Is near-infrared spectroscopy living up to its promises?

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Summary

The first clinical application of near-infrared spectroscopy (NIRS) was made 20 years ago on the head of newborn infants under intensive care. Since then NIRS has yielded much credible and some important clinical research data. The most important results have been obtained using the cumbersome but quantitative techniques for measuring cerebral blood flow, cerebral blood volume, or venous oxygen saturation with manipulation of FiO2 or impeding venous outflow from the brain. The continuous nature of NIRS has been combined with monitoring of arterial pressure to obtain measures of cerebrovascular regulation, but this method has not been applied on a larger scale. Second-generation instruments allow a running estimate of vascular haemoglobin oxygen saturation, named the tissue oxygenation index (TOI), in absolute terms. Applied to the head, this is a surrogate measure of cerebro-venous saturation, an important variable in neuro-intensive care. The precision, however, is insufficient to be useful. In conclusion: clinical application is not in sight.

Introduction

Transillumination of the head of small animals is possible using near-infrared spectroscopy (NIRS). The first clinical research use of NIRS in 1985 was in newborns,1 and quantitative spectroscopy was subsequently performed in 1986.2 In the following years many papers on NIRS in newborns were published. The purpose of this chapter is to provide a short overview and discuss the potential clinical use of NIRS for neonatal intensive care.

Section snippets

Geometry

The newborn infant's head is ideally suited for NIRS. The overlying tissues are relatively thin, which ensures that the signal is dominated by brain tissue – white as well as grey matter. NIRS recordings can be performed with the light being applied to one side of the head and received on the other side (transmission mode) in low-birth-weight infants with biparietal diameters of 6–8 cm. In this situation a large part of the brain is ‘seen’ during the measurement, and the results may be

Trend monitoring of haemoglobin signals

Near-infrared spectroscopy is a perfect candidate for clinical monitoring of the tiny sick preterm neonates. It is non-invasive, gives real-time information, does not interfere with intensive care, does not affect the underlying skin, and does not remove the infants from the nursery.

In principle, NIRS allows on-line trending of changes in O2Hb and HHb, and hence of tHb (the sum of [O2Hb] and [HHb]), which is proportional to changes in cerebral blood volume, which in turn can be used as a

Clinical correlates of cTOI

In a group of 18 severely asphyxiated term infants, those with poor outcome showed a rise in cTOI during the first day of life.22 This is in agreement with the concept of delayed energy failure and ‘luxury’ perfusion. In a group of 20 newborn infants operated for congenital transposition of the great vessels, those with preoperative cTOI <35% tended to have lower developmental scores at follow-up at 2–3 years of age.23 It should be noted that in both studies the differences were seen at the

Conclusion and the future

NIRS has matured as a method to obtain quantitative measures of cerebral blood volume, flow and oxygenation, and has yielded credible and sometimes important information. These methods are manual, however, and too cumbersome for clinical practice or large-scale research. Second-generation instruments directly quantify a tissue oxygenation index, which is a surrogate measure of venous saturation, an important parameter for judging tissue oxygen sufficiency. The precision of these instruments,

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