Wireless Medical Blog
Wireless health – Part 1: How do we transmit all the data?
By Michael Hosemann - Last updated: Friday, June 11, 2010Please excuse me for introducing another term, ‘Wireless’, to the plethora of Mobile Health, Connected Health, Telehealth, etc. Paul explained in an earlier post here a while ago. But I’d like to focus on the mobility aspect of all the health applications, the reason why we want to make these applications wireless: Patients are rarely stationary. They move from their rooms to treatment rooms or operating theatres and back all the time. People also might be recovering at home and are encouraged to go out and exercise but still need to be monitored. Others may just want to have coaching for their daily exercise regime by devices like the Milife. All these scenarios have one thing in common: A cable for transmitting the data would be highly cumbersome and impractical. This is where we ideally just cut the cable and the “wireless” aspect comes in.
Unfortunately, it is not that simple and a bit of forethought is required before a successful system can be rolled out for wireless patient monitoring. I’ll be exploring some important aspects of deploying wireless links for medical and health applications via several posts over the next few weeks. As this can get fairly technical quite quickly, I’ll supplement the blog posts with some white papers for those who would like to know more.
Designing a wireless system starts with gathering the requirements, viz.
- What range do you need? Do you just want avoid having to plug your data-logging watch into your computer or do you want to transmit data while you are outdoors on you bicycle?
- How much data do you want to transmit? Just someone’s location every few hours or many patients’ ECG data at diagnostic grade?
- Shall the batteries last for years or are a few hours of operation enough?
- How reliable does the transmission need to be? Does someone’s life depend on it or will you just try again if something fails?
Once you know what you want, you can match the requirements against the numerous technologies already available. Chances are, you will find a suitable solution amongst existing standard options e.g. Bluetooth, Wifi, cellular systems of all generations, DECT, near-field communication, etc. You can then use commercial off-the-shelf components to implement it. Sometimes you will have to design your own system, though. One case where you might need to think about your own design is high-reliability patient monitoring for hospitals. Just to give an example on reliability, a typical cellphone network is planned on figures of roughly 95% coverage or a few percent of dropped calls. A hospital grade patient telemetry system often has requirements of less than 1 second outage in 20 minutes. That is less than 0.1%! Of course the law of diminishing returns applies here. Going from 99% to 99.9% involves a disproportionally large amount of effort. For a cellular network operator, the return-on-investment is key and a higher reliability network would cost a lot more but bring little or no extra revenue. For a hospital the foremost consideration should be reliability of the system.
Most wireless systems have been designed for specific frequency bands. Frequency bands are allocated by regulators to individual or multiple users under certain conditions. That means that users of many wireless technologies actually have to share the transmission space with other users. This leads to collisions and sometimes poorly working systems. This might be just annoying when your are trying to listen to your favourite internet radio station over your home Wifi. But it could be outright dangerous if a patient suffers a heart condition and this goes unnoticed because the signal can not get through. Hence, care must be taken when selecting a wireless technology and corresponding frequency band to achieve the require reliability. With more and more wireless devices out there, this will become even more crucial. If you’d like to read more about available frequency bands, have a look at this white paper on which frequency band to utilise for a wireless medical system.
Paul Williamson Said,
June 14, 2010 @ 9:19 am
Thanks for your insight into the choices of frequency band for a wireless medical system. It is worth noting that there are significant commercial impacts of the decision. ISM techniques can offer lower product and development cost due to the volume shipments of the technology for other applications.
As you say the challenge is then to optimise your system performance in the chosen spectrum. Not all ISM band radio techniques are equal…