Brief Answer:
It is postural hypotension

Detailed Answer:
Hi,
I am Dr. Prakash HM and I would like to help you.

Looking at your symptoms of dizziness and narrowing peripheral vision which gets exacerbated with exertion or change in position, possibility of postural hypotension or orthostatic hypotension could be thought of.
Its further increase with intravascular volume depletion due to heat confirms the condition.
In this condition visual defects vary from blurred vision to blackouts.

Orthostatic hypotension is not a cardiovascular abnormality. In the elderly it is due to baro-receptors inability to function properly or the inability of blood vessels to constrict blood vessels under stress.

Dizziness can occur in the following situations:
1. After food - due to blood pooling in the gut during digestion
2. Reflexes triggerred due to straining at stools, urination, swallowing or coughing
3. Warm envirnoment
4. Sudden positional change
5. Prolonged recumbency
6. Physical exertion

In patients with treatment for hypertension,
1. antihypertensive agents impair cardiovascular reflexes.
2. Hypertension impairs baro receptors reflex sensitivity.

You need to manage your orthostatic hypotension as sudden dizziness and fall can lead to severe injuries.

Management includes: (Already you are on many drugs: Try non-pharmacological therapies first - Drug interactions are unpredictable)
1. Moving slowly from one position to another - sitting to standing, sleeping to sitting
2. Avoid standing motionless for long time, large meals, hot weather, hot showers, dehydration or isometric exercises.
3. Avoid diuretics and change the drug if on hypertensive therapy
4. Raising the head of bed to higher level
5. Controlled postural exercises
6. Drug therapy is available - If the above fails, you can discuss with your doctor regarding this.

Hope it helps.
Any further queries, happy to help again.
Dr. Prakash HM

---