The Eye of the Stethoscope

There was a time when physicians diagnosed heart ailments merely by listening. They'd place a stethoscope's chest piece against a patient, then apply a wealth of medical experience and acoustical knowledge to detect maladies ranging from murmurs to heart failures.

Now, the venerable stethoscope may be re-claiming its place among medicine's best diagnostic tools. Thanks to the integration of digital processing and display technology, physicians can now use a stethoscope to “see” a heart's behavior, as well as hear it. With an attached PDA-sized display, they can watch the changing waveforms of a beating heart and make preliminary diagnoses in less time and at a fraction of the cost of an echocardiogram or more complicated tests.

“It's a real step forward,” says Dr. Nelson B. Schiller, a cardiologist and nationally recognized expert in echocardiography at the University of California, San Francisco Medical Center. “It's lightweight, convenient and easy to see. And the quality of the data is really good.”

The new technology, developed by HD Medical Group Ltd., could one day serve as a triage tool in hospital emergency rooms. It could also take up residence in doctors' offices, where family physicians might employ it for patient screening. And it has already played a role in crowded Third World medical clinics that don't have easy access to more expensive diagnostic machines.

“You don't need something incredibly complex to do triage,” says Damon Coffman, president and chief technical officer of HD Medical Group, developer of the new stethoscope technology. “If you just need a 'go' or 'no-go' decision, you don't need a full work-up or full diagnostics. This stethoscope will give you an answer without all the added cost and time.”

Digitizing Heartbeats

HD Medical's effort to enhance the stethoscope started when its founder, Arvind Thiagarajan, launched a student project to develop a device about the size of a fountain pen that could recognize heart anomalies. Thiagarajan's idea was for a physician to be able to point the device at a person's chest and have it identify various types of heart murmurs and other cardiac maladies. His studies eventually took him to Singapore, where he garnered funding and, ultimately, to Sri Lanka, where he put the device to the test in poorly equipped medical clinics after the Indian Ocean earthquake and tsunami in 2004.

There, Thiagarajan made an impression on visiting physicians by quickly screening patients in the crowded clinic, ultimately identifying those who were in need of immediate care. “He would place his device on the patient and say, 'this baby has a aortic regurgitant murmur,' or whatever,” says Coffman. “After about three or four times being right, the doctors started looking to him for a quick diagnosis. They recognized that he could go through that entire crowded room of 200 kids in one hour and find the 10 who needed the most immediate help.”

Since that time, Thiagarajan's idea has evolved into a product that might now make its way into the U.S. medical establishment. Today's version, which has changed some details over the past four years, still uses the same basic approach. Unlike a conventional stethoscope, HD's ViScope 100 incorporates a simple cell-phone-type microphone near the bell (the part the doctor places on the patient's chest) to pick up the beating sounds of the heart. Analog signals from the microphone travel to an anti-aliasing filter, are digitized by an A/D converter and are passed on to a Quickfilter Technologies' digital signal processor (DSP). The Quickfilter DSP, in turn, splits the input into four separate signals, which it sends to a TI MSP430 ultra-low-power microcontroller. From there, two signals are sent to the stethoscope's electronic display, one is sent to an optional laptop computer and the final one is re-constituted into an analog audio signal and sent to a speaker in the stethoscope tube, which enables the physician to hear the original heartbeat as it would have sounded before it was digitized. As a result, the doctor not only sees the waveform — he or she hears the heartbeat through the stethoscope's earpieces.

HD engineers say they've worked hard to take Thiagarajan's original vision and evolve it into a product physicians will actually use. To accomplish that, Coffman and HD's engineers focused on simplicity.

“Our first product ended up going out and then being brought back because we realized it was too complex,” Coffman recalls.

Over time, HD's engineering team worked at minimizing the size and maximizing the simplicity of successive product iterations, Coffman says. Ultimately, the display was reduced to a mere 1.5 x 2.5 inches and its software was repeatedly simplified until doctors could obtain each diagnosis quickly and easily.

“Today's product answers the questions, 'does the patient have a (heart) murmur?' and 'if so, what kind of murmur is it?'” Coffman says.

Moreover, HD engineers worked with counterparts at Texas Instruments to maximize battery life. Using TI's MSP430, they were able to reduce current consumption, during use and during standby. The MSP430, which serves as the main controller for the stethoscope, offers active power consumption as low as 160 µA/MHz and standby consumption of 1.5 µA/MHz. As a result, a physician can use the stethoscope through the course of an entire workday without worrying about the battery drain.

TI engineers say HD also made use of the MSP430's feature integration. Because the MSP430 incorporates a number of built-in features, such as LCD drivers, analog peripherals and power management, it served as a means for placing more functionality into a smaller area. It also enabled HD to bring the ViScope 100 to market faster and make software changes for subsequent iterations on the fly.

“When you're a start-up company like HD Medical and you're trying to bring your product to market quickly, you need a component that has the ability to kick-start your development,” says Matt Harrison, director of marketing for medical and high-reliability applications at Texas Instruments. “Having the LCD controller and analog peripherals of the 430 was a big advantage for them from a time-to-market perspective.”

TI engineers say they also worked hand-in-hand with HD on the technical issues surrounding the analog-to-digital and digital-to-analog conversion of the stethoscope's sounds. “Unfortunately, we don't live in a digital world of 'ones' and 'zeroes,'” Harrison says. “We live in a real analog world and it's a tough place to be, especially when you're dealing with acoustic signals and a lot of amplification, as you have in this particular project.”

Making a Comeback

The resulting technology could be a boon to physicians in the U.S., as well as in Third World medical clinics. Cardiologists say it could help resurrect a testing procedure known as phonocardiology, in which physicians record the sound of a patient's heartbeat as a means of gathering information. Once considered a viable means of testing hearts, phonocardiograms fell out of favor because they were costly and difficult to do, and were therefore overshadowed by the echocardiogram and the electrocardiogram.

The visual stethoscope, however, could help bring phonocardiography back because it's now simpler and far less costly than those other methods. HD's ViScope provides about 3 sec — usually three or four heartbeats — worth of audio information on the heart, displayed in waveforms on the unit's screen.

“This brings the phonocardiogram back,” Schiller says. “The display is just like the displays we used to labor over 30 years ago. It's quite familiar to cardiologists who have worked with phonocardiograms.”

Coffman says he expects the devices to be used by a few cardiologists, but more so by family doctors looking for a quick diagnosis, so they can determine whether to send a patient to a specialist. That's why HD already laid plans to add sophistication to the product, incorporating electrocardiogram capabilities, as well as an automated diagnostic procedure for doctors. Coffman says he expects the company to roll out those product generations in the next two years.

In the meantime, cardiologists at the University of California, San Francisco have published research suggesting cardiac function can be better understood by reviewing audio recordings from devices like the ViScope.

“This is just the beginning as far as I'm concerned,” Schiller says. “This could serve as a teaching tool and as a better way of understanding cardiac function. We've only begun scratching the surface here.”