EditorWe read with interest Dr Deogaonkar's group's work on the use of the bispectral index (BIS) for detection of cerebral ischaemia during carotid endarterectomy.1 Multi-channel electroencephalography (EEG) is certainly a valuable tool in detecting cerebral ischaemia, though its complexity requires a neurologist or neurologist-supervised technician for interpretation, which adds considerable expense. If a simple, easy to use processed EEG could be adequate to diagnose ischaemia and provide the answer to the shunt or not shunt clinical question, this device would be a valuable tool, indeed.
We recently were involved in the care of a patient who developed signs of intraoperative cerebral ischaemia during awake carotid endarterectomy performed under cervical plexus block. Some of our awake carotid endarterectomy procedures are converted to a general anesthetic if the patient's mental status deteriorates or if the regional anesthetic is insufficient. Therefore, we use the PSA 4000 with the PSArray2 (PSA) (Physiometrix Inc., North Billerica, MA, USA) as an adjunctive monitor to better titrate the depth of anesthesia. In these cases the PSA is used during the entire procedure. The PSA, like the BIS, uses the EEG and a complex algorithm to derive the Patient State Index (PSI), a number from 0100 that is intended to be a guide to anesthetic depth. Unlike the BIS, which uses a single channel monopolar EEG to derive the BIS value, the PSA utilizes four channel (Fp1, Fpz1, Cz, Pz), multi-regional, multi-frequency, power and coherence relationships and its own unique algorithm to arrive at the PSI value. There have been case reports that the PSA, like the BIS, can detect global cerebral ischaemic events.2 Often, even focal cerebral hypoperfusion manifests with large regional or even global EEG changes.3 Additionally, there are some data that suggest that as few as two channels are necessary to detect cerebral ischaemia during carotid endarerectomy.4
Our patient was a 56-yr-old male with severe left internal carotid artery stenosis (8099% stenosis) and moderate right internal carotid artery stenosis (5079% stenosis). After placing a cervical plexus block and until the time of carotid cross-clamp, the patient was awake and alert, with a PSI value of 9598. The monitor was set to display the raw EEG waves during the case, which were predominantly alpha and beta waves on both the left and right. Shortly after left carotid cross-clamp, the patient's mental status acutely deteriorated and his right hand grip strength was markedly diminished. Interestingly, the PSI value dropped to 85 after 2 min and the raw EEG showed delta and theta waves on both the left and righta dramatic change from just moments before. Haemodynamics were unchanged.
It would appear that the BIS, as Deogaonkar and colleagues state, lacks the sensitivity to adequately detect cerebral ischaemia and detect the restoration of cerebral electrical activity once perfusion is reestablished. This monitor was certainly not designed for such a purpose. However, PSA may offer advantages over the BIS in having multi-channel analysis and the ability to compare raw EEG from the left and right hemispheres, thus the potential for the PSA to detect cerebral ischaemia may be much higher. Further study with the PSA in this area may be worth pursuing.
Temple and Galveston, TX, USA
EditorThank you for the opportunity to respond to the letter regarding our article.1 We are interested to see Culp and colleagues confirm our observation that the BIS does not detect unilateral (ipsilateral) ischemia during awake carotid endarterectomy. It is interesting that more detailed EEG analysis by the PSA device might be more sensitive than BIS. Whichever form of monitoring is utilized, there is nothing more sensitive than the patient's neurological status during awake endarterectomy. It is for this reason that the awake procedure offers the highest sensitivity and specificity for ischaemia while no other monitoring procedure has achieved this sensitivity. We have previously reported the specificity and sensitivity of jugular venous oxygen saturation monitoring during awake carotid endarterectomy.5 Of all the forms of intraoperative monitoring that have been compared with ischaemia in awake patients during carotid endarterectomy the SJV02 had the highest sensitivity and specificity. Transcranial Doppler has also been evaluated but it is not tolerated well in awake patients because of the need to maintain pressure on the temporal bone constantly throughout the procedure.
Careful studies using sophisticated electrophysiology as used in the case reported by Culp and colleagues need to be evaluated in a large series of patients before they can be universally accepted.
Newcastle, UK
References
1 Deogaonkar A, Vivar R, Bullock RE, Price K, Chambers I, Mendelow AD. Bispectral index monitoring may not reliably indicate cerebral ischaemia during awake carotid endarterectomy. Br J Anaesth 2005; 94: 8004
2 Nguyen NK, Lenkovsky F, Joshi GP. Patient state index during a cardiac arrest in the operating room. Anesth Analg 2005; 100: 1557
3 Jordan KG. Emergency EEG and continuous EEG monitoring in acute ischemic stroke. J Clin Neurophysiol 2004; 21: 34152[ISI][Medline]
4 Tempelhoff R, Modica PA, Grubb RL Jr, Rich KM, Holtmann B. Selective shunting during carotid endarterectomy based on two-channel computerized electroencephalographic/compressed spectral array analysis. Neurosurgery 1989; 24: 33944[ISI][Medline]
5 Crossman J, Bannister K, Bithell V, Bullock R, Chambers I, Mendelow AD. Predicting clinical ischemia during awake carotid endarterectomy: use of the SJV O2 probe as a guide for selective shunting. Physiol Meas 2003; 24: 34754[CrossRef][ISI][Medline]