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I'd say "In case of a drop in blood pressure
That being said, I think the source text is very poor. A drop in blood pressure by itself may not need fluid replacement as it may not lead to hypotension... It should say either "In the case of a drop in blood pressure [leading to hypotension]", or "In the case of a substantial drop in blood pressure"...
Secondly, I really doubt that they're administering human serum in children for volume replacement. That would have catastrophic consequences, in my most humble medical opinion....
I do believe that it is a good idea to take this up with the client; I believe that he will be able to put me in contact with the doctors this document is destined for, who may be able to provide further insight.
Thank you to everyone for your invaluable input, I am taking careful note.
SJLD : I agree, the sentence is not my best, but I do intend to improve upon it - in my post I was just hoping to present the problem in a clear way. Thank you for your suggestions!
In case of a decrease of blood pressure - this does not sound idiomatic to my native English (doctor) ear. Perhaps "If hypotension is present..." or "In the presence of hypotension..." or "If the patient is hypotensive..."
Presumably the text is aimed at a professional readership.