Patent Application
20180199869
Embodiments of the subject matter disclosed herein relate to biological probes, sensors, and methods, and in particular, to photoplethysmography probes, sensors, and methods.
Photoplethysmography (PPG) relates to the use of optical signals transmitted through or reflected by blood-perfused tissues for monitoring a physiological parameter of a subject (also referred to as a patient herein). In this technique, one or more emitters are used to direct light at a tissue, and one or more detectors are used to detect the light that is transmitted through or reflected by the tissue. The volume of blood of the tissue affects the amount of light that is transmitted or reflected, which is output as a PPG signal. As the blood volume in a tissue changes with each heartbeat, the PPG signal also varies with each heartbeat.
Pulse oximetry is, at present, the standard of care for continuously monitoring arterial oxygen saturation (SpO2). Pulse oximeters include PPG probes that use red (˜660 nm) and infrared (˜940 nm) light to determine physiological parameters (e.g., blood characteristics) of the subject, including SpO2, pulse rate, and pulsating blood flow (e.g., blood perfusion) at the site of measurement. Conventional pulse oximetry probes are typically mounted to an extremity of the subject (e.g., a finger or ear lobe). However, during severe physiological stress, such as hypotension (low blood pressure), hypothermia (low body temperature), and volumetric shock (low blood volume), the body responds by constricting blood vessels (vasoconstriction) in order to divert blood away from the extremities and the periphery to maximize blood flow to central, vital organs (e.g., the brain, heart, and liver). Vasoconstriction, which may be regulated by the autonomic nervous system (ANS), may result in insufficient pulse amplitude for a pulse oximeter to reliably measure blood or blood circulation characteristics.
In one embodiment, a system for an optical probe comprises a light emitter and a light detector coupled to a substrate, the light emitter and light detector configured to measure blood originating at least partly from an internal carotid artery of a patient; and an attachment mechanism configured to couple the optical probe to a nose of the patient, the light emitter and light detector positioned to be on opposite sides of the nose of the patient at a root of a nasal bridge when the optical probe is worn by the patient.
Thus, blood from the internal carotid artery, which does not undergo substantial ANS regulation, may be monitored, resulting in strong PPG signals even under physiological stress conditions such as hypothermia and hypovolemia.
It should be understood that the brief description above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:
The following description relates to various embodiments of an optical probe, such as the nasal pulse oximetry probe shown in
A pulse oximeter comprises a computerized measuring unit and a probe attached to a patient, typically a finger or ear lobe of the patient. The probe includes a light source for sending an optical signal through tissue of the patient and a photo detector for receiving the signal transmitted through or reflected from the tissue. On the basis of the transmitted and received signals, light absorption by the tissue may be determined. During each cardiac cycle, light absorption by the tissue varies cyclically. During the diastolic phase, absorption is caused by venous blood, non-pulsating arterial blood, cells and fluids in tissue, bone, and pigments. The level of light transmitted at end of the diastolic phase is typically referred to as the “DC component” of the total light transmission. During the systolic phase, there is an increase in light absorption (e.g., a decrease in transmitted light) compared with the diastolic phase due to the inflow of arterial blood into the tissue on which the pulse oximetry probe is attached. A crucial pulse oximetry principle is how the measurement can be focused on the blood volume representing the arterial blood only. In pulse oximetry, this is done by taking the pulsating arterial blood portion (the “AC signal”) from the total transmission signal and normalizing this signal by the “DC” component. The resulting “AC/DC” signal is called the PPG waveform. Pulse oximetry is thus based on the assumption that the pulsatile component of the absorbance is due to arterial blood only.
In pulse oximetry, arterial blood is typically modeled as containing two species of hemoglobin: oxyhemoglobin (HbO2) and reduced hemoglobin (Hb). Oxyhemoglobin is hemoglobin that is fully saturated with oxygen, and reduced hemoglobin is without oxygen. Arterial oxygen saturation measured by pulse oximetry (SpO2) is defined as the percentage of HbO2 divided by the total amount of hemoglobin (HbO2+Hb). In order to distinguish between the two species of hemoglobin, light absorption is measured at two different wavelengths. The probe of a traditional pulse oximeter includes two different light sources, such as light-emitting diodes (LEDs) or lasers, that emit light at two different wavelengths. The wavelength values widely used are 660 nm (red light) and 900 nm (infrared light), as the two species of hemoglobin have substantially different absorption at these wavelengths. Each light source is illuminated in turn at a frequency that is typically several hundred Hz.
The light transmitted through the tissue 102 is received by a detector unit 103, which comprises two photo detectors 104 and 105 in this example. For example, photo detector 104 may be a silicon photodiode, and photo detector 105 may be a second silicon photodiode with different spectral characteristics or an indium gallium arsenide (InGaAs) photodiode. The emitter and detector units form a probe detector subunit 113 of the pulse oximetry system 10. The photo detectors convert the optical signals received into electrical pulse trains and feed them to an input amplifier unit 106. The amplified measurement channel signals are further supplied to a control and processing unit 107, which executes instructions stored in memory to convert the signals into digitized format for each wavelength channel.
The control and processing unit 107 further controls an emitter drive unit 108 to alternately activate the light sources. As mentioned above, each light source is typically illuminated several hundred times per second. With each light source being illuminated at such a high rate compared to the pulse rate of the patient, the control and processing unit 107 obtains a high number of samples at each wavelength for each cardiac cycle of the patient. The value of these samples varies according to the cardiac cycle of the patient, the variation being caused by the arterial blood.
The input amplifier unit 106, the control and processing unit 107, the emitter drive unit 108, and probe detector subunit 113 collectively form a probe 11. As used herein, the term “probe” may refer to the probe 11 and the attachment parts that attach the optical components, the probe 11, to the tissue site. The term “SpO2 sensor” may refer to a unit comprising a probe, an analog front end, and a signal processing unit that calculates SpO2 and other blood characteristics. In a multi-parameter body area network system, the system typically represents a set of multiple sensors, e.g., the different physiological parameter measurements. Therefore, the whole measurement system may comprise of several sensors and their associated probes, and the sensors may communicate to a common hub in which the parameters' information is integrated.
The digitized PPG signal data at each wavelength may be stored in a memory 109 of the control and processing unit 107 before being processed further according to non-transitory instructions (e.g., algorithms) executable by the control and processing unit 107 to obtain physiological parameters. For example, memory 109 may comprise a suitable data storage medium, for example, a permanent storage medium, removable storage medium, and the like. Additionally, memory 109 may be a non-transitory storage medium. In some examples, the system 10 may include a communication subsystem 117 operatively coupled to one or more remote computing devices, such as hospital workstations, smartphones, and the like. The communication subsystem 117 may enable the output from the detector units (e.g., the digitized PPG signal data) to be sent to the one or more remote computing devices for further processing and/or the communication subsystem may enable the output from the algorithms discussed below (e.g., determined physiological parameters) to be sent to the remote computing devices. The communication subsystem 117 may include wired and/or wireless communication devices compatible with one or more different communication protocols. As non-limiting examples, the communication subsystem 117 may be configured for communication via a wireless telephone network, a local- or wide-area network, and/or the Internet.
Algorithms may utilize the same digitized signal data and/or results derived from the algorithms and stored in the memory 109, for example. For example, for the determination of oxygen saturation and pulse transit time (PTT), the control and processing unit 107 is adapted to execute an SpO2 algorithm 111 and a PTT algorithm 112, respectively, which may also be stored in the memory 109 of the control and processing unit 107. Further, a blood pressure algorithm 110, a hypovolemia algorithm 115, and a respiration rate algorithm 116 may also be stored in the memory 109 for determining blood pressure, an indication of hypovolemia, and respiration rate, respectively. The use of such algorithms will be described in more detail below with respect to
As used herein, the terms “sensor,” “system,” “unit,” or “module” may include a hardware and/or software system that operates to perform one or more functions. For example, a sensor, module, unit, or system may include a computer processor, controller, or other logic-based device that performs operations based on instructions stored on a tangible and non-transitory computer readable storage medium, such as a computer memory. Alternatively, a sensor, module, unit, or system may include a hard-wired device that performs operations based on hard-wired logic of the device. Various modules or units shown in the attached figures may represent the hardware that operates based on software or hardwired instructions, the software that directs hardware to perform the operations, or a combination thereof.
“Systems,” “units,” “sensors,” or “modules” may include or represent hardware and associated instructions (e.g., software stored on a tangible and non-transitory computer readable storage medium, such as a computer hard drive, ROM, RAM, or the like) that perform one or more operations described herein. The hardware may include electronic circuits that include and/or are connected to one or more logic-based devices, such as microprocessors, processors, controllers, or the like. These devices may be off-the-shelf devices that are appropriately programmed or instructed to perform operations described herein from the instructions described above. Additionally or alternatively, one or more of these devices may be hard-wired with logic circuits to perform these operations.
As shown in
Probe 200 may include two cushion portions 206a and 206b, one on each end of the length of the probe and connected by a bridge portion 208. As shown, cushion portions 206a and 206b have a thicker amount of sleeve 204 than bridge portion 208 and are somewhat convex. In some examples, cushion portions 206a and 206b may have a more square shape, and in other examples, cushion portions 206a and 206b may have a more rounded shape. Cushion portion 206a houses an emitter unit 210, and cushion portion 206b houses a detector unit 212. In other examples, cushion portion 206a houses the detector unit 212 and cushion portion 206b houses the emitter unit 210. As described with reference to
As described with reference to
As also shown in
The nasal cavity contains terminal branches of the internal carotid artery, namely, the anterior ethmoidal artery. Branches of the internal carotid artery, including the anterior ethmoidal artery, are among the last locations of the human body to experience vasoconstriction under physiological stress conditions because the internal carotid artery supplies blood to the brain. Furthermore, copious amounts of blood are supplied to the nasal cavity in order to warm incoming air before it reaches the lungs. Thus, blood flow through the anterior ethmoidal artery is not subject to substantial ANS regulation, making it an advantageous location from which to measure physiological parameters. The anterior ethmoidal artery 306 branches from the ophthalmic artery 308, which stems from the internal carotid artery 310. As shown in
Turning now to
Bridge portion 408 of the nasal pulse oximetry probe (e.g., probe 200 of
Cushion portions 406a and 406b are affixed to each side of the nose, for example, by an attachment mechanism such as adhesive. As described above with reference to
Turning now to
As described with reference to
As shown in
The amount of light transmitted through a tissue varies according to changes in blood volume at the site. Specifically, the amount of light transmitted through vascular bed decreases during systole (e.g., more light is absorbed) and increases during diastole, resulting in periodic PPG waveforms (e.g., the AC/DC component). Multiple physiological parameters can be extracted from the PPG waveforms measured by a pulse oximetry probe. As an advantage of the copious blood supply to the nasal cavity and flowing through the anterior ethmoidal artery, the resulting PPG data measured by a nasal pulse oximetry probe has a high signal-to-noise ratio. For example, noise may be caused by non-physiological processes such as ambient light and bodily movement, which may be filtered out from the nasal PPG, because these artifact signals remain, in most cases, weak in comparison to the cardiac pulsation.
Turning now to
Method 600 begins at 602 and includes receiving probe output from a nasal pulse oximetry probe (e.g., probe 200 of
At 606, method 600 includes calculating one or more physiological parameters from the probe output. For example, physiological parameters may include SpO2, pulse transit time (PTT), blood circulation at the measurement site, respiration rate, and an indication of overall blood volume. Calculating the physiological parameters may include using algorithms stored in a memory of a control and processing unit (e.g., memory 109 of
At 608, method 600 includes storing the physiological parameters (e.g., as calculated at 606) in the memory and/or displaying the parameters via a display device (e.g., display unit 114 of
includes determining the ratio of ratios where AC is the valley-to-peak amplitude and DC is the baseline of the light transmission, based on probe output from the nasal pulse oximetry probe. Thus, to determine R, PPG waveforms for two wavelengths of light, one red and one infrared (IR), are recorded. At the red wavelength, Hb absorbs more light and HbO2 transmits more light. At the IR wavelength, HbO2 absorbs more light and Hb transmits more light. Thus, the light transmission is higher and the PPG amplitude is correspondingly smaller at the red wavelength (and vice versa at the IR wavelength) when blood has a high oxygen saturation, and light transmission is lower and PPG amplitude larger at the red wavelength (and vice versa at the IR wavelength) when blood has a low oxygen saturation. As can be seen from the above equation for R, a low R value corresponds to a high amount of HbO2 compared to Hb and thus a high arterial oxygen saturation.
At 704, method 700 includes calculating SpO2 using empirical calibration. That is, first, the relationship between arterial oxygen saturation directly measured from an arterial blood sample and R, as calculated at 702, is determined in a volunteer test in laboratory setup. The pulse oximetry calibration is then established so that the SpO2 output at each saturation level is set to show the blood oxygen saturation as closely as possible. The calibration is stored in the pulse oximeter memory, for example, in form of a lookup table or a polynomial function with R as input and SpO2 as output.
At 706, method 700 includes outputting SpO2. For example, SpO2 may be displayed on a display device of the pulse oximeter (e.g., display device 114 of
Turning now to
Referring to
At 804, method 800 includes determining a nasal PTT and/or a finger PTT from the ECG R-peak and a specific time point in the PPG systolic rise at the nasal and/or finger sites. For example, a control and processing unit (e.g., control and processing unit 107 of
The PTT(s) may be used in one or more algorithms to determine continuous blood pressure (BP). There are several ways to use PTT to measure BP. In a first example, as indicated at 806, continuous BP may be determined based on nasal PTT and a population calibration. As described above, the PTT is determined from the ECG R-peak to the systolic rise (maximum derivative) in the nasal PPG. As discussed above, the benefit of this method is that the PTT is least affected by confounding ANS regulation, hypovolemia, hypothermia, and atherosclerosis of peripheral arteries, for example. The BP may be measured by first determining the relationship between PTT and BP in a large patient population. In these calibration runs, the BP is determined, for example, invasively or using non-invasive blood pressure (NIBP) measurement, after which the functional relationship between BP and PTT may be established. This relationship may then be used on patients of the same population characteristics. For example, a calibration equation may be calculated that is different for different patient populations, such as children and adults.
In another example for a continuous BP measurement, as indicated at 808, the continuous BP may be determined based on the PTTs determined for both the nasal and finger sites and the population calibration described above. The PTTs may be calculated using the ECG R-peak time point as a reference. A benefit of this embodiment is that a BP change is larger in absolute value for the finger PTT. As discussed above, however, the finger PTT may be confounded by ANS regulation or other reasons. In this situation, the nasal PTT may be beneficially used to individualize the relationship between the two measured PTTs and the patient's individual BP. As a result, a large part of the patient-to-patient variability in the particular patient population may be removed.
As a further example, as indicated at 810, continuous blood pressure may be determined based on the finger and nasal PTTs and an adaptive calibration. For example, a two point calibration of the measured PTT values against a NIBP measurement at two different levels of BP may be calculated for one patient. The PTT and BP values may be obtained close in time so that confounding factors are likely constant during the two-point calibration interval. The assumption is valid for such confounding factors as hypovolemia, hypothermia, and blood vessel antheroclerosis. In one example, a linear model between BP and the nasal and finger PTT (PTT1 and PTT2, respectively) may be assumed, according to equation A:
The above equation has four unknown coefficients, c11, c12, c21, and c22. In a first step of calibration, the coefficients c11 and c12 are determined so that the correct BP values are obtained at the two NIBP levels. The output parameter C may be used for detecting changed confounding factor situations, e.g., those that may occur due to blood volume changes or temperature changes. At normal patient conditions, the value of C may be set to be a fixed constant at both BP calibration levels. The coefficients c21 and c22 may then be determined in the calibration phase at the two levels of BP. Multiple point calibration may be used to improve the calibration process. In this case, the coefficients c11, c12, c21, and c22 are determined using a statistical optimization analysis, such as linear least square optimization. After calibration, the equation Eq. A is used to calculate BP from the measured PTT values, e.g., PTT2 for the finger and PTT1 for the nasal site. In one example, the constant C may be used to alert about a changed patient condition due to reasons other than BP. When a change in C is observed, the coefficients c11 and c22 may be determined again by requesting NIBP calibration from a user. The NIBP measurement is activated at least at one time point and at one BP level, and the corrections for the coefficients c11 and c12 (as well as c21 and c22) are determined.
The above adaptive method may be improved by using a multiple point original calibration with a non-linear relationship between PTT1 and PTT2 and a large patient population with different (mixed) levels of confounding factors. For example, BP may be calculated as BP=f(PTT1, PTT2, +other variables such as HR), in which f is the non-linear function for BP having PTTs and other physiological parameters as independent variables.
A further example for continuous BP measurement using PTT measurement is indicated at 812, where the continuous BP is determined based on the finger and nasal PTTs using the nasal PTT as a reference (rather than the ECG R-peak as described above). Since PTT to the nasal root is very short (e.g., relative to the finger PTT) at some tens of milliseconds, the nasal plethysmogram may be used as a reference time point for the finger PTT. In this case, the ECG waveform is not needed. The finger PTT is then calculated from the nasal pleth systolic rise to the finger pleth systolic rise, for example, both determined as the maximum derivative of the pleth waveform. This PTT may then be calibrated against NIBP BP determinations using standard methods.
At 814, method 800 includes outputting PTT and/or BP. For example, PTT and/or BP may be displayed on a display device of the pulse oximeter (e.g., display device 114 of
Following 814, method 800 ends.
Turning now to
Method 900 begins at 902 and includes calculating a first respiration rate based on probe output in a time domain. PPG data is collected over time, resulting in the PPG waveform. A control and processing unit (such as control and processing unit 107 of
At 904, method 900 includes calculating a second respiration rate based on microfluctuations in SpO2. In the previous example, data from a PPG waveform measured at one wavelength (typically infrared) may be used to determine respiration rate. In another example, the complete two waveform data, e.g., the red and infrared PPGs, may be used to determine respiration rate. The respiration rate may be determined from the ratio of ratios, R, by first calculating R separately for systolic and diastolic phases of the two PPGs, with R calculations repeated pulse by pulse (e.g., for each heartbeat), as described further below with reference to
Turning briefly to
Continuing to
Returning to
By calculating the respiration rate in more than one way, the accuracy of the resulting value may be improved compared with calculating the respiration rate one way. Further, the respiration rate may be more accurately determined using a nasal pulse oximetry probe positioned at the root of the nasal bridge compared with a pulse oximetry probe positioned at an extremity due to stronger respiration-related variation observed at the nasal site.
Changes in respiration-related variation can be used to determine additional physiological parameters. Turning now to
Method 1000 begins at 1002 and includes transforming probe output to a frequency domain. For example, the probe output obtained over a duration (e.g., 60 seconds) may be analyzed in the frequency domain using a fast Fourier transform (FFT). The resulting spectrum shows the PPG signal plotted as amplitude density against frequency.
Turning briefly to
During hypovolemia, there is an increase in respiratory-induced variation due to the greater impact positive pressure has on a lower blood volume. Therefore, at the respiratory frequency, the first peak 1402b of the spectrum corresponding to the hypovolemic patient (solid line) has a greater amplitude density than the first peak 1402a of the spectrum corresponding to the normovolemic patient (dashed line). Thus, it may be possible to determine hypovolemia in a patient based on the amplitude density at the respiratory frequency (e.g., the respiration-related variation), as described below. In an embodiment, the ratio of the spectral amplitudes at the respiratory frequency and cardiac frequency is determined. This ratio is high for a hypovolemic patient and low for a normo- or hypervolemic patient. Of note, the respiratory frequency peak of transformed PPG signal data acquired with a finger pulse oximeter is weaker and less sensitive to volemia status than the respiratory frequency peak of transformed PPG signal data acquired with a nasal pulse oximeter. Thus, it may be advantageous to determine volemia status with the nasal pulse oximeter.
Returning to
At 1006, the method includes indicating hypovolemia when the amplitude density is greater than a threshold. In another example, hypovolemia is indicated when the amplitude density of the respiratory frequency signal increases by a threshold amount. In still another example, hypovolemia is indicated when the ratio of the respiratory and cardiac spectral peaks is greater than a threshold. An indication of hypovolemia may be output on a display device (e.g., display device 114 of
Method 1100 begins at 1102 and includes obtaining a first probe output with a first amount of positive end-expiratory pressure (PEEP). The probe output may be the DC level of the total light transmission. It is further noted that only the infrared output may be used. PEEP refers to a pressure in the lungs that exists at the end of expiration. Extrinsic PEEP may be applied with a ventilator, and an amount of PEEP may be controlled by settings of the ventilator. PEEP may contribute to decreased venous return, which results in increased blood accumulation, as measured by the pulse oximetry probe at the measurement site.
At 1104, method 1100 includes obtaining a second probe output with a second amount of PEEP. For example, the second amount of PEEP may be higher than the first amount of PEEP, resulting in further blood accumulation. Alternatively, the second amount of PEEP may be less than the first amount of PEEP, resulting in reduced blood accumulation compared with the first amount of PEEP.
Turning briefly to
Returning to
Thus, the systems and methods described herein provide for determining various physiological parameters of a patient, including SpO2, pulse transit time, respiration rate, hypovolemia, and intracranial blood circulation, from an output of a nasal pulse oximetry probe configured to measure blood flowing through the anterior ethmoidal artery. In some examples, in addition to determining the physiological parameters using the output from the nasal pulse oximetry probe, an output from a second pulse oximetry probe located at an extremity of the patient, such as a finger or ear, may also be used. Physiological parameters calculated from the output of the nasal pulse oximetry probe may be compared to physiological parameters calculated from the output of the second pulse oximetry probe to determine a state of the autonomic nervous system, for example. For example, if a pulse wave is detected at the nasal site but not at the extremity (or if the pulse wave is diminished at the extremity), it may be determined that the ANS is regulating blood flow to the extremities. Such a comparison is enabled by the fact that blood vessels in the extremities are subject to vasoconstriction and the anterior ethmoidal artery is not. As a result, the physiological parameter calculations described herein (e.g., with respect to
The technical effect of measuring the blood oxygen saturation using a nasal pulse oximetry probe is a more accurate measurement that is not confounded by vasoconstriction.
An example provides a system for an optical probe, comprising a light emitter and a light detector each coupled to a substrate, the light emitter and light detector configured to measure blood originating from an internal carotid artery of a patient; and an attachment mechanism configured to couple the optical probe to a nose of the patient, the light emitter and light detector positioned to be on opposite sides of the nose of the patient at a root of a nasal bridge when the optical probe is worn by the patient. In examples, the optical probe is configured to measure arterial oxygen saturation at the root of the nasal bridge.
The optical probe may further comprise a first soft cushion-like structure that encompasses the light emitter and a second soft cushion-like structure that encompasses the light detector, each cushion-like structure conforming to the nose of the patient, wherein each cushion-like structure is deformable. In an example, each cushion-like structure includes deformable gel. In one example, the first cushion-like structure covers the light detector and the second cushion-like structure covers the light emitter, each cushion-like structure being at least partially transparent such that light from the light emitter is configured to pass through the first cushion-like structure and through the second cushion-like structure to the light detector.
The attachment mechanism may include a pressure element configured to block surface blood circulation of skin of the patient at the root of the nasal bridge. In examples, the attachment mechanism is an eyeglass-type frame.
The light emitter may include a light emitting diode (LED) configured to emit light having a wavelength of between 760 and 950 nm. In an example, the light emitter includes a first LED having an emission wavelength in a range between 620 and 690 nm and a second LED having an emission wavelength in a range between 760 nm and 950 nm.
The substrate may comprise a T-shape with opposite T-ends of the substrate housing the light detector and light emitter, respectively, and a T-foot of the substrate carrying electrical wirings from the optical probe to a measurement unit along the nasal bridge of the nose.
The optical probe may further comprise a control and processing unit that processes signals from the light detector and calculates physiological parameters, the control and processing unit communicatively coupled to a display unit. In an example wherein the attachment mechanism is an eyeglass-type frame, and the control and processing unit may be coupled to the eyeglass-type frame.
Another example system for an optical probe comprises a light emitter and a light detector shaped to be attached on a nasal root of a patient and configured to measure light transmission through blood originating from an internal carotid artery of the patient and output a light transmission signal; and a control and processing unit including instructions to extract a respiration-related variation from a photoplethysmogram obtained from the light transmission signal. The control and processing unit may be configured to extract a respiration rate and/or an amplitude of the respiration-related variation. Further, the control and processing unit may be configured to detect hypovolemia from the respiration-related variation. Further still, the control and processing unit may be configured to measure the respiration-related variation at two physiological conditions that reflect different venous blood return volumes to a heart of the patient, for which a difference of the respiration-related variation is determined.
Another example of a system for an optical probe comprises a light emitter and a light detector shaped to be attached on a nasal root of a patient and configured to measure light transmission through blood originating from an internal carotid artery of the patient and output a light transmission signal; and a control and processing unit having instructions stored in memory or hardware configured to determine microfluctuations in arterial oxygen saturation based on the light transmission signal obtained over a duration. The control and processing unit may be configured to perform a first arterial oxygen saturation calculation for systole and a second arterial oxygen saturation calculation for diastole and determine the microfluctuations in arterial oxygen saturation based on a difference of the first and second arterial oxygen saturation calculations. In an example, the control and processing unit may be configured to use the microfluctuations in arterial oxygen saturation to calculate a respiration rate of the patient. In another example, the control and processing unit may be configured to use the microfluctuations in arterial oxygen saturation to determine a respiration-related variation.
In another representation, a method comprises receiving probe output from a nasal pulse oximetry probe attached to a subject at a measurement site and calculating one or more physiological parameters from the probe output. In examples, the measurement site is a root of a nasal bridge. In one example, calculating the one or more physiological parameters includes calculating SpO2. In examples, SpO2 may be calculated from a ratio of ratios (R), wherein the ratio of ratios is determined from the probe output measured at two different wavelengths of light. In one example, SpO2 may be calculated using R and an empirical calibration that relates arterial oxygen saturation values directly measured from arterial blood samples and corresponding R values.
In another example, calculating the one or more physiological parameters includes calculating pulse transit time (PTT). For example, calculating PTT may comprise determining a first time from an electrocardiogram (ECG), wherein the first time corresponds to an R-peak time, and a second time from the probe output, wherein the second time corresponds to a time at which blood volume increases at the measurement site. Further, in some examples, PTT may used to calculate continuous blood pressure (BP). For example, continuous BP may be calculated using PTT and a population calibration, wherein the population calibration includes measuring PTT and measuring BP for a plurality of subjects and determining a functional relationship between BP and PTT. In another example, continuous BP may be calculated using PTT and an adaptive calibration. The adaptive calibration may include measuring PTT and BP at two or more BP levels and determining a relationship between BP and PTT.
In an example, calculating the one or more physiological parameters includes calculating blood circulation at the measurement site. In examples, calculating blood circulation comprises obtaining a first probe output at a first amount of positive end-expiratory pressure (PEEP) and obtaining a second probe output at a second amount of PEEP. In one example, calculating blood circulation further comprises determining a difference in respiration-related variation for the first amount of PEEP and the second amount of PEEP from the first probe output and the second probe output, respectively, wherein the difference in respiration-related variation gives an indication of total blood volume.
In another example, calculating the one or more physiological parameters includes calculating respiration rate. In one example, calculating respiration rate comprises obtaining probe output over a duration and identifying a variation in the probe output over the duration, wherein the frequency of the variation is used to calculate respiration rate. The variation in the probe output may include one or more of a baseline modulation, an amplitude modulation, and a frequency modulation. In another example, calculating respiration rate comprises calculating microfluctuations in SpO2 for each heartbeat of the subject, wherein the microfluctuations in SpO2 are determined by calculating a ratio of a first ratio of ratios (R) for a systolic phase and a second R for a diastolic phase of each heartbeat.
In examples, calculating the one or more physiological parameters includes calculating an indication of overall blood volume. In examples, calculating the indication of overall blood volume comprises transforming the probe output to a frequency domain, determining an amplitude density at a respiratory frequency, and determining an amplitude density at a cardiac frequency. In one example, the overall blood volume is indicated to be low responsive to the amplitude density at the respiratory frequency being greater than a first threshold. In another example, calculating the indication of overall blood volume further comprises determining a ratio of the amplitude density at the respiratory frequency and the amplitude density at the cardiac frequency, wherein the overall blood volume is indicated to be low if the ratio is greater than a second threshold.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. The terms “including” and “in which” are used as the plain-language equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc., are used merely as labels and are not intended to impose numerical requirements or a particular positional order on their objects.
This written description uses examples to disclose the invention, including the best mode, and also to enable a person of ordinary skill in the relevant art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
