Dr. Smartwatch:
Medicine’s latest Pandora’s box
WRick Snyder, MD ith the recent announcement of the
excessively hyped Apple Series
4 smartwatch comes an era of
transformative mobile biometric
monitoring. Wearables are ubiquitous
and have been around a while (e.g.,
heart rate monitors), but the field now seems to be growing
exponentially. Analysts indicate that so intense is the demand
for these new gadgets that the health wearable market —
led by smartwatches — could reach $25 billion worldwide
in 2019. Apple says that its smartwatch ability to take an
electrocardiogram could be a game changer for people with
heart conditions, and thus disrupt how we interact with our
patients and treat and monitor their conditions. But the growing
availability of these devices creates its own set of problems for
clinicians who need to recognize the strengths and limitations of
these devices and the information they yield.
TYPES OF WEARABLES
Ambulatory biometric monitors, i.e., “wearables,” fall into two
broad classifications: fitness trackers and disease sensors.
FITNESS TRACKERS
The most common fitness tracker is a heart rate (HR) monitor
most frequently worn to estimate caloric expenditure and titrate
the intensity of exercise. Modern wearables can determine HR
by two common methodologies: electrocardiography (ECG) and
photoplethysmography (PPG).
ECG technology is used in chest straps that are made of an
elastic band; they fit snugly around the chest and contain a small
20 Dallas Medical Journal November 2018
electrode that sits against the skin. These electrode sensors
record the electrical activity given off by the heart and send that
information to a snap-on transmitter on the strap. For this to
work well, the electrode pad must be wet so the minute electrical
signals are well-conducted to the sensor. The transmitter contains
a microprocessor that records and analyzes the electrical signals
to determine HR. Using Bluetooth technology, the transmitter
sends HR data to the user’s smartphone or smartwatch, which
acts as a receiver.
Wrist-based HR monitors use photoplethysmography, which
is the process of using light to measure blood flow. These
devices use the fact that the skin changes in volume with each
cardiac cycle as blood is pumped into the periphery, and that
different wavelengths of light interact differently with these
blood volume changes. These devices have small light-emitting
diodes (LEDs that shine green light on the skin. Photodiode
sensors then measure the
amount of light reflected (or
transmitted), which varies as
a function of the amount of
blood in the tissue with each
cardiac cycle. (See photo 1.)
Photo 1. Underside of
the Apple Watch Series 2
demonstrating two LED
photodiodes and two
photo sensors.