Open source life-critical medical devices are not common because of the liability that manufacturers of the device and medical professionals using the device assume when the device is used on a patient. The current practice to reduce this liability and ensure patient safety is to certify the medical device with a trusted government regulation body. For good reason, certifying life-critical medical device is generally a very intensive (and consequently costly) process. 

This "introduction to the regulations to design, commercialize and distribute an open source medical device in EU" document provides brief overview of what types of open source medical devices require certification. 

The secondary barrier to the development of open source life-critical medical devices is the high level of design complexity required to ensure patient safety during operation. Redundant monitoring and user alert systems are good practice towards ensuring that the user is alerted of any device malfunction in a timely manner. As you can imagine, these redundancies can double or triple the complexity of the hardware and software.

To date, I have not seen a open source life-critical medical device that I would consider to be successful. "Successful" meaning a completely open design, widely reproduced, and considered an acceptable alternative to a commercially available product.

All of this background information is meant to put this effort to develop a open source IV fluid warmer in context. Because in some circumstances, an IV fluid warmer might be considered a life-critical medical device. Will the OpenFluidWarmer project result in one of the first "successful" open source life-critical medical devices? Who knows, but it certainly seems worth a try.