Page 159 - Safety Risk Management for Medical Devices

138 Safety Risk Management for Medical Devices

The Software System is software safety class B if:
2 the software system can contribute to a Hazardous Situation which results in
unacceptable risk after consideration of Risk Control measures external to
the Software System and the resulting possible harm is non-serious injury.
The Software System is software safety class C if:
2 the Software System can contribute to a Hazardous Situation which results in
unacceptable risk after consideration of Risk Control measures external to the
Software System and the resulting possible harm is death or serious injury.”

Naturally these definitions necessitate the need to know what “serious Injury” is.
IEC 62304 [9] sec. 3.23 defines Serious Injury as:
“injury or illness that:

a. is life threatening, or
b. results in permanent impairment of a body function or permanent damage to a
body structure, or
c. necessitates medical or surgical intervention to prevent permanent impairment
of a body function or permanent damage to a body structure
NOTE Permanent impairment means an irreversible impairment or damage to a body
structure or function excluding trivial impairment or damage.”
These definitions are not well aligned with the definitions of seriousness of Harm as
offered in ISO 14971 [3,7] and cited in Table 17.1. One could roughly align the defini-
tion of Serious Injury with a combination of Critical and Serious from ISO 14971 [3].
All Harms have an associated Hazardous Situation. If the Hazardous Situation can
manifest solely due to software failure, then P 1 for that Hazardous Situation is
presumed to be 100% and risks are equal to the P 2 numbers for each Harm severity
class. According to IEC 62304 [9] Section 7.2.2, the safety class of software that is
intended to be a Risk Control is based on the risk that the risk-control-measure is
controlling. If the software Risk Control is controlling the risk of a software item,
then its class would be the same as the class of the controlled software. If the software
Risk Control is controlling the risk of a hardware item, then use the following algo-
rithm to determine the software safety class for the Risk Control software.

1. What Harm(s) can the hardware-failure cause? Utilizing the Harms Assessment
List (HAL), identify the Harm severity class that has the highest probability. If
more than one Harm is identified, choose the highest class among all the Harms.
2. Using Table 15.6 select the software safety class that correlates to the hardware
severity class from Step 1.