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Ann Thorac Surg 1997;63:291-293
© 1997 The Society of Thoracic Surgeons
Cardiovascular Surgical Research Laboratories, Harvard Medical School, Department of Cardiac Surgery, Children's Hospital, 300 Longwood Ave Boston, Ma 02115
To the Editor:
In their study, Daubeney and associates [1] describe a correlation between regional cerebral oxygen saturation measured by a near-infrared spectrophotometric (NIRS) device (INVOS 3100; Somanetics Corp, Troy, MI) and jugular bulb venous saturation in children during cardiac operations. They conclude that this method might potentially be useful for monitoring trends in cerebral intravascular oxygenation, a statement no one would argue with. In fact, some other recent studies [2, 3] have already shown the important predictive value of NIRS data with respect to the neuropsychological outcome after cardiac operations. But in these studies a different NIRS device (NIRO 500; Hamamatsu Corp, Hamamatsu City, Japan) was used and the results were contradictory to the results obtained by Daubeney and associates.
My colleagues and I have recently published a study [4] showing no or a negative correlation between jugular bulb venous saturation and NIRS data during cardiac operations and specifically during cardiopulmonary bypass in adults. The different findings might partially be explained by the different NIRS device and the obtained parameters. The NIRO 500 device we used measures changes in oxygenated and deoxygenated hemoglobin as well as changes in the redox state of cytochrome a,a3. To compare NIRS data obtained by different devices, we monitored 12 patients undergoing bypass and valve operations simultaneously with the INVOS 3100 and the NIRO 500 device placed on the right and left side of the forehead, respectively. During cardiopulmonary bypass there was no correlation between the devices, although the oxygenated and and deoxygenated hemoglobin values obtained by the NIRO 500 device were calibrated to the INVOS 3100 saturation values at the onset of cardiopulmonary bypass and the beginning of rewarming. These two devices therefore seem to measure something completely different. In pediatric patients, it is even more difficult to interpret INVOS 3100 data, because the use of the device is restricted by the company to adults because of the fixed optode placement and the errors resulting from a small skull. In conclusion, the interpretation of the results obtained by Daubeney and associates remains uncertain because (1) NIRS data are only correlated to oxygen saturation in the jugular bulb but no other parameter of operative management, such as pH, temperature, and arterial hemoglobin, which certainly affect cerebral oxygenation, (2) no neuropsychological investigations have been made to validate the data, and (3) the INVOS device, as Daubeney and associates themselves stress, is not valid for measurements in infants and children.
References
Department of Thoracic and Cardiovascular Surgery, Klinikum Karlsburg, Greifswalderstr 11 A, 17495 Karlsburg, Germany
We read with great interest the recently published article by Daubeney and associates [1] as well as the invited commentary by Gunter. We agree with them that near-infrared spectroscopy is a potential tool for continuous perioperative monitoring of tissue oxygenation. Concerning the assessment of sufficient oxygenation of cerebral tissue we believe that hemoglobin oxygenation and cellular oxygenation state are certainly the most important parameters predicting normal postoperative organ function. Looking for a new potential "gold standard" for measurement of cerebral oxygenation, Daubeney and associates have studied the INVOS 3100 device (Somanetics Corp, Troy, MI) in children undergoing cardiac catheterization or cardiac operations.
Concerning the method and the device we would like to make a few comments: One of the disadvantages of the INVOS 3100 device is the spectrum of the parameters measured. The system measures (real time?) only the "tissue oxygenation" (regional cerebral oxygenation saturation) and gives no information about the oxygenation state of hemoglobin. Furthermore, the most interesting parameter, cytochrome aa3 (describing the oxygenation state on cellular level), is not included in the spectrum of parameters measured. These parameters, as Du Plessis and associates [2] have recently stressed, give the most interesting information concerning oxygenation state at the tissue and cellular level.
The continuous measurement of cerebral oxygenation saturation (= tissue saturation) from our point of view does not provide safe information about what we call sufficient oxygenation on the cellular level-and what we need to know! The method may be safe, but the cerebral tissue oxygenation is an even more uncertain parameter than hemoglobin oxygenation for the description of cellular oxygenation state. Only measurement of cytochrome aa3, the terminal enzyme of the respiratory chain, allows an exact assessment of cellular oxygenation state. This measurement is from the technical point of view quite a difficult problem, and the NIRS devices used for measurement of oxygenation state (as we could find in the literature so far) do not measure oxidized or reduced cytochrome aa3 [3], although several authors claim to do this [2, 4]. The relation "[oxidized] cytochrome aa3 minus total cytochrome aa3," which is claimed to be descriptive as a "redox state" concerning the relation between oxidized and reduced cytochrome aa3 in the instruction manual of this device is not a suitable parameter and cannot be used for such an interpretation [3, 5].
The INVOS 3100 device only shows trends in cerebral tissue oxygenation state in individual patients. Therefore it is difficult to compare data received from different patients and even impossible to use these data for statistics.
As far as we know, it is not very easy to handle the INVOS 3100 device perioperatively. This makes it difficult to use the system as a standard monitoring device in the catheterization laboratory or in the operating room.
Comparing the device with other available systems, we would not recommend the INVOS 3100, especially due to the loss of important information.
Today, there is to our knowledge only one NIRS device that measures cytochrome aa3 safely. We have used this system (Multiscan OS 30; NIOS, Essen, Germany) and we found impressive changes in reduced cytochrome aa3 concentrations starting rapidly after interruption of perfusion (increase) and onset of reperfusion (decrease) of myocardial tissue.
Near-infrared spectroscopy will be in the near future a very useful tool for intraoperative measurement of the tissue oxygenation demand/supply relationship and for assessment of oxygen metabolism at the cellular level. We agree with the invited commentary, however, that near-infrared spectroscopy needs additional development and adaptation to clinical requirements as well as clinical validation to become a standard method for oxygenation monitoring of different organs in patients undergoing open heart operations.
References
Division of Cardiology, Children's Hospital of Pittsburgh, 3705 5th Ave At Desoto St Pittsburgh, Pa 15213
We are grateful for the opportunity to reply to the letters of Dr Nollert and Drs Wollert and Eckel. It would seem that we all agree that near-infrared spectroscopy (NIRS) shows great potential for continuous monitoring of cerebral oxygenation status. We would concur with many of the points made by Drs Wollert and Eckel. In particular, I think we would all agree that the most important measure of cerebral oxygenation is the assessment of "cellular" oxygenation rather than "vascular" oxygenation. Thus, the potential of NIRS to measure cytochrome aa3 is particularly exciting. However, it is apparent from the above letters (as well as a review of the literature) that there is no agreement as to which machines, if any, accurately measure tissue oxygenation via the evaluation of cytochrome aa3. Doctor Nollert has used the NIRO 500 device (Hamamatsu Corp) to measure changes in the redox state of cytochrome aa3 as well as changes in oxygenated and deoxygenated hemoglobin. By contrast, Dr Wollert and Dr Eckel believe that only the Multiscan OS 30 system (NIOS, Essen, Germany) is suitable for measurement of cytochrome aa3. One of the great problems of this field of research is that we have no good standards against which measurements of cerebral vascular or cellular oxygenation can be made. Quantitation of cerebral oxygenation by NIRS entails a number of assumptions and, in the absence of such suitable standards, it is very difficult to validate the various NIRS devices now available for research or clinical use.
We had no preset convictions at the onset of our study as to whether the INVOS 3100 can reliably measure intravascular cerebral oxygenation, nor do we have any financial or other interest in this device. However, the machine was available to us, we found it easy to use, and we saw the obvious potential that this device might hold in monitoring children during pediatric cardiac operations. Although we accept the criticism that this device does not measure cytochrome aa3 status, we still thought it would be valuable to see whether this machine could help assess cerebral intravascular oxygenation. Although Dr Nollert points out that we have not performed neuropsychological investigations on these patients, this omission was intentional at this preliminary stage of investigation. Ultimately, NIRS devices will have to prove themselves to be clinically useful, ie, predict adverse neurologic outcome, and this will require very large, carefully performed prospective trials in which NIRS technology is incorporated into the perioperative monitoring of children undergoing cardiac operations. Such studies are very time consuming and expensive and, although they are certainly necessary, it would be inappropriate to plan such studies with the INVOS 3100 device (or any other device) until it could be shown that the device in question measures what it is purported to measure. Although we recognize that jugular bulb venous saturation is not the same as regional cerebral oxygen saturation, it seemed an appropriate starting point to see if changes in regional cerebral oxygen saturation reflect changes in jugular bulb oxygen saturation. Finally, we would disagree with Dr Nollert's contention that the INVOS 3100 system is not valid for use in infants and children because the manufacturer restricts its use to adults. There are many therapies and technologies employed in medicine that are designed for adult use, but subsequently attain use in children. At the present time, use of this device in children is not recommended because no prior studies have been performed to assess its usefulness in the pediatric age group. Although we appreciate that the optode spacing of the INVOS 3100 device was initially thought to be optimal for the adult age group, this does not preclude the possibility that the machine may (unintentionally) be well-designed for use in children. Indeed, one of the earlier criticisms of this device [1, 2] was that the regional cerebral oxygen saturation measurements obtained may be contaminated by signals from the superficial, ie, extracerebral, tissues such as the scalp and skull. It is possible, therefore, that in a child with thinner extracerebral tissues, deeper penetration may be achieved with less contamination by these superficial tissues [2].
In summary, therefore, we fully agree that the ideal monitoring device would be one that reliably measures oxygenation at the cellular level. At the present time there is controversy as to which device, if any, can reliably measure this. We do believe that devices that measure primarily intravascular oxygenation might still have an important role in assessing the cerebral oxygen supply-and-demand relationship and that such devices warrant critical evaluation in both pediatric and adult patients.
References
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