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Ann Thorac Surg 2000;70:337
© 2000 The Society of Thoracic Surgeons

Correspondence

Reply

Yuji Kadoi, MDa, Shigeru Saito, MDa

a Department of Anesthesiology and Reanimatology, Gunma University, School of Medicine, 3-39-22, Showa-machi, Maebashi, Gunma 371-8511, Japan

e-mail: kadoi{at}akagi.sb.gunma-u.ac.jp

To the Editor

We appreciate the comments of Miyamoto and Miyamoto. Near-infrared spectrophotometry has become a tool as a noninvasive, real-time, online monitor for the determination of the cerebral oxygenation state in animals or humans, although some controversy concerning technologic problems remains. The machine, which was able to estimate the changes in HbO2, Hb, and cytochrome aa3 concentrations, was originally designed by Wray and associates [1]. Laser beams are emitted from laser diodes (wavelengths of 775, 826, 850, and 906 nm). Changes in light absorption were transformed into relative changes in HbO2, Hb, and cytochrome aa3 concentrations in the brain tissue. A differential pathlength factor (DPF) was assumed to be 5.93 times the distance between the optical probes in this system [1, 2]. This DPF is the average pathlength of a photon migrating in cerebral tissue. In animal studies, it is possible to obtain the value for nonspecific absorbance by sampling the data under oxygen-free conditions [2]. However, as the extent of nonspecific absorbance is difficult to estimate in human studies [2], it is impossible to obtain the specific absorbance value. Also, it is not reasonable to compare the optical density values from different subjects, because nonspecific absorbance might differ among subjects. Moreover, Matcher and associates reported that they found differences between the results from the algorithms on different NIRS machines, particularly in their estimation of cytochrome oxidase [3]. Pollard and Prough [4] suggested that NIRS machines should be considered as trend monitors for intracranial phenomena. Previous studies showed that rSO2 estimated by our machine indicated the regional cerebral oxygenation state [5, 6]. We do not admit that the unchanged rSO2 in our hypothermic group is a manifestation of the spuriously high oxyhemoglobin content.

It remains controversial whether cerebral autoregulation is intact during CPB [7, 8]. Newman and associates [7] reported that an increase in CBF was observed by an increase in MAP. In contrast, Sadahiro and associates [8] reported that CBF was constant when MAP was higher than 50 mm Hg. As of now, there is no solid agreement as to what is the acceptable level of MAP during CPB.

References

  1. Wray S., Cope M., Delpy D.T., et al. Characterization of the near-infrared absorption spectra of cytochrome aa3 and haemoglobin for the non-invasive monitoring of cerebral oxygenation. Biochem Biophys Acta 1988;933:184-192.[Medline]
  2. Levy W.L., Levin S., Chance B. Near-infrared measurement of cerebral oxygenation. Anesthesiology 1995;83:738-746.[Medline]
  3. Matcher S.J., Elwell C.E., Cooper C.E., et al. Performance comparison of several published tissue near-infrared spectroscopy algorithms. Analy Biochem 1995;227:54-68.[Medline]
  4. Pollard V., Prough D.S. Cerebral near-infrared spectroscopy. Anesth Analg 1996;83:673-674.[Medline]
  5. Saito S., Yoshikawa D., Nishihara F., et al. The cerebral hemodynamic response to electrically induced seizures in man. Brain Res 1995;673:93-100.[Medline]
  6. Saito S., Miyoshi S., Yoshikawa D., et al. Regional cerebral oxygen saturation during electroconvulsive therapy. Anesth Analg 1996;83:726-730.[Abstract]
  7. Newman M.F., Croughwell N.D., White W.D., et al. Effect of perfusion pressure on cerebral blood flow during normothermic cardiopulmonary bypass. Circulation 1996;94(suppl II):353-357.[Abstract/Free Full Text]
  8. Sadahiro M., Haneda K., Mohri H. Experimental study of cerebral autoregulation during cardiopulmonary bypass with or without pulsatile perfusion. J Thorac Cardiovasc Surg 1994;108:446-454.[Abstract/Free Full Text]

Related Article

Monitoring adequacy of brain oxygenation
Tadaomi-Alfonso Miyamoto and Koho-Julio Miyamoto
Ann. Thorac. Surg. 2000 70: 336-337. [Extract] [Full Text] [PDF]




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