High blood pressure is define as a systolic reading greater than 140mmHg and/or diastolic reading (the smaller reading) greater than 90mmHg.  It is normal for a person's blood pressure to vary.  Anxiety, for instance, can result in an artificially elevated blood pressure.  The actual blood pressure when he is relaxed may be normal.  One way to determine if the blood pressure is truly elevated is to do a 24-hour blood pressure measurement on an outpatient/ambulatory basis.

If someone does not have high blood pressure but is given blood pressure lowering medication, his blood pressure may become too low.  This could lead to giddiness or even loss of consciousness. Alternatively, the giddiness and blackouts could be due to a heart rate that is abnormally slow.

Older patients have degeneration of the electrical conduction system in their heart which results in an abnormally slow heart rate.  Some blood pressure lowering drugs like Atenolol slow down the heart rate.  It may lead to giddiness and fainting in someone who already has an underlying abnormality of the electrical conduction system of the heart.

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