Substandard medical care is an ""intervening act"" that relieves a defendant from homicide liability if the medical care

Grossly negligent and intentional mistreatment.

Explanation:

Substandard medical care may be defined as a type of the malpractice medical case. This is the level and type of the health care professional that provide medical assistance in the same community.

The medical care can be intentionally mistreated in which the professional shows negligence in the  patient's treatment. This also relieves the homicide liability and defendant in the intervening act.

Thus, the answer is Grossly negligent and intentional mistreatment.