Patent Application
20180055427
This disclosure relates to a method and apparatus to enhance peripheral venous oxygen measurements, and in particular, to enhance peripheral venous oxygen measurements by controlling the application of pressure, frequency and optionally temperature.
Pulse oximeters are the ubiquitous devices used in hospitals to non-invasively estimate arterial blood oxygen saturation (SpO2) and monitor heart rate (HR). Pulse oximetry uses photoplethysmograph (PPG) signals acquired by a sensor, typically mounted on a finger, toe, or ear-lobe to optically detect blood volume changes in the tissue. The PPG results from the time-varying amount of blood that is pushed into the vascular bed during systole and diastole. Conventional pulse oximetry relies on the pulsatile nature of arterial blood and differential absorption of oxyhaemoglobin and de-oxyhaemoglobin at red (RD) and infrared (IR) wavelengths to estimate SpO2 and HR.
Commercial pulse oximeter probes consist of two high output RD and IR light emitting diodes (LED) and a very sensitive photo-detector (PD). Light energy transmitted through tissue is detected by the PD, which generates the PPG signal. From the PPG signal, the slowly changing (DC) and the rapidly changing (AC) signals are extracted. The DC signal predominantly captures the unchanging light scattering and absorption. The AC signal predominantly captures the varying absorption due to pulsatile arterial blood and is synchronous with HR. Thus, by taking the appropriate AC/DC ratios and calibration, SpO2 can be reliably estimated.
While the predominant application of pulse oximeters has been arterial blood oxygen saturation (SaO2) estimation, the raw PPG waveform (PPGRaw) is rich with relevant physiological information. Indeed, the PPGRaw contains a complex mixture of the influences of arterial, venous, autonomic and respiratory systems on the peripheral circulation. Non-invasive estimation of blood flow change in muscles and bones using PPG was previously reported showing that the AC component of the PPG corresponds to blood flow while the DC corresponds to the blood volume change. Thus, application of PPG is not restricted to SpO2 estimation.
A number of factors have been reported to limit pulse oximeter accuracy, including motion artifacts, environmental noise, skin tone, gender, nail polishes, and ambient light. Another important limiting factor is temperature, which is often overlooked. It is generally accepted that cold digits may provide inaccurate pulse oximeter readings. People with naturally very cold fingers or patients in intensive care units, where room temperature is maintained at 20° C., are examples of cases that can be affected by this problem.
Commercially available pulse oximeters have limited access to raw PPG signal data and thus the rate of new pulse oximeter applications has diminished significantly. Although conventional pulse oximeters use two wavelengths, only the infrared PPG waveforms are typically presented to the user or clinician. In addition, the filtering of a majority of the DC signal removes valuable physiological data, including venous blood absorption information. The current systems do not allow for temperature control of the site on the body being measured.
The pulse oximeter is one of the most widely accepted non-invasive clinical monitoring systems. However, conventional pulse oximeters are restricted to estimation of SaO2 only. Coupling estimation of peripheral venous blood oxygen saturation (SvO2) with estimation of SaO2 would enable assessment of local oxygen extraction (O2E) according to (O2E=SaO2−SvO2) and any related metrics. O2E information could provide an indicator of the adequacy of local tissue perfusion, to aid in early diagnosis of microcirculatory dysfunction in medical conditions, such as sepsis and shock.
Venous blood in the periphery is typically non-pulsatile in nature and thus conventional pulse oximetry cannot be used to measure SvO2. Arteries and veins have significantly different vessel wall mechanical properties. Venous walls are significantly thinner and less elastic than arterial walls. Due to the significant differences in mechanical properties of the two vessels, under low pressure, the veins are up to 10-20 times more compliant compared to arteries. Vessel compliance (C) is the ability of a blood vessel to distend and increase in volume with increasing transmural pressure, as defined:
Transmural pressure (ΔP) is the pressure difference between the inside and outside of the vessel wall. With small changes in pressure, the circulating blood inside the veins thus experiences large volume changes compared to the arteries. This large compliance difference is what causes the respiratory modulations in the venous blood seen in previous studies.
U.S. Pat. No. 7,263,395 discloses an automated artificial pulse generation (APG) system which generates user-defined pulsations that can mechanically modulate blood volumes inside the finger. This system is designed to utilize the arterial-venous compliance difference to induce artificial pulsations predominantly in the venous compartment. A pressure transducer is applied to a first site on a body and an oximeter is applied to a second site. Ideally, the magnitude of the pulsations is controlled such that the venous blood system is modulated without disturbing the arterial system. However, the current methods use an activation signal that has a frequency (above 4 Hz) and pressure (160 mm Hg) close to normal arterial signals. These two signals can easily be confounded and a significant effort is needed to disentangle the two signals. Moreover, the same correlation formula is used to calculate both the venous and arterial oxygen saturation. No mention of regulation of measurement temperature is disclosed or suggested. There is a need to have an activation signal that is sufficiently distinguished from the arterial signal to enable simpler and more robust signal processing.
Thus, there is a need for a pulse oximeter system that overcomes the limitations of current pulse oximeters, including controlling the temperature of the patient's fingers. Moreover, providing a method to generate an artificial pulse which avoids confounded signals and significant effort to disentangle the two signals would enable additional diagnostic and real-time measurement of patient data.
In accordance with an aspect of the present invention, there is provided a method for non-invasively measuring venous oxygen saturation of a subject, including: applying a pressure transducer at a first site on a subject having venous and arterial systems; applying a drive signal to the pressure transducer at a frequency below about 0.83 Hz to cause a series of pulsations of a magnitude in the venous blood volume less than diastolic pressure of the subject such that the arterial system is minimally disturbed; applying an oximeter device at a second site on the subject and optionally controlling the temperature; measuring output signals received from the oximeter device, the output signals containing a component representative of the modulation of venous blood volume due to the series of pulsations; and deriving a measure of venous oxygen saturation of the subject from the frequency response of the output signals.
In accordance with another aspect of the present disclosure, there is provided an apparatus for non-invasively measuring venous oxygen saturation of a subject, including: a pressure transducer capable of applying a series of pulsations to a first site on a subject having venous and arterial systems; a pulse oximeter; optionally a heat source; optionally a temperature sensor or a thermometer; a controller in communication with the pressure transducer, pulse oximeter, optional heat source and optional temperature sensor or thermometer, wherein the controller is capable of applying a drive signal to the pressure transducer at a frequency below about 0.83 Hz to cause a series of pulsations of a magnitude in the venous blood volume less than diastolic pressure such that the arterial system is minimally disturbed, optionally controlling the temperature of the subject at a second site, and measuring output signals from the pulse oximeter, the output signals containing a component representative of the modulation of venous blood volume due to the series of pulsations; and a signal processor capable of extracting a value for venous oxygen saturation from the frequency response of the output signals.
These and other aspects of the present disclosure will become apparent upon a review of the following detailed description and the claims appended thereto.
This disclosure provides a method and apparatus to improve the real-time patient data collection by adding to and improving the signal quality of the data collected by a pulse oximeter. The disclosure enables one to measure patient pulse-oximetry as well as optionally a way to control the patient temperature near the point where the pulse oximeter is attached and a way to artificially generate a low frequency pulse to ascertain the arterial-venous compliance difference in an efficient manner.
Recent work (see Kahn, et. al., “Investigating the Effects of Temperature on Photoplethysmography,” IFAC-PapersOnLine, 2015-Elsevier) has shown large increases in PPG quality and pulse oximeter accuracy when the temperature of fingers is controlled to near body temperature (30° C. to 35° C.).
In an embodiment shown in
An embodiment of a device, shown in
Optoelectronic Sensor: A suitable optoelectronic sensor includes a standard transmission mode sensor used to acquire PPG data from a finger. The dual LED sensor produces 660 nm and 940 nm wavelength light for the RD and IR modes, respectively, and contains a high sensitivity PD.
Artificial Pulse Generator: In accordance with an embodiment, an APG system is shown schematically in
The system is driven, for example, utilizing an air source, such as, compressed air that is stored and if needed restored by an air compression unit. The regulator assures the air pressure transmitted to the pressure cuff is adequate to inflate the cuff, while not allowing so much air pressure to either cause the patient needless discomfort or local tissue trauma. Since the intent of the cuff is to send a pulse this can be accomplished by alternately allowing the cuff to inflate and to deflate. A suitable modulation frequency includes a frequency below about 0.83 Hz, time for this cycle to complete is approximately 50 times a minute; a frequency of from about 0.1 Hz to about 0.83 Hz; a frequency of from about 0.2 Hz to about 0.4 Hz; or a frequency of about 0.2 Hz. The control of this cycle can be done by a small electronic controller. This controller may be part of the overall system controller.
As a result of the APG activation, the original PPG waveform contains an additional pulsatile signal, superimposed on the DC signal. The effect of introducing a pulsatile signal can be considered as an artefact introduced into the PPG signal. This low frequency artefact was spectrally extracted and analyzed for SvO2.
Thermal Glove: The temperature sensor and thermal glove provides the ability to maintain the patient's body temperature near the optoelectronic sensor at a temperature of above about 30° C. or preferably within a temperature range of from about 30° C. to about 35° C. This can be accomplished, for example, as shown in
Data Processing and Control System: This system, an embodiment as shown in accordance with the method of
Where,
The values for arterial and venous saturation can then be found using the following calibration equations. It should be noted that the present system does not use identical coefficients for Rart and Rven as was previously done in prior systems, but rather the formulas below:
SpaO
2=110−25*RArt
SpvO2=110−40*RVen
wherein SpaO2 is the pulse oximeter estimation of SaO2 and SpvO2 is the pulse oximeter estimation of SvO2
The peak magnitude may be calculated using peak and trough detection algorithms or by using FFT based frequency domain analysis.
Determination of calibration equations: In order to ascertain the coefficients used in the calibration equations a study was conducted using a cohort of volunteers in order to collect sufficient data using a system in accordance with the present disclosure and traditional clinical measurement methods as set forth in the Example. An APG system was developed to exploit the significant arterial-venous compliance difference to induce artificial pulsations predominantly in the venous compartment. An embodiment of the APG system is shown schematically in
A study was conducted in the intensive care unit (ICU) of St George's Hospital, Christchurch. Eight healthy adult, male, volunteers (aged 23-37 years) with no pre-existing medical conditions were recruited, and provided signed, informed consent.
A catheter was inserted into a vein on the back of the left hand in order to get SvO2 by standard methods. A portable i-Stat1 blood gas analyzer (Abbott, Princeton, N.J., USA) was employed to analyze catheter drawn blood samples. A standard transmission mode sensor 1 was connected to the custom pulse oximeter (PO) system and was clipped to the index/middle finger of the left hand, as illustrated in
A temperature sensor 3 was attached next to the PO sensor 1 on the skin surface to continuously monitor digit skin temperature. Subjects were asked to refrain from smoking and strenuous physical activities for at least 4 hours prior to the experiment. During the study, subjects were comfortably seated, while resting their left arm on a flat surface at approximately the same height as their heart and with minimum movement. Tests were performed to achieve a wide range of experimentally measured SvO2 values and provide a valid dataset in order to ascertain the calibration equation.
Inflation/deflation of the digit cuff 2 is controlled by a 3-way solenoid valve (EVT 307-5DZ-02F-Q, SMC, Noblesville, Ind., USA). Operation of the solenoid valve is managed by the controller 5. The solenoid is switched on and off with square pulses at 0.2 Hz and 50% duty cycle. The 0.2 Hz frequency is close to typical respiratory frequencies to create artificial venous blood modulations when detecting natural venous blood modulations in the oesophagus using a PPG sensor.
A precision pressure regulator (AS1002F, SMC, Noblesville, Ind., USA) was used to control the pressure of air supplied to inflate the cuff at 40-50 mmHg, which is approximately 50% below typical diastolic arterial pressures. Thus, this cuff compression pressure will predominantly effect the venous compartment. For the deflation portion of the pulse, the air in the cuff was exhausted to the atmosphere with the solenoid valve turning off.
PPG finger sensor control and data acquisition were managed with the PO system, which includes serial communication and a graphical interface. The PO system is based on the CY8CKIT-050 PSoC 5LP Development Kit (Cypress Semiconductor, San Jose, Calif., USA) that has an ARM Cortex M3 microcontroller. The PPG sensor uses 660 and 940 nm wavelengths for the RD and IR LEDs, respectively, and a very sensitive photodetector (PD). Feedback control was used by the PO system microcontroller to adjust LED intensities specific to finger thickness. Light energy received at the PD was converted to current signals and passed through a two stage trans-impedance amplifier for signal conditioning. A de-multiplexer splits the conditioned PPG signals so that the RD and IR PPGs are processed independently. Analog PPG signals in the range 0-5 V were sampled at 50 Hz by a 16-bit analog-to-digital converter (ADC) on the development board. Sampled data were sent to a PC via serial communication and recorded for offline signal processing in MATLAB (R2014a, MathWorks, Natick, Mass., USA).
A two-stage digital filter system was implemented to extract the PPG signals of interest from the raw and amplified PPG data. Zero phase (forward—reverse) filtering was applied at each filter stage to reduce phase distortion and maintain PPG waveform shape.
The first filter stage was an equiripple, low-pass, FIR filter with a cut-off frequency of 10 Hz and attenuation of 50 dB to effectively remove any unwanted, high frequency noise from the PPG signals. The second filter stage had three parallel filters to separately extract high frequency (AC), APG modulated frequency (APG), and very low frequency (DC) signals. An empirically derived 4th order band-pass, Butterworth, IIR filter extracted AC signals, such as the cardiac frequencies, with pass band frequencies of 0.67-4.5 Hz. An empirically derived Butterworth, IIR filter extracted APG signals with pass band frequencies of 0.15-0.67 Hz. An empirically derived Butterworth, IIR filter with a cut-off frequency of 0.15 Hz was used to extract DC signals between 0 and 0.15 Hz.
A Fast Fourier Transform (FFT) analysis was used to determine the relative power in each part of the PPG signal generated from the subjects to calculate the “modulation ratios” (R values):
where,
RArt is the modulation ratio related to arterial blood.
RVen is the modulation ratio related to venous blood.
|AC|HR is the peak Fourier magnitude of the AC signal, related to cardiac frequency.
|APG|0.2 Hz is the peak Fourier magnitude at 0.2 Hz of the APG signal.
|DC|0 Hz is the peak Fourier magnitude at 0 Hz frequency of the DC signal.
The fundamental frequency associated with the peak magnitude was identified from the frequency power spectra of AC, APG, and DC offset signals. Equations 1 and 2 were then used to compute values of RArt and RVen.
A linear model was fitted to the calculated RArt and RVen values and measured blood gas SvO2 to generate the correlation equation in accordance with the present invention:
SpaO
2=110−25*RArt
SpvO
2=110−40*RVen
wherein SpaO2 is the pulse oximeter estimation of SaO2 and SpvO2 is the pulse oximeter estimation of SvO2
The coefficient of determination (r2) was computed to assess the strength of their correlation for both were found to have a sufficiently high degree of correlation. The linear models can now be used for the general population as the calibration curve to estimate SpvO2 and SpaO2. See
This system presents a non-invasive method to induce venous blood modulations using a pneumatic digit cuff, which can be captured by PPG. A calibration model is derived from venous blood modulated PPG signals and measured SvO2 to assess SpvO2 estimate. In addition, the gradient of this model (−40%) is different to the gradient of the conventional pulse oximeter calibration model (−25%). The proposed SpvO2 estimation model can be used as a potential metric and indicator for low O2E, consumption, and tissue hypoxia.
Real-time estimation of peripheral SvO2, using the pulse oximeter method in accordance with the present invention, and SaO2 will enable continuous, real-time monitoring of these physiological conditions. Thus, improvement and application of this concept could aid in diagnosis of medical conditions related to microcirculation dysfunction, such as sepsis and cardiac failure, which are both common in the ICU, as well as length of stay, cost, and mortality.
Although various embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the disclosure and these are therefore considered to be within the scope of the disclosure as defined in the claims which follow.
This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 62/378743, filed Aug. 24, 2016, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62378743 | Aug 2016 | US |
