Transcranial Cerebral Oximetry During Surgical Procedures: A New Matrix Model For Minimization Of Problems Of Interpretation And Intervention In Desaturation Events
G Schwarz, G Litscher
cerebral oxygen saturation cos, near infrared spectroscopy nirs, nirs matrix, transcranial cerebral oximetry
G Schwarz, G Litscher. Transcranial Cerebral Oximetry During Surgical Procedures: A New Matrix Model For Minimization Of Problems Of Interpretation And Intervention In Desaturation Events. The Internet Journal of Neuromonitoring. 2003 Volume 3 Number 2.
Transcranial cerebral oximetry is a method to detect cerebral hypoxia and to avoid cerebral dysfunctions. Regional cerebral oxygen saturation was measured in a 35-year-old female during cerebral aneurysm surgery. For the effective clinical use of near infrared spectroscopy (NIRS) during surgical or endovascular procedures the intrinsic and extrinsic factors which can influence the NIRS data have to be considered. Therefore a NIRS matrix as a short information for the specific management to correct changes of cerebral oxygen saturation (COS) was developed.
The aim of development of the NIRS matrix is to support the patient management specifically adapted to the surgical, neurointerventional and anesthesiological context.
Transcranial near infrared spectroscopy (NIRS) can detect changes in cerebral oxygen metabolism even following minimal physiologic, pathophysiologic, and therapeutic events [1,2,3]. This method has potential applications for monitoring patients at risk of cerebral oxygen desaturation during certain types of surgery  such as cardiac surgery, major vascular surgery and carotid endarterectomy, for patients with surgical positioning issues, for patients older than 60 years  and neuroendovascular procedures . There are prerequisites and limitations associated with NIRS which have to be appreciated for the use of this technique . However, reports about the beneficial aspects of intraoperative NIRS monitoring for early identification of vulnerable episodes  and the improved postoperative neurological outcome  are encouraging.
Using NIRS effectively requires accurate interpretation of the measurement values and this in turn requires correcting for intrinsic and extrinsic factors that can affect the results. We developed a matrix - i.e., the shortest unit of information - for interpreting NIRS data with the aim of integrating NIRS results into procedural care.
Our NIRS matrix for intraoperative applications contains five descriptors which are causally related to one another. The horizontal left-right link at one level contains the changes of cerebral oxygen saturation (COS) at one end. At the other end - as the result of conclusions - there are specific anesthesiologic interventions to consolidate the altered COS-values.
NIRS matrix descriptors
The major determinant is the “change of COS” as an indicator of changes in cerebral oxygen balance. COS changes are described as decrease (in special cases fluctuations of couplets of COS decreases and increases) from baseline.
The next determinant is the “key variable”, which as the value that undergoes marked changes is related to the changes in COS. This determinant results from a group of values obtained with conventional intraoperative monitoring named the basic data (Tab. 1): mean arterial blood pressure (MAP), hemoglobin (Hb), peripheral oxygen saturation (SaO2), partial carbon dioxide pressure (pCO2), core temperature (t). In the logistics chain group A (explained below) one parameter of the basic data (= vital data) becomes the key variable once it shows a marked change (shift into an abnormal range or change from baseline). It is thus necessary to obtain a series of values to detect changes.For the logistics chain groups B and C (explained below) the key variables are not vital parameters obtained from the patient but rather physical (anatomical) (B) or (neuro-) pharmacologic (C) variables that have an effect on the COS.
The key variable is compared to the “associated parameters” which within the groups of basic data described above showed no marked changes (Fig. 1). Each of the basic data can also assume the position of key variable that cause decrease of the COS. For the sake of didactic order the matrix does not use complex representations of synergistic constellations or antagonistic combinations.
The “clinical interpretation” of the most probable underlying event is deduced from the data constellation COS - key variable - associated parameters (Fig. 2).
Finally the matrix deduces the “intervention” most likely to normalize the COS or return it to its baseline level (Fig. 3).
NIRS matrix - Groups of logistics chains
The vertical grouping (Fig. 3) of the horizontal logistic chains was done with causal differentiation:
a)biologic measurement values (vital data),
The logistics chains in group A via key variables contain information on cerebral oxygen supply (MAP, pO2, hemoglobin, pCO2) and oxygen demand (core temperature).
The pathophysiologic chain of the configurations in group B is its common endpoint, the compromised oxygen supply to the brain. Primary causes are anatomical obstructions of blood vessels and patient positioning, most commonly decreased perfusion with rotation of the head in patients with bilateral carotid stenosis or insufficiency of the arterial circle of Willis.
Continuous supply of oxygen to the brain can also be compromised by obstructed venous return due to malpositioned catheters and cannulas or cerebral edema.
A very important unique pattern during endovascular procedures can be observed in patients with arterial vasospasm after subarachnoid hemorrhage. Periodic fluctuations of increases and decreases of COS ( ) may appear from time to time with a relative stable baseline or they can be observed as a regular pattern . In this situation the characteristic increase of mean flow velocity of transcranial Doppler sonography or the angiographic aspect of diminished diameters of cerebral arteries take the role of the key variable.
Additionally, functional factors (especially the key variables of oxygen supply) can accentuate the changes in COS in patients with predisposing anatomical factors.
Group C reflects the influences of neuronal activity on cerebral oxygen consumption.
Inadequate anesthesia may lead to increased oxygen consumption by the waking brain and thus to decreased COS. The beneficial effect of central inhibitory medications such as anesthetic agents on the equilibrization of COS becomes apparent especially with the sometimes necessary suppression of neuronal activity. Cerebral seizures, with their dramatic increase in cerebral neuronal activity (verifiable with EEG), can increase cerebral oxygen consumption and thus lower COS.
NIRS matrix - Explanation of terms
If one parameter of the “basic data” is significantly altered it becomes the “key variable”. The other parameters of the “basic data” which are unchanged or minimally changed are summarized as “associated parameters”.
The complete horizontal linkage of the main determinants of the matrix from COS to “intervention” is called “logistics chain”.
The vertical combinations of corresponding “logistics chains” are summarized to specific “logistics chain groups”.
The results (NIRS matrix) are summarized in Table 2:
We report about simultaneous transcranial cerebral oximetry and laser Doppler flowmetry monitoring during a cerebral aneurysm surgery (a. cerebelli sup.) in a 35-year-old female.
Figure 1 shows an intraoperative recording session of more than 3 hours. LDF-Flux, rSO2 and skin temperature at the recording position of the laser Doppler probe were sampled and documented simultaneously. At “A” (time event marker in Fig. 1; middle panel) the skin incision was performed. The Flux and the rSO2 values increased. At “B”, “C” and “F” (Fig. 1) anesthesia was deepened and burst suppression activity in the electroencephalogram (EEG) appeared.
Only scant changes were seen in the Flux values, which were not reproducible. The rSO2 showed no significant alterations during this period. The marker “D” (Fig. 1) indicates the rupture of the aneurysm. It is clearly demonstrated that the rSO2 values decreased. The Flux also showed a reduction, though with a significant time delay in comparison to the intracerebral oxygenation. Systolic blood pressure decreased from 120 mmHg to 80 mmHg.
Further, there was a period of time in which an extreme reduction of temperature was observed. This was caused by the application of cold red cells transfusion following the rupture of the aneurysm (“E”). The external microcirculation did not follow this trend. Finally, the end of the operation is marked by “G”. Corresponding to the NIRS matrix, in the presented patient the
The effect of the intervention was documented by an immediate increase of COS (rSO2). The temporary overswing may be caused by the decrease of t and changes in the distribution of cerebral blood volume.
Accurate interpretation of intraoperative NIRS data requires integrating of extracerebral influences. The NIRS matrix (Tab. 3) is based on this fact. The matrix corresponds to a configuration of logistics chains in the sense of forward chaining.
Like most of matrices the NIRS matrix for intraoperative use can provide only a rough guideline and the risk of oversimplification is imminent. However, this matrix can support decision making in clinical practice.
For the configuration of the matrix, the individual changes in COS are assigned only one univariate descriptor change according to the key variable from the pool of the basic data. Bivariate and polyvariate links of the key variables and/or links of parameters in the pool of basic data to key variables have to be made individually. Interpretations and correctional interventions should be made on this basis.
According to the experience of some authors [10,11,12] the number of therapeutic interventions to correct COS is not wide spread and this is reflected in the configuration of the matrix.
The most evident problem with NIRS use is the lack of recognized limits for alarm or intervention at the moment. A number of reports indicate that COS decreases of > 25 % signify imminent cerebral ischemia/hypoxia and require immediate intervention. Roberts et al.  reported that awake patients undergoing carotid endarterectomy who had a rSO2 decrease of > 27 % were at risk of developing neurodeficits. Studies on conscious individuals involved in + Gz acceleration , implantable cardioverter–defibrillator testing , and tilt–table syncope testing  consistently found severely impaired cerebral function with a relative rSO2 decline of 25 % or more. Moehle  defined a cerebral oxygen desaturation event as a COS decrease > 20 % from baseline for greater than 3 min in the cardiopulmonary bypass surgical patient.
Although abnormally low absolute rSO2 values have been associated with evidence of cerebral dysfunction , in our opinion NIRS monitoring is mainly a trend monitoring [18,19]. Accordingly we chose for the NIRS matrix the relative changes of COS (uni- or bilateral decrease or fluctuation of alterning patterns of dercreases and increases) as described above related to baseline for indicating significant changes of cerebral oxygen metabolism.
The ambivalence of NIRS data should be kept in mind. Marked drops in COS are very sensitive indicators of potentially threatening changes in cerebral oxygen metabolism. But case studies of dynamic pathologic processes such as intracranial pressure crises or transtentorial herniation  show that the converse does not apply - i.e., that largely stable COS values do not guarantee cerebral integrity.
Despite these limitations NIRS monitoring has considerable potential to improve cerebral monitoring during surgical  and neurointerventional [22,23] procedures. We aim to include our NIRS matrix in a computer-assisted system with automated linkage of different data combinations to provide an immediate monitoring support adapted to the surgical, neurointerventional and anesthesiological context.