Objective To develop a durable, low-cost pulse oximeter probe to interface with the MedeCal, for use in developing countries. The MedeCal The MedeCal is a low-cost medical computer being developed by our clients Amit Nimunkar and Jonathan Baran, in conjunction with Engineering World Health, for use in developing countries. It will analyze and display the probe’s output, and power its optical elements. The MedeCal will be a central processor for multiple medical devices in addition to the pulse oximeter, such as low-cost digital thermometers, electrocardiograms (ECGs), and digital spirometers. MedeCal Pulse Oximeter Probe Team Members: Bogdan Dzyubak, Joe Helfenberger, Jonathan Meyer, Matthew Parlato Advisor: Dr. John G. Webster, PhD.Clients: Amit Nimunkar and Jonathan Baran References United States Most Developed Countries Most Developing Countries Money spent on healthcare per capita $6096 $1,569 $48.86 Money spent on healthcare as a % of GDP 15.4%7.0%5.57% Life Expectancy Figure 1: Comparison of income and life expectancy [1] [1] United Nations. (2007/2008). Human Development Report 2007/2008. Retrieved March 12, 2009, from Table 6: Commitment to Health: Resources, Access, and Service: [2] Engineering World Health. (n.d.). Engineering World Health Website. Retrieved March 12, 2009, from Engineering World Health Website: [3] Fortney, L. (2009, January 31). Dr. (D. Team, Interviewer) [4] “Figure 1.” (2008) Accessed April , at Introduction Theory Plethysmography In tissues, blood volume regularly fluctuates with each heart beat. This volume change affects the amount of light that transmits through the tissue. Monitoring blood flow by measuring fluctuations in light transmittance is called photoplethysmography (Figure 3). Oximetry Oximetry is the measurement of oxygen saturation, which is the percent of oxygenated hemoglobin. Since oxygenated and deoxygenated hemoglobin have unique light absorption curves, it is possible to determine oxygen saturation by comparing light absorption at two wavelengths (red and infrared). Photoplethysmography is used to isolate the blood absorbance from tissue absorbance. Figure 3: Photoplethysmograph [4] Red Infrared Figure 4: Absorption curves of oxyhemoglobin and deoxyhemoglobin. The Problem Hospitals in the developing world often lack necessary medical equipment, such as pulse oximeters. Donated pulse oximeters are rare, and often cannot be repaired or replaced when they break. The only low- cost probes currently on the market are disposable. A Global Healthcare Perspective Design Specifications The primary goals of this project were as follows:  The probe must be durable, and preferably easy to repair, as many hospitals can only afford to buy one.  The probe must be versatile, usable with a wide range of age and ethnic groups.  The final product must cost less than $8.  The probe must interface with the MedeCal.  The probe must be convenient to use, to prevent misuse and disuse.  Oxygen saturation accuracy must be within 5%, preferably 3%. Probe Design Rubber end pieces Wires to MedeCal Modified Clothespin Optical Elements Inside The designed probe costs under $8, thus satisfying our goal. The probe operates in plethysmography mode yielding a clear waveform on both the finger and the ear, with both red and infrared. It has been tested on a range of subjects with different ear and finger sizes, morphologies, and skin colors. A good signal can be achieved in all of these cases with gain adjustment. Results Future Work Future work will include improving the probe’s mechanical design, and testing its performance. Mechanical improvements will include:  Reducing the probe’s spring constant, for long-term monitoring.  Replacing the plastic spring with a metal spring, for improved durability.  Implementing calibration resistors into the probe to correct for deviation in LED wavelengths. This will require modifications to the clients’ circuit. Performance tests will include:  The accuracy of oxygen saturation measurements will be compared with other pulse oximeters on the market. This will be possible when the clients’ circuit is completed.  The probe’s durability will be tested by subjecting the probe to cyclic deformation until failure.  A larger range of ear and finger sizes will be tested, including: Infants through adults with thicker ears Different ear morphologies  More tests on people with different skin colors will be conducted.  The effects of different cardiovascular conditions will be investigated: Decreased perfusion due to cold Oxygenation below 70%, achieved by breathing an air-nitrogen mixture Our probe can be used on both the ear and the finger. This greatly increases the range of patients on which it can be used. The ear offers a reliable blood supply, even under critical conditions, while the finger is more convenient. The probe we constructed is based on a clothespin. Its two prongs are connected with a plastic spring. Rubber sleeves protect the optical elements at the ends of the prongs from mechanical damage and ambient light. The probe is light enough to hang from the ear without exerting excessive pressure, thus avoiding pressure sores, and a decrease in signal quality. The optical elements consist of two LEDs (red and IR) and a photodiode. The photodiode amplifies the signal internally and returns a voltage signal to the MedeCal. ItemCost Photodiode$0.72 Two LEDs (on the same mount) ~ $0.33 x 2 ($15.00 for prototype) Plastic clip$1.50 Cord and serial connector$2.00 Rubber sleeves$1.00 Total$5.88 Acknowledgements Dr. John Webster (mentorship), Chris Esser (circuit design), Eamon Bernardoni (mechanics consultation), Dr. Rod Lakes (mechanics consultation), Amit Nimunkar and Jonathan Baran (encouragement and advice). The probe has been worn for an hour without significant discomfort.