patients with an acute increase in intracranial pressure ( icp ) often present with a decreased level of consciousness secondary to reduced cerebral perfusion pressure ( cpp ) . in these patients , cpp is maintained by increased systemic blood pressure as a result of cushing 's reflex . hence , peri - operative management goals in these patients primarily focus on maintaining the systemic blood pressure ( bp ) . entropy and bispectral index ( bis ) are commonly used electroencephalographic ( eeg ) based depth of anesthesia monitors . it has been shown that changes in eeg patterns correlate well with changes in cpp and hence entropy and bis values may also correlate with cpp . previous reports have shown that sudden decrease in cerebral perfusion was associated with lower bis value . we report two cases where entropy values suddenly increased after cranial decompression in patients with elevated icp . a 25-year - old male asa i , weighing 70 kg , presented for left parietotemporal craniotomy and resection of temporal lobe tumor . he underwent a successful tumor resection and was admitted in the neuro - intensive care unit ( nicu ) post - operatively . on post - operative day 2 , the patient 's level of consciousness deteriorated and he became unresponsive ( glasgow coma scale [ gcs ] 5/15 ) . computed tomography showed a large intracranial hematoma in the left temporal lobe with the midline shift . patient was intubated in nicu and positive pressure ventilation was instituted at a higher rate and then transferred to the operating room for emergency decompression and hematoma evacuation . endotracheal tube ( ett ) was connected to the anesthetic machine ( aisys care station ( ge health care , madison , wi . usa ) ventilator and hyperventilated to keep end - tidal carbon dioxide ( etco2 ) at 30 - 33 mmhg . entropy electrodes were attached on the forehead opposite to the side of surgical incision and were connected to transducer and the module . his baseline entropy ( response entropy ( re)/state entropy ( se ) values were 37/33 without any sedation . since his gcs and baseline entropy value were low , received only 100 mcg of fentanyl and 50 mg rocuronium . 2% xylocaine with adrenaline infiltration for head pin fixation followed by scalp flap infiltration with 0.5% bupivacaine + adrenaline ( 1 in 200,000 ) was done to reduce the pain thereby the anesthetic requirement . entropy values remained low until the cranial decompression . immediately following the removal of the bone flap , entropy temporarily decreased to below ( re / se ) 40/40 but rebounded to 90/80 within 30 s. sevoflurane concentration was then increased and additional doses of propofol were given . entropy decreased to ( re / se ) 37/32 and remained low for the remainder of the procedure [ figure 1 ] . entropy values during cranial decompression in patient 1 * bsr = burst suppression ratio , re = response entropy , se = state entropy following the procedure , patient was transferred to nicu . a 65-year - old female weighing 50 kg , presented to the emergency department with acute hydrocephalus secondary to subarachnoid hemorrhage ( world federation of neurologic surgeon grade 5 , fischer grade 4 ) . she was intubated and arterial line was started and shifted to the operating room for insertion of an external ventricular drain ( evd ) . upon arrival to the operating room , her initial blood pressure was 190/85 with heart rate of 90 beats / min . patient was connected to the ventilator and hyperventilated to achieve an etco2 level of 30 mmhg . patient received 25 mcg of fentanyl , 30 mg of rocuronium and a propofol infusion was started at 20 - 30 mcg / kg / min was started . blood pressure remained elevated and entropy readings were consistent during initial stages of surgical exposure . immediately following the insertion of the evd , both re and se values increased by 20 points ( 58 - 80 for re and 51 - 72 for se ) and there was a mild reduction in blood pressure ( 205/99 - 176/80 ) [ table 1 ] . propofol infusion rate was increased to 50 mcg / kg / min and this subsequently decreased the entropy values to 40 - 60 . post - operatively patient was transferred to nicu and then discharged to a rehabilitation center in her local town . a 25-year - old male asa i , weighing 70 kg , presented for left parietotemporal craniotomy and resection of temporal lobe tumor . he underwent a successful tumor resection and was admitted in the neuro - intensive care unit ( nicu ) post - operatively . on post - operative day 2 , the patient 's level of consciousness deteriorated and he became unresponsive ( glasgow coma scale [ gcs ] 5/15 ) . computed tomography showed a large intracranial hematoma in the left temporal lobe with the midline shift . patient was intubated in nicu and positive pressure ventilation was instituted at a higher rate and then transferred to the operating room for emergency decompression and hematoma evacuation . endotracheal tube ( ett ) was connected to the anesthetic machine ( aisys care station ( ge health care , madison , wi . usa ) ventilator and hyperventilated to keep end - tidal carbon dioxide ( etco2 ) at 30 - 33 mmhg . entropy electrodes were attached on the forehead opposite to the side of surgical incision and were connected to transducer and the module . his baseline entropy ( response entropy ( re)/state entropy ( se ) values were 37/33 without any sedation . since his gcs and baseline entropy value were low , received only 100 mcg of fentanyl and 50 mg rocuronium . 2% xylocaine with adrenaline infiltration for head pin fixation followed by scalp flap infiltration with 0.5% bupivacaine + adrenaline ( 1 in 200,000 ) was done to reduce the pain thereby the anesthetic requirement . entropy values remained low until the cranial decompression . immediately following the removal of the bone flap , entropy temporarily decreased to below ( re / se ) 40/40 but rebounded to 90/80 within 30 s. sevoflurane concentration was then increased and additional doses of propofol were given . entropy decreased to ( re / se ) 37/32 and remained low for the remainder of the procedure [ figure 1 ] . entropy values during cranial decompression in patient 1 * bsr = burst suppression ratio , re = response entropy , se = state entropy following the procedure , patient was transferred to nicu . a 65-year - old female weighing 50 kg , presented to the emergency department with acute hydrocephalus secondary to subarachnoid hemorrhage ( world federation of neurologic surgeon grade 5 , fischer grade 4 ) . she was intubated and arterial line was started and shifted to the operating room for insertion of an external ventricular drain ( evd ) . upon arrival to the operating room , her initial blood pressure was 190/85 with heart rate of 90 beats / min . patient was connected to the ventilator and hyperventilated to achieve an etco2 level of 30 mmhg . patient received 25 mcg of fentanyl , 30 mg of rocuronium and a propofol infusion was started at 20 - 30 mcg / kg / min was started . blood pressure remained elevated and entropy readings were consistent during initial stages of surgical exposure . immediately following the insertion of the evd , both re and se values increased by 20 points ( 58 - 80 for re and 51 - 72 for se ) and there was a mild reduction in blood pressure ( 205/99 - 176/80 ) [ table 1 ] . propofol infusion rate was increased to 50 mcg / kg / min and this subsequently decreased the entropy values to 40 - 60 . post - operatively patient was transferred to nicu and then discharged to a rehabilitation center in her local town . our cases suggest that entropy can be used as a surrogate marker for changes in cpp during cranial decompression in patients with increased icp . advances in anesthesia have led to the development of monitors such as entropy and bis for assessing the depth of anesthesia . these monitors rely on the eeg information gathered from three electrodes from the patient 's forehead . se is computed over the frequency range from 0.8 to 32 hz , which represents the eeg part of the frequency spectrum . re is computed over the frequency range from 0.8 to 47 hz , covering both the eeg and the emg - dominant areas of the spectrum . consequently , rese difference serves at least partly as an indicator of upper facial emg activation . it is well - known that eeg patterns correlate with cerebral blood flow ( cbf ) . when cbf decreases to 25 - 35 ml/100 g / min , eeg first loses its faster frequencies ( 8 - 14 hz ) . with further reductions in cbf ( 18 - 25 ml/100 g / min ) eeg becomes increasingly slower ( 4 - 7 hz ) . when the cbf reduces further to 12 - 18 ml/100 g / min and 10 - 12 ml/100 g / min , eeg becomes increasingly slower ( 1 - 4 hz ) to complete eeg suppression respectively , indicating hypoxic brain cell death . the reduced irregularity and complexity of an eeg waveform in response to reduced cbf is analogous to increasing depth of anesthesia . thus , theoretically , the effect from reduced cbf would cause a similar dose - dependent reduction in entropy values . in our cases , entropy values were very low at the time of electrode placement with very little or no anesthetics given . we believe that this increase in perfusion was reflected as an increase in entropy values . in patients with acutely elevated icp , administration of un - titrated doses of anesthetic may lead to decrease in bp and further reduction of cpp . after cranial decompression , there is an increased cerebral perfusion , which can lead to an improved level of consciousness . hence , depth of anesthesia monitors may be helpful in these cases for a better peri - operative care and better titration of anesthetics . the absolute values from the depth of anesthesia monitors can provide information on the level of consciousness and the relative changes may provide valuable information about changes in cerebral perfusion . although entropy or bis have not been validated for monitoring cerebral perfusion , previous case reports suggest their potential application . in a study by morimoto et al . describes a case where bis values dropped to around 0 - 20 after the induction of general anesthesia . bis levels improved with the management of blood pressure suggesting that lower bis values were probably due to cerebral hypoperfusion . in another report , welsby et al . describes a case where a patient suffered an intra - operative cerebral embolus and bis levels precipitously dropped from 50 - 60 to below 15 - 10 , again reflecting the influence of decreased perfusion on eeg - based anesthetic monitors . there are other case reports describing decreased bis levels in response to cerebral hypoperfusion secondary to ischemic brain injury . in addition , there is a recent increase in interest in nicus for the use of eeg - based monitors to detect ischemia or cerebral hypoperfusion . one of the limitations of our case report is the lack of quantitative measurement of cbf . transcranial doppler is often used to measure velocity and can give an indirect measure of cbf . unfortunately , the urgency of these cases and the lack of access to middle cerebral artery during the case limited its use before and during cranial decompression . furthermore , another important cause for increased intra - operative entropy values in acute neurosurgical conditions is seizure activity . in addition , administration of propofol and sevoflurane could have abolished the seizure activity , causing the decrease in the entropy values . though this is a possibility in our cases but it is highly unlikely given the temporal association with cranial decompression and the accompanying decrease in bp . furthermore , there were no other hemodynamic changes ( e.g. , tachycardia , rise in etco2 ) indicating possible seizure activity . eeg - based anesthetic monitors may not only provide information on level of anesthesia but may have a pivotal role into guiding anesthetic management in patients with decreased cpp . further investigations are needed to assess the utility of these eeg - based monitors as an indicator for detecting changes in cerebral perfusion .
changes in electroencephalogram ( eeg ) patterns correlate well with changes in cerebral perfusion pressure ( cpp ) and hence entropy and bispectral index values may also correlate with cpp . to highlight the potential application of entropy , an eeg - based anesthetic depth monitor , on indicating cerebral perfusion in patients with increased intracranial pressure ( icp ) , we report two cases of emergency neurosurgical procedure in patients with raised icp where anesthesia was titrated to entropy values and the entropy values suddenly increased after cranial decompression , reflecting the increase in cpp . maintaining systemic blood pressure in order to maintain the cpp is the anesthetic goal while managing patients with raised icp . 
 eeg - based anesthetic depth monitors may hold valuable information on guiding anesthetic management in patients with decreased cpp for better neurological outcome .