D2.1 Requirements and Specification - CORBYS

D2.1 Requirements and Specification
Prototypes used for measuring electro dermal activity (sweat) have been made and reported [Tronstad et al,
2008] [Poh et al, 2010]. There are also some commercial systems available, e.g. from Plux 5 .
10.2.1.4 Humidity/sweat sensors
In addition to the information obtained from EDR type measurement, it is also possible to apply humidity
sensors that do not have to be in direct contact with the skin of the patient. Humidity sensors might be used to
measure relative humidity inside the clothing as an indication of sweating, and methods for using two sensors
for measuring sweat rate have also been reported [Salvo et al, 2010]. Humidity sensors are often combined
with temperature to measure relative humidity (e.g. SHT21 6 ). Relative humidity is often used instead of
absolute humidity in situations where the rate of water evaporation is important. SINTEF has carried out
climate chamber testing on a jacket with integrated relative humidity sensors. Changes in relative humidity
are correlated with the change in heart rate and tie in well with subjective reported sweat activity [Seeberg et
al, 2011 (submitted)]. A humidity sensor does not need to be glued to the skin.
10.2.1.5 Respiration
The respiration rate is the number of breaths within a certain amount of time, often breaths per minute. The
ventilation rate is a related parameter, specifically addressing the flow of gas entering or leaving the lungs
with measurement units of volume per time. While respiration can be measured outside the patient’s chest,
measurements of the ventilation require the use of a breathing tube or spirometer. Ventilation measurements
are therefore considered too obtrusive for the CORBYS patient.
A change in respiratory rate or volume might be an indication of physical or psychological stress, and hence
might be a useful input into the CORBYS system. An individual can be aware and adjust his/her respiration
rate, and the rate is also affected by physical or mental activities, talking and so forth, it is therefore difficult to
apply the respiration rate as a single indicator as physiological state. In CORBYS the motion of the thoracic
cage can be measured by impedance plethysmography (with electrodes on both sides), by inductive
plethysmography (inductive sensor that changes values of inductance when stretched across the chest or
abdomen), piezo resistive sensors (resistance changes when stretched across the chest or abdomen), or
capacitive displacement sensors (measuring the change in area between two capacitor plates that slide parallel
to one another). There are commercially available systems that also report the respiratory rate (like the
Hidalgo Equivitale system 7 ), and more stand alone sensors like the respPlux 8 .
10.2.1.6 Pulse oxygen saturation
The oxygen content of arterial blood can be measured by optical techniques. The saturation partial O2
pressure (SpO2) is a measure for the percentage of haemoglobin binding sites in the bloodstream occupied by
oxygen. Since it measures the arterial blood the pulse can also be extracted from the measurements. SpO2
measurements are easily affected by movements and measurements can only be carried out on certain areas
like the earlobe and finger tip (using transmission photo-plethysmography). In normal circumstances the
oxygen level is between 95% and 99%, and is not correlated with psycho-physiological states. SpO2 is
continuously monitored whenever a patient’s oxygenation may be unstable as in intensive care, critical care,
and emergency department areas of a hospital. SpO2 is also checked periodically in the case of congestive
heart failure (CHF) and chronic obstructive pulmonary disease (COPD) patients. For CORBYS patients, the
oxygenation is usually not an issue. It might also be a problem to do accurate measurements during gait
5 www.plux.info/EDA
6 www.sensirion.no
7 www.equivital.co.uk
8 www.plux.info/resp
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