"Concern about blood pressure measurements"

I had an appointment in February at IRH to check the current status of my vascular disease.  When checking my BP both nurses appeared to be quite alarmed.  They asked if I was ok,  I said I was light headed all the time.  They then informed me that my BP was high.

The doctor discussed when they would see me next and advised me to go to my local GP nurse.  I then drove all the way back home before arranging the nurse appointment.  I relayed what had occurred at IRH and the receptionist arranged for an appointment that day.  When the nurse checked again my BP had reduced.  They too looked alarmed and immediately prescribed medication, which has recently been increased.

As my blood pressure has been pretty perfect all my life, I am disturbed that I now have to stay on this for the rest of my life.  I am also confused as to why this would not be investigated... there must be a reason.  Can you tell me what you would deem a critical BP rate ... just in case I reach it ?

Many thanks in advance :-)