WO2004112606A1 - Procedes et appareil pour detecter l'apnee du sommeil en utilisant des mesures de bioimpedance - Google Patents

Procedes et appareil pour detecter l'apnee du sommeil en utilisant des mesures de bioimpedance Download PDF

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Publication number
WO2004112606A1
WO2004112606A1 PCT/IE2004/000088 IE2004000088W WO2004112606A1 WO 2004112606 A1 WO2004112606 A1 WO 2004112606A1 IE 2004000088 W IE2004000088 W IE 2004000088W WO 2004112606 A1 WO2004112606 A1 WO 2004112606A1
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signal
bioimpedance
trans
thoracic
cardiac
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PCT/IE2004/000088
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English (en)
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Ioanis Kapnisakis
Conor Heneghan
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University College Dublin, National University Of Ireland, Dublin
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Publication of WO2004112606A1 publication Critical patent/WO2004112606A1/fr

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/48—Other medical applications
    • A61B5/4806—Sleep evaluation
    • A61B5/4818—Sleep apnoea
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
    • A61B5/0809—Detecting, measuring or recording devices for evaluating the respiratory organs by impedance pneumography
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
    • A61B5/7235—Details of waveform analysis
    • A61B5/7239—Details of waveform analysis using differentiation including higher order derivatives

Definitions

  • the present invention relates to methods for detecting sleep apnea using bioimpedance 5 measurements.
  • trans-cervical electrodes electrodes measuring bioimpedance across the neck region of a patient
  • trans-thoracic electrodes electrodes measuring bioimpedance across the torso of a patient
  • Sleep apnea is commonly defined as the cessation of breathing during sleep. Clinicians usually divide sleep apnea into three major categories - obstructive, central, and mixed sleep apnea. Obstructive sleep apnea (OSA) is characterised by intermittent pauses in breathing during sleep caused by the obstruction and/or collapse of the upper airway. If
  • OSA is typically accompanied by a reduction in blood oxygen saturation, and leads to wakening from sleep in order to breathe.
  • Central sleep apnea CSA
  • CSA Central sleep apnea
  • the apnea episodes should be of ten seconds or longer duration, and occur more than five times per hour (exact definitions vary from specialist to specialist).
  • Sleep apnea is conventionally diagnosed using full polysomnography.
  • the signals typically acquired in full polysomnogram include nasal airflow, ribcage and abdominal respiratory effort, electroencephalogram (EEG) recordings, a left and right electrooculogram, and a chin electromyogram.
  • EEG electroencephalogram
  • Measurements of oro/nasal airflow can be measured using thermal based devices situated below the nose, in which temperature changes in inspiration and expiration are recorded. An example of such a device is shown in Figure 1. Airflow can also be measured using air pressure measurement with a nasal prong.
  • Patent publications include US 6,422,240, which discloses an Oral/Nasal Cannula; US 4,648,407, which discloses a method for detecting and differentiating central and obstructive apneas in newborns; and US 6,155,986, which discloses a method of monitoring of oro-nasal respiration. Conventional techniques for measuring oro/nasal airflow can be unreliable, cumbersome, and prone to patient dislocation.
  • Bioimpedance is used to measure cardiac events, and is disclosed in patent specifications such as US 6,463,326 (Cardiac Pacemakers, "Rate adaptive cardiac rhythm management device using trans- thoracic impedance") who measure trans-thoracic impedance to extract both cardiac and ventilation parameters.
  • An early trans-thoracic bioimpedance monitor is disclosed in US 4,905,705.
  • Electrical bioimpedance is a representation of the relationship between the flow of electric current through body tissue and applied voltage.
  • the measurement of electrical bioimpedance has attracted increasing interest in recent years for a variety of purposes.
  • An electrical bioimpedance measurement is typically performed by passing a very low amplitude high-frequency alternating current (AC) across a section of tissue. Typical figures for amplitude and frequency of this current are 500 ⁇ A and 50 kHz respectively. Since the electrical conductivity of fatty tissue, bone, blood etc. are quite different, the measured impedance can give information about intra-body structure. For example, electrical impedance is higher in fatty tissue than in lean tissue, which allows bioimpedance to be used to measure body composition.
  • Impedance cardiography Another well-developed application of bioimpedance is its use in measuring cardiac output.
  • measurements are made of trans-thoracic impedance, using a pair of current injecting electrodes, and a separate pair of voltage sensing electrodes.
  • Impedance cardiography relies upon the fact that the current seeks the path of least impedance intra-thoracically, which is the blood filled aorta. As the volume of blood in the aorta changes through the cardiac cycle, these changes are reflected in the measured bioimpedance.
  • ICG impedance cardiogram
  • trans-thoracic bioimpedance measurements can also reflect movement of the ribcage and hence can provide information about respiratory effort. Since it is known that obstructive sleep apnea affects both respiratory effort and cardiac inter-beat intervals, it is reasonable that trans-thoracic impedance measurements have utility in the assessment of obstructive sleep apnea. Applicants have discovered that changes in the patency of the upper airway are reflected in changes in bioimpedance measured trans-cervically and applicants have invented novel methods of detecting sleep apnea using bioimpedance measurements.
  • the present invention seeks to alleviate the disadvantages associated with the prior art.
  • the present invention accordingly provides a diagnostic recording apparatus comprising means for measuring at least one electrical impedance measurement from a human and for generating bioimpedance signals; means for analysing said bioimpedance signals to produce output signals and means for providing diagnostic measures of sleep disordered breathing based on said output signals.
  • the apparatus includes signal processing means for filtering out unwanted interference from the bioimpedance signals and for producing processed bioimpedance signals for inputting to the analysing means.
  • the apparatus includes means for recording the processed bioimpedance signals
  • the means for analysing said processed bioimpedance signals comprises a computer algorithm performed within said apparatus.
  • the apparatus includes a display interface which allows direct inspection of the analysis results.
  • the means for analysing said processed bioimpedance signals comprises a computer algorithm performed on an external device, and the apparatus includes means for communicating with said external device.
  • the apparatus includes:
  • the present invention further provides a method of detecting sleep apnea using bioimpedance measurements including the steps of: -
  • step (D) using the estimate of respiratory events obtained from step (C) to detect presence of sleep apnea.
  • the set of electrodes are applied trans-cervically to the patient so as to obtain a trans-cervical bioimpedence signal from the patient.
  • the means for estimating a respiratory signal comprises applying a set of trans- thoracic electrodes to a patient, to obtain a trans-thoracic bioimpedance signal from that patient, over a pre-determined time period.
  • the present invention provides a method of detecting sleep apnea using bioimpedance measurements including the steps of: -
  • step (f) using the estimate of respiratory events obtained from steps (c) and (d) to detect presence of sleep apnea and preferably, classify the episode according to its type.
  • the means for estimating a respiratory signal may comprise a more direct measure such as rib cage movement measured using inductance plethysmography.
  • the signal-processing step at (e) above comprises: -
  • trans-thoracic bioimpedance signal passing the trans-thoracic bioimpedance signal through a filter means to separate the respiration and cardiac signal, thereby obtaining two bioimpedance signals, one being associated substantially with airflow, and the other being associated substantially with heart rate (cardiac output).
  • the signal-processing step at (e) above comprises:
  • This invention uses measurements of electrical bioimpedance in the real-time assessment of sleep apnea. Bioimpedance is measured by injecting high frequency sinusoidal AC current (with amplitude less than 1 mA) with a pair of current injecting electrodes, and then measuring the magnitude and phase of the sinusoidal voltage induced across sections of the body, using a second set of electrodes called the voltage sensing electrodes.
  • a set of trans-thoracic electrodes measures the trans-thoracic impedance and a second set of trans-cervical electrodes measures bioimpedance across the neck.
  • the trans-thoracic electrodes provide estimates of cardiac activity, respiratory effort, and airflow and the measurement provided by the trans-cervical electrodes reflects information primarily about respiratory effort.
  • This signal processing comprises firstly separating respiration and cardiac information using a filter.
  • This may be an adaptive filter, in adaptive line enhancement configuration, which effectively subtracts out the cardiac signal from the trans-thoracic bioimpedance signal, thereby obtaining a residual bioimpedance signal associated substantially only with respiration and by using the difference operator to obtain an estimation of airflow.
  • both trans-cervical and trans-thoracic bioimpedance measurements are made simultaneously and are used to detect the presence of apnea.
  • the trans-thoracic bioimpedance measurement is optimised to provide good respiratory signals with no motion artefacts and good cardiac signals.
  • the respiratory events of an obstructive nature are distinguished from those of a central nature through assessment of the spectral content of the impedance signals over the time periods of interest.
  • the trans-cervical bioimpedance measurement is optimised to provide good respiratory effort signal. Having obtained the bioimpedance measurements, evidence of apnea can be found by looking for
  • the measurement system of the present invention can be integrated with other signals typically used in sleep apnea diagnosis such as pulse oximetry, electroencephalograms, electrocardiogramns, electromyograms, and electrooculograms.
  • the method of the present invention ideally comprises the steps of:
  • acquiring multiple bio-impedance signals from a human over a period of time preferably, including measurements from a trans-thoracic and trans cervical configuration
  • step g joint processing of the signals mentioned in steps c,d,e, and f to identify periods 30 of obstructive and central apnea and hypopnea; h. characterizing of the results of the processing in step g in terms of clinically accepted measures such as apnea-hypopnea index;
  • step d processing of the signal obtained in step d to estimate hemodynamic parameters including stroke volume, cardiac output/index, systemic vascular resistance/index (SVR/SVRI), velocity index (VI), thoracic fluid content (TFC), systolic time ratio (STR), left ventricular ejection time (LVET), pre-ejection period (PEP), left cardiac work index (LCW LCWI), and heart rate; and
  • hemodynamic parameters including stroke volume, cardiac output/index, systemic vascular resistance/index (SVR/SVRI), velocity index (VI), thoracic fluid content (TFC), systolic time ratio (STR), left ventricular ejection time (LVET), pre-ejection period (PEP), left cardiac work index (LCW LCWI), and heart rate; and
  • step (i) processing of the sequence of inter-beat times derived in step (i) in order to assess cyclic variations in heart rate known to be associated with sleep disordered breathing
  • Figure 1 is a photograph of an oro/nasal airflow cannula of the prior art shown positioned under a patient' s nose;
  • Figure 2 is a schematic diagram showing the position of the trans-thoracic electrodes and the trans-cervical electrodes on a patient in accordance with one embodiment of the method of the present invention
  • Figure 3 is a graphical representation showing raw signals obtained from trans- thoracic impedance magnitude, trans-cervical impedance magnitude, ribcage respiratory effort, abdominal rib cage effort, and oro/nasal pressure;
  • Figure 4(a) is a graphical representation showing the measured trans-thoracic raw bioimpedance magnitude ( ⁇ ) over a period of 65 seconds
  • Figure 4(b) is a graphical representation showing the derivative of the raw bioimpedance signal over the same time period. The peaks in the signal ("E- points") were obtained using a simple threshold technique, and are marked with an x;
  • Figure 4(c) is a graphical representation showing estimated instantaneous heart rate over the same time period using the measured E points from Figure 5(b).
  • Instantaneous heart rate is obtained by simply taking 1 EE, where EE is the time between adjacent E points.
  • Figure 5(a) is a graphical representation showing the derivative of the trans- thoracic bioimpedance signal over a period of approximately 90 seconds [d(ICG)/ d t].
  • the peaks in the signal (“B-points") were obtained using a simple threshold technique, and are marked with an x. Some manual scoring was also necessary to remove spurious peaks;
  • Figure 5(b) is a graphical representation showing estimated instantaneous heart rate over the same time period using the measured E points from Figure 6(a).
  • Instantaneous heart rate is obtained by simply taking 1/EE, where EE is the time between adjacent E points;
  • Figure 5(c) is a graphical representation showing corresponding nasal airflow over the same time period. The patient experienced two apneic episodes over this time period. It can be seen that the apnea leads to characteristic bradycardia/tachycardia patterns.
  • Figure 6 is a block diagram showing the Adaptive Line Enhancement (ALE) technique.
  • ALE Adaptive Line Enhancement
  • Figure 7(a) is a graphical representation showing the estimated airflow signal over a period of 200 seconds. The signal is closely correlated with the measured airflow.
  • Figure 7(b) is a graphical representation showing the measured airflow signal using a nasal prong over the same period as Figure 7(a).
  • Figure 8 displays a block diagram of the automated apnea detection scheme.
  • Figure 9 displays a block diagram of the peak detector used to obtain the E peaks of the cardiac signal.
  • two sets of bioimpedance measurements were made using an Electrical Bioimpedance Amplifier.
  • This amplifier measures bioimpedance by using a lOO ⁇ A sinusoidal current at one of four frequencies (12.5, 25, 50 or 100 kHz).
  • Figure 2 shows the location of the electrodes on a patient.
  • the current is injected into the measurement tissue volume using a pair of current electrodes, labelled I ln + and I ⁇ n - as shown in Figure 2.
  • the corresponding AC voltage caused by this current can be measured at the body surface using the voltage sensing electrodes. More robust measurements can be achieved by injecting an equal in-phase current of 50 ⁇ A at two locations.
  • the bioimpedance amplifier can also average the differential voltage sensed across two different paths.
  • the bioimpedance amplifier detects the magnitude and phase of the sinusoidal current at the frequency of interest.
  • the trans- thoracic measurements were carried out using a 25kHz current and the trans-cervical measurements were carried out using a 100 kHz current.
  • the described components of the system provide four electrical signals:
  • the trans-thoracic bioimpedance magnitude is referred to as the trans-thoracic bioimpedance signal
  • the trans-cervical bioimpedance magnitude is referred to as the trans-cervical bioimpedance signal.
  • Figure 3 shows examples of typical raw impedance magnitude signals measured from human volunteers with sleep apnea. It also shows respiratory effort at the ribcage and abdomen, as well as oro/nasal airflow, measured by nasal prong.
  • Signal processing of the measured bioimpedance signals provides means to obtain useful information about the presence of obstructive or central apnea events.
  • the following notation will be used to describe the measured signals:
  • FIG. 4a shows measurements of trans-thoracic impedance magnitude over a time period of approximately one minute. This raw signal does not convey much visually useful information; however, the derivative of the signal ( Figure 4b) clearly shows the time of left ventricle ejection, as described in the previous section. The corresponding instantaneous heart rate (in beats per minute) is shown.
  • Figure 5 shows the cardiac activity as assessed by this technique during an episode of apnea, interspersed with breathing. Characteristic patterns can be seen in the heart rate, which themselves can be used to assess for apnea.
  • the trans-thoracic bioimpedance signal Z[n] consists of a slowly- varying almost- periodic signal that represents a respiratory effort signal which typically has low harmonic content and is concentrated in a narrow low-frequency band (typically 0.15-0.4 Hz for adult humans).
  • the trans-thoracic bioimpedance also contains an approximate wideband signal c[n], that represents the cardiac signal.
  • the spectral content of the cardiac signal typically extends from around 1 Hz (which is the fundamental frequency) to several Hz (the harmonics of the fundamental cardiac frequencies).
  • the trans-thoracic bioimpedance signal can be modeled as a mixture of a periodic (respiration) and wideband (cardiac) signal.
  • a periodic and a broadband signal is a special configuration of an adaptive noise canceller termed Adaptive Line Enhancement (ALE).
  • ALE Adaptive Line Enhancement
  • an adaptive noise canceller attempts to "subtract" out the effect of the signal in a reference input u[n] from the signal in a primary input
  • the primary input is taken to consist of two signals, which are uncorrelated with each other, and the reference signal consists of a signal that is somehow correlated with one of the signals in the primary input.
  • L is the length of the filter.
  • the adaptive algorithm attempts to recursively find an optimum filter, which minimises an error signal as follows.
  • the optimum choice of filter coefficients minimises the mean square error between the primary input signal d[n] and the estimated is minimised.
  • E[ ] denotes the expectation operator.
  • LMS Least Mean Squares
  • a modification of the general adaptive noise canceller can be used in order to cancel a periodic interference from a wide band signal or to detect a narrow-band signal buried in broad-band noise without the use of an external reference input.
  • the technique is called Adaptive Line Enhancement (ALE), and is particularly suited to our application.
  • ALE Adaptive Line Enhancement
  • a typical ALE signal processing system is shown in Figure 6. The underlying idea is that when a signal consists of a periodic a[n] and wide band component c[ ⁇ ], it is possible to detect the periodic component by creating a reference input which is a delayed version of the primary input. The delay should be longer than the correlation time of the wide-band component, which can be estimated a priori.
  • the resultant signal will then indicate changes in the trans-thoracic impedance magnitude which are solely due to cardiac changes and y[n] will indicate changes in the trans-thoracic impedance magnitude which are due to respiration.
  • techniques for estimating cardiac activity i.e., determining heart rate, left ventricle ejection time, and other cardiac parameters
  • FIG. 8 A schematic representation of the method is shown in Figure 8.
  • the trans-thoracic signal is passed through an ALE in order to separate the cardiac and respiration information from the impedance signal.
  • the error output of the ALE contains the cardiac information and the high frequency noise of the impedance signal.
  • the cardiac signal is passed through a low pass filter to eliminate noise (FIR 64 taps, 3 Hz cut-off frequency).
  • the cardiac signal passes through a first-order differentiator to obtain the first derivative of the signal.
  • the normalized signal is then passed through a threshold-based peak detector in order to find the positions of the E peaks.
  • the threshold can be calculated based on the root mean square of the signal, as shown in Figure 9.
  • the net outcome of the peak detector is a set of locations in time corresponding to the E peaks (which are the point of maximum cardiac outflow). This provides an estimate of the heart rate.
  • This cardiac signal can also be processed to give information about the cardiac stroke volume.
  • the respiration signal In parallel to cardiac signal processing, we process the respiration signal Firstly, we create a sliding window that contains a ten-second segment of data. In order to eliminate the baseline wander the trend (linear fit to data) of the segment is removed. Then the signal segment is normalized by dividing by the standard deviation of the last 60 seconds of the data. The third power of the normalized signal segment is calculated, in order to enhance the difference of the standard deviation during normal breathing and the standard deviation during apnea. For that segment, the standard deviation is estimated. If the standard deviation is below a predefined threshold (in our case, set to 0.6) the window is assumed to contain data that corresponds to apnea. Otherwise the data window is said to contain normal breathing. We then measure the time between two successive normal breathing marks, and if that time is greater than 10 seconds then that segment is assumed to contain an apnea episode.
  • a predefined threshold in our case, set to 0.6
  • the method of the invention (using a set of bioimpedance measurements) allows the clinician to assess the presence of sleep apnea though provision of measurements relating to
  • Cardiac activity e.g., heart rate
  • Ribcage respiratory effort • Airflow estimates
  • the method of the invention has the advantage that it uses bioimpedance to identify apnea episodes, and to classify them as obstructive or central in nature.
  • the method of the invention also has the advantage that it provides a new technique for assessing oro-nasal airflow, which is independent of nasal thermistors or pressure measurement. It also allows for use of a single transduction technique (impedance measurements), which can simplify the design of an apnea monitoring system.

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Abstract

La présente invention concerne procédé et un appareil de détection de l'apnée du sommeil par des mesures de bioimpédance. A cet effet, on applique au patient un ensemble d'électrode de façon à obtenir de ce patient, pendant une période définie, un signal de bioimpédance transcervicale. On mesure ensuite le signal de bioimpédance transcervicale de façon à obtenir de l'information concernant les événements respiratoires de ce patient pour la période définie. On estime le signal de respiration au moyen d'un organe permettant d'estimer un signal respiratoire, et on utilise l'estimation des événements respiratoires obtenue pour détecter la présence d'une apnée du sommeil. L'invention concerne également un appareil mettant en oeuvre ce procédé.
PCT/IE2004/000088 2003-06-24 2004-06-24 Procedes et appareil pour detecter l'apnee du sommeil en utilisant des mesures de bioimpedance WO2004112606A1 (fr)

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IES20030467 IES20030467A2 (en) 2003-06-24 2003-06-24 Methods for detecting sleep apnea using bioimpedance measurements
IES2003/0467 2003-06-24

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US10492720B2 (en) 2012-09-19 2019-12-03 Resmed Sensor Technologies Limited System and method for determining sleep stage
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US8414498B2 (en) 2003-05-12 2013-04-09 Cheetah Medical, Inc. System, method and apparatus for measuring blood flow and blood volume
US8388545B2 (en) 2005-02-15 2013-03-05 Cheetah Medical, Inc. System, method and apparatus for measuring blood flow and blood volume
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