Systems and Methods for Monitoring Subjects in Potential Hypoxic Distress

Abstract

A method of monitoring a subject for the risk of Acute Mountain Sickness (AMS) includes obtaining real-time pulse arterial oxygen saturation (SpO.sub.2) measurements from the subject. The SpO.sub.2 measurements are transformed into a novel metric known as Accumulated Hypoxic Debt (AHD). The AHD metric is used as the independent variable in a longitudinal generalized linear mixed model to calculate the probability D that the subject is at risk of AMS. Based on the probability D, appropriate courses of action may be communicated to the subject via the output device of a wearable or portable monitor.

Claims

1. A method for real-time monitoring of an individual, comprising: (a) providing a pulse arterial oxygen saturation (SpO.sub.2) sensor and a portable computing device to an individual; (b) using the sensor, measuring the SpO.sub.2 value of the individual in real-time once a second; (c) using a processor in the computing device, obtaining an average real-time SpO.sub.2 value over a time interval between 1 and 60 seconds; (d) using the processor, subtracting the average real-time SpO.sub.2 value from 90% to obtain a real-time SpO.sub.2 difference; (e) using the processor, multiplying the real-time SpO.sub.2 difference by the time interval and converting the product to % hours to obtain a real-time hypoxic debt value; (f) storing the real-time hypoxic debt value in a memory of the computing device; (g) repeating steps (b)-(f) for a plurality of the time intervals; (h) using the processor, retrieving a plurality of stored real-time hypoxic debt values from the memory; (i) using the processor, summing the plurality of retrieved real-time hypoxic debt values to obtain accumulated hypoxic debt (AHD); (j) using the processor, calculating a probability (D) of experiencing acute mountain sickness as D=e.sup.1.94+0.017(AHD)/1+e.sup.1.94+0.017(AHD); and (k) using an output device, communicating the probability D to the individual.

2. The method of claim 1, wherein step (b) includes measuring the SpO.sub.2 value of the individual in real-time when the individual is located above an altitude of about 2500 meters.

3. The method of claim 1, wherein step (c) includes using the processor to obtain an average real-time SpO.sub.2 value over a time interval of 15 seconds.

4. The method of claim 3, wherein step (e) includes using the processor to multiply the real-time SpO.sub.2 difference by the time interval of 15 seconds and converting the product to % hours to obtain a real-time hypoxic debt value.

5. The method of claim 4, wherein step (g) includes repeating steps (b)-(f) for a plurality of the 15 second time intervals.

6. The method of claim 5, wherein steps (b)-(k) are repeated over a period of 48 hours.

7. The method of claim 1, wherein step (k) includes communicating the probability D to the individual using a visual display.

8. The method of claim 7, wherein step (k) includes using the output device to communicate a course of action to the individual.

9. The method of claim 8, wherein step (k) includes communicating one or more of the probability D and the course of action in one or more of a numerical format, a color-coded format and a format using words.

10. A method for real-time monitoring of an individual, comprising: (a) providing a pulse arterial oxygen saturation (SpO.sub.2) sensor and a portable computing device to an individual; (b) using the sensor, measuring the SpO.sub.2 value of the individual in real-time once a second when the individual is located above an altitude of about 2500 meters; (c) using a processor in the computing device, obtaining an average real-time SpO.sub.2 value over a 15 second time interval; (d) using the processor, subtracting the average real-time SpO.sub.2 value from 90% to obtain a real-time SpO.sub.2 difference; (e) using the processor, multiplying the real-time SpO.sub.2 difference by the time period of 15 seconds and converting the product to % hours to obtain a real-time hypoxic debt value; (f) storing the real-time hypoxic debt value in a memory of the computing device; (g) repeating steps (b)-(f) for a plurality of 15 second intervals; (h) using the processor, retrieving a plurality of stored real-time hypoxic debt values from the memory; (i) using the processor, summing the plurality of retrieved real-time hypoxic debt values to obtain accumulated hypoxic debt (AHD); (j) using the processor, calculating a probability (D) of experiencing acute mountain sickness as D=e.sup.1.94+0.017(AHD)/1+e.sup.1.94+0.017(AHD); and (k) using an output device, communicating the probability D to the individual.

11. A non-transitory computer-readable medium with instructions stored thereon that, when executed by a processor, a memory, a pulse arterial oxygen saturation (SpO.sub.2) sensor, and an output device, perform the steps comprising steps (b)-(k) of claim 1.

12. A non-transitory computer-readable medium with instructions stored thereon that, when executed by a processor, a memory, a pulse arterial oxygen saturation (SpO.sub.2) sensor, and an output device, perform the steps comprising steps (b)-(k) of claim 10.

13. A system for real-time monitoring of an individual for a risk of Acute Mountain Sickness, comprising: a pulse arterial oxygen saturation (SpO.sub.2) sensor configured to extract SpO.sub.2 measurements from the individual; a portable computing device configured to be worn or carried by the individual and connected to the SpO.sub.2 sensor; the portable computing device include a processor, a memory, and an output device; wherein the processor is configured to (a) obtain an average real-time SpO.sub.2 value over a 15 second time interval; (b) subtract the average real-time SpO.sub.2 value from 90% to obtain a real-time SpO.sub.2 difference; (c) multiply the real-time SpO.sub.2 difference by the time period of 15 seconds and convert the product to % hours to obtain a real-time hypoxic debt value; (d) store the real-time hypoxic debt value in the memory of the computing device; (e) retrieve a plurality of stored real-time hypoxic debt values from the memory; (f) sum the plurality of retrieved real-time hypoxic debt values to obtain accumulated hypoxic debt (AHD); (g) calculate a probability (D) of experiencing acute mountain sickness as D=e.sup.1.94+0.017(AHD)/1+e.sup.1.94+0.017(AHD); and (h) communicate the probability D to the individual by sending the probability D to the output device.

14. A method of detecting Acute Mountain Sickness (AMS) in a human, comprising: obtaining SpO.sub.2 measurements from the human; detecting whether the human is at risk of AMS by transforming the SpO.sub.2 measurements to accumulated hypoxic debt (AHD); and using the AHD as an independent variable in a longitudinal generalized linear mixed model to calculate the probability D that the human is at risk of AMS.

15. The method of claim 14, wherein the probability D equals e.sup.1.94+0.017(AHD)/1+e.sup.1.94+0.017(AHD).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals.

[0022] The single FIGURE is a schematic diagram of one embodiment of an individualized hypoxia monitor.

DETAILED DESCRIPTION

[0023] A novel apparatus and method utilizes real-time monitoring and analysis of an individual's pulse arterial oxygen saturation (SpO.sub.2) to predict the risk of the occurrence of Acute Mountain Sickness (AMS) during the first 48 hours at altitude (above 2500 meters). For soldiers, the first 48 hours at altitude is the riskiest. A novel, useful, unconventional and non-routine metric is accumulated hypoxic debt (AHD) in units of % hours.

[0024] To determine an individual's AHD, first, an individual's real-time SpO.sub.2 is measured by a sensor worn by the individual. The real-time SpO.sub.2 may be measured, for example, once a second. The measured real-time SpO.sub.2 is then averaged across a relatively short time interval to obtain an average real-time SpO.sub.2 measurement for that short time interval. In one embodiment, the short time interval is 15 seconds. In other embodiments, the short time interval may be longer or shorter than 15 seconds.

[0025] The SpO.sub.2 difference is 90% minus the average real-time SpO.sub.2 calculated over the short time interval. The SpO.sub.2 difference is then multiplied by the length of the short time interval (for example, 15 seconds (15/3600 hours)) to obtain a single hypoxic debt amount in % hours. Then, the single hypoxic debt amounts are summed over the total time interval during which SpO.sub.2 measurements have been taken to thereby obtain the AHD in % hours.

[0026] The AHD is then used in a longitudinal generalized linear mixed model to assess the risk of experiencing AMS. The preferred model is of the form shown in Equation 1. below:

[00001]$\begin{array}{cc}{y}_i=\underset{j=0}{\overset{M}{.Math.}}.Math.{}_j.Math.x_{\mathrm{ij}}+{\mathrm{.Math.}}_i.& \mathrm{Equation}.Math..Math.1\end{array}$

[0027] In the model, x is the real-time measured AHD. The AHD is used to calculate y, which is the risk of experiencing AMS. The model was developed using sixteen healthy nonsmoking unacclimatized lowlanders (M=11, F=5, age=236 yrs, weight=7413 kg; meanSD) that ascended to the summit of Pikes Peak (PP) at 4300 meters and wore a physiologic status monitor (Equivital EQ-02) that measured pulse arterial oxygen saturation (SpO.sub.2) every 15 seconds for the first 20 hours of altitude exposure. An Environmental Symptoms Questionnaire was utilized to measure the prevalence and severity of AMS after 4, 8, 12 and 20 hours of exposure. Data was filtered such that all volunteers had the same number of physiologic measurements. AHD (% hr) was calculated by multiplying the real-time SpO.sub.2 difference [90%actual SpO.sub.2] by the time period (15 sec), converting this to % hours of hypoxic debt and then summing the hypoxic debt amounts over the total time period.

[0028] In Equation 1., the regression coefficient fl equals 0.017 and the residual variable equals 1.94. Thus, the percent probability D of experiencing AMS is given by Equation 2. below:

D=(e.sup.1.94+0.017(AHD))/(1+e.sup.1.94+0.017(AHD))Equation 2.

[0029] AHD was a significant predictor (P=0.002) of the occurrence of AMS over time at altitude. Every 10% hour increase in AHD increased the odds of getting AMS by 18.4% (odds ratio, 1.184; confidence interval, 1.065-1.316) [16]. The model has been externally validated in a set of ten volunteers exposed to either 3000 meters or 4000 meters and had the ability to correctly diagnose AMS 86% of the time during the first 24 hours of altitude exposure.

[0030] Using the model, Table 1. below shows the risk (%) of Acute Mountain Sickness calculated from Accumulated Hypoxic Debt (AHD) (% hr). The bracketed ranges are the 95% confidence intervals.

TABLE-US-00001 TABLE 1 Accumulated Hypoxic Debt (% hr) Risk of AMS (%) 10 18.4 [6.5-31.6] 20 40.2 [13.4-73.3] 30 66.0 [20.7-228.0] 40 196 [128-300] 50 232 [137-395] 60 275 [145-520]

[0031] A very important benefit of the novel, unconventional, non-routine hypoxic debt metric is its universal nature. For instance, it can be utilized at any altitude. Individuals will accumulate more hypoxic debt the higher the altitude and less hypoxic debt the lower the altitude. In addition, it can be utilized in men and women. Women tend to demonstrate higher SpO.sub.2 measurements at altitude and AHD takes that variability into account. The metric also takes into account the desaturation that occurs with physical activity at altitude. Individuals engaging in vigorous physical activity experience a 5%-10% desaturation depending on the altitude, which effectively puts individuals at a higher altitude for a given period of time. Heavy physical exercise at altitude, therefore, is typically associated with higher values of AMS. In addition, medication, such as acetazolamide, stimulates ventilation, which is accounted for by the hypoxic debt metric. Last, this hypoxic debt metric captures the importance of AHD over time at altitude. Individuals do not immediately experience AMS upon arrival at altitude. AMS develops after about 4 hours of altitude exposure, peaks around 18-22 hours of exposure and then subsides after 36 hours of exposure. The real-time component of the hypoxic debt metric is useful in tracking the time course of AMS.

[0032] FIG. 1 is a schematic diagram of one embodiment of an individualized hypoxia monitor or apparatus 10. Embodiments of the apparatus 10 may include a portable computing device, for example, a wrist-worn device and/or a smartphone. Apparatus 10 either includes or is in communication with an oximeter (SpO.sub.2 sensor) 18. Sensor 18 is connected to a processor 12. Memory 14 and battery 16 (or other power supply) are connected to the processor 12. User input and control devices 20 and output devices 22 are connected to the processor 12. The user input and control devices 20 may include, for example, keyboards (virtual or real), touch screens, microphones, movable switches, ports and jacks, such as USB ports, memory card slots, such as SD card slots, etc. Output devices 22 may include, for example, visual displays, speakers, vibrating devices, antennas, ports and jacks, such as USB ports, memory card slots, such as SD card slots, etc.

[0033] Suitable SpO.sub.2 sensors 18 are available from, for example, Equivital, Inc., Cambridge, UK; Athena GTX, Des Moines, Iowa: Masimo, Inc., Irvine, Calif.; and Nonin, Inc., Plymouth, Minn. These companies may also provide portable and/or wearable devices that incorporate the required capabilities of processor 12, memory 14, battery 16 and input and output devices 20, 22. Many currently available smart phones models would also be suitable and can be connected to sensor 18 wirelessly via a Bluetooth connection.

[0034] It is not necessary that the user input any information into apparatus 10 other than the real-time SpO.sub.2 measurements provided by sensor 18. Using the real-time SpO.sub.2 measurements, the processor 12 calculates the AHD and then uses the AHD to calculate the probability D of experiencing AMS using Equation 2. It is important to note that even if a human being could manually perform the calculations performed by the processor 12, the time required to manually calculate the real-time AHD at small time intervals, such as 15 seconds, and then manually calculate the probability D of experiencing AMS using Equation 2 would be prohibitive and would so interfere with the individual's other activities as to make doing so virtually worthless. Thus, a computer processor is a necessary and integral component of apparatus 10.

[0035] Apparatus 10 may provide the probability D to the user via an output device 22, such as a visual display. In addition, the apparatus 10 may provide a visual indication of the category of the probability P, such as MILD, MODERATE, or SEVERE, for example. In addition to or as a substitute for a textual display, apparatus 10 may visually display a color code, such as green for MILD, yellow for MODERATE, or red for SEVERE. In some embodiments, apparatus 10 may provide instructions and appropriate courses of action to the individual, for example, on a visual display. For example, if the probability D is less than 30%, then a message such as CONTINUE ACTIVITIES may be displayed. If D is more than 30% but less than 50%, then a message such as STOP ASCENDING may be displayed. If D is more than 50%, then a message such as DESCEND IMMEDIATELY may be displayed. When AHD is 64.3 hours, then the probability D is greater than 30%. In one embodiment, if the probability D is greater than 30%, then a change in the individual's level of activity is indicated. Other instructions and other ranges of probabilities may be used.

[0036] Embodiments of the invention have been described to explain the nature of the invention. Those skilled in the art may make changes in the details, materials, steps and arrangement of the described embodiments within the principle and scope of the invention, as expressed in the appended claims.

REFERENCE LIST

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