The traditional stethoscope is ubiquitously used in the chain of medical care. However, in isolation it is only capable of assessing respiration and heart rate; blood pressure measurements are possible when the stethoscope is used in conjunction with a sphygmomanometer. A traditional stethoscope head contains a diaphragm that mechanically amplifies audio signals in the 0.01 Hz to 3 kHz range. For medical use, operators fix the head of the stethoscope adjacent to the phenomenon being observed (e.g. against the chest to measure respiration). The diaphragm transmits the sound coupled into the stethoscope head from the features (such as the heart or lungs) into a set of ear pieces. The operator then interprets this sound and manually records this measurement. Studies have shown that these measurements have a strong dependence on the level of training for the operators, as well as the audio environment in which the measurements are taken.
Electronic stethoscopes have attempted to address the limitations of traditional stethoscopes in loud environments, such as the emergency department. They convert the mechanical vibrations incident on the diaphragm into electronic signals that can be readily amplified and transmitted to the earpiece worn by the operator. However, the human operator is still required to interpret the audio signals to deduce physiometric parameters such as heart rate and respiration rate.
In contrast, ultrasound imaging equipment has been developed to automate some of this data collection and interpretation. For example, ultrasound imagers can extract adult or fetal heart rate from recorded images or Doppler ultrasound. These imagers measure high frequency echoes that penetrate and reflect off of tissues within a body. A number of strategies have been developed to modulate the frequency of the sound to perform tomography using these ultrasound instruments. For example, high frequencies generate higher resolution images at shallower depths (e.g. subcutaneous tissue, lungs, vasculature) and lower frequencies generate lower resolution images at deeper depths (e.g. visceral organs). Ultrasound is used for a variety of diagnostic imaging purposes including examination and monitoring of infection, trauma, bowel obstruction, cardiac disorder, pregnancy staging, and fetal health. Though its versatility would make the ultrasound a particularly effective tool for use in point-of-care medicine, in the developing world, in wilderness expeditions, and in spaceflight, the typically high cost, power-requirements, and size of ultrasound equipment have prevented its adoption for many scenarios.
Furthermore, unlike stethoscopes, current ultrasound imagers require substantial training to use, yet still suffer from substantial inter-operator variability. These limitations have allowed ultrasound to augment, but not replace, stethoscopes.