A normal resting ECG cannot capture every standing-related change

The key may be how heart rate and blood pressure respond over time after rising, not a one-time ECG.

Postural orthostatic tachycardia syndromeExcessive sustained heart-rate rise on standing can cause dizziness without a resting ECG abnormality.
Orthostatic hypotensionA blood-pressure fall on standing can produce overlapping symptoms.
Medication or dehydration effectDiuretics, stimulants, fever or poor fluid intake may change positional symptoms.
Post-viral autonomic disturbanceAutonomic symptoms can follow illness, though the history and examination must support the link.

Not every palpitation is a standing-related syndrome

Palpitations occurring without postural triggers may need different rhythm or metabolic assessment.

Intermittent cardiac arrhythmiaShort episodes may be missed by a standard ECG performed between events.
Iron deficiency or anemiaReduced oxygen-carrying capacity can aggravate palpitations and fatigue.
Medication or dehydration effectDiuretics, stimulants, fever or poor fluid intake may change positional symptoms.
Postural orthostatic tachycardia syndromeExcessive sustained heart-rate rise on standing can cause dizziness without a resting ECG abnormality.

Dehydration, low blood volume, anemia and some medicines may contribute to orthostatic symptoms.

Medication or dehydration effectDiuretics, stimulants, fever or poor fluid intake may change positional symptoms.
Orthostatic hypotensionA blood-pressure fall on standing can produce overlapping symptoms.
Iron deficiency or anemiaReduced oxygen-carrying capacity can aggravate palpitations and fatigue.
Post-viral autonomic disturbanceAutonomic symptoms can follow illness, though the history and examination must support the link.

Fainting, exertional chest pain or concerning rhythm symptoms require a clinician to exclude more serious causes.

Intermittent cardiac arrhythmiaShort episodes may be missed by a standard ECG performed between events.
Orthostatic hypotensionA blood-pressure fall on standing can produce overlapping symptoms.
Postural orthostatic tachycardia syndromeExcessive sustained heart-rate rise on standing can cause dizziness without a resting ECG abnormality.
Iron deficiency or anemiaReduced oxygen-carrying capacity can aggravate palpitations and fatigue.