Title: Method and apparatus for monitoring the autonomic nervous system
Abstract: An apparatus and method for detection and monitoring of autonomic nervous system (ANS) activity in humans, primarily in the field of sleep research. The present invention discloses a portable, simple, and cost-effective electronic device containing hardware and software that permits real-time monitoring of a pulsatile blood volume waveform obtained through use of a photoplethysmographic (optical volume detecting) probe, thereby allowing signal conditioning, waveform slope analysis, display, recording, and output of pulse transitional slope data representative of activity in the ANS.
Patent Number: 7,024,234 Issued on 04/04/2006 to Margulies, et al.
| Inventors:
|
Margulies; Lyle Aaron (5546 - 34th Ave. NE., Seattle, WA 98105);
Harrell; David B. (5122 - 103rd St. SW., Mukilteo, WA 98275);
Riggins; Michael (320 NE. 57th St., Seattle, WA 98105)
|
| Appl. No.:
|
666121 |
| Filed:
|
September 19, 2003 |
| Current U.S. Class: |
600/324; 600/310 |
| Current Intern'l Class: |
A61B 5/02 (20060101) |
| Field of Search: |
600/324,323,310,500,501
|
References Cited [Referenced By]
U.S. Patent Documents
| 4859057 | Aug., 1989 | Taylor et al.
| |
| 5299188 | Mar., 1994 | Hotta et al.
| |
| 5520176 | May., 1996 | Cohen.
| |
| 5605151 | Feb., 1997 | Lynn.
| |
| 5999846 | Dec., 1999 | Pardey et al.
| |
| 6091973 | Jul., 2000 | Colla et al.
| |
| 6228033 | May., 2001 | Koobi et al.
| |
| 6272378 | Aug., 2001 | Baumgart-Schmitt.
| |
| 6319205 | Nov., 2001 | Goor et al.
| |
| 6322515 | Nov., 2001 | Goor et al.
| |
| 6358201 | Mar., 2002 | Childre et al.
| |
| 6363270 | Mar., 2002 | Colla et al.
| |
| 6402698 | Jun., 2002 | Mault.
| |
Other References
Meoli et al.; Hypopnea in Sleep-Disordered Breathing in Adults; SLEEP. vol. 24,
No. 4, 2001; pp. 469-470.
Pitson et al.; Value of beat-to-beat blood pressure changes, detected by pulse
transit time, in the management of the obstructive sleep apnoea/hypopnoea syndrome;
Eur Respir J ; 1998; 12: 685-692.
Bakewell; The Autonomic Nervous System; Update in Anasthesia; Issue 5; 1995;
Article 6; pp. 1-2.
Pagani et al.; Detection of Central and Obstructive Sleep Apnea in Children Using
Pulse Transit Time; International Symposium ISMDA-2002 Proceedings; Rome, Italy;
Oct. 10-11, 2002; Berlin: Springer-Verlag; pp. 144-158.
|
Primary Examiner: Winakur; Eric F.
Assistant Examiner: Natarajan; Vivek
Attorney, Agent or Firm: Garrison & Associates PS, Garrison; David L.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims benefit of U.S. Provisional Application Ser. No. 60/412,310
entitled Method and Apparatus for Monitoring the Autonomous Nervous System, filed
Sep. 20, 2002.
Claims
We claim:
1. An apparatus for monitoring human autonomic nervous system activity using
pulsatile blood volume waveform signals, said apparatus comprising:
a photoplethysmographic probe having a light emitting element and an opposing
light detecting element, and having an output signal indicating changes in blood
volume on at least one alpha andrenergic receptor site of a human body;
a processor element, responsive to said output signal indicating changes in blood
volume, said processor element defining a time interval for calculation of slope
of blood volume waveform continuously and in real time and reducing said waveform
signals to a slope value within said time interval;
said processor element containing an algorithm for normalization of the slope value;
said processor element containing an artifact rejection algorithm for eliminating
from further processing slope values less than one; and
amplifier and filter circuitry for rendering output signals representative of
said slope values.
2. The apparatus of claim 1, wherein the photoplethysmographic probe is adapted
for application on a finger.
3. The apparatus of claim 1, wherein the photoplethysmographic probe is adapted
for indirect application to the alpha andrenergic receptor site, whereby no direct
contact with a body part is required.
4. The apparatus of claim 3, further comprising a display for indicating information
representative of pulsatile blood volume waveform signals.
5. The apparatus of claim 3, further comprising a display for indicating information
representative of slope values.
6. The apparatus of claim 3, further comprising a display for indicating information
representative of a slope ratio.
7. The apparatus of claim 1, further comprising at display for visual indication
of output signals.
8. The apparatus of claim 1, further comprising an electronic storage medium
for data storage capability.
9. The apparatus of claim 1, further comprising at least one data part for downloading
output signals.
10. A method for identification of human autonomic nervous system activity, the
method comprising the steps of:
disposing a photoplethysmographic probe proximate to a single alpha andrenergic
receptor site of a human body part;
obtaining an electrical signal from said probe representative of pulsatile blood
volume within said body part;
deriving a pulsatile blood volume waveform as a function of amplitude and time;
defining a time interval for calculation of a slope of the pulsatile blood volume waveform;
applying an algorithm that continuously provides real-time calculation of the
slope along said waveform within said time interval;
dividing peak amplitude values by a time constant and eliminating slope values
less than 1, whereby artifact elimination is achieved;
normalizing slope values; and providing information representative of slope values,
whereby autonomie nervous system activity is monitored.
11. The method of claim 10 further comprising the step of applying signal filtration
means, wherein undesirable low and high frequency signal components are eliminated.
12. The method of claim 10 further comprising the step of monitoring the pulsatile
blood volume amplitude.
13. The method of claim 10 further comprising the step of amplifying and filtering
slope values, whereby improved sensitivity and accuracy is achieved.
14. The method of claim 10 further comprising the step of providing an output
display of visual information representative of slope values.
15. The method of claim 10 further comprising the step of providing data output
representative of input data and slope values.
16. The method of claim 10 further comprising the step of providing a means for
storing data representative of input data and slope values.
Description
TECHNICAL FIELD
This invention relates to medical devices, and more particularly to physiological
monitoring methods and devices used for detection of autonomic nervous system (ANS)
activity in the field of sleep research. The present invention discloses a portable,
simple, and cost-effective electronic sleep diagnostic device containing hardware
and software that permits recording and signal conditioning of a pulsatile blood
volume waveform obtained through use of a photoplethysmographic (optical volume
detecting) probe, thereby allowing analysis pulse transitional slope data that
is representative of activity in the autonomic nervous system (ANS).
BACKGROUND OF THE INVENTION
Cardiovascular risk is directly linked to sleep related breathing
disorders (SRBD). The number of U.S. laboratories that study sleep, roughly 2,792,
is incredibly low when compared to the number of Americans estimated to have a
chronic SRBD, just over 40 million. The average number of beds per lab is 3.6 bringing
the total number of beds in which to do a sleep study to roughly 10,000. This means
that to test all 40 million Americans, there would be 4,000 patients that would
be seen per bed. If sleep tests were run 365 days per year, the result is an astounding
11 years of conclusive tests needed to be run to test the current population of
individuals suffering form SRBD. The length of time increases as one considers
the actual number of days per year sleep labs actually test patients, plus the
amount of tests that need to be re-run due to inconclusive testing, plus the number
of patients that continually need to be retested to see if their treatment is functioning
properly. Given this scenario, it is no shock that wait times for patients to be
scheduled for a sleep test can typically range from six weeks to six months. The
problem will only increase, as "it is estimated that nearly 80 million Americans
will have a sleep problem by the year 2010 and 100 million will have one by the
year 2050." Clearly then, the problem with wait time for testing should be addressed
immediately to relieve pent up demand.
The current "gold standard" for testing sleep related breathing disorders is
full polysomnography. Full polysomnography is, however, quite labor intensive,
requires considerable instrumentation and is therefore rather expensive to conduct.
As a result, many sleep laboratories have found it difficult to keep up with the
demand for this test, and a long waiting list becomes the norm. Given that obstructive
sleep apnea (OSA) is quite prevalent, leads to serious complications and that treatment
options exist, it is important that individuals suffering from the disease are identified.
The need to study the ANS has been realized in academia for a considerable time.
It is known in the field of microneurography that rapid-eye movement (REM) sleep
is associated with profound sympathetic activity. It has also been found that arousals
from non-rapid-eye movement (NREM) elicits K complexes that are associated with
sympathetic activity. The sympathetic division of the ANS prepares a body for movement.
Arousals require movement and hence an arousal requires sympathetic activation.
Generally, patients with OSA, a type of SRBD, have extremely disrupted
sleep and terribly high daytime somnolence. Obstructive sleep apnea events are
always accompanied by an acute rise in systolic blood pressure (rises in systolic
blood pressure are associated with sympathetic activation), even when the usual
EEG criteria for arousals are not met (a recognizable cortical electroencephalographic
arousal). The duration of the apnea of individuals that demonstrate EEG arousal
and those that do not meet the usual criteria for defining an arousal have been
found to be identical. The pleural pressure peak, at the end of apnea, is identical
between the two types of arousals, as are the EEG frequencies. These findings suggest
that monitoring the cardiac changes of sleep is a more accurate measurement.
It has been demonstrated that apneic episodes result in progressive increases
in sympathetic nerve activity. The increases are most marked toward the end of
the apnea, when a patient moves. These findings are exactly what is excepted of
sympathetic activation and its relationship to arousals in patients with SRBD.
Because cardiovascular control during sleep is primarily dictated by brain
states that produce profound variation in ANS activity, many studies have been
conducted to monitor the ANS. Since the data shows clearly that monitoring the
ANS or cardiac changes in sleep yields more accurate data defining an arousal in
sleep, it is clear that diagnostic studies must include ANS or cardiac monitoring.
It has been shown that in transitions from NREM to REM sleep, heart rate accelerations
precede the EEG arousals marking the onset of REM. Therefore, not only does monitoring
ANS activity give the clinician a possibly more accurate study, but also changes
in ANS activity precede that information being observed via the EEG electrodes.
There are two existing technologies that attempt to monitor the ANS, namely
pulse transit time (PTT) and peripheral arterial tonometry (PAT). Neither PTT nor
PAT can lay claim to monitoring the ANS without adding additional sensors. PTT
requires the use of ECG electrodes that may be difficult for a patient to self-apply
due to skin cleaning and shaving requirements. PAT requires a very costly gauntlet-type
device with a single-use finger pressure cuff. Also, the addition of extra sensors
adds to noise artifact and difficulty in patient use. It is therefore an object
of the present invention to provide an improvement over existing PTT and PAT technology
through a more economical and more easily used device without need of additional sensors.
Several disclosures have been made in the prior art that teach methods and
devices for diagnosis and monitoring of sleep breathing disorders using physiological
data obtained from pulse oximetry-derived waveforms.
U.S. Pat. No. 5,398,682 to Lynn (Mar. 21, 1995) discloses a method and apparatus
for the diagnosis of sleep apnea utilizing a single interface with a human body
part. More specifically, a device is disclosed for diagnosing sleep apnea by identifying
the desaturation and resaturation events in oxygen saturation of a patient's blood.
The slope of the events is determined and compared against various information
to determine sleep apnea.
U.S. Pat. No. 6,363,270 B1 to Colla, et al. (Mar. 26, 2002) discloses a method
and apparatus for monitoring the occurrence of apneic and hypopneic arousals utilizing
sensors placed on a patient to obtain signals representative of at least two physiological
variables, including blood oxygen concentration, and providing a means for recording
the occurrence of arousals. Obtained signals pass through conditioning circuitry
and then processing circuitry, where correlation analysis is performed. A coincident
change in at least two of the processed signals are indicative of the occurrence
of an arousal that in turn indicates an apneic or hypopneic episode has occurred.
A patient thus can be diagnosed as suffering conditions such as obstructive sleep apnea.
U.S. Pat. No. 6,529,752 B2 to Krausman and Allen (Mar. 4, 2003) discloses a
method and apparatus for counting the number of sleep disordered breathing events
experienced by a subject within a specified time period. Such a counter comprises:
(1) an oxygen saturation level sensor for location at a prescribed site on the
subject, (2) an oximetry conditioning and control module that controls the operation
of the sensor and converts its output data to oxygen saturation level data, (3)
a miniature monitoring unit having a microprocessor, a memory device, a timer for
use in time-stamping data, a display means and a recall switch, and (4) firmware
for the unit that directs: (i) the sampling and temporary storage of the oxygen
saturation level data, (ii) the unit to analyze using a specified method the temporarily
stored data to identify and count the occurrence of the subject's disordered breathing
events, and to store the time of occurrence of each of these events, and (iii)
the display means to display specified information pertaining to the counts in
response to the actuation of the recall switch.
U.S. Pat. No. 6,580,944 B1 to Katz, et al. (Jun. 17, 2003) discloses a method
and apparatus for identifying the timing of the onset of and duration of an event
characteristic of sleep breathing disorder while a patient is awake. Chaotic processing
techniques analyze data concerning a cardiorespiratory function, such as oxygen
saturation and nasal air flow. Excursions of the resulting signal beyond a threshold
provide markers for delivering the average repetition rate for such events that
is useful in the diagnosis of obstructed sleep apnea and other respiratory dysfunctions.
The above references all make use of oxygen saturation data obtained through
pulse oximetry to determine arousals and/or sleep breathing disorders. Each necessarily
requires additional analysis and calculation of blood oxygen concentrations in
order to render information useful specifically in the diagnosis and monitoring
of sleep breathing disorders. It is therefore another object of the present invention
to provide a more simplified method of obtaining and analyzing physiological data
that accurately represents ANS activity.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to overcome one or more of the problems
with the prior art. In one preferred embodiment the present invention provides
a method and apparatus for improved monitoring of ANS activity using a single patient sensor.
A variety of breathing disturbances may occur during sleep, including snoring,
hypoventilation, apnea, increased upper-airway resistance, and asthma related conditions.
This project proposes development of a novel device that can noninvasively and
accurately detect frequent brief micro arousals that are not well identified by
conventional airflow, respiratory effort, pulse oximetry and EEG methods. These
subcortical events result from increased respiratory effort and cause disruption
of nocturnal sleep, leading to excessive daytime somnolence.
Since microarousals have been associated with changes in autonomic system outflow,
this invention provides for a small, portable device that analyzes the shape of
the arterial finger pulse, thereby detecting on a beat by beat basis changes in
vascular tone directly attributable to microarousals. The present invention uses
a photoplethysmographically derived arterial blood volume waveform for monitoring
changes in peripheral arterial vascular tone, in conjunction with A/D converters
and a microcontroller for analyzing the morphology of the pulsatile signal.
The method of the present invention provides for detection of microarousals that
compares favorably with detection by pulse transit time (PTT) devices, EEG analysis,
ECG analysis, esophagal pressure (Pes) or some combination of these methods. Although
PTT and peripheral arterial tonometry (PAT) have both been receiving much attention
as techniques for detecting changes in the ANS during sleep studies, PAT is relatively
expensive and PTT has implementation problems caused by motion artifact.
It is a further object of the present invention to provide an apparatus that
utilizes
transmitted light intensity from an existing FDA approved pulse oximeter probe
so that no additional device is attached to the patient. Valuable diagnostic information
can then be extracted through electronic processing of this existing data.
Normalization is a method to correct for the photoplethysmographic
pulse signal morphological changes based on finger position (as opposed to actual
changes of autonomic activity.) PTT and PAT lack a means for signal normalization
and therefor cannot correct for finger position changes. Normalization provides
immunity to artifact caused by both elevation changes of the finger probe, and
changes in blood flow due to arterial compression during patient positional changes.
It is therefor another object of the present invention to provide a means of normalization
in order to ensure appropriate artifact suppression.
Since pulse oximeters use an alternating flashing of two different wavelength
LEDs, the present invention is intended to synchronize with the desired LED in
order to examine the transmitted intensity due to a single wavelength. Alternatively,
certain models of oximeter OEM modules provide an analog or digital output that
can be utilized directly by the present invention.
Another objective is to provide algorithms for slope detection, peak to peak
height, and normalization may be performed either with firmware within the present
invention apparatus, or by software after the data is downloaded into a polysomnograph
or other data processing device.
It is a further objective of the present invention to provide a means of data
storage and transfer, and to provide a method of displaying the observed changes
in slope. Alternative embodiments display these changes as a waveform, light bars,
and/or numerical information.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic representation of a typical pulse oximeter sensing
configuration on a finger.
FIG. 2 shows a graphic representation of the components of vascular tissue that
contribute to light absorption plotted as absorption versus time.
FIG. 3 shows a graphic representation of a single peripheral pulse waveform
plotted as volume versus time.
FIG. 4 shows comparative physiological waveforms following administration of
vasoactive agents.
FIG. 5 shows a second derivative waveform consisting of a, b, c and d waves
in systole, and an e wave in diastole.
FIG. 6 shows a graphic representation of changes in Normalized Slope plotted
as slope ratio versus heart beats while subject performs Valsalva maneuver.
FIG. 7 shows a sleep stage hypnogram of an hour and a quarter sleep study.
FIG. 8 shows a block diagram of the present invention apparatus.
FIG. 9 shows a block diagram of the present invention method.
DETAILED DESCRIPTION OF THE INVENTION
A variety of breathing disturbances may occur during sleep, including snoring,
hypoventilation, apnea, increased upper-airway resistance, and asthma related conditions.
The present invention discloses a method and apparatus that can noninvasively and
accurately detect frequent brief "microarousals" (small amplitude subcortical disturbances
that disrupt normal sleep) that are not well identified by conventional airflow,
respiratory effort, pulse oximetry and EEG methods. These subcortical events result
from increased respiratory effort and cause disruption of nocturnal sleep, leading
to excessive daytime somnolence.
Microarousals can be detected using data obtained from the absorbance
of visible or infrared light in a finger or other body part of a patient, and by
analyzing changes in the obtained peripheral blood volume waveform that are indicative
of microarousals. Specifically, sufficient information is contained in slope variations
of the rising edge of the pulsatile blood volume waveform to allow analysis of
changes in the autonomic nervous system (ANS). This technology is herein referred
to as pulse transitional slope (PTS). Both ANS and hemodynamic responses occur
during obstructive sleep apnea and are influenced by apnea, hypopnea, hypercapnea,
and arousal.
Analysis of the noninvasive blood pressure pulse wave has been shown to
be useful for evaluation of vascular load and aging. Pressure transducers located
at a palpable artery, such as the carotid, femoral, or radial artery provided a
detailed waveform of pressure versus time. This continuous pulse wave tracing contains
precise waveshape, frequency, and inflection information easily discernable by
the human eye that is not available from only systolic and diastolic pressure numerics.
The progression from pressure transducers to photoplethysmography allows detection
of the pulse wave at sites not easily palpated, including the finger and earlobe.
Photoplethysmography detects the changes in the amount of light absorbed by hemoglobin,
which corresponds to changes in blood volume. Changes in amplitude of the photoplethysmographic
wave have been used to evaluate arterial compliance, but the wave contour itself
was not used, as is disclosed by the present invention.
Plethysmography is the measurement of volume changes of tissue or
an organ. Photoplethysmography measures blood volume changes in a tissue using
the fractional change in light transmission. One of the most common applications
of this technology is the noninvasive measurement of the oxygen saturation of the
hemoglobin in red blood cells through a technique called pulse oximetry. FIG. 1
shows a typical pulse oximeter sensing configuration on a finger. Typically, two
different wavelengths of light (e.g. 660 and 805 nm) are applied to one side of
a finger and the received intensity is detected on the opposite side after experiencing
some absorption by the intervening vascular tissues. The amount of absorption (and
conversely transmission) is a function of the thickness, color, and structure of
the skin, tissue, bone, blood, and other tissues that the light traverses.
The present invention is specifically directed to alpha andrenergic receptor
sites, the activation of these receptors at certain locations on the body resulting
in physiological responses such as peripheral vascular resistance, mydriasis, and
contraction of pilomotor muscles, which are representative of sympathetic nervous
system activity. The preferred locations generally include the fingers and the
big toe (other sites are under investigation), due to a desirable lack of beta
or parasympathetic receptors at those locations on the body.
The transmitting light comes from light emitting diodes (LEDs), typically in
the visible red and the invisible infrared (IR) spectrums. The optical receiver
may be a photodiode, photoresistor, or solar cell. By using two different wavelengths,
each with different absorbance characteristics in oxygenated and deoxygenated blood,
the intensity ratio between the two received signals can be analyzed, and not just
the intensity. Therefore the attenuating tissues mentioned earlier do not affect
the ratio of the intensities, which via a look-up table can determine the oxygen
saturation percent in the finger vasculature.
FIG. 2 shows the components of vasculature tissue that contribute to light absorption.
The static or dc component of the received optical signal represents light absorption
by the tissue, venous blood, pigments and other structures. The present invention
is concerned with the ac, or pulsatile component because the focus is on examining
the wave shape of the systolic portion of the blood volume waveform. Electronically,
the dc component is removed with a simple resistor-capacitor high pass circuit
that has a -3 dB frequency of around one Hertz.
The amount of light passing through the finger is called transmittance, T, and
is defined by:
T=I/Io
where Io is the intensity of the incident light and I is the intensity of the
transmitted light.
The amount of light of a specified wavelength absorbed by a substance is directly
proportional to both the length of the light path and the concentration of the
material within the light path. The absorbance, A, is defined as the negative logarithm
of the transmittance, or:
A=-log
T=-log
I/Io=aCL
where a is a constant called the extinction coefficient and is dependent on
the wavelength of the light passing through the substance and on the chemical nature
of the substance. C is the concentration of the substance and L is the path length
of the absorbing material.
The present invention makes use of just one of the wavelengths from the pulse
oximeter probe, since the objective is to observe only relative changes in the
pulse wave shape, which in turn is derived from systolic blood volume changes in
the finger. Since a pulse oximeter probe is part of all portable sleep diagnostic
screening devices, it is a further object of the present invention to tap into
the received light intensity signal of an existing probe, thereby alleviating the
need for any additional patient sensors.
FIG. 3 shows a typical peripheral pulse waveform. Pulse height is the number
of A/D counts between the minimum and maximum excursions of each pulse, while the
slope is also calculated in A/D counts for a fixed period of time beginning about
40 ms after a minimum is detected.
The first and second derivative waveforms of the photoplethysmographic waveform
have characteristic contours, and the contour of the second derivative facilitates
the interpretation of the original waves. The analysis of the second derivative
of a fingertip photoplethysmogram waveform has been shown to be a good indicator
of the effects of vasoconstriction and vasodilation by vasoactive agents, as well
as an index of left ventricular afterload as shown in FIGS. 4A,
4B and
4C.
FIGS. 4A,
4B and
4C show waveform tracings demonstrating the
results of administration of vasoactive agents. FIG. 4A shows the ECG parameter,
FIG. 4B shows corresponding PTG and SDPTG waveforms, and FIG. 4C shows corresponding
AoP and AoF waveforms. An increase in the late systolic component of aortic pressure
(AoP) and PTG after intravenous injection of 2.5 mg AGT and a deepened d-wave in
relation to the height of the a-wave (decreased d/s) are seen in SDPTG. On the
other hand, NTG produces marked reduction in late systolic components of aortic
pressure and PTG, with d-waves becoming shallower in relation to the height of
a wave (increased d/a). AoF indicates ascending aortic flow velocity. Augmentation
index (AI) is defined as the ratio of the height of the late systolic peak to that
of the early systolic peak, two components of the ascending aortic pressure at
the anacrotic notch.
Selected Abbreviations and Acronyms
AGT=Angiotensin
AI=Augmentation Index
NTG=Nitroglycerin
PTG=Photoplethysmography
SDPTG=Second Derivative Wave of Fingertip Photoplethysmography, where
the a through d components of the second derivative wave are described in FIG.
5. The second derivative waveform consists of a, b, c, and d waves in systole and
an e-wave in diastole.
Pulse transitional slope (PTS) technology as applied in the present invention
expands on this concept of using photoplethysmographically derived waveforms to
assess changes in vascular tension, whether caused by apnaeic obstruction or the
more subtle microarousals that are not detectable by cortical means. A normalized
slope is calculated by dividing the height achieved during 40 ms of rise time by
the maximum height of the pulse waveform (=height of late systolic peak). A normalized
slope can be calculated in real time by a microprocessor controlled device as opposed
to the post processing (analysis after recording) required by second derivative
methods. This will allow use of the present invention technology in labs performing
overnight polysomnograph studies in addition to the intended use for home sleep screening.
Since vasoactive drugs have a distinct and predictable affect on the AI when
measured by photoplethysmographic methods, by extension the body's own hormonal
control of the arterial system shows comparable changes in the pulse waveform when
measured using similar techniques.
The present invention provides a portable, simple, and cost effective sleep diagnostic
method and apparatus capable of detecting arousals and microarousals without adding
EEG electrodes or additional patient sensors beyond those worn during a typical
home study.
Since microarousals have been associated with changes in autonomic system outflow,
an object of the present invention is to provide a small, portable device that
analyzes the shape of the arterial finger pulse, thereby detecting on a beat by
beat basis changes in vascular tone directly attributable to microarousals. The
present invention uses a photoplethysmographically derived arterial blood volume
waveform for monitoring change in peripheral arterial vascular tone in conjunction
with A/D converters and a microcontroller for analyzing the morphology of the pulsatile signal.
Detection of microarousals by the present invention compares favorably
with results achieved using pulse transit time (PTT) devices, EEG analysis, ECG
analysis, esophagal pressure (Pes), and combinations of these methods. Although
PTT and peripheral arterial tonometry (PAT) have both been receiving much attention
as techniques for detecting changes in the ANS during sleep studies, PAT is relatively
expensive and PTT has implementation problems caused by motion artifact.
Efficacy of the present invention has been verified through monitoring of
test subjects performing a "Valsalva Maneuver," which is the quickest and most
dramatic method of producing ANS discharge—a resulting increase in intrapulmonic
pressure produced by forcible exhalation against the closed glottis. This produces
a sympathetic discharge with subsequent vascular constriction.
A typical response to the Valsalva Maneuver is shown in FIG. 6. The normalized
slope increases significantly, around 30% on the average which we postulate to
be caused by increased rate of heart tissue conduction, increased contraction force,
and increased rigidity in the arterioles. FIG. 6 shows changes in Normalized Slope
produced by the present invention during a Valsalva Maneuver. The increase in ANS
outflow begins around heart beat
59, indicated by the sharp rise in the
normalized slope of he pulsatile arteriole waveform.
Further testing was conducted using daytime nap studies—Several short
daytime nap studies were performed on sleep deprived volunteers for the purpose
of scoring the sleep stages during these naps and looking for correlations between
the stages and recorded normalized PTS slopes. None of the subjects were known
to have sleep disordered breathing. Volunteers were monitored with two central
lobe electroencephalographic EEG electrodes, two occipital EEG electrodes, two
electrooculogram (EOG) electrodes, a chin electrode, a nasal air flow device, two
respiratory airflow belts, and a PTS apparatus of the present invention, which
provided a normalized slope value on a beat to beat basis.
A typical recording of the normalized slope (on a scale of 0 to 100, where 100
is vertical) versus the sleep stages is shown in FIGS. 7A and 7B. The sleep stages
were scored by a registered polysomnographic technologist (RPSGT) from the EEG,
EOG, and respiratory waveforms. FIGS. 7A and 7B show a sleep stage hypnogram of
an hour and a quarter sleep study. FIG. 7A shows sleep ratio percentages through
the duration of the study. FIG. 7B shows a graph that has been scored from EEG,
EOG, and respiratory waveforms according to the sleep scoring convention of the
American Sleep Academy. Point A is the beginning of stage
3 sleep, corresponding
to point B on the normalized pulse slope diagram. Area C is stage
4, and
a definitive corresponding area of reduced slope values can be seen in the area
labeled D. As sleep becomes lighter, rising from at point E to stage
3 and
then stage
2, a corresponding rise in slope can be seen starting at point F.
A block diagram of a preferred embodiment of the present invention apparatus
is
shown in FIG. 8. The device is battery powered
81, with sufficient capacity
for a 12 hour overnight study; analog circuitry for voltage regulation and power
supply conditioning; the device has an interface
82 for an OEM supplied
finger pulse oximetry probe
83; a low frequency front-end filter
84
for probe input signals; an input signal pre-amplifier
85; a high frequency
filter
86; a gain-controlled signal amplifier stage
87; a bar graph
display for indication of a pulse signal
88; a polygraph output
89
for pulse signal data; a means of digital processing via a microprocessor or microcontroller
810 to provide slope detection and peak to peak height determination of
each systolic finger pulse, mathematical normalization of the slope, digital to
analog (D/A) conversion of the slope value for polysomnographic display, and digital
control of the finger probe gain, and having a status indicator LED
812;
user controls
811 including start/stop and transmit functions; a polygraph
output
813 for slope ratio data; a bar graph display
814 for visual
indication of slope ratio information; means to permit 12 hours (minimum) of data
storage, such as on-board multi-media card storage
815; and analog and/or
digital outputs
816 for providing output of the pulsatile waveform and a
DC level representative of the normalized slope, and slope ratio data.
The present invention provides a constant excitation to the pulse oximeter finger
probe LED to evaluate the overall concept of slope detection without actually using
the OEM's pulse oximeter circuit board. In an alternative embodiment, a pulse oximeter
printed circuit board (PCB) is incorporated as a daughter board (internal to the device).
Normalization is a method to correct for the photoplethysmographic
pulse signal morphological changes based on finger position (as opposed to actual
changes of autonomic activity.) Currently used ANS activity monitoring methods
such as PTT and PAT lack the capability for normalization of incoming data and
therefor cannot correct for finger position changes. The present invention includes
a process for normalization, and thus provides immunity to artifact caused by both
elevation changes of the finger probe, and changes in blood flow due to arterial
compression during patient positional changes.
The obtained photoplethysmographic signal can be normalized to minimize changes
in peak to peak signal amplitude that are not due to ANS activity. In other words,
if there is a vertical peak to peak percentage increase of the pulsatile waveform
(and consequent increase in slope), but otherwise no waveform distortion, the percentage
increase is likely to have been caused by a changing of position (relative to the
heart) of the finger probe. If this same increase in peak to peak height occurred,
but there was also a slight shift in the waveform shape, becoming slightly more
of a square wave from its sinusoidal shape, then the increased slope is likely
to be the effect of ANS activity, such as increased heart contractility.
The present invention normalizes the slope of each blood pressure pulse by dividing
the slope by the peak to peak height of that same pulse. For each pulsatile beat
with a constant period and shape over a relatively short period of time, the normalization
will remove variations due to height only. Both the height and peak will actually
be measured in terms of analog-to-digital (A/D) counts in the PTS unit's microcontroller.
Research on finger vascular tone has shown that normalized pulse volume, also derived
photoplethysmographically, appears to be superior to the conventional pulse volume.
The present invention is intended to make use of an existing single photoplethysmmographic
(optical volume detecting) probe, and therefore existing pulse oximetry technology.
Since pulse oximeters use an alternating flashing of two different wavelength LEDs,
the present invention synchronizes with the desired LED in order to examine the
transmitted intensity due to a single wavelength. Alternatively, certain models
of oximeter OEM modules provide an analog or digital output that can be utilized
directly by the present device.
In a preferred embodiment, the apparatus contains an autogain circuit to prevent
the pulsatile waveform from clipping during changes in finger height, large blood
pressure changes, and between patients with different thickness and skin color
fingers. There are also several peak detection algorithms for detecting the beginning
of the systolic rise time and the beginning of diastole. These algorithms provide
the minimal quantizing noise, something that can occur when attempting to lock
onto the rounded peak of a waveform. With sufficiently fast sampling and the correct
threshold for detecting a zero slope (peak) the circuit was designed to not trigger
on noise and yet be sensitive enough to be very close to the peak and not loose
accuracy due to detection well beyond the peak.
Alternative embodiments of the present invention provide algorithms for
slope detection, peak to peak height, and normalization in the form of firmware
within the device, or by software after the data is downloaded into the polysomnograph.
Alternative methods of data storage and transfer are also possible, including multimedia
card storage, computer hard drive storage, serial input/output interface with other
devices, and various forms of telemetry and phone transmission. Various embodiments
for displaying pulse rate and slope ratio can include waveform displays, light
bars, and numerical information.
FIG. 9 shows a block diagram of the method of the present invention. Shown are
the steps of disposing a photoplethysmographic probe proximal to a single body
part; deriving a continuous pulsatile blood pressure waveform as a function of
amplitude and time; defining a time interval for calculation of a slope of the
pulsatile blood pressure waveform; performing continuous calculation of the slope
of each blood pressure waveform over a defined time interval; processing input
data to divide peak amplitude values by a given time constant; eliminating from
further calculation slope values of less than one; signal processing, conditioning,
and artifact rejection; amplifying and filtering normalized slope values; and providing
output information representative of pulse and slope ratio in the form of a display,
electronic data output, and data storage.
INDUSTRIAL APPLICABILITY
The present invention has applicability to the field of medical devices, and
more particularly to a physiological monitoring method and device used for detection
of autonomic nervous system (ANS) activity in the field of sleep research.
In compliance with the statute, the invention has been described in language
more
or less specific as to sleep diagnostic medical devices. It is to be understood,
however, that the invention is not limited to the specific means or features shown
or described, since the means and features shown or described comprise preferred
ways of putting the invention into effect.
Additionally, while this invention is described in terms of being used
for sleep diagnostic studies, it will be readily apparent to those skilled in the
art that the invention can be adapted to other uses for other forms of medical
and non-medical monitoring of the autonomic nervous system as well, and therefore
the invention should not be construed as being limited to sleep study applications.
The invention is, therefore, claimed in any of its forms or modifications within
the legitimate and valid scope of the appended claims, appropriately interpreted
in accordance with the doctrine of equivalents.
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