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Ann Thorac Surg 2003;76:1522
© 2003 The Society of Thoracic Surgeons
a Department of Surgery, SUNY at Stony Brook, HSC T19, Room 80, Stony Brook, NY 11794 USA.
e-mail: tbilfinger{at}notes.cc.sunysb.edu
e-mail: irampil{at}notes.cc.sunysb.edu
In this issue of The Annals, the group from the Bristol Heart institute report on their observations of changes in cerebral perfusion from manipulations to the heart during OPCAB surgery [1].
Since the inception of cardiac surgery, practitioners have been aware that certain surgical maneuvers placed the cerebral circulation at risk. Despite an alleged clinical benefit from the new setting of OPCAB surgery, these concerns persist. Real-time information about cerebral perfusion, and ultimately cerebral function, has been felt desirable, as it would permit intervention before long-term sequellae occur. Near infrared spectroscopy (NIRS) has many features, making it theoretically appealing for that purpose to the heart surgeon. Wavelengths of the near infrared spectrum easily pass the scalp and skull to illuminate brain tissue allowing noninvasive measurement of hemoglobin absorption bands. In 1977 Jobsis described absorption changes in these bands, which are related to changes in the concentration and oxygen saturation of hemoglobin in brain tissue [2]. Since that time, a number of additional chromophores, usually with less reliability than hemoglobin, have been measured.
Understandably attractive, NIRS technology requires several simplifying assumptions, which are all altered by heart surgery, rendering the interpretation of results difficult: (1) NIRS requires the use of several wavelengths of light in order to differentiate hemoglobin species. In-vivo these wavelengths differ in their absorption paths lengths through tissue. (2) Changes in blood volume also differentially alter transmission path lengths. (3) In-vivo passage through scalp, skull, CSF, and brain involves more complex light scattering than the simple assumptions made by the processing algorhitms. It is, therefore, difficult to predict how much of an observed change in the measurements is due to brain versus scalp blood flow, or simultaneous changes in flow and volume, for example. Furthermore, hemodynamic changes, as encountered during heart surgery, affect brain regions differently, a phenomenon basically not accounted for by the instrument that treats the brain as homogenous mass. (4) Finally, despite a call in 1993 by the NIH consensus panel on NIRS technology for outcome studies, none are yet available [3].
We are faced with a device with potential, but scarce data on whether the signal variations should be important for the clinician or just the laboratory investigator. Likewise, there is a paucity of appropriate validation of the measurements. This leaves us with a piece of work addressing an important and very relevant question, the results of which for the time being stand alone and await further validation.
References
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  M.-C. Taillefer and A. Y. Denault Cerebral near-infrared spectroscopy in adult heart surgery: systematic review of its clinical efficacy: [La spectroscopie cerebrale par infrarouge en cardiochirurgie chez l'adulte : une etude systematique de son efficacite clinique] Can J Anesth, January 1, 2005; 52(1): 79 - 87. [Abstract] [Full Text] [PDF]
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