Vital signs

5 min read

A multi-parameter Sensor Electronics Module that measures ECG, heart rate, breathing and temperature has been developed for medical and military applications. Berenice Baker reports

As The Engineer goes to press, three British explorers are midway through the Catlin Arctic Survey, a gruelling 100-day, 1,000km polar expedition across the frozen wastes of the Arctic.

Their aim is to study the effects of climate change on the floating sea ice of the Arctic Ocean, but thanks to a UK-developed body-monitoring system the expedition is also providing valuable insights into how the human body copes in extreme environments. And in a move sure to excite armchair explorers everywhere, this data, which includes measurements of the trio’s heart rates, breathing rates and core temperature, can be viewed in real time on the Catlin Arctic Survey website.

Manufactured by Cambridge-based Hidalgo, the so-called Equivital system originally came out of work Hidalgo did for the US Department of Defense to produce a wearable physiological monitoring system for soldiers. As well as military applications, a modified version of this system is now used for applications in tele-health, occupational welfare, sports performance analysis and physical endurance research.

Described by its developer as a multi-parameter Sensor Electronics Module (SEM), the system measures a number of primary vital signs such as ECG, heart rate, breathing and temperature, and reports them in real time. It combines these with contextual information that indicates recent activity and body posture and then runs algorithms on them to produce what Hidalgo dubs a ‘welfare index’. For example, a high heart rate following activity would be expected, but in a prone, inactive patient would indicate that medical attention is required.

Justin Pisani, chief technical officer at Hidalgo, said: ‘The welfare index is a very simple traffic-light type output where green is OK, yellow is bad, red is really bad, and there are even states beyond that when it is really bad but not worth trying.

‘That’s really important because if you consider a traditional medical device, you put it on a patient and the data is sent to a machine that either logs them or displays the waveforms on a monitor. You either need to analyse that data offline after you have collected it or have a doctor look at the data immediately to say if it is good or bad.’

The body-worn SEM comprises a batch of sensors in a single module, which can be worn in a variety of positions according to user preference. It is usually placed in the soft part under the sternum where it is close to the main cardio-respiratory organs, but can be worn on the side or in certain instances it can be used remotely.

The SEM also supports peripheral sensors, including a ‘core pill’ that is swallowed to measure internal body temperature, which it then transmits to the Equivital module. This is being used on the Catlin survey to see the effects of extreme cold, but is also important for monitoring heat in military personnel and first responders.

Pisani added: ‘Heat is a big issue for occupational welfare applications. Soldiers running around with lots of kit quite often suffer from heat stress, which is probably one of the biggest peace-time killers of people in that sort of emergency services role. The data from the pill is aggregated with the other data being recorded to give the state of thermal strain on that individual.’

The SEM also contains the processor that runs the algorithms: a personal area network (PAN) that can transmit to a receiving device and a standard micro-SD memory card, such as those used in digital cameras.

Pisani said: ‘We log all our data within the device and then it has the ability to connect via the PAN to a phone or a comms gateway. If the comms gateway is there, it can send either the full data or a summary of it. Army operations, for example, just send the traffic-light data over the comms gateway, but we actually store on the card the full waveforms of that person like a little black box. If something happens, you can get a full tracing of what happened to that person’s physiology as they were wounded or succumbed to some trauma event.

‘Being rather gruesome, it’s not that useful to the guy who is wearing it, because the main thing for him is whether he has been dealt with and recovers. But it helps further the medical arm in that it helps you see how people respond to certain events, which could lead to future versions triggering alarms earlier.’

For emergency service use, Equivital can use a number of different communication protocols, including the Tetra network used in the UK. For other applications, it uses SMS or Equivital’s native protocol-in-packet data mode over the GSM network.

The device automatically decides which mode to work in depending on the available bandwidth and state of health of the wearer. For instance, where sufficient bandwidth is available, in most instances just basic status information is broadcast. But if the sensors detect that the welfare status is going away from normal, Equivital automatically drops into a higher level of disclosure.

The data is processed using a standard 8-bit RISC micro processor. ‘We set out a design target that banned the use of DSPs and processors with operating systems from day one, mainly because we knew we’d get nowhere near our power budget if we did that,’ said Pisani. ‘The human body works in our favour there, in that the frequency of events such as heart beat and breathing are, in electronics terms, pedestrian compared to radio and the like.’

Equivital is powered by a Li-ion mobile-phone type battery and also supports AAA batteries for use in the field where mains power is hard to come by. Future models will use lithium-polymer battery technology.

One of the biggest challenges Hidalgo had to overcome when designing Equivital was sweat, which is full of salt and very conductive — leading to intermittent conduction variance. This variable resistance is a particular problem when measuring an ECG of a millivolt.

Hidalgo is launching Generation 2 of Equivital in the first quarter of next year. The company already has most of the electronics and technology at PCB test level and is in the process of finalising the mechanical form factors. A complete integrated model will be built and tested in summer. The first generation of Equivital used the same hardware for all its different applications, but Hidalgo is planning to diverge medical from military in the next iteration. Pisani said: ‘We can produce a much more streamlined and lightweight product for the military and first-responder environment. The medical device is going the other way — we are adding more peripheral sensors and integrating with other wireless devices such as blood-pressure cuffs.’

The way in which acceleratory forces are tested on the device also varies with application. For military applications, it is put behind body armour and 7mm bullets are shot at it. For the medical applications, it is tested on a patient falling relatively gently to the ground.

Although there are a number of other physiological monitoring systems available on the market — including Toumaz’s Sensium system and Foster-Miller’s Watchdog — Pisani said that the real-time aspect of Equivital sets it apart from competitors. ‘Most medical telemetry devices work on spooling the data to a PC and allowing secondary algorithms to run over the top of it to deduce whether it’s good or bad,’ he added. ‘That’s OK when the PC is next to the bed and bandwidth isn’t an issue, but we can cost-effectively and with low power consumption process that data on the device in real time.’

Pisani said that Equivital is the only device in the market that has full US and European medical accreditation, so the base design will not change significantly for the new model. The company works with Cambridge University Teaching Hospital, part of the Addenbrookes, in clinical research and has a relationship with the Institute of Environmental Medicine in Boston, a division of the US Army research facility, which provides data in relation to the environment and the effect of activity on soldiers.