@tatiana987 A 24-hour ambulatory blood pressure (ABPM) monitor is not designed to reliably capture fainting (syncope) events, and when it shuts itself off during such an episode, it is usually because the device cannot obtain a valid reading, not because “nothing happened” to your blood pressure.
So yes — important physiological changes may have occurred, but the monitor simply failed to record them.
That limitation is often under-acknowledged in clinical notes, which is what you’re experiencing now.
Why a fainting episode may NOT be recorded
1. ABPM devices are intermittent, not continuous*

Most ABPMs:
Measure BP every 15–30 minutes during the day
Every 30–60 minutes at night
If you faint between readings, there is no data point captured.
A faint that lasts seconds to minutes can easily occur completely outside the measurement window.

2. Why the monitor may shut itself off:

During syncope or near-syncope, several things can happen that break the measurement algorithm:
a) Very low blood pressure
If systolic BP drops too low (often < 70–80 mmHg):
The cuff cannot detect Korotkoff oscillations.
The device interprets this as a measurement failure.
Many monitors are programmed to:
Abort the reading.
Log an “error”.
Or stop cycling after repeated failures.

b) Irregular heart rhythm

If you had:
Bradycardia.
Tachycardia.
Atrial fibrillation.
Or pauses.
The oscillometric algorithm may fail completely, especially during sudden hemodynamic instability.

3. Movement and collapse
During a fall or loss of consciousness:

Arm position changes.
Muscle tone drops.
The cuff may slip or kink.

Any of these cause the monitor to:

Cancel the reading.
Power down after repeated invalid attempts.

4. Cuff size really does matter:

You are correct here.
Too large a cuff → falsely LOW readings.
Too small a cuff → falsely HIGH readings.
If the cuff was oversized and your BP dropped suddenly:

The monitor may not detect pressure changes at all.
Leading to shutdown or “no data”
This is not user error — it is a known limitation.
What was likely happening physiologically during the faint
In the real world, fainting at your age and in that context often involves one or more of the following:

Sudden BP drop (orthostatic or neurally mediated hypotension).
Transient cerebral hypoperfusion
Heart rate changes (too slow, too fast, or irregular).
Medication-related autonomic effects (Repatha timing is notable).
None of these require chronic hypertension or hypotension to occur.
You can have “normal” BP on paper and still experience dangerous episodic drops.
Why the conclusion “perfect BP” is flawed

The statement:
“The 24-hour monitor provides the most reliable data”
is only true for average BP trends, not for:
Syncope.
Falls.
Medication reactions.
Autonomic dysfunction
ABPM cannot rule out episodic hypotension.

A more accurate statement would be:
“The monitor did not capture an event — therefore no conclusion can be drawn about BP during that episode.”

Unfortunately, that nuance is often lost.

Important clinical point (this matters):

Unexplained fainting + falls at advanced age is NEVER benign, even if:

Office BP looks “perfect”.
ABPM averages look normal.
This scenario usually warrants other. testing, such as:

Orthostatic BP testing.
Continuous ECG (Holter or event monitor).
Autonomic testing.
Medication review
ABPM alone is not sufficient.

Bottom line:

The monitor shutting down does NOT mean nothing happened.
It usually means the device could not measure during instability.
The “perfect BP” conclusion is over-interpreted.
Your concern is medically justified.

Source's:

https://pmc.ncbi.nlm.nih.gov/articles/PMC7356999/ — explains what ABPM is and how it’s used for hypertension care. �
PMC

https://www.aafp.org/pubs/fpm/issues/2020/0500/p19.html — describes how ABPM takes repeated measurements, not continuous data.

https://www.researchgate.net/publication/362529150_Feasibility_of_Blood_Pressure_Measurement_With_a_Wearable_Watch-Type_Monitor_During_Impending_Syncopal_Episodes — highlights that diagnosing syncope with ambulatory monitors is possible but imperfect and that criteria are still evolving. �
ResearchGate

https://pmc.ncbi.nlm.nih.gov/articles/PMC10227244/ — . �
PMC

About Cuff Size:

https://www.acc.org/Latest-in-Cardiology/Articles/2023/06/05/19/18/Why-Is-Cuff-Size-So-Important-and-Other-Factors-That-Affect-Accurate-BP-Measurement — explains how incorrect cuff size changes systolic/diastolic readings. �
American College of Cardiology

https://www.ama-assn.org/public-health/prevention-wellness/4-big-ways-bp-measurement-goes-wrong-and-how-tackle-them — covers general inaccuracies in BP measurement.