Wearable electronics provide better insight into the heart's health

One method of investigating the heart is for the patient to undergo an electrocardiogram (ECG). But an ECG will only give a snapshot of how the heart is performing at that point in time. This deficiency has been well known for decades – the first ECG was made by Dutch physiologist Willem Einthoven in 1895. A solution was developed in the last years of the 1940s, when US biophysicist Norman Holter invented the concept of telemetric cardiac monitoring; a technique that entered the medical mainstream in the 1960s. Holter's invention – the Holter monitor – supplemented the ECG technique by collecting data over a longer period on a device attached either to a belt or hung around the patient's neck. Earlier devices used audio cassettes to store the data, but more recent models have taken advantage of solid state storage. Advances in technology have also allowed the Holter monitor to be reduced in size. Whilst Holter monitors have given doctors a better insight into the health, or otherwise, of a patient's heart, the devices have a restricted data capture window. Now, researchers are working on ways to provide more data to doctors. A paper just published in the American Journal of Medicine has compared the performance of the Holter monitor with that of a recently introduced device called Zio Patch, developed by Californian company iRhythm. In the study, conducted by Scripps Health, 146 patients referred for evaluation of cardiac arrhythmia were equipped with a Holter monitor, to record data over a 24 hour period, and with a Zio Patch, which captures data over 14 days. Both devices were looking for evidence of a range of heart defects. According to the study, the Zio Patch detected 96 arrhythmia events, compared with the 61 identified by the Holter monitor. Zio Patch, which has already been approved by the US Food and Drug Administration, is an adhesive device measuring 12.5 x 5cm which does not require external leads. Because it is small and water resistant, those wearing Zio Patches can follow their normal routine. Once the monitoring period is completed, the patch is analysed and a report sent to the appropriate doctor. However, iRhythm is not the only company pursuing this agenda; researchers from the Fraunhofer Institute for Photonic Microsystems IPMS in Dresden have developed an ECG recorder that reads and analyses long term ECGs in everyday conditions, then transmits the results to the doctor in real time by radio. Since complaints or certain events – for example sports – can affect the results, doctors often prefer to read resting or exercise ECGs under controlled conditions. However, when the patient is exhibiting intermittent symptoms – cardiac arrhythmia, for example – these are not always recorded in an ECG and doctors therefore look to record data continuously for least 24 hours. The Fraunhofer IPMS team has developed a three channel ECG recorder called SmartVital for these applications. SmartVital is not only said to be small, light and easy to use, but also to provide a real time analysis of the ECG signals, based on a number of methods. If the evaluation software identifies an abnormality in the ECG, the recorder sends the appropriate section to the doctor via a gateway. Apart from avoiding a visit to the doctor, the quality of the readings is said to be increased by recording and analysing the patient's physical activity. A motion sensor included in SmartVital allows it correlate changes in the ECG with physical stress, while disturbances due to motion artefacts can be identified. But ECGs are not the only way to determine cardiac health; blood pressure is another useful input into a doctor's diagnosis. Dr Chris McLeod, principal research fellow, cardiovascular instrumentation, at Imperial College, says he's been looking for better ways to measure blood pressure on a regular basis. "You can have a wearable device which sits on the wrist or wherever and measure blood pressure using a cuff. That's fine for measurement of systemic circulation, even though there are large errors, particularly in those who aren't well." According to Dr McLeod, in those with impaired circulation or who are obese, the relationship between pressure measured by a cuff and actual arterial pressure is significantly different. "If you're trying to diagnose based on that, it's an error." The better way to do it, in his opinion, is with a transducer in the bloodstream. "It depends on how valuable the measurement is in helping a clinician treat the patient better, but if the patient is ill and there are positive risk benefits, a lot of clinicians think they will be able to better look after patients if they have accurate measurements." Dr McLeod has based his solution on surface acoustic wave technology. "I've been looking at physical measurement all my career," he noted, "and have always been on the lookout for a new way of measuring blood pressure." Talking to a colleague involved in the automotive industry, he discovered a tyre pressure measurement system in which a wireless transponder interrogated a pressure sensor in the tyre every time it passed an antenna. "That's absolutely perfect for a medical device," he continued. "There is no battery and no moving parts; it's a very simple structural device and interrogation can be undertaken from outside of the body." The pressure transducer is, he says, 'a few mm by a few mm' and sits on a blood vessel's inside wall – in this case, the aorta. "Putting something of that size in a blood vessel which may be 25mm in diameter won't disturb the flow. And the vessel will grow over the sensor so it becomes embedded in the aorta's wall." A further benefit is that such devices can be fitted relatively easily. "People are implanting other passive devices – stents, for example – via catheter," Dr McLeod continued, "and we plan to fit the sensor in the same way, using a mini stent." Operation of the device is simple. "You supply energy to the sensor through an antenna. When you turn off the excitation, some of the resonant frequency will be transmitted and can be detected outside of the body. If you make your device pressure sensitive, then you can measure pressure," he explained. The interrogator is a variation on the theme of a mobile phone, with sophisticated measurement electronics, storage and onward communications ability. "We can measure blood pressure at rates of up to 1kHz," he pointed out, "and can capture a complete waveform on every beat. We have also included a motion sensor, so blood pressure can be correlated against activity. It measures blood pressure continuously, so it will pick up all the nuances." The device is programmable, so doctors can set a suitable sampling regime and alarms and determine what happens when alarms are triggered. The project has now evolved to the point where the first usable sensors are about to be delivered. "These will quickly go into the test lab and I hope we can be in trials within 12 months," he concluded.