Non-invasive optical monitoring of free and bound oxygen in humans

Background: Possibilities of detecting oxygen - both in its free form, as gas in the lungs, and in its bound form, as oxygenated
hemoglobin - have been explored in this thesis. Perfusion and oxygenation of vital organs (e.g., heart, brain and kidneys) may
be severely compromised in critical illness or major trauma, which is why blood is rapidly diverted to those organs to improve
chances of survival. Blood vessels in less important organs (e.g., skin, skeletal muscles and intestines) are constricted, leading
to reduced regional perfusion and oxygenation in these organs. Non-invasive measurements of changes in tissue perfusion and
oxygenation, in e.g., the forearm, might give an early indication of clinical deterioration.
Preterm infants are very vulnerable patients. Their organs, in particular the lungs, are not fully developed, and the respiratory
distress syndrome (RDS) frequently occurs. The intestines may be affected by necrotizing enterocolitis (NEC).
Complementary diagnostic and surveillance methods of RDS and NEC are desirable.
Aims: The overall aim of this thesis, which includes Studies I-IV, was to develop and evaluate non-invasive optical techniques,
based on light at different wavelengths, to complement future bedside surveillance in critically illness or severe injury, for
adults as well as for infants.
Methods: Changes in tissue oxygenation by near-infrared spectroscopy (I-II), blood perfusion by laser Doppler imaging (I)
and blood volume by tissue viability imaging (I) in skeletal muscle and skin were studied, and continuous-wave and timeresolved
near-infrared spectroscopy were compared (II) in healthy volunteers subjected to various defined regional
physiological perturbations.
For the first time, gas in scattering media absorption spectroscopy (GASMAS) was used to detect alveolar water vapor (III-IV)
and oxygen gas (IV), as well as intestinal water vapor (III) in newborn infants.
Main results: Near-infrared spectroscopy, laser Doppler imaging and tissue viability imaging provided valuable information
on physiological changes in the microcirculation (I). Continuous-wave and time-resolved near-infrared spectroscopy
techniques were both able to determine changes in tissue oxygenation, but the time-resolved technique provided more realistic
values with smaller inter-individual differences (II). Alveolar (III-IV) and intestinal signals of water vapor (III), were readily
detected, together with alveolar signals of oxygen gas (IV), non-invasively in newborn infants.
Conclusions: Optical techniques, being non-invasive and providing data in real-time, are attractive as potential tools for
surveillance in critical illness or severe injury, in particular concerning the oxygenation. As an overall conclusion, we believe,
that fully developed time-resolved near-infrared techniques have the potential to become an additional monitoring method of
choice for surveillance of critically ill or severely injured patients.
Likewise, GASMAS has great potential for future monitoring of critically ill preterm or full-term infants, and might,
ultimately, reduce the current use of X-ray imaging in these most vulnerable patients.