endotracheal tube, pneumothorax, and pleural effusion as well as conditions influenced by gravity
such as collapsed, fluid filled or distended lung areas, with DSS reflecting the first two conditions and
NSS the latter.
•
Functional Lung Spaces
(FLS) (9.10) reflecting the distribution and percentage of lung areas with
non-negligible impedance change during breathing. As such they are meant to represent the
percentage of ventilated lung regions. They are related to what in literature is referred to as
Functional Lung Size [1] or size of the available lung volume, i.e. the percentage of the aerated
remaining lung.
The EIT methodology has been an object of study for decades and there is plenty of literature on the subject; a
summary description can be found for example in Costa et al.[2]. For more information on its clinical
applications, please also refer to reviews available in literature, e.g. Frerichs et al. [3], Putensen et al.[4], Lobo et
al.[5].
4.3 Particular features of Sentec EIT
Sentec EIT selects the thorax and lung models best adapted to the individual patient from a set of predefined,
CT-derived thorax and lung models (9.1). These models are used for
EIT image
reconstruction and to determine
corresponding
Thorax
and
Lung ROIs
respectively
Thorax
and
Lung Contours
. In Adults/Children
configuration these models mainly depend on body mass index (BMI) for both genders. In Neonates/Infants
configuration, where only one CT-derived thorax and lung model is used, the LuMon System accounts for the
displacement of the electrodes around the thorax from their assumed default positions. It is herewith
emphasized that, with the exception of the
Global Dynamic Image
(9.4), the LuMon System solely evaluates
impedance values within the
Lung ROI
.
Sentec EIT also features a position sensor that continuously evaluates position (
rotation
and
inclination
) and permits the clinician to assess the influence of gravity on lung mechanics and ventilation
distribution in the lungs (9.2).
As fully functional electrodes are important for the generation of
EIT data
, the LuMon System continuously
evaluates the skin contact quality of all 32 electrodes integrated in the belt
and categorizes the electrodes into
those having adequate, poor, or insufficient contact, respectively impedance coupling to the skin, the latter
referred to as
failing electrodes
(9.3). The LuMon System uction algorithm
can compensate up to six
failing electrodes
. When the belt to skin contact quality changes significantly or a
change in the number of
failing electrodes
occurs, the LuMon System performs a calibration of its
measurement setup and monitoring is briefly interrupted. In case of too many
failing electrodes
monitoring is
not possible. Once data quality respectively signal quality (9.3) improves, for example, when there are fewer
failing electrodes
, monitoring resumes.
4.4 Limitations of EIT/Sentec EIT
The following clinical situations or factors may affect
EIT raw data
and thereof derived
EIT images
,
waveforms
and
indices
and may limit the accuracy of thereof derived
parameters
such as the
Respiratory Rate
(RRi).
• Mal-positioned or not properly applied belt e.g. belt not positioned as recommended or not
enough contact agent applied on belt (8).
• Conditions hindering sufficient belt to skin contact quality such as bandages in the area around the
thorax where the striped fabric of the belts has to be placed.
• Conditions causing an abrupt or gradual deterioration of the belt to skin contact quality e.g.
(excessive) patient movement/manipulation or gradual loosening of the belt fit around the chest.
• Conditions (transiently) causing impedance changes within the
EIT sensitivity region
not related to
breathing e.g. administration of fluids or movement of liquid and tissue (e.g. diaphragm) within the
EIT sensitivity region
.
• Use of
TB-I Analysis Mode
in very irregularly and/or very weakly breathing patients, in which the
maximal
Lung Impedance
change within
Analysis Intervals
may not be
breathing-related
and, hence,
breathing-related EIT images and indices
generated in
TB-I mode
may not always be physiologically
meaningful (8.5, 9.7).
• Use of Sentec EIT in patients after lung resection or having thoracic malformations in this case, the
models for thorax and lung
may deviate significantly from reality, a fact that should be considered in
the interpretation of the results.
• Use of Sentec EIT when it is contraindicated (3.4).
• Interferences caused by other devices such as high-frequency (HF) surgical equipment or devices
emitting strong electromagnetic fields.
• The subdivision of
Silent Spaces
in
Dependent Silent Spaces
and
Non-Dependent Silent Spaces
as
well as the vertical and horizontal components of the
Center of Ventilation
(CoV(v) and CoV(h)) are
less meaningful if the
inclination
is big enough that the gravity vector is more or less perpendicular to
the examined thorax cross-section being defined by the
belt plane
. This is, for example, the case if the
patient is standing or sitting upright.
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