The palpitation, if related to an arrhythmia may be the root cause for the low blood pressure. The monitor may be a good idea, since palpitations can be better evaluated if the heart rhythm can be seen before during and after an episode.
The cause for palpitations and low blood pressure can be a slight electrolyte imbalance or dehydration which may have been corrected with the fluids administered. If dehydration is avoided the episode may never repeat itself. This is the reasoning behind the cardiologist choosing to wait before using the Holter monitor. You can request that he pursue that evaluation if your instincts tell you it's best.
The other causes for palpitations and low blood pressure, such as infection were likely ruled out in ER. You didn't mention other medications but there are several that can either lower blood pressure, or raise the heart rate to the extent that she would experience symptoms like you described. There is still a chance that her thyroid medication is connected. Though her levels did not rise, a higher dose, or change in metabolism which affected how she processed the medication, could cause a temporary reaction with symptoms like you describe.
The bottom line is that palpitations are benign in the majority of patients. Since she had some symptoms, a Holter monitor may be used to rule out any congenital heart defects that could lead to arrhythmia and palpitations. The Cardiologist is choosing to wait and evaluate the need for such testing, but you can make a follow up appointment and ask that, because of her symptoms and her history of Down's Syndrome he go ahead with the test now.
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