Instrumentation & Measurement Magazine 24-2 - 15

evaluate lying position [21], [27], bed exits (described further
in the following section) [28]-[31], and movement to evaluate restless sleep [27], [32]-[34]. In-bed pressure monitoring
systems have also been used to identify lying position and
movement patterns in both adults [21], [27] and neonates [32].

Bed Exit Characterization
Bed-exits can provide mobility information about an older
adult who is potentially at risk for falls. The current widely
used clinical method for bed-exits is a sit-to-stand evaluation,
which is a test performed by a medical professional in a hospital setting. Remotely obtaining this kind of information may
reduce the strain on the healthcare system and provide key diagnostic information to medical professionals more rapidly
and frequently. PSM have been used to fully characterize bed
exits by measuring four things: bed exit timing, bed exit symmetry, bouncing (how smooth the transition is) [15], and the
center of pressure trajectory (conjuring the momentum to get
up) [16]. PSM placed under a mattress have been used to automatically monitor this sit-to-stand transition experimentally
and in hospital settings [15], [16], [28].

Breathing
As mentioned, breathing patterns can also be an indicator
of sleep quality. PSM have been used to monitor breathing
rate over the course of the night in [12], [34]-[36]. Breathing monitoring also has applications in palliative care, where
Cheyne-Stokes respiration has been associated with end of life.
PSM have been used to evaluate this unique breathing pattern
[37], [38].
Sleep disordered breathing (SDB) has also been investigated. One of the most common SDB conditions is sleep
apnea-hypopnea syndrome (SAHS) and presents as periods
of little (hypopnea) to no (apnea) breathing overnight [39].
These periods of reduced or absent air flow can be categorized
as either central sleep apnea (CSA), where breathing and chest
movement completely stops because the brain does not send
appropriate signals, or obstructive sleep apnea (OSA), where
breathing stops due to a physical blockage in the airway but
the chest keeps moving. OSA is commonly accompanied by
the common snoring sound.
Our research group has investigated sleep apnea detection, mainly focusing on CSA, using pressure sensitive mats
(PSM) placed between the mattress and bedframe (Fig. 2c)
[32], [39]-[44].

Heart Rate Monitoring
More recently, novel PS are being developed that are able to
monitor increasingly small pressure changes. Heart rate is a
very important vital sign that can be monitored in both healthy
and unhealthy individuals. Heart rate assessment can provide
information about the state of the electrical system of a person's heart (i.e., an evaluation of the pacemaking system that
maintains heart rate, which is controlled by action potentials),
how the heart reacts to changing oxygen needs of the body, as
well as the impact of different medications on the heart. Heart
April 2021	

Fig. 2. PS placement: (a) below bed legs, (b) on top of mattress below the bed
sheet, and (c) between the mattress and the bed frame.

rate is typically measured by counting the number of heart
beats per minute (bpm), with a standard heart rate being about
60 bpm, and the regularity of these beats. Remote monitoring
of this can provide continuous heart information with very
little effort exerted by the user. Novel PSM have been able to
detect heart rate and its variation with respect to time successfully in [12], [13], [29], [39].

Internal Fluid Monitoring
More recently, internal changes in fluid have been investigated
using PSM [18]. The ability to track fluid within the body has
implications in nocturia (indicated by frequent bathroom visits) and congestive heart failure, where fluid pools in the ankle
region with gravity while an individual is standing and then
redistributes throughout the body after laying down, facilitating elimination by the kidneys.

IEEE Instrumentation & Measurement Magazine	15



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