Noise-Immune Stethoscope

Army medics and civilian paramedics often have to function in difficult surroundings. Noise, for example, can be a major hindrance in the diagnostic process, when the patient's heartbeat is barely audible above ambient noise. This could be during medical evacuation from a battlefield by helicopter, or when dealing with non-military medical emergencies on a highway, or at a noisy public event. Conventional passive acoustic stethoscopes work fine as long as the ambient noise levels are under 80-85 decibels (comparable to a busy city street). Unfortunately, once levels reach 90 decibels effective auscultation is not possible. Modern electronic stethoscopes have raised the maximum tolerable noise-level to between 90 and 95 decibels with the help of earphones, effective sound insulation, and optimized sensor design. But they do not go far enough.

     However, help could be at hand in the form of the noise-immune stethoscope that deploys ultrasound-imaging technology. This device ensures the sound produced by the heart or lungs can be clearly detected above environmental noise, regardless of intensity.

     Work on this stethoscope was conducted by Adrian Houtsma, a former MIT researcher and emeritus professor of acoustics from the Technical University of Eindhoven in the Netherlands, and Ian Curry, both of the US Army Aeromedical Research Laboratory (USAARL) in Alabama, together with John Sewell and William Bernhard of Active Signal Technologies in Maryland.

     The project to find a more effective stethoscope was born out of a medical need identified by the US Army. The first hour after sustaining an injury-the 'critical hour'-is when diagnosis and treatment must take place if combat casualties are to stand a good chance of survival and early recovery. Auscultation is an important diagnostic tool for assessing the condition of the heart muscle, valves and major arteries. In addition, auscultation of the lungs is invaluable when confirming the placement of endotracheal tubes, or to diagnose such conditions as a collapsed lung, asthma or pulmonary edema. Unfortunately, these patients are often transported by helicopter where noise levels prohibit the use of traditional stethoscopes.

Above the Noise
      The noise-immune stethoscope works by transmitting a sound signal at 2.3 megahertz into the patient's body. It is high enough to ensure that ambient noise will not interfere with the signal. The signal is then reflected back to the stethoscope at a slightly different frequency. This phenomenon, called the Doppler effect, is caused by the velocity of the internal organs, and the difference in frequencies of these two sound waves determines the motion of the internal organs. The resulting sound (produced when this frequency differential is converted into an audible signal) is markedly different from that of conventional stethoscopes. An acoustic stethoscope produces a 'lub-dub' sound from a heartbeat, with the first beat being the strongest. On the other hand, an ultrasound stethoscope will yield a 'ta-da-ta' pattern, with the second beat being the strongest.

     However, this invention is not without its critics. While complimentary of the technology, one professional in the field of medical technology argues that users of the noise-immune stethoscope will need specialized training-thanks to the non-conventional way it works-to interpret the ultrasound information. Another, a British physicist, sees the need of a steady hand and stable surface when operating this device, since any movement could itself cause a Doppler shift.

     Nevertheless, early trial results are encouraging. In a formal field test conducted at Fort Campbell, KY, by the 160th Special Operations Army Regiment, the stethoscope proved its effectiveness, says Dr Houtsma, the USAARL stethoscope project's team leader.

     "As expected, this trial showed that, when used in Black Hawk helicopters, the noise-immune stethoscope provided a clear and useful signal in ultrasound mode," explains Dr Houtsma.

     The ultrasound stethoscope is currently undergoing FDA and Airworthiness Rating (AWR) approval.

A Netherlands-based business and technology writer, Joe Figueiredo contributes to business magazines and trade journals. Contact him via www.fits.scarlet.nl.