Cerebral oxygenation monitoring in neonates: improving and validating instrumentation

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Author
Date
2017Type
- Doctoral Thesis
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Abstract
The brain is a very vulnerable organ and damage to it is often followed by severe
implications such as long-term disabilities and it may even lead to death. Cerebral
oximetry by near-infrared spectroscopy (NIRS) has repeatedly been cited as promis-
ing technology, potentially enabling clinicians to prevent these outcomes. Preterm
neonates are likely to suffer from complications leading to brain damage and may
thus benefit strongly from NIRS monitoring. Although a number of commercial NIRS
oximeters are clinically approved, the method has not yet been widely established.
Two of the major reasons for this are poor precision of instruments and that tissue
oxygen haemoglobin saturation (StO 2 ) obtained from different oximeters and sensors
are incomparable. This thesis addresses these two problems and provides solutions.
OxyPrem was developed with the objective to provide an instrument with increased
precision to clinicians. Two versions, OxyPrem v1.2 and v1.3, are introduced in this
thesis and their performance is validated in vivo and in vitro. Both sensors are based
on symmetric arrangement of light sources and detectors and employ a self-calibrating
algorithm.
OxyPrem v1.2 performed excellently in vivo in a precision assessment on the forearm
of adults (repeatability = within-subject standard deviation (S w ) = 1.7 %). Repeata-
bility in preterm neonates was S w = 3.3% which is still good, taking into account that
S w in neonates is typically higher than in adults.
An improved version of the instrument, OxyPrem v1.3, was assessed in vivo in
neonates as well. The study demonstrated S w as good as 2.8 %. Simultaneous mea-
surements with another OxyPrem v1.3 sensor and a pulse oximeter revealed unstable
physiology in some neonates. In a second analysis without these confounding sub-
jects, S w improved drastically to 1.9 % which is amongst the best precision values ever
achieved for NIRS oximeters.
To overcome the lack of comparability of different oximeters and sensors, we have
performed several studies with liquid phantoms simulating optical properties of neona-
tal brain tissue. We first conducted experiments with a simple, homogeneous phantom
and then refined the set-up to model a more realistic two-layer geometry resembling
skull and brain. Our studies showed substantially different StO 2 readings provided by
different oximeters which, however, were mostly linearly related. With the improved
set-up, we characterized a large number of commercially available oximeters and sen-
sors and provided coefficients for their pairwise linear relation. The method showed
good repeatability and helps establishing comparability.
As neonates are a very heterogeneous group, we investigated the effect that a varia-
tion in total haemoglobin concentration (c tHb ) has on StO 2 readings by NIRS oxime-
ters. We found strong influence of c tHb on StO 2 while only OxyPrem v1.3 proved to
be largely immune to this effect, which causes substantial uncertainty to readings of
other instruments.
As the presented phantom set-up is very versatile, we additionally investigated sev-
eral other effects with slight adaptations. These showed that StO 2 readings were un-
affected by a thin superficial layer, while sensitivity decreased substantially for a layer
with 16 mm thickness. In another experiment, we did not observe a change in StO 2
readings for very thin clear layers such as oil on the skin of neonates, whereas thicker
layers must be avoided. Partial placement of sensors on top of hair and birth marks
may seriously flaw StO 2 of sensors without symmetric source-detector arrangement
and self-calibrating algorithm.
In summary, this thesis provides solutions to two of the problems mentioned most
often in association with cerebral oxygenation monitoring by NIRS. We have intro-
duced OxyPrem v1.2 and v1.3 and demonstrated superior precision of the instruments
in vivo. OxyPrem v1.3 proved to be largely immune to variation in c tHb , which reduces
this considerable uncertainty in StO 2 readings to a minimum. By a series of in vitro
experiments with liquid phantoms we were able to establish comparability of different
instruments and systematically assessed several types of influences to StO 2 readings. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000175281Publication status
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Publisher
ETH ZurichSubject
Tissue Oxygenation; Near-Infrared Spectroscopy; Phantom Model; RepeatabilityOrganisational unit
03750 - Rudin, Markus (emeritus)
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