Comparison of Near-Infrared Spectroscopy and Somatosensory Evoked Potentials for the Detection of Cerebral Ischemia During Carotid Endarterectomy
To the Editor:
When used appropriately, the INVOS 3100A is a valuable tool for monitoring cerebral perfusion during carotid endarterectomy. Over the last 4 years we have monitored over 600 carotid endarterectomies, with a stroke rate of less than 2% and a shunt insertion rate of only 10%. All our patients are monitored with this device.
Beese et al1 used somatosensory evoked potentials (SEP) to predict shunt requirements. However, no patient suffered permanent neurological injury, and it cannot be assumed that SEP would have forewarned neurological damage.
They experienced “considerable variability” in the oximetry reading using the INVOS 3100A when the probe was placed on the forehead. We demonstrated in 19942 that the forehead is inappropriate for this purpose. The sensitivity to hypoxemia is greatly improved (as might be expected) when the probe is placed over the relevant middle cerebral artery territory. With a “correctly” positioned probe in the parietal area, we demonstrated a highly significant (r=0.92, P<0.001) correlation between jugular venous oxygen saturation and oximetry readings; however, the INVOS 3100A is virtually useless when positioned over the frontal cortex in carotid surgery. We have never experienced a patient refusing to have a small area of scalp shaved for this purpose.
Beese et al have not produced any evidence to invalidate the opposite conclusion: that near-infrared spectroscopy detects cerebral hypoxia during carotid endarterectomy and is the simplest apparatus to use and interpret.
- Copyright © 1999 by American Heart Association
We are grateful for the opportunity to reply to the comments of Nemoto and Chant and collegues on our study.R1 Both letters reflect the heated debate concerning the usefulness of near-infrared spectroscopy (NIRS) in the setting of carotid endarterectomy under general anesthesia. Although Nemoto as well as Chant and collegues are convinced of the clinical value of the INVOS 3100A, they are adding little evidence to support their conviction besides “personal experience.” However, one of the basics when introducing a new monitoring device is to answer the question of whether aquisition of data will alter the treatment plan. Therefore, it is essential to define a threshold under which neuronal damage is likely to occur and intervention is appropriate. So far, no threshold values, confirmed by the individual neurological outcome of a sufficient number of patients, were found for the INVOS 3100A. The hypothesis that this device reliably detects cerebral ischemia has yet to be validated. To ask for an invalidation (letter of Chant and collegues) can only thereafter be accomplished.
No single neuromonitoring device will detect all causes of perioperative cerebral damage with a 100% sensitivity and specificity, and we never claimed that this can be accomplished by the monitoring of SEP. However, the loss of cortical waveform after cross-clamping of the internal carotid artery is a reliable marker for severe clamp-related ischemia and the need for placement of an intraluminal shunt.R2 R3 Our own preliminary validation studyR4 showed that a loss of cortical waveform N20P25 was followed by neurological deficits in 7 of 9 nonshunted patients. The significance of a clamp-related SEP loss was also demonstrated in a study of patients undergoing the same procedure under regional anesthesia.R5 A prospective analysis of our database, including more than 1500 patients, revealed that a preserved cortical SEP predicts uneventful recovery at emergence from general anesthesia in 99.4% of cases (Dinkel, unpublished data). Therefore we believe that SEP is useful as a reference method for new forms of neuromonitoring such as NIRS.
Nemoto states that there is a correlation between changes in SEP and reduction of “regional cerebral saturation” (rSo2).R6 It should be noted that in their study only 9 patients were monitored by means of INVOS 3100A. Two patients showed a reduction of SSEP amplitude after carotid cross-clamping, but in only 1 patient did a >50% reduction occur. However, in both patients a decrease of ≈11% in rSo2 was noted. We have seen patients with rSo2 changes of >11% without concomittant SEP loss as well as patients with minimal or even no changes with accompanying SEP loss (see the Figure⇑). Nemoto and collegues are merging a complete loss and a <50% reduction of SEP. Although the latter criterion is not recognized as an indicator of cerebral ischemia that requires intervention, they claim that a decrease of >10% of the displayed rSo2 value should dictate the need for a shunt. Given the few patients studied and the inherent risk of shunt placement, we do not think this recommendation is justified.
We agree with Nemoto and Chant et al that the placement of the sensor is of importance. We stated that the sensor location was “according to the manufacturer instructions over the ipsilateral forehead,” and this implies that the lower margin of the sensor was ≈2 cm above the eyebrow, thus not covering the frontal sinus. Chant and collegues were trying to provide evidence that only placement of the sensor over the temporal lobe is correct by correlating the rSo2 readings with jugular venous oxygen saturation in 14 patients.R7 However, the significance of a good correlation between the readings of the INVOS and jugular venous oximetry is unclear, since the latter form of neuromonitoring is acknowledged as unreliable in carotid surgery.R8 On this point we agree with Nemoto, although we doubt his statement that during cross-clamping of the internal carotid artery the entire venous outflow comes from the other hemisphere. Interestingly, a recently published studyR9 provides evidence that lateral placement will not improve the performance of INVOS 3100 in patients with severe cerebral ischemia indicated by clamp-related EEG changes.
Dr Nemoto is correct when he states that blood pools between the sensor and the brain reduce the effectiveness of the INVOS measurement. By means of routinely obtained CT scans prior to surgery, we excluded that artifact.
We did not cite articles concerning jugular venous oximetry for the lack of correlation with NIRS but for the variability of the obtained values for rSo2 prior to any intervention. We cannot share Dr Nemoto’s experience regarding the values he stated as normal in adults. Although the possibility of unknown underlying pathology cannot be excluded, it seems unlikely that this is true for many patients who will be outside the narrow “normal range” of 65 to 75. It is our concern that a single displayed rSo2 number will give a false sence of certainity. The fact that a device which claims effective reduction of extracranial contamination will give rSo2 readings of >70% in cadavers with removed brains renders the interpretation of single numbers questionable.R10
Due to the various etiologies of neurological damage during and soon after carotid endarterectomy, a multimodal monitoring approach in patients undergoing this procedure under general anesthesia is desirable.R11 There is growing evidence that transcranial Doppler will be a useful adjunct to electrophysiological monitoring.R12 The same will have to be proved for NIRS before implementation of this evolving technology will lead to an improvement of patient care.
Beese U, Langer H, Lang W, Dinkel M. Comparison of near-infrared spectroscopy and somatosensory evoked potentials for the detection of cerebral ischemia during carotid endarterectomy. Stroke. 1998;29:2032–2037.
Haupt WF, Horsch S. Evoked potentials in carotid surgery: a review of 994 cases. Neurology. 1992;42:835–838.
Williams IM, Picton A, Farrell A, Mead GE, Mortimer A, McCollum CN. Light-reflective cerebral oximetry and jugular bulb venous oxygen saturation during carotid endarterectomy. Br J Surg. 1994;81:1291–1295.
Larson CP, Ehrenfeld WK, Wage JG, Wylie EJ. Jugular venous oxygen saturation as an index of adequacy of cerebral oxygenation. Surgery. 1967;62:31–39.
Arnold M, Sturznegger M, Schaeffler L, Seiler RW. Continuous intraoperative monitoring of middle cerebral artery blood flow velocities and electroencephalography during carotid endarterectomy: a comparison of the two methods to detect cerebral ischemia. Stroke. 1997;28:1345–1350.