Blood pressure is a complex interaction between cardiac output and peripheral vascular tone. It varies from minute to minute and hour to hour, and it is probably related to neurovascular tone more so than cardiac output. The low numbers of blood pressure are usually not significant unless he feels the effects of this, which are usually not fatigue, etc, but more likely to be dizziness, lightheadedness or passing out. If these occur he may need a decrease in the medications doses or decrease in the frequency of the medication.
Unless he is on a diuretic, or has poor oral intake in a hot setting then volume depletion is usually not a cause and volume repletion will probably not help.
Another question related to this condition. My father's weight fluctuates from 113 lbs to 130 lbs. At 130 lbs he has severe congestive heart failure symptoms and typically needs to be on an aggressive managed diuretic. When he gets out of the hospital after such an episode - with all of the water removed - his weight is only 113 lbs.
Based on that range, what would be a reasonable target weight for such a person? We could simply aim for a mid point, but I'm wondering if there are general guidelines to use for establishing a target.