What does cerebral oximetry measure?

* E-mail: kyle.pattinson{at}nda.ox.ac.uk

Editor—We feel that the recent paper by Yoshitani and colleagues1 raises important issues. They demonstrated that, in patients undergoing elective hip arthroplasty, normovolaemic haemodilution caused no change in jugular bulb saturations () but decreased cerebral saturations () when measured with a cerebral oximeter (INVOS 4100S). Two explanations were offered for the unexpected disparity in readings. First, they postulated that subtle changes in regional might not be revealed with the global measure provided by . Second, they proposed that changes in near infrared path length induced by haemodilution could affect the readings.

We feel the interpretation of the results deserves a more detailed appraisal. The INVOS 4100S gives a single readout for regional cerebral oxygen saturations using an algorithm based upon the Beer–Lambert law. In this study, both subject groups showed no change in but a decrease in . The disparity in readings could be consistent with a systematic error. We suspect the algorithm incorporated within the INVOS 4100S may not take into account the fall in haemoglobin concentration. The fall in oxygenation index has actually been found to correlate with blood loss in healthy volunteers.2

Cerebral oximetry does not take into account changes in the relative proportions of blood in the arterial or venous part of a capillary bed. The proportion of arterial blood in the cerebral capillaries had been estimated at 28%.3 Hypoxia can induce changes in the cerebral arterial to venous volume ratio4 and it would not be unreasonable to assume that other factors may also affect the cerebral arterial to venous volume ratio. A change in the relative proportions of arterial and venous blood in the cerebral capillaries may alter cerebral oximetry readings without a ‘real’ effect on cerebral tissue oxygenation.

We have previously expressed concerns5 about the interpretation of cerebral oximetry in a clinical setting, and Yoshitani's paper has raised a most important issue concerning the effect of haemodilution upon the readings. A change in could be attributable to many factors. In the clinical setting, changes in should be interpreted with caution, particularly when there is significant blood loss.

K. T. S. Pattinson1,*, C. H. E. Imray2 and A. D. Wright3

1 Oxford, UK
2 Coventry, UK
3 Birmingham, UK


 
* E-mail: nkneji{at}mva.biglobe.ne.jp

Editor—We wish to thank Pattinson and colleagues for their comments on our recent article and are pleased to take this opportunity to reply. We demonstrated that there was a discrepancy between and values during normovolaemic haemodilution1 and two possible explanations were offered (as above). Pattinson and colleagues5 have suggested that our results deserve a more detailed appraisal and that various factors may have an effect on values. Validation of near infrared spectroscopy measurements has not been established.

In our study, we demonstrated that there was a significant correlation between haemoglobin concentrations and values. The results indicated haemoglobin concentration had a significant effect on values. Kurth and Uher6 suggested that there was a significant negative correlation between haemoglobin concentrations and optical path length in an experimental model. We believe that optical path length had a strong effect on values. As suggested above, the algorithm incorporated in the INVOS 4100S might not take into account the fall in haemoglobin concentration. The algorithm is not open for scrutiny (Somanetics, Troy, MI, USA). If a modified Beer–Lambert law, in which optical path length should be constant, was incorporated in INVOS 4100S, prolongation of optical path length would cause enhancement of changes in . Therefore, if optical path length became longer with haemodilution, there would be an overestimation of changes in .

Previous studies have demonstrated that various factors, such as haemoglobin concentrations, extracranial blood flow and changes in cerebral arterial to venous blood volume ratio had an effect on near infrared spectroscopy measurements.79 However, it was difficult to evaluate the degree of effects of such factors on values. We have only investigated the effect of haemoglobin concentration on values. However, we need to evaluate the degree of effects of other factors on in the near future.

K. Yoshitani* and M. Kawaguchi

Osaka, Japan

References

1 Yoshitani K, Kawaguchi M, Iwata M, et al. Comparison of changes in jugular venous bulb oxygen saturation and cerebral oxygen saturation during variations of haemoglobin concentration under propofol and sevoflurane anaesthesia. Br J Anaesth 2005; 94: 341–6[Abstract/Free Full Text]

2 Torella F, Cowley R, Thornilley MS, McCollum CN. Monitoring blood loss with near infrared spectroscopy. Comp Biochem Physiol A Mol Integr Physiol 2002; 132: 199–203[CrossRef][ISI][Medline]

3 McCormick PW, Stewart MG, Goetting MG, Balakrishnan G. Regional cerebrovascular oxygen saturation measured by optical spectroscopy in humans. Stroke 1991; 22: 596–602[Abstract]

4 Wolff CB, Imray CHE. Partitioning of arterial and venous volumes in the brain under hypoxic conditions. Adv Exp Med Biol 2003; 540: 19–23[ISI][Medline]

5 Pattinson K, Clutton-Brock T, Imray C. Validity of near-infrared cerebral spectroscopy. Anaesthesia 2004; 59: 507–8[CrossRef][Medline]

6 Kurth CD, Uher B. Cerebral hemoglobin and optical pathlength influence near-infrared spectroscopy measurement of cerebral oxygen saturation. Anesth Analg 1997; 84: 1297–305[Abstract]

7 Kishi K, Kawaguchi M, Yoshitani K, et al. Influence of patient variables and sensor location on regional cerebral oxygen saturation measured by INVOS 4100 near-infrared spectrophotometers. J Neurosurg Anesthesiol 2003; 15: 302–6[CrossRef][ISI][Medline]

8 Germon TJ, Young AE, Manara AR, Nelson RJ. Extracerebral absorption of near infrared light influences the detection of increased cerebral oxygenation monitored by near infrared spectroscopy. J Neurol Neurosurg Psychiatry 1995; 58: 477–9[Abstract]

9 Watzman HM, Kurth CD, Montenegro LM, Rome J, Steven JM, Nicolson SC. Arterial and venous contributions to near-infrared cerebral oximetry. Anesthesiology 2000; 93: 947–53[CrossRef][ISI][Medline]





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