Hypothermia amplifies somatosensory-evoked potentials in uninjured rats

Intraoperative somatosensory evoked potential (SEP) monitoring: an updated position statement by the American Society of Neurophysiological Monitoring

Somatosensory evoked potentials (SEPs) are used to assess the functional status of somatosensory pathways during surgical procedures and can help protect patients' neurological integrity intraoperatively. This is a position statement on intraoperative SEP monitoring from the American Society of Neurophysiological Monitoring (ASNM) and updates prior ASNM position statements on SEPs from the years 2005 and 2010. This position statement is endorsed by ASNM and serves as an educational service to the neurophysiological community on the recommended use of SEPs as a neurophysiological monitoring tool. It presents the rationale for SEP utilization and its clinical applications. It also covers the relevant anatomy, technical methodology for setup and signal acquisition, signal interpretation, anesthesia and physiological considerations, and documentation and credentialing requirements to optimize SEP monitoring to aid in protecting the nervous system during surgery.

Effects of focal cortical cooling on somatosensory evoked potentials in rats

Although the focal brain cooling technique is widely used to examine brain function, the effects of cortical temperature at various levels on sensory information processing and neural mechanisms remain underexplored. To elucidate the mechanisms of temperature modulation in somatosensory processing, this study aimed to examine how P1 and N1 deflections of somatosensory evoked potentials (SEPs) depend on cortical temperature and how excitatory and inhibitory inputs contribute to this temperature dependency. SEPs were generated through electrical stimulation of the contralateral forepaw in anesthetized rats. The SEPs were recorded while cortical temperatures were altered between 17-38 °C either without any antagonists, with a gamma-aminobutyric acid type A (GABAA) receptor antagonist (gabazine), with an aminomethylphosphonic acid (AMPA) receptor antagonist (NBQX), or with an N-Methyl-D-aspartic acid (NMDA) receptor antagonist ([R]-CPP). The effects of different gabazine concentrations (0, 1, and 10 µM) were examined in the 35-38 °C range. The P1/N1 amplitudes and their peak-to-peak differences plotted against cortical temperature showed an inverted U relationship with a maximum at approximately 27.5 °C when no antagonists were administered. The negative correlation between these amplitudes and temperatures of ≥ 27.5 °C plateaued after gabazine administration, which occurred progressively as the gabazine concentration increased. In contrast, the correlation remained negative after the administration of NBQX and (R)-CPP. These results suggest that GABAergic inhibitory inputs contribute to the negative correlation between SEP amplitude and cortical temperature around the physiological cortical temperature.

Commonly Overlooked Factors in Biocompatibility Studies of Neural Implants

Biocompatibility of cutting-edge neural implants, surgical tools and techniques, and therapeutic technologies is a challenging concept that can be easily misjudged. For example, neural interfaces are routinely gauged on how effectively they determine active neurons near their recording sites. Tissue integration and toxicity of neural interfaces are frequently assessed histologically in animal models to determine tissue morphological and cellular changes in response to surgical implantation and chronic presence. A disconnect between histological and efficacious biocompatibility exists, however, as neuronal numbers frequently observed near electrodes do not match recorded neuronal spiking activity. The downstream effects of the myriad surgical and experimental factors involved in such studies are rarely examined when deciding whether a technology or surgical process is biocompatible. Such surgical factors as anesthesia, temperature excursions, bleed incidence, mechanical forces generated, and metabolic conditions are known to have strong systemic and thus local cellular and extracellular consequences. Many tissue markers are extremely sensitive to the physiological state of cells and tissues, thus significantly impacting histological accuracy. This review aims to shed light on commonly overlooked factors that can have a strong impact on the assessment of neural biocompatibility and to address the mismatch between results stemming from functional and histological methods.

Effects of Changes in Body Temperature on Perioperative Bleeding in Adolescent Idiopathic Scoliosis Surgery

Perioperative bleeding is a critical challenge in adolescent idiopathic scoliosis (AIS) surgery. Preventing hypothermia is associated with decreased development of coagulopathy, blood transfusion rate in various surgery groups. We hypothesized that blood loss would be reduced in patients who were kept normothermic by implementation of aggressive warming methods in AIS. This randomized-controlled study included patients aged 12-18 years who were scheduled to undergo elective scoliosis deformity correction surgery. The patients were divided into two groups: the study group (Group S) was heated aggressively with three different heaters including compressed-air blower heater, intravenous fluid heating, and a heating bed, while the control group (Group C) received only heating with a standard compressed-air blower heater. Tympanic, esophageal, and axillary body temperatures were measured, and hemoglobin and arterial blood gas analyses were repeated during the anesthesia period. Daily bleeding-coagulation parameters were recorded on postoperative days 0, 1, and 2. Forty-eight patients were randomized, and 39 patients were included into the final analysis. The total amount of intraoperative bleeding (p = 0.027) was significantly lower, and duration of surgery (p = 0.025) and length of hospital stay (p = 0.002) were significantly shorter in Group S. Significant linear relationships were found between the core body temperature and the amount of bleeding (β = 0.0001; p = 0.009), operation time (β = 0.003; p = 0.015), and length of hospital stay (β = 0.027; p = 0.044) with linear logistic regression analysis. We reported that normothermia was preserved in the multiheated group, which diminished blood loss, operation time, and the length of hospital stay. Consequently, we suggested that active heating should be applied in AIS surgeries. ClinicalTrials.gov (NCT04686214).

The effect of Glibenclamide on somatosensory evoked potentials after cardiac arrest in rats

BACKGROUND

Science continues to search for a neuroprotective drug therapy to improve outcomes after cardiac arrest (CA). The use of glibenclamide (GBC) has shown promise in preclinical studies, but its effects on neuroprognostication tools are not well understood. We aimed to investigate the effect of GBC on somatosensory evoked potential (SSEP) waveform recovery post CA and how this relates to the early prediction of functional outcome, with close attention to arousal and somatosensory recovery, in a rodent model of CA.

METHODS

Sixteen male Wistar rats were subjected to 8-min asphyxia CA and assigned to GBC treatment (n = 8) or control (n = 8) groups. GBC was administered as a loading dose of 10 μg/kg intraperitoneally 10 min after the return of spontaneous circulation, followed by a maintenance dosage of 1.6 μg/kg every 8 h for 24 h. SSEPs were recorded from baseline until 150 min following CA. Coma recovery, arousal, and brainstem function, measured by subsets of the neurological deficit score (NDS), were compared between both groups. SSEP N10 amplitudes were compared between the two groups at 30, 60, 90, and 120 min post CA.

RESULTS

Rats treated with GBC had higher sub-NDS scores post CA, with improved arousal and brainstem function recovery (P = 0.007). Both groups showed a gradual improvement of SSEP N10 amplitude over time, from 30 to 120 min post CA. Rats treated with GBC showed significantly better SSEP recovery at every time point (P < 0.001 for 30, 60, and 90 min; P = 0.003 for 120 min). In the GBC group, the N10 amplitude recovered to baseline by 120 min post CA. Quantified Cresyl violet staining revealed a significantly greater percentage of damage in the control group compared with the GBC treatment group (P = 0.004).

CONCLUSIONS

Glibenclamide improves coma recovery, arousal, and brainstem function after CA with decreased number of ischemic neurons in a rat model. GBC improves SSEP recovery post CA, with N10 amplitude reaching the baseline value by 120 min, suggesting early electrophysiologic recovery with this treatment. This medication warrants further exploration as a potential drug therapy to improve functional outcomes in patients after CA.

Somatosensory Evoked Potentials and Neuroprognostication After Cardiac Arrest

Improved understanding of post-cardiac arrest syndrome and clinical practices such as targeted temperature management have led to improved mortality in this cohort. Attention has now been placed on development of tools to aid in predicting functional outcome in comatose cardiac arrest survivors. Current practice uses a multimodal approach including physical examination, neuroimaging, and electrophysiologic data, with a primary utility in predicting poor functional outcome. These modalities remain confounded by self-fulfilling prophecy and the withdrawal of life-sustaining therapies. To date, a reliable measure to predict good functional outcome has not been established or validated, but the use of quantitative somatosensory evoked potential (SSEP) shows potential for this use. MEDLINE and EMBASE search using words "Cardiac Arrest" and "SSEP," "Somato sensory evoked potentials," "qSSEP," "quantitative SSEP," "targeted temperature management in cardiac arrest" was conducted. Relevant recent studies on targeted temperature management in cardiac arrest, plus studies on SSEP in cardiac arrest in the setting of hypothermia and without hypothermia, were included. In addition, animal studies evaluating the role of different components of SSEP in cardiac arrest were reviewed. SSEP is a specific indicator of poor outcomes in post-cardiac arrest patients but lacks sensitivity and has not clinically been established to foresee good outcomes. Novel methods of analyzing quantitative SSEP (qSSEP) signals have shown potential to predict good outcomes in animal and human studies. In addition, qSSEP has potential to track cerebral recovery and guide treatment strategy in post-cardiac arrest patients. Lying beyond the current clinical practice of dichotomized absent/present N20 peaks, qSSEP has the potential to emerge as one of the earliest predictors of good outcome in comatose post-cardiac arrest patients. Validation of qSSEP markers in prospective studies to predict good and poor outcomes in the cardiac arrest population in the setting of hypothermia could advance care in cardiac arrest. It has the prospect to guide allocation of health care resources and reduce self-fulfilling prophecy.

Effects of whole body skin cooling on human cognitive processing: a study using SEPs and ERPs

The present study investigated the effect of whole body skin cooling on somatosensory ascending processing by utilizing somatosensory-evoked potentials (SEPs) and motor execution, as well as inhibitory processing by event-related potentials (ERPs). Fourteen healthy participants wearing a water-perfused suit performed two sessions (sessions 1 and 2) consisting of SEPs and ERPs with somatosensory Go/No-go paradigms under two conditions (cold stress and control) on different days. In session 2, under the cold stress condition, whole body skin cooling was achieved by circulating 20°C water through the suit for 40 min, whereas 34°C water was perfused in the other sessions. The mean skin temperature decreased from 35.0 ± 0.5°C (session 1) to 30.4 ± 0.9°C (session 2) during whole body skin cooling, but the internal temperature was maintained. Whole body skin cooling delayed the peak latencies of N20, P25, and P45 components at C4' of SEPs (all: P < 0.05). Moreover, the peak latencies of P14, N18, and P22 components at Fz of SEPs and the Go-P300 component of ERPs were delayed (all: P < 0.05). In contrast, the peak amplitudes of all individual components of SEPs as well as N140 and P300 of ERPs remained unchanged. These results suggest that passive whole body skin cooling delays neural activities on somatosensory processing and higher cognitive function.

Real-time quantitative monitoring of cerebral blood flow by laser speckle contrast imaging after cardiac arrest with targeted temperature management

Brain injury is the main cause of mortality and morbidity after cardiac arrest (CA). Changes in cerebral blood flow (CBF) after reperfusion are associated with brain injury and recovery. To characterize the relative CBF (rCBF) after CA, 14 rats underwent 7 min asphyxia-CA and were randomly treated with 6 h post-resuscitation normothermic (36.5-37.5℃) or hypothermic- (32-34℃) targeted temperature management (TTM) (N = 7). rCBF was monitored by a laser speckle contrast imaging (LSCI) technique. Brain recovery was evaluated by neurologic deficit score (NDS) and quantitative EEG - information quantity (qEEG-IQ). There were regional differences in rCBF among veins of distinct cerebral areas and heterogeneous responses among the three components of the vascular system. Hypothermia immediately following return of spontaneous circulation led to a longer hyperemia duration (19.7 ± 1.8 vs. 12.7 ± 0.8 min, p < 0.01), a lower rCBF (0.73 ± 0.01 vs. 0.79 ± 0.01; p < 0.001) at the hypoperfusion phase, a better NDS (median [25th-75th], 74 [61-77] vs. 49 [40-77], p < 0.01), and a higher qEEG-IQ (0.94 ± 0.02 vs. 0.77 ± 0.02, p < 0.001) compared with normothermic TTM. High resolution LSCI technique demonstrated hypothermic TTM extends hyperemia duration, delays onset of hypoperfusion phase and lowered rCBF, which is associated with early restoration of electrophysiological recovery and improved functional outcome after CA.

Isoflurane but Not Halothane Prevents and Reverses Helpless Behavior: A Role for EEG Burst Suppression?

BACKGROUND

The volatile anesthetic isoflurane may exert a rapid and long-lasting antidepressant effect in patients with medication-resistant depression. The mechanism underlying the putative therapeutic actions of the anesthetic have been attributed to its ability to elicit cortical burst suppression, a distinct EEG pattern with features resembling the characteristic changes that occur following electroconvulsive therapy. It is currently unknown whether the antidepressant actions of isoflurane are shared by anesthetics that do not elicit cortical burst suppression.

METHODS

In vivo electrophysiological techniques were used to determine the effects of isoflurane and halothane, 2 structurally unrelated volatile anesthetics, on cortical EEG. The effects of anesthesia with either halothane or isoflurane were also compared on stress-induced learned helplessness behavior in rats and mice.

RESULTS

Isoflurane, but not halothane, anesthesia elicited a dose-dependent cortical burst suppression EEG in rats and mice. Two hours of isoflurane, but not halothane, anesthesia reduced the incidence of learned helplessness in rats evaluated 2 weeks following exposure. In mice exhibiting a learned helplessness phenotype, a 1-hour exposure to isoflurane, but not halothane, reversed escape failures 24 hours following burst suppression anesthesia.

CONCLUSIONS

These results are consistent with a role for cortical burst suppression in mediating the antidepressant effects of isoflurane. They provide rationale for additional mechanistic studies in relevant animal models as well as a properly controlled clinical evaluation of the therapeutic benefits associated with isoflurane anesthesia in major depressive disorder.

Multimodel quantitative analysis of somatosensory evoked potentials after cardiac arrest with graded hypothermia

Cardiac arrest (CA) is one of the most prominent causes of morbidity and mortality in adults. Therapeutic hypothermia (TH) is a recommended treatment to improve survival and functional outcome following CA, however, it is unclear what degree of TH is most beneficial. It has been suggested that TH of 33°C provides no survival or outcome benefits over TH of 36°C. Additionally, there is a lack of verified objective quantitative prognostic tools for comatose CA patients under TH. In this study, we calculated three quantitative markers of somatosensory evoked potentials (SSEP) to examine their potential to track recovery in the early period following CA under graded TH. A total of 16 rats were randomly divided among 4 temperature groups (n=4/group): normothermia (N0, 36.5-37.5°C), hypothermia 1 (H1, 30-32°C), hypothermia 2 (H2, 32-34°C) and hypothermia 3 (H3, 34-36°C). All rats underwent a 15min baseline SSEP recording followed by 9min asphyxial-CA, resulting in severe cerebral injury, and immediate temperature management following resuscitation for 6 hours. SSEP recordings were maintained in 15 min intervals from 30min-4hrs after resuscitation. The N10 amplitude, N10 latency and quantitative SSEP phase space area (qSSEP-PSA) were calculated for the early recovery period and normalized to their respective baselines. Functional recovery was determined by the neurological deficit scale (NDS). N10 amplitude was significantly larger in H1, H2 and H3 compared to N0. N10 latency was significantly longer in H1 than all temperature groups and all hypothermia groups had significantly longer latencies than N0. qSSEP-PSA had significantly better recovery in H1 and H2 than N0. Animals with good outcome (72hr NDS>50) had better recovery of all markers. N10 amplitude was significantly correlated with N10 latency and qSSEP-PSA. The results importantly demonstrate that quantified SSEPs have the potential to objectively track recovery following CA with graded TH.

Neonatal somatosensory evoked potentials persist during hypothermia

AIM

Treatment with therapeutic hypothermia has challenged the use of amplitude-integrated electroencephalography in predicting outcomes after perinatal asphyxia. In this study, we assessed the feasibility and gain of somatosensory evoked potentials (SEP) during hypothermia.

METHODS

This retrospective study comprised neonates from 35 + 6 to 42 + 2 gestational weeks and treated for asphyxia or hypoxic-ischaemic encephalopathy at Helsinki University Hospital between 14 February 2007 and 23 December 2009. This period was partly before the introduction of routine therapeutic hypothermia, which enabled us to include normothermic neonates who would these days receive hypothermia treatment. We analysed SEPs from 47 asphyxiated neonates and compared the results between 23 normothermic and 24 hypothermic neonates.

RESULTS

Our data showed that hypothermia led to SEP latencies lengthening by a few milliseconds, but the essential gain for predicting outcomes by SEPs was preserved during hypothermia. Of the 24 hypothermic neonates, bilaterally absent SEPs were associated with poor outcome in 2/2 neonates, normal SEPs were associated with good outcomes in 13/15 neonates and 5/7 neonates with unilaterally absent or grossly delayed SEPs had a poor outcome.

CONCLUSION

Our findings indicated that SEPs were a reliable tool for evaluating the somatosensory system in asphyxiated neonates in both normothermic and hypothermic conditions.

Evoked potentials recorded during routine EEG predict outcome after perinatal asphyxia

OBJECTIVE

To evaluate the added value of somatosensory (SEPs) and visual evoked potentials (VEPs) recorded simultaneously with routine EEG in early outcome prediction of newborns with hypoxic-ischemic encephalopathy under modern intensive care.

METHODS

We simultaneously recorded multichannel EEG, median nerve SEPs, and flash VEPs during the first few postnatal days in 50 term newborns with hypoxic-ischemic encephalopathy. EEG background was scored into five grades and the worst two grades were considered to indicate poor cerebral recovery. Evoked potentials were classified as absent or present. Clinical outcome was determined from the medical records at a median age of 21months. Unfavorable outcome included cerebral palsy, severe mental retardation, severe epilepsy, or death.

RESULTS

The accuracy of outcome prediction was 98% with SEPs compared to 90% with EEG. EEG alone always predicted unfavorable outcome when it was inactive (n=9), and favorable outcome when it was normal or only mildly abnormal (n=17). However, newborns with moderate or severe EEG background abnormality could have either favorable or unfavorable outcome, which was correctly predicted by SEP in all but one newborn (accuracy in this subgroup 96%). Absent VEPs were always associated with an inactive EEG, and an unfavorable outcome. However, presence of VEPs did not guarantee a favorable outcome.

CONCLUSIONS

SEPs accurately predict clinical outcomes in newborns with hypoxic-ischemic encephalopathy and improve the EEG-based prediction particularly in those newborns with severely or moderately abnormal EEG findings.

SIGNIFICANCE

SEPs should be added to routine EEG recordings for early bedside assessment of newborns with hypoxic-ischemic encephalopathy.

Therapeutic hypothermia protocols

The application of targeted temperature management has become common practice in the neurocritical care setting. It is important to recognize the pathophysiologic mechanisms by which temperature control impacts acute neurologic injury, as well as the clinical limitations to its application. Nonetheless, when utilizing temperature modulation, an organized approach is required in order to avoid complications and minimize side-effects. The most common clinically relevant complications are related to the impact of cooling on hemodynamics and electrolytes. In both instances, the rate of complications is often related to the depth and rate of cooling or rewarming. Shivering is the most common side-effect of hypothermia and is best managed by adequate monitoring and stepwise administration of medications specifically targeting the shivering response. Due to the impact cooling can have upon pharmacokinetics of commonly used sedatives and analgesics, there can be significant delays in the return of the neurologic examination. As a result, early prognostication posthypothermia should be avoided.

Effects of aerobic exercise under different thermal conditions on human somatosensory processing

The present study aimed to investigate the effects of aerobic exercise on human somatosensory processing recorded by somatosensory evoked potentials (SEPs) under temperate [TEMP, 20°C and 40% relative humidity (RH)] and hot (HOT, 35°C and 30% RH) environments. Fifteen healthy subjects performed 4 × 15-min bouts of a moderate cycling exercise [mean power output: 156.5 ± 7.7 (SE) W], with a 10-min rest period and received a posterior tibial nerve stimulation at the left ankle before and after each exercise bout; SEPs were recorded in five sessions; 1st (pre), 2nd (post-1st exercise bout), 3rd (post-2nd exercise bout), 4th (post-3rd exercise bout), and 5th (post-4th exercise bout). The peak latencies and amplitudes of the P37, N50, P60, and N70 components at Cz were evaluated. The latencies of P37, N50, P60, and N70 were significantly shorter with the repetition of aerobic exercise, and these shortened latencies were significantly greater in the HOT condition than in the TEMP condition (P37: 3rd, P < 0.05, and 5th, P < 0.01; P60: 4th, P < 0.05, and 5th, P < 0.01; N70: 4th, P < 0.05, and 5th, P < 0.001). No significant differences were observed in the amplitudes of any SEP component under either thermal condition. These results suggest that the conduction velocity of the ascending somatosensory input was accelerated by increases in body temperature, and aerobic exercise did not alter the strength of neural activity in cortical somatosensory processing.

Effects of passive heat stress on human somatosensory processing

Herein, we investigated the effects of passive heat stress on human somatosensory processing recorded by somatosensory-evoked potentials (SEPs). Fifteen healthy subjects received a median nerve stimulation at the left wrist under two thermal conditions: Heat Stress and normothermic Time Control. The latencies and amplitudes of P14, N20, P25, N35, P45, and N60 at C4′ and P14, N18, P22, and N30 at Fz were evaluated. Under the Heat Stress condition, SEPs were recorded at normothermic baseline (1st), early in heat stress (2nd), when esophageal temperature had increased by ∼1.0°C (3rd) and ∼2.0°C (4th), and after heat stress (5th). In the Time Control condition, SEPs were measured at the same time intervals as those in the Heat Stress condition. The peak latencies and amplitudes of SEPs did not change early in heat stress. However, the latencies of P14, N20, and N60 at C4′ and P14, N18, and P22 at Fz were significantly shorter in the 4th session than in the 1st session. Furthermore, the peak amplitudes of P25 and N60 at C4′, and P22 and N30 at Fz decreased with increases in body temperature. On the other hand, under the Time Control condition, no significant differences were observed in the amplitudes or latencies of any component of SEPs. These results suggested that the conduction velocity of the ascending somatosensory input was accelerated by increases in body temperature, and hyperthermia impaired the neural activity of cortical somatosensory processing.

Electrophysiological Monitoring of Brain Injury and Recovery after Cardiac Arrest

Reliable prognostic methods for cerebral functional outcome of post cardiac-arrest (CA) patients are necessary, especially since therapeutic hypothermia (TH) as a standard treatment. Traditional neurophysiological prognostic indicators, such as clinical examination and chemical biomarkers, may result in indecisive outcome predictions and do not directly reflect neuronal activity, though they have remained the mainstay of clinical prognosis. The most recent advances in electrophysiological methods--electroencephalography (EEG) pattern, evoked potential (EP) and cellular electrophysiological measurement--were developed to complement these deficiencies, and will be examined in this review article. EEG pattern (reactivity and continuity) provides real-time and accurate information for early-stage (particularly in the first 24 h) hypoxic-ischemic (HI) brain injury patients with high sensitivity. However, the signal is easily affected by external stimuli, thus the measurements of EP should be combined with EEG background to validate the predicted neurologic functional result. Cellular electrophysiology, such as multi-unit activity (MUA) and local field potentials (LFP), has strong potential for improving prognostication and therapy by offering additional neurophysiologic information to understand the underlying mechanisms of therapeutic methods. Electrophysiology provides reliable and precise prognostication on both global and cellular levels secondary to cerebral injury in cardiac arrest patients treated with TH.

Effect of hypothermia on cortical and thalamic signals in anesthetized rats

Beneficial effects of hypothermia on subjects with neuro-pathologies have been well demonstrated in both animal studies and clinical trials. Although it is known that temperature significantly impacts neurological injuries, the underlying mechanism remains unclear. We studied the effect of temperature modulation on neural signals in the cortex and the thalamus in uninjured brains of anesthetized rats. Six rats were divided into a hypothermic (32 to 34 °C, n=3) and a hyperthermic group (38.5 to 39.5 °C, n=3). EEG, and extracellular signals from somatosensory cortex and the ventral posterolateral nucleus of thalamus were recorded at different temperature phases (normothermia (36.5 to 37.5 °C) and hypothermia or hyperthermia). During hypothermia, similar burst suppression (BS) patterns were observed in cortical and thalamic signals as in EEG, but thalamic activity was not completely under suppression when both EEG and cortical signals were electrically silent. In addition, our results showed that hypothermia significantly increased the burst suppression ratio (BSR) in EEG, cortical and thalamic signals by 3.42, 3.25, 7.29 times respectively (P<0.01), and prolonged the latency of neuronal response in cortex to median nerve stimulation from 9 ms to 16 ms (P<0.01). Furthermore, during normothermia, the correlation coefficient between thalamic and cortical signals was 0.35±0.02 while during hypothermia, it decreased to 0.16±0.03 with statistical significance (P<0.01). These results can potentially assist in better understanding the effects of hypothermia.

Effect of mild hypothermic cardiopulmonary bypass on the amplitude of somatosensory-evoked potentials

BACKGROUND

Several neurophysiological techniques are used to intraoperatively assess cerebral functioning during surgery and intensive care, but the introduction of hypothermia as a means of intraoperative neuroprotection has brought their reliability into question. The present study aimed to evaluate the effect of mild hypothermia on somatosensory-evoked potentials' (SSEPs) amplitude and latency in a cohort of cardiopulmonary bypass (CPB) patients as the temperature reached the steady-state.

MATERIALS AND METHODS

The amplitude and latency of 4 different SSEP signals--N9, N13, P14/N18 interpeak, and N20/P25--were evaluated retrospectively in 84 patients undergoing CPB during normothermic (36°C±0.43°C) and mild hypothermic (32°C±1.38°C) conditions. SSEPs were recorded in normothermia immediately after the induction of anesthesia and in hypothermia as the temperature reached its steady-state, specifically, when the nasopharyngeal temperature was equivalent to the rectal temperature (±0.5°C). A paired-samples t test was performed for each SSEP to test the differences in latencies and amplitudes between normothermic and hypothermic conditions.

RESULTS

Compared with normothermia, hypothermia not only significantly increased the latency of all SSEPs, N9 (P<0.001), N13 (P<0.001), P14/N18 (P<0.001), and N20/P25 (P<0.001), but also the amplitude of N9 (P<0.001) and N20/P25 (P<0.001).

CONCLUSIONS

The increased amplitude in particularly of cortical SSEPs (N20/P25), detected specifically during steady-state hypothermia, seems to support the clinical utility of this methodology in monitoring the brain function not only during cardiac surgery with CPB, but also in other settings like therapeutic hypothermia procedures in an intensive care unit.

Neuroanesthesiology update

We review topics pertinent to the perioperative care of patients with neurological disorders. Our review addresses topics not only in the anesthesiology literature, but also in basic neurosciences, critical care medicine, neurology, neurosurgery, radiology, and internal medicine literature. We include literature published or available online up through December 8, 2013. As our review is not able to include all manuscripts, we focus on recurring themes and unique and pivotal investigations. We address the broad topics of general neuroanesthesia, stroke, traumatic brain injury, anesthetic neurotoxicity, neuroprotection, pharmacology, physiology, and nervous system monitoring.

The effects of local and general hypothermia on temperature profiles of the central nervous system following spinal cord injury in rats

Local and general hypothermia are used to treat spinal cord injury (SCI), as well as other neurological traumas. While hypothermia is known to provide significant therapeutic benefits due to its neuroprotective nature, it is unclear how the treatment may affect healthy tissues or whether it may cause undesired temperature changes in areas of the body that are not the targets of treatment. We performed 2-hour moderate general hypothermia (32°C core) or local hypothermia (30°C spinal cord) on rats that had received either a moderate contusive SCI or laminectomy (control) while monitoring temperatures at three sites: the core, spinal cord, and cortex. First, we identified that injured rats that received general hypothermia exhibited larger temperature drops at the spinal cord (-3.65°C, 95% confidence intervals [CIs] -3.72, -3.58) and cortex (-3.64°C, CIs -3.73, -3.55) than uninjured rats (spinal cord: -3.17°C, CIs -3.24, -3.10; cortex: -3.26°C, CIs -3.34, -3.17). This was found due to elevated baseline temperatures in the injured group, which could be due to inflammation. Second, both general hypothermia and local hypothermia caused a significant reduction in the cortical temperature (-3.64°C and -1.18°C, respectively), although local hypothermia caused a significantly lower drop in cortical temperature than general hypothermia (p<0.001). Lastly, the rates of rewarming of the cord were not significantly different among the methods or injury groups that were tested; the mean rate of rewarming was 0.13±0.1°C/min. In conclusion, local hypothermia may be more suitable for longer durations of hypothermia treatment for SCI to reduce temperature changes in healthy tissues, including the cortex.

Robust discrimination between EEG responses to categories of environmental sounds in early coma

Humans can recognize categories of environmental sounds, including vocalizations produced by humans and animals and the sounds of man-made objects. Most neuroimaging investigations of environmental sound discrimination have studied subjects while consciously perceiving and often explicitly recognizing the stimuli. Consequently, it remains unclear to what extent auditory object processing occurs independently of task demands and consciousness. Studies in animal models have shown that environmental sound discrimination at a neural level persists even in anesthetized preparations, whereas data from anesthetized humans has thus far provided null results. Here, we studied comatose patients as a model of environmental sound discrimination capacities during unconsciousness. We included 19 comatose patients treated with therapeutic hypothermia (TH) during the first 2 days of coma, while recording nineteen-channel electroencephalography (EEG). At the level of each individual patient, we applied a decoding algorithm to quantify the differential EEG responses to human vs. animal vocalizations as well as to sounds of living vocalizations vs. man-made objects. Discrimination between vocalization types was accurate in 11 patients and discrimination between sounds from living and man-made sources in 10 patients. At the group level, the results were significant only for the comparison between vocalization types. These results lay the groundwork for disentangling truly preferential activations in response to auditory categories, and the contribution of awareness to auditory category discrimination.

The pathophysiological mechanisms of the onset of death through accidental hypothermia and the presentation of "The little match girl" case

Hypothermia and death caused by hypothermia may be found in a number of fiction works, mainly in novels. In the well-known story “The Little Match Girl” by Hans Christian Andersen, one can notice that the descriptions of the phenomena occurring before the girl’s death are in fact a literary presentation of the pathophysiological mechanisms of the onset of death through accidental hypothermia. This essay presents the medical aspects of the story written by Andersen.

Effect of hypothermia on the thalamocortical function in the rat model

Neuroprotective effects of hypothermia are well documented in many injuries of the central nervous system in animal models as well as clinical studies. However, the underlying mechanisms are not fully understood. An important yet unexplored background issue is the effect of hypothermic cooling on the regional functionality of the healthy CNS. In a pilot study with the rat model, we seek to characterize the effect of moderate bodily cooling on the thalamo-cortical (T-C) function. Multiunit activity (MUA) and local field potentials (LFPs) were recorded from the thalamus (VPL nucleus) and the somatosensory cortex (S1) for normothermic, mild hypothermic and mild hyperthermic conditions in healthy rats and the thalamo-cortical dynamics was characterized with Granger Causal Interaction (GCI). The GCI indicated that the thalamic driving of the cortical activity significantly increases in strength with bodily cooling and weakens with mild heating. These results could have important implications towards understanding of hypothermia.

Regional hypothermia inhibits spinal cord somatosensory-evoked potentials without neural damage in uninjured rats

Both the therapeutic effects of regional hypothermia (RH) and somatosensory-evoked potentials (SSEP) have been intensively studied; however, the in vivo relationship between the two remains unknown. The primary focus of the current study was to investigate the impact of RH on SSEP in uninjured rats, as well as the neural safety of RH on neuronal health. An epidural perfusion model was used to keep local temperature steady by adjusting perfusion speed at 30°C, 26°C, 22°C, and 18°C for 30 min, respectively. Total hypothermic duration lasted up to 3 h. Neural signals were recorded at the end of each hypothermic period, as well as before cooling and after spontaneous rewarming. In addition, the Basso, Beattie, and Bresnahan (BBB) Locomotor Rating Scale was used to evaluate the effects of RH pre- and post-operative, combined with hematoxylin and eosin (H&E) and Fluoro-Jade C (FJC) staining. The results showed a marked declining trend in SSEP amplitude, as well as a significant prolongation in latency only during profound hypothermia (18°C). The BBB scale remained consistent at 21 throughout the entire process, signifying that no motor function injury was caused by RH. In addition, H&E and FJC staining did not show obvious histological injury. These findings firmly support the conclusion that RH, specifically profound RH, inhibits spinal cord SSEP in both amplitude and latency without neural damage in uninjured rats.