiratory neural drive (EAdi);
an airway pressure detector (<NUM>) for measuring an airway pressure (Paw) corresponding to the detected lowest level of the measured respiratory neural drive (EAdi), wherein the measured airway pressure (Paw) corresponding to the detected lowest level of the measured respiratory neural drive (EAdi) represents a transpulmonary pressure for distending lungs; and
a calculator (<NUM>) for determining the respiratory feature of the spontaneously breathing patient,
wherein the calculator (<NUM>) uses the measured airway pressure (Paw) corresponding to the detected lowest level of the measured respiratory neural drive (EAdi) for determining the respiratory feature, and
wherein the respiratory feature is selected from elastance of a lung and resistance of a lung.