(Stroke. 1999;30:895-897.)
© 1999 American Heart Association, Inc.
Letters to the Editor |
Comparison of Near-Infrared Spectroscopy and Somatosensory Evoked Potentials for the Detection of Cerebral Ischemia During Carotid Endarterectomy
Department of Surgery, University Hospital of South Manchester, Manchester, UK
Key Words: evoked potentials, somatosensory • intraoperative
monitoring
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.
|
References
1.
Beese U, Langer H, Lang W, Dinkel M. Comparison of
near-infrared spectroscopy and somatosensory evoked potentials for the
detection of cerebral ischaemia during carotid
endarterectomy. Stroke. 1998;29:2032–2037.
2. 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.[Medline] [Order article via Infotrieve]
Response
Department of Anaesthesiology
Department of Surgery, Division of Vascular Surgery, University of Erlangen-Nürnberg, Erlangen, Germany
Key Words: evoked potentials,
somatosensory • intraoperative monitoring
We are grateful for the opportunity to reply to the comments of Nemoto and Chant and collegues on our study.1 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.2 3 Our own preliminary validation study4 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.5 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).6 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.7 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.8 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
study9 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.10
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.11 There is growing evidence that transcranial Doppler will be a useful adjunct to electrophysiological monitoring.12 The same will have to be proved for NIRS before implementation of this evolving technology will lead to an improvement of patient care.
References
1. 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.
2. Dinkel M, Schweiger H, Goerlitz P. Monitoring during carotid surgery: somatosensory evoked potentials vs carotid stump pressure. J Neurosurg Anesthesiol. 1992;4:167–175.[Medline] [Order article via Infotrieve]
3.
Haupt WF, Horsch S. Evoked potentials in carotid
surgery: a review of 994 cases. Neurology. 1992;42:835–838.
4. Dinkel M, Kamp HD, Schweiger H. Somatosensorisch evozierte Potentiale in der Karotischirurgie. Anasthesist. 1991;40:72–78.[Medline] [Order article via Infotrieve]
5.
Markand ON, Dilley RS, Moorthy SS, Warren C.
Monitoring of somatosensory evoked responses during carotid
endarterectomy. Arch Neurol. 1984;41:375–378.
6. Cho H, Nemoto EM, Yonas H, Balzer J, Sclabassi RJ. Cerebral monitoring by means of oximetry and somatosensory evoked potentials during carotid endarterectomy. J Neurosurg. 1998;89:533–538.[Medline] [Order article via Infotrieve]
7. 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.
8. 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.
9. de Letter JA, Sie TH, Moll FL, Algra A, Eickelboom BC, Ackerstaff RGA. Transcranial cerebral oximetry during carotid endarterectomy: agreement between frontal and lateral probe measurements as compared with an electroencephalogram. Cardiovasc Surg. 1998;6:373–377.[Medline] [Order article via Infotrieve]
10. Schwarz G, Litscher G, Kleinert R, Jobstmann R. Cerebral oximetry in dead subjects. J Neurosurg Anaesthesiol. 1996;8:189–193.[Medline] [Order article via Infotrieve]
11. Ackerstaff RGA, van de Vlasakker CJW. Monitoring of brain function during carotid endarterectomy: an analysis of contemporary methods. J Cardiothorac Vasc Anesthesiol. 1998;12:341–347.[Medline] [Order article via Infotrieve]
12.
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.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||