Cerebral oximetry is a method of measuring brain tissue oxygen saturation noninvasively by near-infrared spectroscopy (NIRS), the data from which may be important clinically. This technique has been validated in clinical studies, as well as in experimental animals (1–3). However, doubts have been expressed concerning the accuracy of the measured values. Because there is no “gold standard,” validation of the values has been difficult. Several investigators have suggested that cerebral oximetry by NIRS is affected significantly by changes in extracranial blood flow, despite the capacity of detecting tissue hypoxia deep in the scalp (4–6). Furthermore, there are a variety of commercially available NIRS instruments for clinical use. They use different technologies and methods to provide cerebral oxygenation data. Although previous studies have tried to compare various instruments, an appropriate analysis has not been used to assess the degree of agreement among these devices (7–9). There seems to be little agreement, for example, as to how to establish the criteria of cerebral oxygenation for each device. Currently at least two commercially available instruments—the INVOS 4100 (Somanetics, Troy, MI) and the NIRO 300 (Hamamatsu Photonics, Hamamatsu, Japan)—can display cerebral oxygen saturation. To determine whether these two devices produce similar cerebral oxygen saturation data, we compared the values obtained by these two instruments during the fluctuation of cerebral blood flow induced by CO2 challenge tests.
Nineteen patients (7 women and 12 men, aged 22 to 76 yr) scheduled for elective orthopedic or gynecologic surgery during general anesthesia were enrolled in the study. After institutional approval, informed consent was obtained from each patient. Patients with a history of cerebrovascular disease were excluded. No preanesthetic medication was administered. Anesthesia was induced with fentanyl 4 μg/kg IV, propofol 2 mg/kg IV, and vecuronium 0.2 mg/kg IV, and the trachea was intubated. The lungs were mechanically ventilated to maintain Paco2 between 35 and 45 mm Hg. Anesthesia was maintained with 1% sevoflurane and 67% nitrous oxide in oxygen. The usual monitoring equipment was used, including a radial artery catheter for direct arterial blood pressure measurement and a nasopharyngeal thermistor for monitoring body temperature, which was maintained between 35.5°C and 36.5°C by using a warming blanket.
Regional cerebral oxygen saturation (rSo2) and tissue oxygen index (TOI) were measured with the INVOS 4100 and the NIRO 300, respectively. For the measurement of rSo2, the cerebral oximeter probe was placed on the right forehead, with the caudal border approximately 1 cm above the eyebrow and the medial edge at the midline. The INVOS 4100 uses two wavelengths of near-infrared light (730 and 810 nm) and measures the ratio of oxyhemoglobin (Hbo2) to total hemoglobin (HbT), which is a percentage value of rSo2. The sensor contains a light-emitting diode (LED) and two detectors located 30 and 40 mm from the LED, allowing removal of the extracranial contribution of scattered light by the application of a subtraction algorithm. The proximal detector receives less of a signal from deeper brain tissue, whereas the distal detector measures the saturation of all of the tissue, including skin, muscle tissue, skull, and brain. If the signal detected by the proximal detector is subtracted from that of the distal detector, the ratio of the two can give a mean value for cerebral saturation.
The NIRO 300 monitor uses four wavelengths of near-infrared right (775, 825, 850, and 904 nm), and the sensor contains a laser diode and three detectors placed at 4 or 5 cm from the source of emitting light. It can measure a TOI (%), which is the ratio of Hbo2 to HbT. The NIRO 500, the earlier model of the NIRO 300, monitored only changes in Hb concentration and the redox state of cytochrome oxydase with a modified Beer-Lambert equation (10). Furthermore, the NIRO-300 uses the specially resolved spectrometer (SRS), which combines the multidistance measurements of optical attenuation and makes it possible to calculate the absolute concentration of Hbo2 and Hb in the tissue. Then, the TOI (%) is rapidly calculated. In contrast with the modified Beer-Lambert equation, the values derived by SRS are not affected by differential path-length factors.
A 2-MHz pulsed Doppler ultrasound device (Companion TCD System; EME, Überlingen, Germany) was used for transcranial measurements of blood flow velocity of the right middle cerebral artery (MCA). Insonation of the MCA was initiated at a depth of 45 mm. Confirmation of MCA identity was achieved by increasing insonation depth to visualization of the bidirectional flow pattern typical of the bifurcation of the internal carotid artery into the MCA and anterior cerebral artery. After individual adjustment of Doppler variables such as gain, sample volume, and power of ultrasound, the probe was handheld and fixed during the measurement.
At least 15 min after the beginning of surgery, during which stable hemodynamic variables were maintained, a series of measurements of rSo2 and TOI were performed at the following points after an accommodation period of approximately 15 min: 1) during normocapnia (Paco2, 35–45 mm Hg) (the sensor of the INVOS 4100 was placed on the right forehead, and rSo2 was measured 5 min after the attachment; just after the measurement of rSo2, the sensor of the NIRO 300 was replaced on the same place and TOI was measured); 2) during hypocapnia (Paco2, 25–35 mm Hg) induced by adjusting the respiratory rate (TOI was measured with the sensor of the NIRO 300 as it was during normocapnia; the sensor of the INOVS 4100 was then replaced, and rSo2 was measured 5 min after the attachment); 3) during normocapnia (Paco2, 35–45 mm Hg) (rSo2 was measured with the sensor of the INVOS 4100 as it was during hypocapnia, and TOI was measured similarly with the replaced sensor of the NIRO 300); and 4) during hypercapnia (Paco2, 45–55 mm Hg) induced by adjusting the respiratory rate (TOI was measured, and then rSo2 was similarly measured). Hemodynamic variables, including arterial blood gases and the mean blood flow velocity of the MCA, were also measured at a midpoint of each period.
Data were expressed as mean ± sd. Hemodynamic data were analyzed by one-way analysis of variance for repeated measures. The Bonferroni test was used for post hoc pairwise comparisons. The values and percentage changes in measured rSo2 and TOI were compared by using regression analysis. The analysis of agreement is based on the methods proposed by Bland and Altman (11). These assess the extent of the agreement between two different methods of clinical measurements by calculating the mean of the differences between the values obtained by the two methods: bias and the sd of the differences.
Mean arterial pressure, heart rate, body temperature, and Hb remained unchanged during the period of CO2 challenge tests (Table 1). The values of TOI and rSo2 and the percentage values of TOI and rSo2 of control were similar during hypocapnia (Table 2). In contrast, during hypercapnia, percentage values of TOI of control were significantly lower than percentage values of rSo2 (P < 0.05). The blood flow velocity of MCA significantly decreased during hypocapnia and increased during hypercapnia compared with normocapnia (P < 0.05). There was a significant positive correlation between the values of TOI and rSo2 (r = 0.58, P < 0.01) and between the percentage values of TOI and rSo2 of control (r = 0.85, P < 0.01) (Figs. 1 and 2).
Data of TOI and rSo2 were plotted according to Bland and Altman (11). Bland and Altman analysis revealed a bias of −0.5% with 2 sd of 15.6% when comparing the rSo2 values with TOI values and a bias of −3.4% with 2 sd of 15.2% when comparing percentage values of rSo2 of control with percentage values of TOI of control. The test suggested unacceptable disagreement.
NIRS is a method of continuous noninvasive monitoring of cerebral oxygenation (12,13). Previous studies of commercially available devices for clinical use mainly compared the INOVS 3100 and the NIRO 500. The INVOS 3100, approved by the United States Food and Drug Administration, displays rSo2 directly. The NIRO 500 displays a change in HbT concentration and Hbo2 concentration and does not show the percentage value. It was difficult to compare the two apparatuses directly. Several studies (8,9) processed the NIRO 500 data to derive rSo2 to compare the INVOS 3100 and the NIRO 500. The NIRO 500 measures the degree of change of Hbo2 concentration from an unknown baseline. On the other hand, the INOVS 3100 measures the ratio of Hbo2 to HbT at one time, not the degree of change. The comparison would not have been made accurately in the previous studies. The NIRO 300, which uses a new NIRS method of SRS, provides a TOI as a percentage value in rSo2 in addition to the originally measured values of the NIRO 500. TOI is an absolute one-time value and is not the degree of the change. TOI, therefore, leads to a more accurate comparison among the INVOS 4100, the new model of the INVOS 3100, and the NIRO 300.
The results of this study showed that there was a good correlation between the values of rSo2 and TOI and the percentage changes of rSo2 and TOI during the CO2 challenge test. However, Bland and Altman analysis revealed an unacceptable disagreement between the INVOS 4100 and the NIRO 300 during the CO2 challenge test. A good correlation does not necessarily show an agreement between the values and their relative changes with the INVOS 4100 and the NIRO 300, but it indicates that their values change proportionally during the CO2 challenge test (10). Although these instruments could measure CO2-induced cerebrovascular reactivity individually (14,15), it seems likely that their values were not equivalent. The disagreement of measured data between the INVOS 4100 and the NIRO 300 can probably be explained by several factors. First, there is a difference in technical method between the INVOS 4100 and the NIRO 300. The INVOS 4100 measures the ratio of light absorption of Hbo2 and HbT and calculates a value for the mean cerebral saturation, avoiding quantification of change in Hb concentration. Servic et al. (16) described a method for determining the ratio of the absorption coefficients by using two wavelengths first. A further variant of this type of instrument is the INVOS 4100. However, the NIRO 300 provides an absolute measurement of tissue Hb concentration by using the SRS method. The multidistance approach of four wavelengths brings about an absolute measurement of cerebral Hb oxygen saturation. The difference of the two devices might create the discrepancy between the measured values. Second, the effect of extracranial blood flow on rSo2 varies with the circumstances. Germon et al. (5) demonstrated that the INVOS 3100 cerebral oximeter was affected significantly by changes in extracranial blood flow oxygenation, which may affect its reliability in clinical practice. However, Cho et al. (6) reported that the responses to CO2 change of the NIRO 500 tended to be greater than those of the INVOS 3100. They suggested that the subtraction method of the INVOS 3100 that effectively removes contamination by skin and bone leads to the fewer responses of the NIRO 500. Although the effect of extracranial blood flow on the data is still controversial (17), this might lead to the difference between the two devices. Third, instruments using LEDs as light sources, such as the INVOS, are more prone to error because the broad emission bandwidth (30 to 40 nm) means that an average extinction over this range must be used in the calculation. On the other hand, the laser diodes used in the NIRO 300 have a narrow emission wavelength that is only a few nanometers wide (12). This might affect the measured values of the two devices. The sensors for the INVOS 3100 and the NIRO 500 were placed on opposite sides at the same time in previous studies (7–9). The emerging light from the sources of the INVOS 3100 with wide bandwidth may interfere with the emerging light of the NIRO 500. Al-Rawi et al. (18) indicated that the NIRO 300 detector connection to the patient, being of a low voltage, was more prone to electrical interference from other monitoring equipment. The intensity of the emerging light would not be detected accurately. Therefore, we did not attach the sensors of these devices on opposite sides in this study.
NIRS methods in reflectance mode were accompanied by a path-length factor. The modified Beer-Lambert method used by the INVOS 4100 first needs to estimate path length. This factor appears in both the numerator and denominator of the ratio and cancels itself out, so that rSo2 is considered not to be affected by the path-length factor. In the SRS method used by the NIRO 300, multidistance measurements of optical attenuation make it possible to be free from the path-length factor. The NIRO 500—a previous model of the NIRO 300—was limited in clinical use by the path-length factor and measured only the changes in Hb concentration from an unknown baseline. The SRS method makes it possible to quantify the absolute concentration of oxy- and deoxyhemoglobin and to calculate TOI. Although the measured values of the INVOS 4100 and the NIRO 300 may not be affected by path length, the difference of calculation method between the two devices might explain the discrepancy of measured values.
In this study, to exclude the effect of interference by the emerging light of the INVOS 4100 and to measure rSo2 and TOI at the same side, the sensor of the INVOS 4100 was replaced with the NIRO 300 sensor. Therefore, the measured values of the INVOS 4100 and the NIRO 300 might be influenced by the replacement of the sensor, although it was replaced at the same site by marking the position. However, there were no significant differences between the measured values at first and second normocapnia. Furthermore, in several cases, after the last measurement of hypercapnia, patients were returned to normocapnia and the rSo2 was measured. There were no significant differences among the values at the three time points of normocapnia. Therefore, we believe that the influence of sensor replacement on the results would be small.
In summary, we compared the INVOS 4100 and the NIRO 300 in detecting cerebrovascular CO2 reactivity. The INVOS 4100 and the NIRO 300 showed a significant linear relation between the values of rSo2 and TOI in response to CO2 alteration. However, Bland and Altman analysis revealed that the individual values and the relative changes of the two apparatuses were not equivalent. These findings suggest that the degree of reduction of these values in response to improper oxygenation may differ depending on the device used. Although little is known about the ability of rSo2 to detect dangerous levels of desaturation in a clinical setting, we may need to determine it for each device.
The authors thank Professor Norio Kurumatani, Department of Hygiene, Nara Medical University, for his assistance in statistical analysis and preparing the manuscript.
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© 2002 International Anesthesia Research Society
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