Near-infrared spectroscopy (NIRS) is a tool to non-invasively evaluate tissue oxygenation in term and preterm infants. As reported previously [1–3], there are several instruments commercially available which use different techniques to measure tissue oxygenation, e.g. in the brain.

NIRS quantifies the interaction of near-infrared photons with biological tissue, which can be described by two different properties: the light absorption and the reduced scattering coefficient (μ_a and μ_s’). Since deoxy- and oxyhemoglobin (HHb, O₂Hb) are the most relevant chromophores absorbing light of the near-infrared spectrum their concentrations can be calculated from μ_a. NIRS is able to determine μ_a by the diffusion approximation or changes in μ_a by the modified law of Lambert and Beer if the optical path length and geometrical properties are known. An important factor in these calculations is the brain water content (BWC) because water also absorbs near-infrared light, although to a lesser extent than O₂Hb and HHb.

As far as the underlying algorithms for the determination of measures of tissue oxygenation are revealed at all, most NIRS devices do not allow adjustment for maturational changes in BWC and their underlying algorithms may assume BWCs as low as 75%.

Considering that due to physiological maturation BWC varies from 75% to 95% [4–6], one has to expect that this maturational change in BWC will have an effect on readings of cerebral oxygenation determined by NIRS, which might be therapeutically relevant in the clinical setting.

We aimed to quantify the impact of different assumptions for brain water content (75% for adults, 85% for term infants and 95% for very preterm infants [6]) in a series of measurements of cerebral oxygenation in preterm infants.

This prospective observational study was approved by the ethics committee of Tuebingen University Hospital and written informed parental consent was obtained in all infants.

NIRS is increasingly used in neonatal intensive care. Instruments suitable and approved for continuous monitoring in this age group give readings on a measure of cerebral oxygenation based on several assumptions that may not hold true in a given infant – and hence displayed values of cerebral oxygenation may not be appropriate in certain individuals.

Our study addressed the question whether the assumption of different BWCs consistently influences O₂Hb, HHb, tHb and rcStO₂ readings in a realistic sample of clinically stable preterm infants using computations based on absolute coefficients μ_a and μ_s’ determined by multi-distance FD-NIRS and the diffusion equation for homogeneous, semi-infinite media.

A clinically relevant overestimation of “true” rcStO₂ by up to 8% may result if BWC is incorrectly assumed to be only 75% in an extremely immature infant with a true BWC of 90-95%. This influence of different BWCs within the physiological range encountered in the neonatal intensive care unit on parameters of cerebral oxygenation is disregarded by most manufacturers of NIRS devices and also neglected by many clinicians who rely on readings of parameters of cerebral oxygenation for guiding cardiovascular therapy.

This systematic overestimation of rcStO₂ by 4% (1%-8%), is probably not important in settings where rcStO₂ trend monitoring is used e.g., during surgical interventions, and whenever relative changes of cerebral oxygenation in relation to a ‘normal’ baseline are observed to indicate cardiovascular interventions. Neonatal applications of cerebral oxygenation monitoring frequently lack a ‘normal baseline’ as rcStO₂ monitoring is used in extremely preterm [7–9] and asphyxiated infants after resuscitation [10]. Furthermore, neonatal cerebral oxygenation monitoring is intended for days rather than just a few hours (e.g., [9]). As indicated in an European collaborative phase 2 trial of rcStO₂-monitoring in extremely preterm infants, neonatologists are indeed interested in long-term continuous rcStO₂ monitoring and, in the absence of a ‘normal baseline’, do rely on absolute rcStO₂ readings [7]. Furthermore, suggested treatment algorithms indicate cardiovascular interventions if absolute cut-off values of rcStO₂ are exceeded [8]. Whenever absolute readings of rcStO₂ are relied upon for clinical decision making, a systematic over-/underestimation of rcStO₂ may be of clinical importance.

For various reasons, including inhomogeneity of the tissue and issues of probe placement (underlying blood vessels, skin, background absorbers, different scattering properties, hair and texture, etc.), the signal-to-noise ratio is poorer and the limits of agreement after repeated repositioning of the NIRS-probe are greater in rcStO₂-monitoring than in SaO₂-monitoring using pulse oximetry (reviewed in [11]). Bland-Altman bias analyses revealing poor agreement with 95% limits of agreement of up to -17% to +17% have been reported previously [12]. More recently, using the ISS Oxiplex TS which was also used in this study, Arri et al. demonstrated that the test retest variability of rcStO₂ measurements was approximately 5% for preterm infants [13], similar to test retest variability of 5% reported by Sorensen using the NIRO 300 [14]. Based on this more recent data, a systematic overestimation of rcStO₂ by 4% (1%-8%) due to incorrect assumptions of BWC is considerable. Moreover, this systematic bias will add to the imprecision of the method and, in contrast to random factors, it is a systematic error that will not be overcome by averaging.

Our findings may also be of importance in the interpretation of longitudinal studies: Previously reported longitudinal data suggested that rcStO₂ values decrease in preterm infants during the first 6 weeks of life despite stable cerebral blood flow index, which was interpreted as an increase in metabolic rate of oxygen [15]. In this study, rcStO₂ was calculated based on the probably incorrect assumption of a constant BWC of 75% throughout the study period. The results of our simulation suggest that incorrect underestimation of BWC early on, may have contributed to the findings and that the postnatal decrease in rcStO2 and the increase in the metabolic rate of oxygen may have been overestimated.

It is obvious that smaller differences between assumed and actual BWC will result in smaller deviations of O₂Hb, HHb, tHb, and rcStO₂ from reality. In fact, depending on postmenstrual and postnatal age most BWC-values will range between 80% and 90% [6]. Furthermore, our data are only applicable to the wavelengths used herein. Different wavelengths with different ratios between the extinction coefficients for O₂Hb, HHb and water will result in different degrees of deviation from reality if BWC is not taken into account. In general, the higher the extinction coefficient of water in relation to that of O₂Hb or HHb at a given wavelength, the more relevant will be the impact of the difference between assumed and actual BWC.

Effects of different assumptions of BWC on rcStO₂ readings of different devices will depend on the wave lengths used (as outlined above) and on the underlying algorithms for determination of rcStO₂. In contrast to the instrument used for our study, unfortunately, many manufacturers of NIRS oximeters did not publish their algorithms and it is unknown how they deal with the water assumption.

We have previously described [16] that introducing a water term into equations describing the relation between the absorption coefficient, μa, and the slope of the decrease in light intensity using multi-distance FD-NIRS resulted in minor changes in StO₂-measurements of the neonatal head if a constant BWC of 90% was assumed. However, this introduction of a water term resulted in large changes (absolute change in StO₂ of up to 18% or relative change up to 30%) if the water content was assumed to be 70% in StO₂-measurements on the adult arm. The present data complement our previous results, accounting for different assumptions for BWC in the range encountered between extremely preterm infants and early childhood. Those different assumptions for BWC will systematically bias results of HHb, O₂Hb, tHb and StO₂ measurements, overestimating StO₂ if too low BWC is assumed. Although the median bias introduced by incorrect assumptions of BWC may be small (Table 2), in the occasional infant overestimation of StO₂ may be clinically relevant.

Developmental changes in BWC should be considered in the clinical setting, especially in preterm infants, because a median difference in rcStO₂ of 4% and a difference in rcStO₂ of up to 8% in individual patients could change therapeutic decisions with potential long-term consequences.

Changing assumptions of BWC resulted in systematic modifications of computed O₂Hb and HHb and in consecutive clinically relevant changes in rcStO₂ of up to 8%. Disregarding maturational changes in BWC is another factor contributing to inadequate accuracy of absolute measures of cerebral oxygenation by standard NIRS devices. Neonatologists should be aware of the fact that rcStO₂ will be overestimated by up to 8% if algorithms for calculating the measure of cerebral oxygenation are based on adult BWC.