BS EN 60601-1:2006+A1:2013 is the current standard in which ME Systems are covered in section 14 (page 226). There is also some useful information in Annex I.
Section 14 starts off with:
After installation or subsequent modification, an ME SYSTEM shall not result in an unacceptable RISK.
Only HAZARDS arising from combining various equipment to constitute an ME SYSTEM shall be considered.
NOTE RESPONSIBLE ORGANISATIONS are reminded that the assembly of ME SYSTEMS and modifications during the actual service life require evaluation to the requirements of this standard.
An ME SYSTEM shall provide:
– within the PATIENT ENVIRONMENT, the level of safety equivalent to ME EQUIPMENT complying with this standard; and
– outside the PATIENT ENVIRONMENT, the level of safety equivalent to equipment complying with their respective IEC or ISO safety standards.
So for example an ECG system used in a cardiac interventional suite may have a section within the patient environment that meets 60601-1 but have an operator workstation designed for use in a control room outside of the patient environment.
The section in the control room, outside the patient environment, would not need to conform to 60601-1 (only 60950 if PC based) but would need appropriate safety measures taking into account the connections between the parts inside the patient environment.
This would typically involve using optical coupling between the PC parts in the control room and the patient environment parts (with 4KV isolation).
Some would consider just sticking a isolation transformer on the PC in the control room but this may not eliminate all risks associated with the direct electrical connections between the two systems, as it only addresses the leakage current. (What happens if a fault occurs on the PC data lines and applies mains to the data port? Will RFI/EMI be coupled into the patient environment?)
Of course you would also need to refer to other collateral standards such as 60601-1-2 (EMC) and any others, such as 60601-2-43 for interventional x-ray systems and so on.