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\begin{document}

\title{Using Wearables to Manage Atrial Fibrillation: Pushing the Boundaries
with Consumer Devices}



\author[1]{Marco Perez}%
\affil[1]{Stanford University}%


\vspace{-1em}



  
  \date{May 19, 2024}


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\selectlanguage{english}
\begin{abstract}
The irregular pulse notification (IPN) algorithm on the Apple Watch
(Apple Inc., Cupertino, CA) was not designed for use by atrial
fibrillation (AF) patients. It is not FDA cleared for use in AF
patients. Before this study by Dr. Wasserlauf and colleagues, there were
no studies of its accuracy in AF patients. Yet, many AF patients could
not resist the temptation to use the feature. In the Apple Heart Study1,
even after making it clear that patients with AF were not eligible for
the study, 174 (18\%) of the participants who received an irregular
pulse notification and connected with a study visit doctor confessed
that they knew they already had AF and were excluded from the study.
These participants were just too curious to pass up the opportunity to
see what the new technology was all about.%
\end{abstract}%



\sloppy


Using Wearables to Manage Atrial Fibrillation: Pushing the Boundaries
with Consumer Devices

Marco Perez, MD\textsuperscript{1}

\textsuperscript{1}Stanford Center for Cardiovascular Research, Stanford
University, Stanford, CA

Word Count, excluding references: 1489

Corresponding author:

Marco Perez, MD

300 Pasteur Dr. M/C 5773

Stanford, CA 94305-5773

mvperez@stanford.edu

phone: 650-723-9363

The irregular pulse notification (IPN) algorithm on the Apple Watch
(Apple Inc., Cupertino, CA) was not designed for use by atrial
fibrillation (AF) patients. It is not FDA cleared for use in AF
patients. Before this study by Dr. Wasserlauf and colleagues, there were
no studies of its accuracy in AF patients. Yet, many AF patients could
not resist the temptation to use the feature. In the Apple Heart
Study\textsuperscript{1}, even after making it clear that patients with
AF were not eligible for the study, 174 (18\%) of the participants who
received an irregular pulse notification and connected with a study
visit doctor confessed that they knew they already had AF and were
excluded from the study. These participants were just too curious to
pass up the opportunity to see what the new technology was all about.

Patients pushing boundaries of what ``should'' be done with consumer
devices is not a novel concept. Patients with hypertension have been
using home blood pressure monitors to optimize their own
antihypertensive regimens for years. Clinical trials have shown that
empowering patients to self-measure blood pressure and to self-adjust
their medications may even outperform traditional management
strategies\textsuperscript{2}. Similarly, diabetics who titrate their
own insulin based on frequent or continuous glucose monitoring can do so
safely and effectively\textsuperscript{3}. More recently, as continuous
glucose monitoring devices have become less expensive and easier to use,
the number otherwise healthy people who are using them to optimize their
diet is growing. While the devices have been studied in obese
non-diabetic patients\textsuperscript{4}, their utility in the healthy
population has not yet been validated.

It is natural that some AF patients will seek a similar sense of
empowerment. Until recently, the only way an asymptomatic AF patient
could confirm they were in AF was by going to a doctor's office to get
an ECG. Savvy patients might measure their own pulse, or perhaps use a
home blood pressure monitoring device that could measure pulse
irregularity. However, most AF patients were in the dark until consumer
ECG devices became available\textsuperscript{5}. With the ability to
measure ECGs at home and confirm AF, patients suddenly had opportunities
to explore novel management strategies.

The Irregular Pulse Notification Algorithm on the Apple Watch was the
first tool to use PPG on a smartwatch to detect AF. The technological
leap here was that monitoring for AF became something that could be done
passively and for prolonged periods. Shortly afterwards, other
smartwatches followed with similarly designed
algorithms\textsuperscript{6, 7}. However, these tools were meant to
detect AF in consumers who had never been diagnosed with AF. The
algorithm was designed to be highly specific with very low false
positive rates. To do this, the algorithm relied on multiple passive
measurements of pulse irregularity, which was made practical by the
wearable nature of the smartwatch. Although the false positive rate of
the IPN algorithm was not measured in the Apple Heart Study, it had to
be lower than the total positive rate, which was 0.52\% over a median
monitoring time of 117 days\textsuperscript{1}.

Despite these caveats, patients with AF saw an opportunity to push the
boundaries of this screening tool. This could prove useful for patients
who had asymptomatic AF, or who wanted more data around onset of
paroxysms of AF. The potential to use AF notifications to manage rate
and rhythm controlling medications was obvious to some patients. One of
the challenges was that the IPN algorithm had never been validated in
patients with known AF. Even clinicians who might want to help patients
design personalized, albeit unvalidated, treatment strategies could not
tell their patients how accurate the algorithm might be.

In this study by Dr. Wasserlauf and colleagues, questions around the
accuracy of the IPN algorithm in AF patients are addressed {[}Wasserlauf
et al., \emph{JCE} {]}. They provided patients who already had
insertable cardiac monitors or cardiac implanted electronic devices and
a history of non-permanent AF with Apple Watches running the IPN
application. Over the course of 6 months, 11 patients had an episode of
AF lasting at least one hour while the watch was worn. Eight of these
subjects had successfully-detected AF episodes lasting \textgreater{} 1
hour with the smartwatch, resulting in a sensitivity of 72\% and
specificity of 100\%. This was a relatively small study and therefore
the caveats of small sample size and number of events apply. In
addition, the gold standards in this study (insertable cardiac monitors
and cardiac implanted electronic devices) are also prone to some degree
of misclassification\textsuperscript{8}. That said, the strength of this
study is in its novelty: while many others had looked at the sensitivity
and specificity of smartwatch and stand-alone ECG applications, no other
study had looked at the accuracy of a PPG-based irregular pulse
algorithm on a wearable device in AF patients.

To understand the limitations that the existing algorithm has for use in
AF patients, one needs to delve into details of the design of the IPN
algorithm\textsuperscript{9}. The smartwatch makes one-minute-long pulse
measurements once every two hours, as long as the user is not moving.
However, if the device detects motion, a pulse check may not occur for
prolonged periods of time. The algorithm then classifies the pulse as
regular or irregular and if the latter is detected, the frequency of
pulse checks increase to once every 15 minutes. A total of 5 out of 6
pulse checks must then be classified as irregular before a notification
is delivered. Again, with motion detected by the smartwatch, this could
prolong the verification process and extend the time-to-notification
even further. This means that short episodes of AF are less likely to
result in a notification. For large-scale screening, the low sensitivity
of detecting short episodes may be acceptable, opting instead for high
levels of specificity. However, for an AF patient who might want to
consider waiting for episodes of AF before taking medications, the IPN
algorithm is not ideal. Finally, a wearable device can't detect AF if it
is not being worn. In this study, about half of the AF episodes occurred
while the participant was not wearing the smartwatch. Currently,
limitations in battery life and charging habits limit wear times and
sampling frequency, though these may improve in future generation
devices.

Another limitation to the existing irregular pulse algorithms is that
they can be triggered by long periods of frequent premature atrial or
ventricular contractions\textsuperscript{10}. In the Apple Heart Study,
in participants who received a notification and subsequently wore an ECG
patch, approximately 16\% of the patients who received an irregular
pulse notification had a rhythm other than AF. This number in theory
could be higher in patients with a known history of AF, since high
burden of PACs are associated with AF, though this did not appear to be
an issue in this small sample.

Whether we as a clinical providers want our patients using wearables or
not, patients with AF are already using these devices to monitor their
AF. Apple released the AF History feature in 2022
(https://www.apple.com/legal/ifu/afhf/1-0/afhf-1-0-en\_EN.pdf), which is
designed for AF patients to measure AF burden, defined as the proportion
of time that a patient has an irregular pulse. The History algorithm is
based on frequent sampling and artificial intelligence distinguishing AF
from other rhythms. The major limitation of this feature is that the
burden measurement is reported only on a weekly basis. The algorithm was
likely intentionally designed this way precisely to prevent patients
from using the feature to make immediately reactive decisions about
medication use.

Patients with AF who want to empower themselves to manage their disease
with more granular detail will need to look for algorithms that are more
sensitive to short periods of AF with more frequent sampling and lower
thresholds for AF classification. These ideal algorithms will also need
to provide passive and near-real-time monitoring of AF, and distinguish
AF from frequent ectopy. There is currently no such device with this
algorithm, much less one that has been validated, but the technologic
limitations can be overcome.

The onus will be on the medical community to help our patients figure
out the best ways to optimize their AF management in a safe and
effective manner. Dr. Passman, senior author of this study, is leading a
team to conduct REACT-AF, an NIH-sponsored, open-label, randomized
clinical trial testing the safety of using a modified irregular pulse
notification algorithm to guide anticoagulation use in patients with AF.
Patients randomized to the intervention arm will only take their
previously prescribed DOAC therapy for a limited period if their
smartwatch alerts them to an episode of AF. The hope is that this
strategy will not only be noninferior to standard management, but that
there will be fewer bleeds.

REACT-AF is pushing existing boundaries of wearable devices. However,
this is the direction the next generation of patients, who are
technologically savvy and who demand empowered self-care, want to go.
Our job as a clinical community is to help our patients push these
boundaries safely.

REFERENCES

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