US 20090107502 A1
Method and apparatus for non-invasively determining the time onset (Tonset) and end (Tend) of patient inspiratory efforts. A composite pressure signal is generated comprising the sum of an airway pressure signal, a gas flow pressure signal obtained by applying a gain factor (Kf) to a signal representing gas flow rate and a gas volume pressure signal obtained by applying a gain factor (Kv) to a signal representing volume of gas flow. Kf and Kv values are adjusted to result in a desired linear trajectory of composite pressure signal baseline in the latter part of the exhalation phase. The current composite pressure signal is compared with (i) selected earlier composite pressure signal values and/or (ii) value expected at current time based on extrapolation of composite pressure signal trajectory at specified earlier times and/or (iii) the current rate of change in the composite pressure signal with a selected earlier rates of change. The differences obtained by the comparison are compared with selected threshold values. Tonset is identified when at least one of the differences exceeds the threshold values.
20. A method for cycling off the inflation phase of a mechanical ventilator comprising:
measuring the average interval between patient inspiratory efforts in a patient in a suitable number of elapsed breaths (TTOT) with said average being updated at suitable intervals;
identifying onset of current inspiratory effort;
monitoring time from said onset of inspiratory effort; and
generating a signal that causes the ventilator to cycle off when time elapsed since onset of inspiratory effort exceeds a specified fraction of TTOT.
21. The method of
22. A method for cycling off the inflation phase of a ventilator in pressure support ventilation comprising:
measuring the interval between successive inspiratory efforts in a suitable number of elapsed breaths (TTOT);
measuring inspiratory flow rate at specified times in those elapsed breaths which triggered ventilator cycles, said specified times corresponding to a specified fraction of the TTOT, measured from the onset of inspiratory effort of said each breath or from the trigger time of the ventilator;
calculating the average of the flow values obtained at said specified times in said elapsed breaths; and
generating a signal that causes the ventilator to cycle off when inspiratory flow in the current inflation phase decreases below said average flow value.
23. The method of
38. A device for estimating a desirable duration of inflation phase of a ventilator, comprising:
circuitry to identify inspiratory efforts of a patient;
means to calculate the time difference between patient inspiratory efforts (patient TTOT);
means for displaying a value corresponding to a specified fraction of patient TTOT, said specified fraction being a user input or a default value between 0.3 and 0.5.
39. The device of
40. The device of
41. The device of
42. A device for determining the desirable inspiratory flow threshold for terminating inflation cycles in the pressure support ventilation mode comprising:
circuitry for estimating desirable duration of inflation phase of the ventilator;
means to measure inspiratory flow in recently elapsed breaths after said desirable duration has elapsed from the ventilator's trigger time, or from the onset of inspiratory effort preceding triggered breaths, or from a specified point in between the latter two points; and
means for displaying the value of said measured flow.
43. The device of
44. The device of
45. The device of
46. The device of
This application claims priority under 35 USC 119(e) from U.S. Provisional Patent Application No. 60/391,594 filed Jun. 27, 2002.
This invention relates to assisted mechanical ventilation.
With assisted ventilation (e.g. assist volume cycled ventilation, pressure support ventilation and proportional assist ventilation) ventilator cycles are triggered by the patient and are intended to coincide with patient's inspiratory effort. In practice, however, the ventilator cycle never begins at the onset of patient's inspiratory effort (trigger delay) and the end of the ventilator's inflation phase only rarely coincides with the end of inspiratory effort (cycling-off errors).
Non-synchrony between patient and ventilator is extremely common. Leung et al found that, on average, 28% of patient's efforts are ineffective (Leung P, Jubran A, Tobin M J (1997). Comparison of assisted ventilator modes on triggering, patient effort, and dyspnea. Am J Respir Crit Care Med 155:1940-1948). Considering that ineffective efforts are the extreme manifestation of non-synchrony, less severe, yet substantial (e.g. first two breaths,
In current mechanical ventilators, triggering occurs when flow becomes inspiratory (i.e. >0) and exceeds a specified amount, or when airway pressure decreases below the set PEEP (positive end-expiratory pressure) level by a specified amount. Trigger delay has two components. One component is related to ventilator trigger response and sensitivity. Thus, if the response of the ventilator is poor, triggering may not occur immediately when the triggering criteria are reached. Alternatively, the threshold for triggering may be set too high by the user. The component of trigger delay attributable to ventilator response and sensitivity is given by the interval between zero flow crossing (arrow,
The second component of trigger delay is the time required, beyond the onset of inspiratory effort (Tonset), for expiratory flow to be reduced to zero (interval between first vertical line and the arrow,
Cycling-off errors result from the fact that, except with Proportional Assist Ventilation, current ventilator modes do not include any provision that links the end of ventilator cycle to end of the inspiratory effort of the patient. In the most common form of assisted ventilation, Volume Cycled Ventilation, the user sets the duration of the inflation cycle without knowledge of the duration of patient's inspiratory effort. Thus, any agreement between the ends of ventilator and patient inspiratory phases is coincidental. With the second most common form, Pressure Support Ventilation, the inflation phase ends when inspiratory flow decreases below a specified value. Although the time at which this threshold is reached is, to some extent, related to patient effort, it is to the largest extent related to the values of passive resistance and elastance of the patient. In patients in whom the product [resistance/elastance], otherwise known as respiratory time constant, is high, the ventilator cycle may extend well beyond patient effort, while in those with a low time constant the cycle may end before the end of patient's effort (Younes M (1993) Patient-ventilator interaction with pressure-assisted modalities of ventilatory support. Seminars in Respiratory Medicine 14:299-322; Yamada Y, Du H L (2000) Analysis of the mechanisms of expiratory asynchrony in pressure support ventilation: a mathematical approach. J Appl Physiol 88:2143-2150).
In U.S. Pat. No. 6,305,374 B1, an approach is described to identify the onset and end of patient's inspiratory effort during non-invasive bi-level positive pressure ventilation (BiPAP). This approach relies exclusively on the pattern of flow waveform to make these identifications. Thus, current values of flow are compared with an estimated value based on projections from preceding flow pattern. If the difference exceeds a preset amount, a phase switch is declared. While this method may yield reasonably accurate results in the intended application (treatment of obstructive sleep apnea patients with non-invasive BiPAP), a number of considerations suggest that its use in critically ill, intubated, ventilated patients may not provide accurate results:
1) Implicit to the use of flow as a marker of respiratory muscle pressure output is the assumption that flow pattern reflects changes in alveolar pressure inside patient's lung. This is where respiratory muscle pressure is exerted. This assumption, however, is true only if airway pressure is constant. Since airway pressure is one of the two pressure values that determine flow (flow-(airway pressure-alveolar pressure)/resistance), it is clear that changes in airway pressure can alter flow even if there is no change in respiratory muscle pressure. In non-invasive bi-level support, airway pressure, one of the two pressure values that determine flow, is reasonably constant during both inspiration and expiration, even though the absolute level is different in the two phases. If one of the two pressure values is constant during a given phase, it is reasonable to assume that changes in flow during that phase reflect changes in the other pressure, namely alveolar pressure. This condition does not apply in intubated, mechanically ventilated patients. In most modern intensive care ventilators, airway pressure is actively controlled during expiration through adjustments of the PEEP/exhalation valve mechanism. The pattern of such active changes in airway pressure during expiration varies from one ventilator brand to another and in the same ventilator from time to time depending on the state of the PEEP/exhalation valve mechanism. Under these conditions, changes in flow trajectory during expiration cannot be assumed to reflect changes in alveolar pressure trajectory. Likewise, during inspiration airway pressure is far from being constant, regardless of the mode used. Thus, changes in inspiratory flow profile cannot be used to reflect similar changes in alveolar pressure. The use of flow to infer end of effort during the inflation phase is accordingly not plausible.
2) When passive elastance (E) and resistance (R) are constant over the entire tidal volume range, the product R/E, or respiratory time constant, is also constant over the entire period of expiration. Because the time constant governs the pattern of lung emptying, a constant R/E produces a predictable exponential flow pattern in the passive system. With a predictable pattern it is possible to make forward extrapolations, or predictions, for the sake of identifying a deviation from the expected passive behaviour. Such deviation may then be used, with reasonable confidence, to infer the development of an additional active force, such as the onset of inspiratory muscle effort. When E and R are not constant throughout the breath, R/E may change from time to time causing changes in flow trajectory (Δflow/Δt) that are not related to muscle pressure. Under these conditions, deviation in Δflow/Δt from previous values cannot reliably signify a change in pressure generated by respiratory muscles. Patients with obstructive sleep apnea, the intended population of U.S. Pat. No. 6,305,374 B1, have generally normal lungs; R and E are expected to be constant over the tidal volume range, particularly when expiratory airway pressure is higher than atmospheric (i.e. the usual case when BiPAP is applied). In critically ill, intubated ventilated patients, this is not the case. Resistance is not constant, primarily because these patients are intubated and the resistance of the endotracheal tube is flow-dependent (the higher the flow, the higher the resistance). The relation between resistance and flow varies from one tube to the other. Furthermore, tidal volume in these patients often extends into the volume range where elastance is not constant. Thus, as the lung is emptying, either or both elastance and resistance may be changing, causing changes in respiratory time constant during the same expiration. Under these conditions, changes in flow trajectory need not reflect changes in respiratory muscle pressure. This considerably decreases the sensitivity and specificity of flow pattern as a marker of inspiratory effort.
3) Changes in respiratory muscle pressure (Pmus) are not exclusively used to change flow. According to the equation of motion, specifically applied to intubated patients:
Where, E is passive respiratory system elastance, K1 is the laminar component of passive respiratory system resistance, K2 is the resistance component related to turbulence (mostly in the endotracheal tube), and Paw is airway pressure which is determined by the pressure at the exhalation/PEEP valve (Pvalve), flow and Rex, that is resistance of the exhalation tubing (Paw=Pvalve−flow*Rex). In this equation expiratory flow is negative. When Pmus changes, as at Tonset, the flow trajectory should change. However, a change in flow trajectory also results in changes in volume and Paw trajectories. According to Equation 1, these changes will oppose the change in flow. For example, if expiratory flow decreases at a faster rate, volume decreases at a slower rate than in the absence of Pmus. At any instant after Tonset, elastic recoil pressure, which is related to volume, is higher, and this promotes a greater expiratory flow. The same can be said for the effect of changes in flow trajectory on Paw trajectory; a lower expiratory flow decreases Paw, which promotes more expiratory flow. How much of the change in Pmus is used to change the flow trajectory depends on the magnitude of the opposing forces. In particular, a higher passive elastance and/or a higher Rex tends to reduce the fraction of the change in Pmus used to change flow trajectory. Furthermore, for a given Pmus expended to change the flow trajectory, the actual change in trajectory is determined by resistance (i.e. K1 and K2). When E, Rex K1 and K2 are all low, a modest change in dPmus/dt results in a sharp change in flow trajectory. As these characteristics become more abnormal, the change in flow trajectory, for a given dPmus/dt, progressively is attenuated.
In the example of
The middle waveform (
With more severe mechanical abnormalities (top waveform,
In summary, the use of flow to identify respiratory phase transitions is entirely unsuitable for identification of inspiratory to expiratory transitions during mechanical ventilation in critically ill patients (because of the highly variable Paw during inflation), and has poor sensitivity and specificity for identifying expiratory to inspiratory transitions in these patients because of the frequent use of active exhalation valves, the presence of variable time constant during expiration and the often marked abnormalities in elastance and resistance.
In one aspect, the present invention provides a method for detecting the onset of inspiratory effort (Tonset) in a patient on mechanical ventilation, comprising the steps of:
(a) monitoring airway pressure, rate of gas flow, and volume of gas flow of the patient;
(b) applying a gain factor (Kf) to the signal representing rate of gas flow to convert the gas flow signal into a gas flow pressure signal;
(c) applying a gain factor (Kv) to the signal representing volume of gas flow to convert the gas volume signal into a gas volume pressure signal;
(d) generating a composite pressure signal (signal) comprising the sum of airway pressure signal, gas flow pressure signal, and gas volume pressure signal, with all signals, having a suitably adjusted polarity;
(e) adjusting Kf and Kv to result in a desired linear trajectory of composite pressure signal baseline in the latter part of the exhalation phase;
(g) comparing differences obtained from such comparison(s) made in step (f) with selected threshold values; and
(h) identifying Tonset when at least one of the differences exceeds the threshold values.
The composite pressure signal may contain a fourth component, consisting of the square of the rate of gas flow to which a gain factor (Kf2) is applied to convert the fourth signal to a pressure signal. Kf2 may also be used to adjust the trajectory of the composite pressure signal baseline in the latter part of the exhalation phase. Kf2 may be assigned a value corresponding to the K2 constant of an endotracheal tube in the patient. The values of Kv, Kf and/or Kf2 may be adjusted to result in a specified slope or pattern of the composite pressure signal during part or all of the expiratory phase.
A default value of Kf may be used while the value of Kv is adjusted to obtain a desired baseline composite pressure signal trajectory. Alternatively, a default value of Kv is used while the value of Kf is adjusted to obtain a desired baseline composite pressure signal trajectory.
The Kf or Kv value used may be a known or estimated value of the respiratory system resistance or elastance, respectively, of the patient.
The current composite pressure signal value may be compared with the composite pressure signal value at the most recent point where the composite pressure signal began a new rising phase and Tonset is identified when the calculated difference exceeds a set threshold value.
Tonset detection may be precluded in the early part of the exhalation phase.
The amplitude of the composite pressure signal may be monitored through the inspiratory phase and the end of inspiratory effort (Tend) is identified from a reduction in signal amplitude or signal slope below a specified value, which may be a specified fraction of the highest value obtaining during the inspiratory phase. Tend detection may be precluded in the early part of the inflation phase. The generated signals corresponding to Tonset may be used to trigger ventilation cycles and/or signals corresponding to Tend may be used to cycle off ventilation cycles.
In another aspect of the invention, there is provided a method for detecting the onset of inspiratory effort (Tonset) in a patient on mechanical ventilation, comprising the steps of:
(a) monitoring airway pressure and rate of gas flow of the patient,
(b) applying a gain factor (Kf) to the signal representing rate of gas flow to covert the gas flow signal into a gas flow pressure signal,
(c) generating a composite pressure signal comprising the sum of airway pressure signal and the gas flow pressure signal,
(e) comparing differences obtained from such comparison(s) made in step (d) with selected threshold values, and
(f) identifying Tonset when at least one of the differences exceeds said threshold values.
In this aspect of the invention, the composite pressure signal may incorporate a third component consisting of the square of the rate of gas flow, to which a gain factor (Kf2) is applied to convert the third signal to a pressure signal. The selected Kf may be a known or assumed value of respiratory system resistance.
The generated signal representing Tonset may be used to trigger ventilation cycles.
The present invention further includes, methods for determining a suitable threshold value for identifying the onset of inspiratory effort from the composite pressure signal obtained according to the procedures described above.
In one such method, suitable for use where the composite pressure signal includes the sum of the airway pressure signal, gas flow pressure signal and gas volume pressure signal, and, optionally, the fourth component, comprises:
monitoring the composite pressure signal over suitable intervals preceding onset of inspiratory effort, in a suitable number of elapsed breaths;
identifying peaks and troughs in the composite pressure signal over the duration of the intervals;
measuring the changes in signal amplitude between successive peaks and troughs, the amplitudes reflecting the range of amplitudes of noise included in the composite pressure signal; and
determining from the detected range of noise amplitude, a value that exceeds the prevailing noise value, such value then being used prospectively to distinguish between true inspiratory efforts and noise.
Another such method, suitable for use where the composite pressure signal includes the sum of the airway pressure signal, gas flow pressure signal and gas volume pressure signal, and optionally, the fourth component, or where the composite pressure signal includes the sum of the airway pressure signal and the gas flow pressure signal, and optionally, the third component, comprising:
monitoring the composite pressure signal over suitable interval preceding onset of inspiratory effort in a suitable number of elapsed breaths;
determining slope of the composite pressure signal in successive subintervals within the intervals;
measuring the range of slope in the subintervals, such range reflecting the range of slope change in composite pressure signal related to noise; and
determining from the detected range of slope changes, a difference in slope that exceeds the prevailing noise level, the resulting value then being used prospectively to distinguish between changes in composite pressure signal slope due to inspiratory efforts and those due to composite pressure signal noise.
An alternative to the latter method comprises:
monitoring the composite pressure signal over suitable intervals preceding the onset of inspiratory effort, in a suitable number of elapsed breaths;
comparing signal amplitude at discrete points within such intervals with values predicted to occur at such times from the signal pattern in previous intervals, the difference in signal amplitude reflecting the range of difference related to composite pressure signal noise; and
determining from the detected range of differences, a value that exceeds the prevailing noise level, such value then being used prospectively to identify differences between current and predicted values that reflect true inspiratory effort.
In another aspect of the present invention, there is provided a method for cycling off the inflation phase of a mechanical ventilator, which comprises:
measuring the average interval between successive inspiratory efforts in a patient in a suitable number of elapsed breaths (TTOT);
identifying onset of inspiratory effort by utilizing any of the procedures provided in accordance with the present invention or otherwise;
monitoring the time from the onset of inspiratory effort; and
generating a signal that causes the ventilator to cycle off when time elapsed since onset of inspiratory effort exceeds a specified fraction of TTOT.
The time to generate a signal to cycle off the ventilator may be calculated from the trigger time of current ventilation cycle plus a specified fraction of TTOT.
In a further aspect of the present invention, there is provided a method for cycling off the inflation phase of a ventilator in pressure support ventilation, comprising:
measuring the interval between successive inspiratory efforts in a suitable number of elapsed breaths (TTOT);
measuring inspiratory flow rate at specified times in the elapsed breaths which triggered ventilator cycles, the specified times corresponding to a fraction of the TTOT, measured from the onset of inspiratory effort of each breath or from the trigger time of the ventilator;
calculating the average of the flow values obtained at such specified times in the elapsed breaths; and
generating a signal that causes the ventilator to cycle off when inspiratory flow in the current inflation phase decreases below said average flow value.
The results concerning patient ventilator interaction may be displayed in suitable format, including but not limited to a monitor, digital or electrical output ports, or printed material. Such results may include, but not limited to, display of the composite pressure signal, Tonset and Tend markers and displays regarding trigger delay, cycling-off errors, patient respiratory rate, number and frequency of ineffective efforts, and frequency and duration of central apneas, desirable duration of inflation phase, and flow at a specified fraction of TTOT of the patient in the pressure support ventilation mode.
In accordance with another aspect of the present invention, there is provided an apparatus for detecting the onset of inspiratory effort (Tonset) in a patient on mechanical ventilation, comprising:
circuitry for measuring airway pressure, rate of gas flow and volume of gas flow of the patient;
amplifier to apply a gain factor (Kf) to the signal representing rate of gas flow to convert the signal into a gas flow pressure signal;
amplifier to apply a gain factor (Kv) to the signal representing volume of gas flow to convert the signal into a gas volume pressure signal;
summing amplifier that generates a composite pressure signal comprising the sum of airway pressure signal, the gas flow pressure signal and the gas volume pressure signal, with all signals having suitably adjusted polarity;
means to permit adjustment of Kf and Kv to provide a desired trajectory of composite pressure signal baseline in the latter part of the exhalation phase;
circuitry to direct the composite pressure signal to a Tonset identification circuitry during a suitable period in the expiratory phase, the identification circuitry comprising circuitry to detect a change in trajectory; and
means for generating a signal corresponding to Tonset when measured change in composite pressure signal trajectory exceeds a specified threshold.
In the device of the invention, an additional signal may be generated to be summed by the summing amplifier being generated by multiplying the flow signal by the absolute value of the flow signal and applying a gain factor (Kf2) to the resulting square flow signal using an amplifier and Kf2 is also used to adjust the trajectory of the composite pressure signal baseline in the latter part of the exhalation phase. Kf2 may be assigned a value corresponding to the K2 constant of the endotracheal tube in place in the patient.
The Kf value may be fixed at a default value while adjustment of signal trajectory is made using Kv and/or Kf2. Alternatively, Kv is fixed at a default value while adjustment of signal trajectory is made using Kf and/or Kf2.
In one embodiment of the invention, the summing amplifier input related to volume of flow is omitted.
The device provided herein may include circuitry that precludes Tonset identification during an adjustable period after the end of the inflation phase of the ventilator.
The Tonset identification circuitry may comprise circuitry to obtain the rate of change of composite pressure signal amplitude and to obtain the difference between the current rate of change and the rate of change of the composite pressure signal amplitude at a specified earlier time and to generate a Tonset signal when the difference exceeds a set threshold value.
The Tonset identification circuitry may comprise circuitry to measure the difference between the current composite pressure signal amplitude and the composite pressure signal amplitude at a specified earlier time and to generate a Tonset signal when the difference exceeds a set threshold value.
In the device of the invention, Kv and/or Kf and/or Kf2 may be adjusted to produce a horizontal or slightly downward sloping composite pressure signal baseline in the latter part of expiration and the Tonset identification circuitry may comprise circuitry to measure the difference between current composite pressure signal amplitude and composite pressure signal amplitude at the most recent point where the composite pressure signal began rising and to generate a Tonset signal when the difference exceeds a set threshold value.
The composite pressure signal may be gated to circuitry to identify end of inspiratory effort (Tend), such circuitry comprising:
circuitry to identify the highest amplitude (peak) of the composite pressure signal reached during the current inspiratory effort;
circuitry to detect when amplitude of the composite pressure signal decreases below a specified value beyond the time at which the peak occurred; and
circuitry to generate a signal corresponding to Tend when the amplitude of the composite pressure signal decreases below the specified value, which may be a specified fraction of the peak amplitude of the composite pressure signal. Circuitry may be provided to preclude detection of Tend during a specified period following ventilator triggering.
Signal corresponding to Tonset maybe used to trigger ventilator cycles and/or signal corresponding to Tend may be used to cycle off inflation phases of the composite pressure signal.
The output of the device may be used for closed-loop control of ventilation setting. Functions executed by electrical circuitry may be executed in whole or in part by digital techniques.
In a further aspect of the present invention, there is provided a device for estimating a desirable duration of the inflation phase of a ventilator, comprising:
circuitry to identify inspiratory efforts of the patient, which may be a device according to the invention or by other suitable circuitry;
means to calculate the time difference between patient inspiratory efforts (patient TTOT); and
means for displaying a value corresponding to a specified fraction of patient TTOT, such specified fraction being a user input or a default value between 0.3 and 0.5.
In this device, a signal may be generated to cycle off the inflation phase of the ventilator when the desirable duration has lapsed after ventilator triggering.
A signal may be generated to cycle off the inflation phase of the ventilator when the desirable duration has elapsed after onset of inspiratory effort in current breaths or after a point intermediate between onset of effort and ventilator triggering.
A user input may be provided for inputting patient TTOT or its reciprocal, patient respiratory rate, and the input then is used by the device, in lieu of device-determined patient TTOT, to determine desirable duration of inflation phase.
In an additional aspect of the invention, there is provided a device for determining the desirable inspiratory flow threshold for terminating inflation cycles in the pressure support ventilation mode, comprising:
circuitry for estimating desirable duration of inflation phase of the ventilator, by using the device provided herein or by any other suitable alternative;
means for measuring inspiratory flow in recently elapsed breaths after the desirable duration has elapsed from the ventilator trigger time, or from the onset of inspiratory effort preceding triggered breaths, or from a specified point in between the two points; and
means for displaying the value of said measured flow.
In such device, the value of the measured flow may be communicated to the cycling mechanism of the ventilator to effect termination of the inflation phase when the measured flow, or a reasonable approximate thereof, is reached during the inflation phase.
The values relating to patient ventilator interaction determined in the devices provided herein may be calculated and displayed in suitable format, including but not limited to a monitor, digital or electrical output ports. The values may be any of those discussed above.
The present invention, therefore, concerns a novel method and apparatus to, non-invasively, determine the true onset (Tonset) and end (Tend) of patient's inspiratory efforts. Such method/device can be used simply as a monitor, informing the user of the presence and magnitude of trigger delays, ineffective efforts and cycling-off errors. The user can then take appropriate action to reduce the non-synchrony. Alternatively, the method/device can be coupled with the cycling mechanisms of the ventilator, whereby onset and end of ventilator cycles are automatically linked to onset and end of patient's efforts, thereby insuring synchrony without intervention by the user.
One aspect of the current invention is to minimize the cycling-off errors either by directly identifying the end of patient's inspiratory effort or by insuring that the ventilator's inflation phase does not extend beyond the physiologic limit of the duration of inspiratory effort.
The present invention contemplates novel methods and devices for specific and timely identification of respiratory phase transitions within the patient for use in monitoring patient-ventilator interaction or to effect switching of ventilator cycles. These methods/devices represent a progression in complexity that address the problems inherent in the prior art ventilation procedures described above.
In the simplest of these methods, a signal is generated (signal X) that incorporates changes in both the flow and airway pressure (Paw) signals. Thus,
where, Kf is a constant that converts flow to pressure. Kf may be an estimated or assumed value of patient's resistance (including endotracheal tube). There are two advantages to this approach: First, the signal becomes relatively immune to changes in flow trajectory produced via changes in pressure at the exhalation/PEEP valve mechanism (#1 in Background above). Thus, if pressure at the exhalation/PEEP valve increased near the end of expiration (to maintain PEEP), flow will decrease at a faster rate. Without the Paw component, this effect may appear as an inspiratory effort. With inclusion of Paw in the signal, changes in flow and Paw tend to cancel out. The extent to which this compensation is complete depends on how close Kf is to actual patient resistance. In the absence of a known value, a default value may be used, for example 15 cmH2O/l/sec, representing average resistance (including ET tube) in critically ill, mechanically ventilated patients. With such a default value, correction is not perfect, but the signal is more specific (than flow) in reflecting Tonset. Second, by including Paw in the signal, the signal incorporates that component of Pmus that was dissipated against Rex (see #3 in Background). For example, if Paw decreases at Tonset (because of the lower expiratory flow), this decrease is summed with the component related to flow, resulting in a sharper change in signal trajectory. With this approach, however, signal baseline prior to inspiratory effort is not flat, but, as in the case of flow, rises in a non-linear fashion. Forward extrapolation continues to be required to identify phase transition. Thus, the uncertainty associated with forward extrapolation is not eliminated but the change in signal trajectory is sharper, resulting in a more timely detection of Tonset for the same selected detection threshold (i.e. difference between actual and predicted signal required for identification). Furthermore, this approach continues to be unsuitable for detection of inspiration to expiration transitions (Tend).
A further improvement is achieved by incorporating a component related to volume in the signal (signal Y). Thus:
where, Kv is a factor that converts volume to pressure. With this treatment, the increase in the flow term during expiration (note that flow is negative) is offset by the decrease in the volume term. This tends to linearize, and decrease the slope of (flatten) the signal in the interval prior to Tonset, reducing the uncertainty associated with extrapolation, while the change in trajectory at Tonset is rendered more acute on account of incorporating representation of all actions resulting from the change in Pmus (see #3 in Background). In the best case scenario, where Kv is identical to passive elastance, Kf is identical to passive resistance, and there are no non-linearities in the passive pressure-flow and pressure-volume relations, signal Y would be identical to the actual Pmus waveform, with a flat baseline and a crisp rising phase at Tonset (i.e. as in the Pmus panel of
A further improvement is achieved by allowing for non-linearity in the pressure-flow relation. In mechanically ventilated patients, the non-linear element is almost exclusively due to endotracheal tube characteristics. Thus, a suitable alternate approach is to partition the flow component in two parts, one related to the endotracheal tube and the other related to a laminar component of resistance (Kf). Such signal is referred to as signal Z. Thus:
where Kf2 may be the commercially available K2 value of the endotracheal tube in place. This treatment essentially eliminates any artifactual baseline instability related to non-linear pressure-flow behaviour, further reducing the need for extrapolation and enhancing the crispness of the transition.
As indicated earlier, precise estimates of E and K1 are impossible to obtain during assisted ventilation. Passive E and R (including K1) may be available from earlier determinations in which the patient was made passive. These values may be different from the current values, either because the ventilation conditions under which measurements were made were different, or true E and R (i.e. K1) may have changed in the interim. Some techniques can be used to estimate E and R during conventional assisted ventilation, but these are not very reliable. An important issue, therefore, is the impact of differences between the Kv and real E, and between Kf and real resistance, on the baseline of the generated signals and on the sharpness of the transition.
As may be expected, when there are no errors (i.e. Kv=E and Kf=K1, middle line,
It follows that the use of known values of E and K1, obtained from previous direct measurement, offers advantages over the use of flow. However, under some conditions (i.e. baseline sloping upward) extrapolation techniques (or comparisons between current and previous rates of signal change) are required, and this may delay detection of phase transition.
A further novel aspect of this invention is to completely ignore patient values of E and K1 and to simply select empiric values of Kv and Kf that result in a flat or slightly downward sloping baseline in the latter part of expiration. It is clear from
The above approach does not address the possibility of non-linear passive pressure-volume relation in the tidal volume range (i.e. non-constant elastance). When this is present, and it is common in mechanically ventilated patients, the respiratory system is stiffer in the higher part of the tidal range. When Kv, which is a constant, is adjusted to produce a flat or slightly decreasing signal in the latter part of expiration the signal is not flat in the early part of expiration. In the presence of non-constant elastance (higher elastance at higher volumes) the signal shows a rising phase in the early part of expiration that continues until volume reaches the range of constant elastance. This artifactual rising phase may cause false identification of a new inspiratory effort. This problem is averted by “blinding” the Tonset detection circuitry to the signal during the early part of expiration. This can be done, for example, by gating the signal to the Tonset detection circuitry only after a certain delay from onset of expiratory flow (Tonset window delay). Alternatively, the Tonset detection circuitry may continue to detect Tonset during this period but the resulting identification is gated out during this period. Detection of these false triggers can be easily recognized visually by their consistent relation to end of ventilator cycle. The magnitude of the delay (blinding or blanking period) can then be adjusted accordingly. Alternatively, software algorithms can be developed to detect triggering signals with a consistent relation to end of ventilator cycle and automatically adjusting the width of the window.
The approach of blinding the Tonset detection circuitry to the signal over a time zone close to ventilator cycling-off, where flow is changing rapidly, also helps weed out false triggers related to other artifacts that commonly occur in the signal at this time (see Cycling-off Artifacts,
It should be pointed out that the selected values of Kv and Kf may have little to do with actual patient elastance and resistance. These values are simply used to facilitate detection of phase transitions. As such the actual value of the signal does not reliably reflect actual Pmus, and such signals cannot be used to reliably estimate the work of breathing or quantitative level of pressure output by the patient.
A number of approaches can be used to identify a change in signal trajectory indicative of E→I transition (Tonset). Some of these include:
Regardless of which approach is used to identify Tonset (a-d, above, or other approaches), a threshold must be set for the magnitude of change that must be reached for Tonset to be declared. Several methods can be used to select such threshold. Some of these include:
The preceding account focussed primarily on identification of E→I transitions. However, once Kv and Kf are selected to produce a nearly flat baseline during expiration, the shape of the signal during inspiration (but not necessarily its amplitude, see above) provides a reasonable approximation of the shape of inspiratory muscle output (Pmus) (for example, see
At times, the signal undergoes a transient artifactual reduction soon after ventilator triggering. An extreme example is shown in
At times, true Tend occurs soon after ventilator triggering. This is because inspiratory muscle activity can be inhibited if inspiratory flow is high, and the ventilator frequently delivers excessive flow soon after triggering. For this reason, the procedure described above for Tend identification may, if used to cycle off the ventilator, result in medically unacceptable inflation times. A back-up procedure is, therefore, required to insure that the duration of inflation phase is physiologically appropriate. The same procedure can be used to insure that the inflation phase does not extend beyond physiologically sound limits. The following is the rationale and method for ensuring that the duration of the inflation phase remains within physiologic limits.
In spontaneously breathing subjects and patients, the duration of the inspiratory phase (T1) ranges between 25% and 50% of respiratory cycle duration (TTOT). In studies by the inventor using proportional assist ventilation (PAV), with which the duration of the ventilator's inflation phase mirrors the patient's own T1, the ratio of T1 to TTOT (T1/TTOT ratio) was also found to be between 0.25 and 0.5. Therefore, one approach to insure that the duration of the inflation phase is within the physiologic range in modes in which end of ventilator cycle is not automatically synchronized with the patient is to constrain the duration of the inflation phase to be between 0.25 and 0.5 of the total cycle duration of patient's own efforts (to be distinguished from duration of ventilator cycles). Accordingly, in another aspect of this invention, the end of the ventilator cycle is constrained to occur within this physiological range. Implementation of this procedure requires knowledge of the true respiratory rate of the patient (as opposed to ventilator rate). The true rate of the patient is the sum of ventilator rate, the number of ineffective efforts occurring during the ventilator's exhalation phase (arrows 1 to 3,
Once the true respiratory rate of patient is known, it becomes possible to calculate the real duration of respiratory cycles of the patient (TTOT=60/respiratory rate) and determine the range of inflation times consistent with a physiologic T1/TTOT. For example, if patient's rate is 30/min, TTOT is 2.0 seconds and the physiologic range for the inflation phase is 0.5 to 1.0 second, reflecting a T1/TTOT range of 0.25 to 0.50. Thus, according to this aspect of the invention, average TTOT is determined using any of a number of possible methods. The desirable duration of the ventilator's inflation phase is then determined by multiplying TTOT by a user selected physiologic T1/TTOT ratio or a suitable default value (e.g. 0.4). In another implementation of this method, a timer is reset at the onset of a new Tonset or a new ventilator cycle. The ventilator ignores other cycling-off commands so long as time elapsed since the last Tonset or onset of ventilator cycle, is less than a set value (e.g. 0.3 of TTOT). Similarly, to guard against excessively long ventilator cycles, the timer may send a cycling-off command once time, since the last Tonset, or onset of inflation phase, exceeds a set fraction of average TTOT (e.g. 0.45). The fractions used for minimum and/or maximum cycling-off time can be fixed within the ventilator or adjustable by the user.
An adaptation of this last aspect of the invention is particularly suited for pressure support ventilation (PSV). Because there is often some breath by breath variability in TTOT, setting the end of ventilator cycle to a fixed fraction of average TTOT results in some cycles having higher, and other cycles having lower, T1/TTOT ratios. In this aspect of the invention, only applicable to PSV, rather than causing the ventilator to cycle-off at a predetermined time from the last Tonset, the ventilator is cycled off when inspiratory flow reaches a specified amount, with this specified amount selected to provide, on average, the specified T1/TTOT. This aspect of the invention is implemented as follows: The interval between successive inspiratory efforts (TTOT) is determined in several elapsed ventilator cycles. The level of inspiratory flow at the specified T1/TTOT fraction is noted. For example, if the specified (desired) fraction is 0.4, and TTOT is 3.0 seconds, flow is measured at 1.2 second after the preceding Tonset which triggered a ventilator cycle or, optionally, after the trigger time of the relevant ventilator cycle. The average of several such determinations, in several elapsed breaths, is used as the cycling-off flow threshold in subsequent breaths. With this approach, current cycles destined to have long TTOT automatically have longer inflation cycles. This is so because there is normally a correlation between the duration of inspiratory muscle activity and the TTOT of individual breaths. Thus, in breaths destined to have a long TTOT, inspiratory activity tends to last longer and this, in PSV, delays the point at which a specified cycling-off flow threshold is reached.
The information provided by the present invention can be utilized in a number of ways: First, the time of Tonset, generated by the current invention, can be used to trigger ventilator cycles by providing an appropriate signal to the ventilator's triggering mechanism. Second, the end of the ventilator inflation phase can be made to coincide with the end of patient effort, as identified by the present invention, through appropriate connections to the cycling-off mechanism of the ventilation. Third, cycling-off can be made to occur at specified times or, in the case of pressure support ventilation, at a specified flow rate, after Tonset or after the onset of ventilator cycle. In this application, the user enters a desired T1/TTOT ratio. The appropriate time, or flow, to cycle-off is then determined from the inputted T1/TTOT ratio and the value of average patient TTOT, obtained using the present invention. Fourth, cycling off may occur at the identified Tend, conditional on this not violating a specified minimum T1/TTOT ratio.
Whether or not it is used to synchronize the ventilator with patient effort, the information provided by the current invention can be displayed to the user to assist him/her in adjusting ventilator settings to, indirectly, improve patient ventilator interaction. In this connection, the information may be printed out on command or be displayed on a monitor. The signal itself can be displayed in real time along with other useful signals such as flow and airway pressure. In addition, numerical values concerning patient ventilator interaction can be displayed. Some recommended values include:
The numerical values may be accompanied by displayed suggestions on how to adjust ventilator settings to reduce the undesirable aspects of current interaction.
The procedures of the present invention as described in details above may be implemented in a device which may be constructed as a freestanding device to be attached externally to a ventilator, or may be incorporated within the ventilator. In either case, the operation of the device requires inputs related to pressure and airflow in the ventilator circuit.
Pressure and flow signals are routinely generated in modern ventilators using an approach similar to that of
The signal 31 generated by the summing amplifier 16 is further processed by two circuits, one for detecting the onset of inspiratory effort (Tonset identification circuit 32) and one for detecting the end of inspiratory effort (Tend identification circuit 33). The overall purpose of the first circuit 32 is to measure the increase in the amplitude of the signal 31 during periods in which the signal 31 is rising, within a specific time window in the breath determined by a Tonset window circuit 34. This time window begins after a specified delay 35 from the point at which expiratory flow decreases below a specified value (e.g. −0.2 l/sec) during expiration. As seen in the diagram of the first circuit 32, the signal 31 is differentiated using a differentiator 36. The differentiated signal 37 is filtered using an appropriate low pass filter (e.g. 5 Hz) 38 to remove high frequency noise. The filtered differentiated signal 39 is passed through two comparators. One comparator 40 sends an enabling positive signal 41 when the filtered differentiated signal 39 is positive and the other comparator 42 sends an enabling positive signal 43 when the filtered differentiated signal 39 is negative. The unfiltered signal 37 is integrated 44 when two gates 45,46 are enabled. The first gate 45 is enabled when the filtered differentiated signal 39 is positive, as detected by the positive comparator 40. The second gate 46 is enabled during the specified time window during expiration, as detected by the Tonset window circuit 34 and conveyed to the gate by an enabling signal 47. The integrator 44 is reset whenever the filtered differentiated signal 39 becomes negative as detected by the negative comparator 42. In this fashion integration begins anew only when the signal is rising within the specified time window. The integrator output 48 is received by a comparator 49 which sends out a signal 50, indicating Tonset, when integrator output exceeds a specified threshold set by an external EI threshold adjust 51.
The specific design used for detection of onset of effort in this implementation 32 is selected because it offered an optimal combination of sensitivity and specificity (i.e. sensitive yet not prone to false triggering). It is clear, however, that other designs for detecting a change in signal trajectory are possible. For example, the filtered differentiated signal 39, representing current rate of change in signal, can be delayed by a specified amount (e.g. 200 msec). A comparator (not shown) compares the current and delayed forms of the filtered differentiated signal. A signal, indicating onset of effort, is generated when the difference exceeds a threshold value. Alternatively, the signal itself 31 may be delayed by a specified amount (e.g. 200 msec). A comparator (not shown) compares the current and delayed forms of the actual signal and generates a signal, indicating onset of effort, when the difference exceeds a threshold value. Other approaches are possible within the scope of this invention.
For identifying the Tend 33, the signal 31 is first differentiated 52 and the differentiated signal 53 is reintegrated 54. The integrator is reset at the onset of inspiratory effort (Tonset) using the signal 50 generated from the Tonset identification circuit 32. In this fashion, any baseline offset in the signal 31 is eliminated and the output of the integrator 55 reflects only the increase in signal 31 amplitude from Tonset. Integrator output 55 is connected to a peak detector circuit 56, which is also reset by the Tonset signal 50. The output of the peak detector 57 is attenuated 58 with a suitable attenuation factor (e.g. 50%). Optionally, the attenuation factor may be individually adjusted by the user through an external input 59. A comparator 60 sends a signal 62 when current integrator output 55 decreases below the attenuated peak detector output 61. In this fashion the end of inspiratory effort is detected when the current integrator output 55 decreases below a set percent of the peak level reached during the current inspiratory effort.
At times, the signal 31 or 55 transiently decreases at the time of ventilator triggering (Ventilator Trigger Artifact,
The number and types of user inputs may vary depending on how comprehensive the device is and the extent to which user involvement is desired by the manufacturer. In the most comprehensive analog embodiment shown in
Some inputs may be deleted by using fixed default values within the device. For example, the Kf adjust input 75 may be deleted and a fixed value of 10.0 is used. A fixed Tonset delay value of, for example, 0.3 second may be used, eliminating the Tonset window delay input 35. A suitable default signal attenuation value (e.g. 50%) may be used replacing the corresponding input 59. Likewise, a Tend window delay of 0.2 second may be used eliminating the Tend window delay 73. Clearly, the more fixed the settings are the less reliable the performance of the device may become. However, this may be acceptable under some circumstances with the potential benefit of simplifying the operation of the device. An alternative would be to have the device operate with default settings unless changed by the user.
Other inputs may also become unnecessary if simpler forms of the signal 31 are generated. For example, signal component related to the non-linear flow function 25 may be eliminated according to Equation 3. In this case the Kf2 adjust input 76 is deleted. Likewise, the signal component related to volume 20 may be eliminated, according to Equation 2, with corresponding deletion of the Kv adjust input 77. Again, the simpler the device, the less reliable its performance will become but this may be acceptable under certain circumstances. In its simplest form, all the user needs to do is to set the E-I threshold input 51.
Certain internal signals need to be displayed to allow the user to adjust the input settings, while others provide the user with the results of monitoring. These signals can be displayed on a monitor 78 included in a freestanding device. Alternatively, if the device is incorporated inside the ventilator, the monitor of the ventilator can be used for this purpose. A third embodiment involves directing the device's outputs to an analog to digital converter and displaying the outputs on a separate computer.
The following output signals are necessary for adjusting the input settings:
a) The main signal itself 31.
b) The output of the integrator 48 in the Tonset circuit (32).
The use of these two signals for the sake of input adjustment is described below under OPERATION (below).
Additionally, the signals representing flow 9, pressure 15 and volume 79 may be displayed on the monitor for general monitoring purposes.
Signals representing the onset of inspiratory effort 50 (Tonset) and end of inspiratory effort 74 (Tend) are also displayed on the monitor. In the event these signals are to be used to actively control the cycling of the ventilator, they are communicated to the ventilator's cycling mechanism.
Additional information of value in guiding ventilator setting is most conveniently generated by a small microprocessor. A block diagram of a preferred embodiment (103) is provided in
In another embodiment of the output processor 103 patient respiratory rate (or TTOT) is inputted to the processor, replacing the “Calculate Patient Rate” function 83. This input is then used to calculate the “Desirable T1” 87 and “Flow at Desired T1” 90. Patient respiratory rate may be determined by the user from inspection of chest movements or by observing the flow tracing on the monitor, or automatically using computational methods other than the ones described in the above embodiment 103.
When the device is built inside the ventilator, the pressure 15 and flow 9 signals are permanently connected to the device. For freestanding systems, the first step is to connect the flow meters and pressure ports to the inspiratory 2 and expiratory 3 lines close to the ventilator (
Tonset Window circuit 34: In this circuit the flow signal is connected to a Schmitt trigger circuit (left half of the Tonset window circuit 34) characterized by hysteresis. With the indicated values of the different circuit components, the Schmitt circuit sends out a constant voltage (8 volts) whenever flow decreases below −0.2 l/sec 96. The signal 96 remains on until-flow rises to >0.2 l/sec. In this application, the onset of the signal 96 indicates the beginning of the exhalation phase and is also used to mark the end of ventilator cycle (Toff). The output of the Schmitt trigger circuit is connected to a delay circuit with an externally adjustable delay time 35. The output of the delay circuit 97 is received by an AND gate 98. The AND gate 98 also receives the output of the Schmitt trigger circuit 96 directly and sends a signal when the Tonset window is open, as indicated by the output of the Schmitt trigger circuit 96 but only after the specified delay 35 has elapsed, as indicated by a positive output from the delay circuit 97. In turn, the output of the AND gate 47 (referred to as Q signal in
Ttrigger detection circuitry: There are many ways by which the time at which the ventilator was triggered can be detected. In this embodiment Ttrigger was detected when the rate of increase in pressure exceeded 15 cmH2O/second OR flow increased beyond 0.4 l/second. To this end, a differentiator 66 was used to obtain Δpressure/Δt 67. Next, a comparator 68 produces a positive signal 99 when Δpressure/Δt 67 exceeds a set value of 15 cmH2O/second. In another circuit 69 a comparator generates a positive signal 100 when flow (9) exceeds 0.4 l/second. Two diodes 101,102 function as an OR gate so that a positive signal (Ttrigger, 64) is generated when either the Δpressure/Δt or flow exceed the set respective thresholds.
Tend Window circuit 63: This circuit has four components. First, the Q signal 47, representing time window for Tonset detection, is inverted using an inverter 104. The positive phase of this inverted Q signal 105 defines the maximum period during which Tend can be located. The second component is an AND gate 106 which receives the inverted Q signal 105 and the Ttrigger signal 64 and sends a positive signal 107 when both its inputs are positive. The positive edge of this signal 107 activates a flip-flop switch 108, which is the third component of the Tend Window circuit. The flip-flop switch 108 is reset by the Q signal 47. The fourth component is a delay circuit 110 with an adjustable external control 111. The delay circuit 110 receives the output of the flip-flop switch 109. After the set delay, the delay circuit 110 sends out a positive signal 112, which persists until the beginning of the Q signal 47. The output of the delay circuit 112 is one of the two inputs to the main AND gate 72 which generates the Tend signal 74.
The other components of the Tend circuit 33, as shown in
The circuitry used in this preferred embodiment is clearly not the only way by which the functions and results contemplated by the current invention can be implemented. Other circuit designs can be used to accomplish the same objectives and these are within the scope of this invention.
In the illustrated embodiment for calculating target flow to cycle off pressure support ventilation (90,
Each of these options has advantages and disadvantages. In practice, the difference in net result should be small. However, some manufacturers or users may prefer one or the other or even a completely different option.
The resulting output of such microprocessor (
Whereas the preferred embodiment described herein utilized electrical circuitry to generate the Signal and to determine Tonset and Tend, it is clear that any and all the functions executed by electrical circuitry for the current application can be readily executed by digital technology.
As in the case of the analog implementation, the digital implementation can be simplified to different degrees depending on user and manufacturer preferences. The outputs of the device may also be expanded or reduced to meet user needs.
In summary of this disclosure, the present invention provides a method and apparatus for detecting the onset and the end of inspiratory effort in a patient on mechanical ventilation. Modifications are possible within the scope of the invention.