Detection of ventricular activity using unipolar and bipolar signals.

Abstract

To provide a method for detection of ventricular activity using unipolar and bipolar signals.SOLUTION: A method includes receiving a bipolar signal sensed by a pair of electrodes at a location in a heart of a patient. A unipolar signal is received, which is sensed by an electrode at the location in the heart. A derivative of the received bipolar signal is computed. A derivative of the received unipolar signal is computed. A ratio between the derivative of the bipolar signal and the derivative of the unipolar signal is evaluated at local minima of the derivative of the unipolar signal. When the ratio is smaller than a preset threshold ratio value, an occurrence of ventricular activity is indicated.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates generally to electrophysiological signals, and more particularly to methods for assessing electrical propagation within the heart. [Background technology]

[0002] Recording unipolar and bipolar electrical signals from the heart has been previously suggested in the patent literature. For example, US Patent Application Publication No. 2018 / 0042505 provides a method including receiving a bipolar signal from a pair of electrodes proximate the myocardium of a human subject and receiving a unipolar signal from one of the pair of electrodes. The method further includes demarcating a window of interest (WOI) for the unipolar and bipolar signals, calculating within the WOI the number of occurrences of a local unipolar minimum derivative and a local unipolar minimum derivative of the unipolar signal, and calculating within the WOI the bipolar derivative of the bipolar signal at the number of occurrences. The method also includes evaluating a ratio between the bipolar derivative and the local unipolar minimum derivative, assigning the number of occurrences as myocardial excitations when the ratio is greater than a preset threshold ratio value, counting the number of excitations, and classifying the unipolar signal according to the number.

[0003] As another example, US Patent Application Publication No. 2015 / 0208938 describes bipolar and unipolar electrograms recorded from electrodes of a probe and differentiated with respect to time. A peak in the differentiated bipolar electrogram is identified. An activity window is defined that includes the bipolar activity around the peak. Extreme negative values ​​in the differentiated unipolar electrogram within the activity window are reported as the onset of unipolar activation. In one aspect, annotations are selected from minimum value candidates in the differentiated unipolar electrogram within the activity window by eliminating candidates that do not correlate with activity in the bipolar electrogram.

[0004] US Patent Application Publication No. 2015 / 0208942 describes a diagnostic catheterization method of the heart that includes recording bipolar and unipolar electrograms from electrodes on a catheter at locations within the heart and defining a window of interest in which the rate of change of the potential of the bipolar electrogram exceeds a predetermined value. Annotations are established in the unipolar electrogram, the annotations indicating the maximum rate of change of the potential of the unipolar electrogram within the window of interest. The annotations are assigned quality values, and a three-dimensional map of a portion of the heart is generated that includes the annotation values ​​and the quality values.

[0005] US Patent Application Publication No. 2013 / 0281870 describes a method for characterizing an electrocardiogram, including receiving a first unipolar signal from a first location on the heart and a second unipolar signal from a second location on the heart. A bipolar signal is generated from the first and second unipolar signals and analyzed to describe a period during which the first and second locations generate a bipolar complex. The method also includes analyzing the first unipolar signal within this period to determine an activity time of the first location. Summary of the Invention [Means for solving the problem]

[0006] Exemplary embodiments of the invention described herein provide a method including receiving a bipolar signal sensed by a pair of electrodes at a location within a patient's heart. A unipolar signal sensed by electrodes at said location within said heart is received. A derivative of the received bipolar signal is calculated. A derivative of the received unipolar signal is calculated. A ratio of the derivative of the bipolar signal to the derivative of the unipolar signal is evaluated at a local minimum of the derivative of the unipolar signal. An occurrence of ventricular activity is indicated when the ratio is less than a preset threshold ratio value.

[0007] In some exemplary embodiments, receiving the bipolar signal includes receiving a plurality of bipolar signals from a plurality of electrode pairs of a multi-electrode catheter, and further includes comparing a time difference between the plurality of bipolar signals, and indicating an occurrence of ventricular activity when the time difference is less than a predetermined value.

[0008] In some exemplary embodiments, receiving the unipolar signal includes receiving a plurality of unipolar signals from a plurality of electrodes of a multi-electrode catheter, and the method further includes indicating an occurrence of ventricular activity when a time difference between the unipolar signals is less than a predetermined value.

[0009] In an exemplary embodiment, receiving a unipolar signal includes receiving a unipolar signal from one of a pair of electrodes.

[0010] In another exemplary embodiment, calculating the ratio includes defining a time interval that includes a window of interest and calculating the ratio within the window of interest.

[0011] In some exemplary embodiments, the method further includes alerting a user to an onset of ventricular activity.

[0012] In some exemplary embodiments, the method further includes presenting to a user the bipolar signal having annotations indicative of ventricular activity.

[0013] In another exemplary embodiment, the method further includes analyzing the time-series bipolar signals and indicating an occurrence of ventricular activity only when the ratio remains less than a preset threshold ratio value for at least a predetermined duration.

[0014] According to an exemplary embodiment of the present invention, there is further provided an apparatus including an interface and a processor, the interface configured to receive a bipolar signal from a pair of electrodes disposed at a location within the patient's heart and a unipolar signal from an electrode disposed at a location within the heart, the processor configured to (a) calculate a derivative of the received bipolar signal, (b) calculate a derivative of the received unipolar signal, (c) evaluate a ratio between the derivative of the bipolar signal and the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal, and (d) indicate an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value.

[0015] A more complete understanding of the present disclosure will be obtained from the following detailed description of the embodiments of the present disclosure when read in conjunction with the drawings. [Brief description of the drawings]

[0016] The invention will be more fully understood from consideration of the following detailed description taken in conjunction with the drawings, in which: [Figure 1] 1 is a schematic, pictorial illustration of a catheter-based electrophysiological (EP) mapping system, in accordance with an exemplary embodiment of the present invention; [Diagram 2] 2 is a graph of bipolar and unipolar electrocardiogram (ECG) signals recorded using the system of FIG. 1 and a graph of the calculated respective ratios of the unipolar and bipolar ECG signals, in accordance with an exemplary embodiment of the present invention; [Diagram 3] 1 is a flow chart that generally illustrates a method and algorithm for detecting far-field ventricular activity, in accordance with an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Overview Electrophysiological (EP) signals acquired from the heart using catheters may typically be a combination of near-field and far-field bioelectric signals. Clinically, however, near-field signals are usually more significant. Furthermore, some far-field signals are considered interference that may corrupt the near-field sensing signal. For example, when acquiring signals from a catheter positioned in the atrium of the heart, a physician needs to know when ventricular far-field activity is invading the near-field signal.

[0018] In this context, near-field bioelectric signals are signals received from cardiac tissue at localized contact points between the diagnostic electrodes and the tissue. Far-field bioelectric signals are signals from regions distant from the contacted tissue region. Typically, such far-field bioelectric signals propagate by conduction through blood and are sensed by diagnostic electrodes on the catheter, which acquire the near-field signals via contact with tissue.

[0019] Exemplary embodiments of the invention described below provide methods and apparatus for detection of transient far-field ventricular activity. In some exemplary embodiments, a processor receives a bipolar signal sensed by a pair of electrodes of a catheter placed in contact with tissue to be diagnosed (such as tissue of the left atrium of the heart) and a unipolar signal sensed by an electrode of the same catheter or another catheter placed in contact with cardiac tissue.

[0020] Ventricular activity is typically indicated by abrupt changes in the unipolar signal, but more gentle changes in the bipolar signal. Thus, if the unipolar signal has a strong slope, but at the same time the bipolar signal has a small slope, this behavior is suspected to result from transient far-field ventricular activity.

[0021] In some exemplary embodiments, the processor first calculates the derivatives of the received bipolar and unipolar signals, and then calculates a ratio between the derivatives of the bipolar and unipolar signals. By comparing the calculated ratio to a preset threshold, the processor identifies an occurrence of interfering far-field ventricular activity, for example, when the ratio is lower than a preset threshold ratio value. The processor then indicates the occurrence of ventricular activity to a user. In exemplary embodiments, this ratio is a dynamic ratio that the processor can update based on, for example, the noise level in the acquired signals.

[0022] In some exemplary embodiments, the electrode sensing the unipolar signal is one of the electrode pairs described above, while in other exemplary embodiments, the unipolar signal is received using one or more ECG surface leads.

[0023] In some exemplary embodiments, the processor defines a time interval that includes a window of interest and calculates a ratio of the received unipolar and bipolar signals within the window of interest. Alternatively or additionally, the processor calculates a ratio of the slope of the received unipolar and bipolar signals within the window of interest.

[0024] In some cases, local bioelectric signals may show similarities to far-field bioelectric signals. In some exemplary embodiments, to distinguish between local and far-field activity, a processor analyzes bioelectric signals from multiple electrode pairs of a multi-electrode catheter (such as Biosense Webster's PentaRay™ catheter). If the signals result from far-field activity, the timing of the signals should be similar across different electrode pairs, whereas if the signals result from local activity, the signals will be dispersed in time, e.g., separated from each other in time by more than a predetermined value.

[0025] In some exemplary embodiments, the processor is further configured to alert or inform a physician that ventricular activity has been detected, which the system can further use to select or exclude beats from the acquisition.

[0026] The disclosed method for detecting ventricular activity relies on the theory that ventricular activity may typically occur when there is a large sensed unipolar signal or a large change in the sensed unipolar signal while simultaneously there is a small bipolar signal or a small change in the bipolar signal, respectively, or more quantitatively, when the ratio of either of the two pairs of signals exceeds a given threshold.

[0027] In an exemplary embodiment, the processor analyzes the time-series bipolar signals, and if a ratio of a derivative of the bipolar signal to a derivative of at least one of the respective unipolar signals remains less than a preset threshold ratio value for at least a predetermined duration, the processor indicates an occurrence of ventricular activity, and only if so, ventricular activity occurs.

[0028] Typically, the processor is programmed with software containing specific algorithms that enable the processor to perform each of the processor-related steps and functions outlined above.

[0029] The disclosed method for detecting ventricular activity can improve the value of diagnostic catheterization by allowing the physician to discard corrupted bioelectrical sensing signals.

[0030] System Description Figure 1 is a schematic, pictorial diagram of a catheter-based electrophysiological (EP) mapping system 21, in accordance with an exemplary embodiment of the present invention. Figure 1 illustrates a physician 27 using an electroanatomical mapping catheter 29 to perform electroanatomical mapping of a heart 23 of a patient 25. The mapping catheter 29 includes at its distal end one or more arms 20, each of which is coupled to a bipolar electrode 22 that includes adjacent electrodes 22a and 22b.

[0031] During a mapping procedure, the positions of the electrodes 22 are tracked while they are within the patient's heart 23. For that purpose, electrical signals are passed between the electrodes 22 and external electrodes 24. For example, three external electrodes 24 may be coupled to the patient's chest and another three external electrodes may be coupled to the patient's back (for ease of illustration, only one external electrode is shown in FIG. 1).

[0032] Based on the signals, and taking into account the known locations of the electrodes 24 on the person's body, the processor 28 calculates an estimated location of each electrode 22 within the patient's heart 23. Respective electrophysiological data, such as intracardiac ECG traces, are additionally acquired from tissue of the heart 23 by using the electrodes 22. The processor may thus associate any given signal received from the electrodes 22, such as an electrophysiological signal, with the location from which the signal was acquired. The processor 28 receives the resulting signals via the electrical interface 35 and uses the information contained in these signals to construct an electrophysiological map 31 and an ECG trace 40 for presentation on the display 26.

[0033] Processor 28 typically comprises a general-purpose computer with software programmed to perform the functions described herein. The software may be downloaded in electronic form to the computer, for example over a network, or alternatively or additionally may be provided and / or stored on a non-transitory tangible medium, such as magnetic, optical or electronic memory. In particular, processor 28 executes dedicated algorithms disclosed herein, including the process illustrated in FIG. 3, which enable processor 28 to perform the steps of the present disclosure, as further described below.

[0034] The exemplary diagram shown in FIG. 1 has been chosen purely for conceptual clarity. Other types of electrophysiological sensing catheter geometries may be employed, such as the Lasso® catheter (manufactured by Biosense Webster, Inc., Irvine, California). In addition, a contact sensor may be fitted to the distal end of the mapping catheter 29 to transmit data indicative of the physical quality of the electrode contact with the tissue. In an exemplary embodiment, measurements of one or more electrodes 22 may be discarded if their physical contact quality is indicated as poor, and measurements of other electrodes may be considered valid if their contact quality is indicated as sufficient.

[0035] Detection of ventricular activity using unipolar and bipolar signals. 2 is a graph of bipolar (60) and unipolar (62) electrocardiogram (ECG) signals recorded using system 21 of FIG. 1 according to an exemplary embodiment of the invention, and a calculated ratio (64) of the unipolar ECG signal to the bipolar ECG signal, respectively. Bipolar signal 60 is received from a pair of electrodes 22 of catheter 29, and unipolar signal 62 is received from a selected electrode from the electrode pair. The signals shown are received over a time interval 50. Two windows of interest (WOIs) 55 are shown within interval 50, both of which are defined by processor 28. Processor 28 calculates a ratio 64 between the bipolar derivative signal and the bipolar derivative signal within WOI 55, and compares calculated ratio 64 to a preset threshold (or dynamic threshold) 67 to identify far-field ventricular activity. In the illustrated exemplary embodiment, processor 28 indicates the occurrence of far-field ventricular activity (i.e., when ratio 64 falls below threshold 67) by annotations 61, 63, and 65 overlaid on the ratio graph (64). However, additional graphical measures and actions taken by processor 28 (such as tagging the signal within the WOI for future processing) are possible.

[0036] 3 is a flow chart that generally illustrates a method and algorithm for detection of far-field ventricular activity according to an exemplary embodiment of the present invention. The algorithm according to the presented exemplary embodiment executes a process that begins with the processor 28 receiving the bipolar and unipolar ECG signals from the catheter 29 in a data receiving step 70. The processor 28 then defines a time interval 50 that includes the WOI 55 in a data selection step 72. The processor 28 then calculates a ratio 64 between the bipolar and unipolar derivative signals in a ratio calculation step 74 for multiple electrode pairs. In a check step 76, the processor 28 checks for each electrode pair whether the ratio 64 is below a preset threshold 67. If the answer is "no" for at least a given fraction of the electrode pairs considered, the process returns to step 70 and returns to a new signal. If the answer is "yes" for a sufficient fraction of the electrode pairs, the process continues.

[0037] The processor compares the timing of the valid fractions of the bioelectric signals to distinguish between local and far-field bioelectric activity. If, at check step 78, the analysis indicates that the timing of the signals is dissimilar across different electrode pairs, the processor concludes that the signal arises from local activity and the process returns to step 70 to collect a new signal.

[0038] If the time difference between the signals is less than a predetermined value, processor 28 concludes that the signal results from transient far-field activity and indicates the detected ventricular activity in a ventricular activity indication step 80, for example, by performing annotations 61, 63, and 65. Finally, using a variety of possible audiovisual means, processor 28 alerts physician 82 to the detected ventricular activity. In some embodiments, the processor compares the received different unipolar signals, and if the processor finds that the time difference between the different unipolar signals is less than a predetermined duration, the processor indicates the occurrence of ventricular activity.

[0039] The exemplary flow chart shown in Figure 3 has been selected solely for the purposes of conceptual clarity. This embodiment may also include additional algorithmic steps, such as receiving a plurality of bipolar and unipolar ECG signals, and receiving an indication of the degree of physical contact between the electrodes and the tissue to be diagnosed from a contact force sensor. This step, and other possible steps, have been intentionally omitted from the disclosure herein to provide a more simplified flow chart.

[0040] Although the exemplary embodiments described herein relate primarily to cardiac diagnostic applications, the methods and systems described herein may also be used in other medical applications, such as in defibrillators and pacemakers.

[0041] It will be understood that the above exemplary embodiments have been mentioned by way of example, and that the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes both combinations and subcombinations of the various features as described hereinabove, as well as variations and modifications thereof not disclosed in the prior art that would occur to one skilled in the art upon reading the foregoing description.

[0042] [Embodiment] (1) A method for detecting ventricular activity using unipolar and bipolar signals, comprising: receiving a bipolar signal sensed by a pair of electrodes at a location within the patient's heart; receiving a unipolar signal sensed by an electrode at the location within the heart; calculating a derivative of the received bipolar signal; calculating a derivative of the received unipolar signal; evaluating a ratio of the derivative of the bipolar signal to the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal; indicating an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value. (2) The method of embodiment 1, wherein receiving the bipolar signal includes receiving a plurality of bipolar signals from a plurality of electrode pairs of a multi-electrode catheter, comparing a time difference between the plurality of bipolar signals, and indicating an occurrence of ventricular activity when the time difference is less than a predetermined value. (3) The method of embodiment 1, wherein receiving the unipolar signal includes receiving a plurality of unipolar signals from a plurality of electrodes of a multi-electrode catheter, and indicating an occurrence of ventricular activity when a time difference between the unipolar signals is less than a predetermined value. (4) The method of embodiment 1, wherein receiving the unipolar signal includes receiving the unipolar signal from one of the electrodes of the pair. (5) The method of embodiment 1, wherein calculating the ratio includes defining a time interval that includes a window of interest and calculating the ratio within the window of interest.

[0043] (6) The method of embodiment 1, further comprising alerting a user to an indication of the onset of said ventricular activity. (7) The method of embodiment 1, further comprising presenting to a user the bipolar signal having annotations indicating the ventricular activity. (8) The method of embodiment 1, comprising analyzing a time-series bipolar signal and indicating an occurrence of said ventricular activity only when said ratio remains less than a preset threshold ratio value for at least a predetermined duration. (9) An apparatus for detecting ventricular activity using unipolar and bipolar signals, comprising: an interface configured to receive a bipolar signal from a pair of electrodes placed at a location within a patient's heart and to receive a unipolar signal from an electrode placed at the location within the heart; 1. A processor comprising: calculating a derivative of the received bipolar signal; calculating a derivative of the received unipolar signal; evaluating a ratio of the derivative of the bipolar signal to the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal; and a processor configured to: indicate an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value. (10) The device of embodiment 9, wherein the processor is configured to receive a plurality of bipolar signals by receiving the bipolar signals from a plurality of electrode pairs of a multi-electrode catheter, compare a time difference between the plurality of bipolar signals, and indicate an occurrence of ventricular activity when the time difference is less than a predetermined value.

[0044] (11) The device of embodiment 10, wherein the processor is configured to receive a plurality of unipolar signals by receiving the unipolar signals from a plurality of electrodes of a multi-electrode catheter, and to indicate an occurrence of ventricular activity when a time difference between the unipolar signals is less than a predetermined value. (12) The apparatus of embodiment 9, wherein the processor is configured to receive the unipolar signal from one of the electrodes of the pair. (13) The apparatus of embodiment 9, wherein the processor is configured to define a time interval including a window of interest and to calculate the ratio of the unipolar signal to the bipolar signal within the window of interest. (14) The device of embodiment 9, wherein the processor is further configured to alert a user of an indication of an occurrence of the ventricular activity. (15) The device of embodiment 9, wherein the processor is further configured to present the bipolar signal to a user with annotations indicating the ventricular activity.

[0045] (16) The apparatus of embodiment 9, wherein the processor is further configured to analyze the time-series bipolar signals and indicate an occurrence of the ventricular activity only when the ratio remains less than a preset threshold ratio value for at least a predetermined duration.

Claims

1. 1. A method of operating a system having a processor for detecting ventricular activity using unipolar and bipolar signals, comprising: receiving a bipolar signal sensed by a pair of electrodes at a location within a patient's heart; receiving, by the processor, a unipolar signal sensed by an electrode at the location within the heart; the processor calculating a derivative of the received bipolar signal; the processor calculating a derivative of the received unipolar signal; the processor evaluating a ratio of the derivative of the bipolar signal to the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal; and the processor indicating an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value; 4. The method of claim 3, wherein receiving the bipolar signal comprises the processor receiving a plurality of bipolar signals from a plurality of electrode pairs of a multi-electrode catheter, the processor comparing a time difference between the plurality of bipolar signals, and the processor indicating an occurrence of ventricular activity when the time difference is less than a predetermined value.

2. A method of operating a system having a processor for detecting ventricular activity using unipolar and bipolar signals, comprising: receiving a bipolar signal sensed by a pair of electrodes at a location within a patient's heart; receiving, by the processor, a unipolar signal sensed by an electrode at the location within the heart; the processor calculating a derivative of the received bipolar signal; the processor calculating a derivative of the received unipolar signal; the processor evaluating a ratio of the derivative of the bipolar signal to the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal; and the processor indicating an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value; The method of claim 1, wherein receiving the unipolar signal comprises the processor receiving a plurality of unipolar signals from a plurality of electrodes of a multi-electrode catheter, and the processor indicating an occurrence of ventricular activity when a time difference between the unipolar signals is less than a predetermined value.

3. The method of claim 1 or 2, wherein receiving the unipolar signal includes the processor receiving the unipolar signal from one of the electrodes of the pair.

4. 3. The method of claim 1, wherein calculating the ratio comprises: the processor defining a time interval that includes a window of interest; and the processor calculating the ratio within the window of interest.

5. A method as described in claim 1 or 2, further comprising the processor alerting a user to an indication of the occurrence of ventricular activity.

6. The method of claim 1 or 2, comprising the processor presenting to a user the bipolar signal having annotations indicating the ventricular activity.

7. A method as described in claim 1 or 2, comprising: the processor analyzing temporally continuous bipolar signals; and the processor indicating the occurrence of ventricular activity only when the ratio remains less than a predetermined threshold ratio value for at least a predetermined duration.

8. 1. An apparatus for detecting ventricular activity using unipolar and bipolar signals, comprising: an interface configured to receive a bipolar signal from a pair of electrodes placed at a location within a patient's heart and to receive a unipolar signal from an electrode placed at the location within the heart; 1. A processor comprising: calculating a derivative of the received bipolar signal; calculating a derivative of the received unipolar signal; evaluating a ratio of the derivative of the bipolar signal to the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal; indicating an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value; 11. The apparatus, wherein the processor is configured to: receive a plurality of bipolar signals by receiving the bipolar signals from a plurality of electrode pairs of a multi-electrode catheter; compare a time difference between the plurality of bipolar signals; and indicate an occurrence of ventricular activity when the time difference is less than a predetermined value.

9. An apparatus for detecting ventricular activity using unipolar and bipolar signals, comprising: an interface configured to receive a bipolar signal from a pair of electrodes placed at a location within a patient's heart and to receive a unipolar signal from an electrode placed at the location within the heart; 1. A processor comprising: calculating a derivative of the received bipolar signal; calculating a derivative of the received unipolar signal; evaluating a ratio of the derivative of the bipolar signal to the derivative of the unipolar signal at a local minimum of the derivative of the unipolar signal; indicating an occurrence of ventricular activity when the ratio is less than a preset threshold ratio value; 11. The apparatus, wherein the processor is configured to receive a plurality of unipolar signals by receiving the unipolar signals from a plurality of electrodes of a multi-electrode catheter, and to indicate an occurrence of ventricular activity when a time difference between the unipolar signals is less than a predetermined value.

10. 10. The apparatus of claim 8 or 9, wherein the processor is configured to receive the unipolar signal from one of the electrodes of the pair.

11. 10. The apparatus of claim 8 or 9, wherein the processor is configured to define a time interval including a window of interest and to calculate the ratio of the unipolar signal to the bipolar signal within the window of interest.

12. 10. The apparatus of claim 8 or 9, wherein the processor is further configured to alert a user of an onset of said ventricular activity.

13. The apparatus of claim 8 or 9, wherein the processor is further configured to present the bipolar signal to a user with annotations indicative of the ventricular activity.

14. 10. The apparatus of claim 8 or 9, wherein the processor is further configured to analyze the time-series bipolar signals and to indicate an occurrence of the ventricular activity only when the ratio remains less than a preset threshold ratio value for at least a predetermined duration.

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