With great interest, we read the article entitled ‘Near-infrared spectroscopy in vegetables and humans – An observational study’ by Kahn and Anyanwu in the European Journal of Anaesthesiology.1 The authors conducted a prospective study in both human and inanimate subjects using noninvasive tissue oximetry to compare obtained measurement results between the two groups. They concluded that predicting cerebral function from baseline tissue oximetry values is a fraught exercise and that data are prone to misinform in the wrong context. With the latter, one can agree, while the former statement is obsolete, as it is essentially impossible to predict neurologic function solely using static observations. Moreover, near-infrared spectroscopy (NIRS) is widely used in the food industry to objectify the presence of pigments in various fruits and vegetables with the goal of assessing their maturation.2,3
Noninvasive tissue oximetry is a continuous measurement method based on NIRS, utilising light in several wavelengths to interact with chromophores in the human capillary bed before emerging to the surface. The algorithm is specifically designed to calculate relative absorption of photons by chromophores in human tissue, including oxygenated and deoxygenated haemoglobin (Hb). The clinician performing tissue oximetry, or in this case cerebral oximetry, is faced with the challenge of adequate interpretation of obtained measurement values. Up till now, several studies aimed to determine tissue threshold values associated with clinically apparent decline of neurologic function, presumably resulting from ischaemic tissue damage in the brain. Efforts to do so resulted in absolute as well as relative thresholds to guide preventing and reversing desaturation episodes, including intervention algorithms as first proposed by Murkin and Arango.4 Literature followed that defined how cerebral oximetry guided care where brain perfusion was at risk, for example during cardiac and thoracic procedures requiring cardiopulmonary bypass, or with surgery in the beach-chair position.5
In the study of Kahn and Anyanwu, the authors applied the same measurement technique, comparing the displayed saturations between humans and inanimate objects. The fact that certain fruits and vegetables have absorptions in the near-infrared region similar to human Hb, does not detract from the ability of NIRS to reliably measure Hb oxygenation in human tissue. Furthermore, from a methodological perspective, the primary research question should determine the study design and guide selection of appropriate measurement tools and techniques, that is, there is no Hb in fruits and vegetables. The major culprit in their study design therefore is not necessarily performing measurements in an ‘off-label’ application, but rather the use of an instrument validated to approximate a certain physiologic construct, where that same construct is simply nonexistent. Consequently, application of a measurement technique in this case will not provide meaningful results, but rather is a fraught exercise prone to misinterpretation.
Every noninvasive clinical measurement technique, including tissue oximetry, measures a direct or indirect derivative of the construct of interest. In the case of cerebral oximetry, the technique was validated to a weighted value of sampled blood oxygenation from arterial and venous jugular bulb sites. Since noninvasive techniques provide results representing the outcome of interest, knowledge of factors affecting the measurement physiology is a prerequisite, and together with careful interpretation, valid conclusions can be drawn to ultimately guide clinical treatment.
During cerebral oximetry monitoring, the presence of the intrinsic homeostatic autoregulatory system is such a factor. Cerebral autoregulation ensures constant cerebral blood flow despite fluctuations in perfusion pressure.6 Without sufficient knowledge on the underlying physiologic processes, interpretation of the measured derivative parameters is essentially useless. For example, during cardiac surgery with cardiopulmonary bypass, cerebral oximetry may show values similar to pre-operative readings while the autoregulatory system is severely impeded through hypercapnia, forcing a high cerebral blood flow with a concomitant risk of hyperperfusion. This, in turn, predisposes the patient to an increased risk of complications including cerebral oedema.7
Therefore, cerebral oxygenation readings, or any measurement values derived from non-invasive assessments, are essentially meaningless when one is unaware of the physiology behind the measurement method. Similar accounts when admissible elaboration and justification of the techniques used are lacking and ‘off-label’ application is given equal footing. The clinician remains responsible for good judgement of obtained measurement values and assessing whether pre-existing conditions for performing the measurement are met. In fact, as the authors rightfully noted in the conclusion, NIRS readings are used in conjunction with other clinical parameters to assess patient condition.
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1. Kahn RA, Anyanwu A. Near-infrared spectroscopy in vegetables and humans – an observational study. Eur J Anaesthesiol
2. Nicolaï BM, Beullens K, Bobelyn E, et al. Nondestructive measurement of fruit and vegetable quality by means of NIR spectroscopy: a review. Annu Rev Food Sci Technol
3. Litscher G, Schwarz G. Transcranial cerebral oximetry – is it clinically useless at this moment to interpret absolute values obtained by the INVOS 3100 cerebral oximeter? Biomed Tech
4. Murkin JM, Arango M. Near-infrared spectroscopy as an index of brain and tissue oxygenation. Br J Anaesth
2008; 103 (Suppl 1):i3–i13.
5. Green DW, Kunst G. Cerebral oximetry and its role in adult cardiac, noncardiac surgery and resuscitation from cardiac arrest. Anaesthesia
2017; 72 (Suppl 1):48–57.
6. Lassen NA. Cerebral blood flow and oxygen consumption in man. Physiol Rev
7. van Mook WN, Rennenberg RJ, Schurink GW, et al. Cerebral hyperperfusion syndrome. Lancet Neurol