The product should have a consumer cost of $200-$250 to be competitive with other stethoscopes and oximeters currently on the market. Stethoscopes are typically around $150, while pulse oximeters cost $30-$50 dollars.
Production cost per unit should be ~$80
Stethoscope Properties:
Stethoscope should be able to detect high and low frequency body sound responses, with a range of 20-1500 Hz.
Stethoscope should be non-invasive.
Oximeter Properties:
A display should present oxygen saturation (SpO2) measurements.
The display should be easily readable in brightly lit areas. Hospital areas are lit as brightly as 200 lux, or approximately 670 candelas so the oximeter should be readable in lighting of this intensity.
Device should have a portable power supply mounted on it able to power the oximeter and any associated circuitry or computing devices.
Pulse oximeter should be able to be powered for 20 hours, comparable to existing portable pulse oximeters.
The oximeter should provide usable data within 10 seconds so that measurement speed is comparable with oximeters currently on the market.
Product Accuracy:
Stethoscope should be able to reproduce auscultation sounds such that it should make no diagnostic difference to physicians using the industry standard 3M-Littmann Cardiology III.
The oximeter’s measurements should be of comparable accuracy to currently available pulse oximeters, with a bias error of ±2% or less, and a precision of ±3% or less.
The oximeter should retain this accuracy through operator applied pressure fluctuations in the range of 60-80 mmHg.
The measurement accuracy should remain consistent for a temperature range of 0-45°C.
Data measurements should not be affected by the amount of ambient light present. The normal range of light intensity in hospital areas ranges from approximately 335-670 candelas so the oximeter should retain accuracy within this range.
The stethoscope and the oximeter should not interfere with each others functions, for example with electromagnetic noise or acoustic impedance.
Safety:
The patient and operator should be insulated from currents or voltages.
The product should not heat above typical body temperature range to avoid burns.
The light source should not pose a risk for damaging the tissue (eyes, skin, etc.) of the patient or clinician.
The device should not contain any sharp edges or corners.
Ease of Use and Ergonomics:
The product must be easy and quick for operators to use.
Clinicians who regularly use stethoscopes should be able to use the product’s stethoscope function with no extra training or information. This confirms the need for the stethoscope to maintain its normal acoustic properties despite the addition of new features.
The addition of the display should not interfere with a clinician’s ability to have a firm and comfortable hold on the stethoscope’s chestpiece.
The product should be comfortable and convenient for clinicians to wear around their neck as they perform their normal daily tasks, much like a traditional stethoscope. To ensure the device is not cumbersome to clinicians the final product should not weigh more than 250-500 grams and the display should no larger than 13 square centimeters.
Final Design
Figure 1: Final Overall Prototype Design
Our final design is a prototype steth-oximeter that consists of a stethoscope with a transmittance finger clip pulse oximeter on a retractable reel. The retractable reel is located at bifurcation site between the binaurals and the tubing. The prototype was developed using a MDF® 747 Dual Head Lightweight Stethoscope but our oximeter and reel are designed to be attachable to most typical stethoscope used by clinicians. The transmittance finger clip is a custom made 3D printed plastic device with a spring-loaded clip. The reel is also a custom made 3D printed plastic device that incorporates a spiral spring and a ratchet and pawl. These mechanisms used to control the winding and unwinding of a USB cord around an inner spool. The cord can be pulled and unwound when the oximeter is needed and retracted back into the reel when measurements are completed.
Figure 2: Design of Spiral Spring and Spool System
Figure 3: Design of Ratchet and Pawl
The signal obtained from the photodetector is small and therefore required amplification and filtering. The following circuit incorporates a low-pass filter and band-pass filter to achieve a noise-free signal from three cycling phases. Arduino code was written and an established algorithm was used to analyze the signal and determine an SpO2 value. An LCD was wired to the Arduino to display the final SpO2 value.
Figure 4: Design of Circuit for Filtering and Amplifying Photodiode Signal