Intraoperative Monitoring: Recent Advances in Motor Evoked Potentials.

Advances in electrophysiological monitoring have improved the ability of surgeons to make decisions and minimize the risks of complications during surgery and interventional procedures when the central nervous system (CNS) is at risk. Individual techniques have become important for identifying or mapping the location and pathway of critical neural structures. These techniques are also used to monitor the progress of procedures to augment surgical and physiologic management so as to reduce the risk of CNS injury. Advances in motor evoked potentials have facilitated mapping and monitoring of the motor tracts in newer, more complex procedures.

Intraoperative neurophysiological monitoring during spine surgery with total intravenous anesthesia or balanced anesthesia with 3% desflurane.

Total intravenous anesthesia (TIVA) with propofol and opioids is frequently utilized for spinal surgery when somatosensory evoked potentials (SSEPs) and transcranial motor evoked potentials (tcMEPs) are monitored. Many anesthesiologists would prefer to utilize low dose halogenated anesthetics (e.g. 1/2 MAC). We examined our recent experience using 3% desflurane or TIVA during spine surgery to determine the impact on propofol usage and on the evoked potential responses. After institutional review board approval we conducted a retrospective review of a 6 month period for adult spine patients who were monitored with SSEPs and tcMEPs. Cases were included for the study if anesthesia was conducted with propofol-opioid TIVA or 3% desflurane supplemented with propofol or opioid infusions as needed. We evaluated the propofol infusion rate, cortical amplitudes of the SSEPs (median nerve, posterior tibial nerve), amplitudes and stimulation voltage for eliciting the tcMEPs (adductor pollicis brevis, tibialis anterior) and the amplitude variability of the SSEP and tcMEP responses as assessed by the average percentage trial to trial change. Of the 156 spine cases included in the study, 95 had TIVA with propofol-opioid (TIVA) and 61 had 3% expired desflurane (INHAL). Three INHAL cases were excluded because the desflurane was eliminated because of inadequate responses and 26 cases (16 TIVA and 10 INHAL) were excluded due to significant changes during monitoring. Propofol infusion rates in the INHAL group were reduced from the TIVA group (average 115-45 µg/kg/min) (p<0.00001) with 21 cases where propofol was not used. No statistically significant differences in cortical SSEP or tcMEP amplitudes, tcMEP stimulation voltages nor in the average trial to trial amplitude variability were seen. The data from these cases indicates that 1/2 MAC (3%) desflurane can be used in conjunction with SSEP and tcMEP monitoring for some adult patients undergoing spine surgery. Further studies are needed to confirm the relative benefits versus negative effects of the use of desflurane and other halogenated agents for anesthesia during procedures on neurophysiological monitoring involving tcMEPs. Further studies are also needed to characterize which patients may or may not be candidates for supplementation such as those with neural dysfunction or who are opioid tolerant from chronic use.

Lidocaine infusion adjunct to total intravenous anesthesia reduces the total dose of propofol during intraoperative neurophysiological monitoring.

Total intravenous anesthesia (TIVA) with propofol and opioids is frequently utilized for spinal surgery where somatosensory evoked potentials (SSEP) and motor evoked potentials (tcMEP) are monitored. Lidocaine infusions can contribute to antinociception and unconsciousness, thus allowing for a reduction in the total dose of propofol. We examined our recent experience with lidocaine infusions to quantify this effect. After institutional review board approval, we conducted a retrospective review of propofol usage in propofol-opioid TIVA (with and without lidocaine) for spine cases monitored with SSEP and tcMEP over a 7 months period. The propofol infusion rate, cortical amplitudes of the SSEP (median nerve, posterior tibial nerve), amplitudes and stimulation voltage of the tcMEP (adductor pollicis brevis, tibialis anterior) were evaluated. The savings of propofol and sufentanil were estimated based on utilization in 50 milliliter (ml) bottles and 5 ml ampules, respectively. 129 cases were evaluated. Propofol infusion rates were reduced with lidocaine infusion from an average of 115-99 µg/kg/min (p = 0.00038) and sufentanil infusions from an average of 0.36-0.29 µg/kg/h (p = 0.0059). This reduction in propofol infusion was also seen when the cases were divided into anterior cervical, posterior cervical, or posterior thoraco-lumbar procedures. No significant differences in the cortical SSEP or tcMEP amplitudes or the tcMEP stimulation voltages used were observed. No complications were associated with the use of the lidocaine infusion. The total estimated drug savings included 104 50 ml bottles of propofol and 5 5 ml ampules of sufentanil. These cases indicate that a lidocaine infusion can be effectively utilized in spine surgery with SSEP and tcMEP monitoring as a means to reduce propofol and sufentanil usage without a negative effect on the monitoring.

Methohexital in total intravenous anesthesia during intraoperative neurophysiological monitoring.

Total intravenous anesthesia (TIVA) is usually recommended during spinal surgery when transcranial motor evoked potentials (tcMEPs) are used to monitor. A shortage of propofol has prompted a search for an alternative sedative-hypnotic agent. We explored the use of methohexital as an alternative. TIVA was provided for two adult patients having spinal surgery using an infusion of methohexital. TcMEPs and somatosensory evoked potentials were acquired to monitor neurological function and electroencephalogram was used to titrate the methohexital dose. Two cases are presented in which the anesthesia and monitoring that was provided were successful. These cases indicate that methohexital can be a suitable alternative to propofol in some patients.

Muscle relaxant use during intraoperative neurophysiologic monitoring.

Neuromuscular blocking agents have generally been avoided during intraoperative neurophysiological monitoring (IOM) where muscle responses to nerve stimulation or transcranial stimulation are monitored. However, a variety of studies and clinical experience indicate partial neuromuscular blockade is compatible with monitoring in some patients. This review presents these experiences after reviewing the currently used agents and the methods used to assess the blockade. A review was conducted of the published literature regarding neuromuscular blockade during IOM. A variety of articles have been published that give insight into the use of partial pharmacological paralysis during monitoring. Responses have been recorded from facial muscles, vocalis muscles, and peripheral nerve muscles from transcranial or neural stimulation with neuromuscular blockade measured in the muscle tested or in the thenar muscles from ulnar nerve stimulation. Preconditioning of the nervous system with tetanic or sensory stimulation has been used. In patients without neuromuscular pathology intraoperative monitoring using peripheral muscle responses from neural stimulation is possible with partial neuromuscular blockade. Monitoring of muscle responses from cranial nerve stimulation may require a higher degree of stimulation and less neuromuscular blockade. The role of tetanic or sensory conditioning of the nervous system is not fully characterized. The impact of neuromuscular pathology or the effect of partial blockade on monitoring muscle responses from spontaneous neural activity or mechanical nerve stimulation has not been described.

Neurophysiologic monitoring in neurosurgery.

This article focuses on the application of neurophysiologic monitoring in uniquely neurosurgical procedures. Neurophysiologic monitoring provides functional testing and mapping to identify neural structures. Once identified, the functionality of the central and peripheral nervous system areas at risk for neurosurgical injury can be monitored. It discusses the use of motor-evoked potentials, sensory evoked potentials, electromyography and electroencephalography to assess neurologic change.

American Society of Neurophysiologic Monitoring and American Society of Neuroimaging joint guidelines for transcranial doppler ultrasonic monitoring.

The American Society of Neurophysiologic Monitoring (ASNM) and American Society of Neuroimaging (ASN) Guidelines Committees formed a joint task force and developed guidelines to assist in the use of transcranial Doppler (TCD) monitoring in the surgical and intensive care settings. Specifically, these guidelines: (1) delineate the objectives of TCD monitoring; (2) characterize the responsibilities and behaviors of the sonographer during monitoring; (3) describe methodological and ethical issues uniquely relevant to monitoring. The ASNM and ASN strongly support the positions that (1) acquisition and interpretation of intraoperative TCD ultrasonograms be performed by qualified individuals, (2) service providers define their diagnostic criteria and develop on-going self-validation programs of these performance criteria in their practices. We agree with the guidelines of other professional societies regarding the technical and professional qualifications of individuals responsible for TCD signal acquisition and interpretation (Class III evidence, Type C recommendation). On the basis of current clinical literature and scientific evidence, TCD monitoring is an established monitoring modality for the: (1) assessment of cerebral vasomotor reactivity and autoregulation; (2) documentation of the circle of Willis functional status; (3) identification of cerebral hypo- and hyperperfusion, recanalization and re-occlusion; and (4) detection of cerebral emboli (Class II and III evidence, Type B recommendation).

Intraoperative autologous transfusion of hemolyzed blood.

During two cases of lumbar spine surgery with instrumentation, we used intraoperative autologous transfusion (IAT), resulting in hemolysis during collection and hemoglobinuria and coagulation abnormalities after transfusion. Hemolysis during IAT collection can lead to hemoglobinuria and binding of nitric oxide, leading to vasoconstriction. The literature suggests that stroma from damaged cells and contact of the blood with the IAT device can lead to coagulation abnormalities and other morbidities, including adult respiratory distress syndrome.

Multimodality monitoring of the central nervous system using motor-evoked potentials.

PURPOSE OF REVIEW: This review was conducted to examine the role of motor-evoked potential monitoring in spine and central nervous system surgery to determine whether other monitoring modalities such as the wake-up test or somatosensory-evoked potentials can be eliminated. RECENT FINDINGS: The current literature suggests that motor-evoked potential, despite some advantages, still requires that other monitoring modalities such as somatosensory-evoked potentials or electromyography be used to provide optimal monitoring. SUMMARY: The literature supports the use of multimodality monitoring using all of the electrophysiological techniques that can provide intraoperative information about the neural structures at risk during the surgery.

Electrophysiologic monitoring during surgery to repair the thoraco-abdominal aorta.

Prevention of paraplegia during the repair of thoraco-abdominal aortic aneurysms and dissections present a substantial challenge to the operative team. The value of intraoperative electrophysiological monitoring (IOM) is to identify spinal cord ischemia that occurs during the procedure and guide the intraoperative management to reduce the risks of paralysis. The usefulness of IOM techniques requires an understanding of spinal cord blood flow and the spinal cord physiology, the surgical technique and their interaction. This paper will integrate these factors to review the laboratory and clinical experience with somatosensory evoked responses (SSEP) and motor evoked potentials (MEP) during thoraco-abdominal aorta surgery.

Electrophysiologic monitoring in neurosurgery.

Electrophysiologic techniques have become common in the neurosurgical operating room. This article reviews the methods used for mapping neural structures or monitoring during surgery. Mapping methods allow identification of target structures for surgery, or for identifying structures to allow avoidance or plot safe pathways to deeper structures. Monitoring methods allow for surgery on nearby structures to warn of encroachment, thereby reducing unwanted injury.

Using EEG to monitor anesthesia drug effects during surgery.

The use of processed electroencephalography (EEG) using a simple frontal lead system has been made available for assessing the impact of anesthetic medications during surgery. This review discusses the basic principles behind these devices. The foundations of anesthesia monitoring rest on the observations of Guedel with ether that the depth of anesthesia relates to the cortical, brainstem and spinal effects of the anesthetic agents. Anesthesiologists strive to have a patient who is immobile, is unconscious, is hemodynamically stable and who has no intraoperative awareness or recall. These anesthetic management principles apply today, despite the absence of ether from the available anesthetic medications. The use of the EEG as a supplement to the usual monitoring techniques rests on the observation that anesthetic medications all alter the synaptic function which produces the EEG. Frontal EEG can be viewed as a surrogate for the drug effects on the entire central nervous system (CNS). Using mathematical processing techniques, commercial EEG devices create an index usually between 0 and 100 to characterize this drug effect. Critical aspects of memory formation occur in the frontal lobes making EEG monitoring in this area a possible method to assess risk of recall. Integration of processed EEG monitoring into anesthetic management is evolving and its ability to characterize all of the anesthetic effects on the CNS (in particular awareness and recall) and improve decision making is under study.

Monitoring of the brain and spinal cord.

IOM has become commonly used by many surgeons to enhance their intraoperative decision making and reduce the morbidity and mortality of selected procedures. The ability to perform these tests rests on the anesthesiologist's ability to provide the patient with an anesthetic plan that provides comfort and monitoring. When events occur, the anesthesiologist's knowledge and ability to manipulate the patient's physiologic condition become integral to the decision making. A good understanding of the neural anatomy, impact of physiology, and anesthetic medications can allow effective IOM and good team decision making when changes in IOM occur.

Implementing a simpler approach to mission-based planning in a medical school.

Changes in the education, research, and health care environments have had a major impact on the way in which medical schools fulfill their missions, and mission-based management approaches have been suggested to link the financial information of mission costs and revenues with measures of mission activity and productivity. The authors describe a simpler system, termed Mission-Aligned Planning (MAP), and its development and implementation, during fiscal years 2002 and 2003, at the School of Medicine at the University of Texas Health Science Center at San Antonio, Texas. The MAP system merges financial measures and activity measures to allow a broad understanding of the mission activities, to facilitate strategic planning at the school and departmental levels. During the two fiscal years mentioned above, faculty of the school of medicine reported their annual hours spent in the four missions of teaching, research, clinical care, and administration and service in a survey designed by the faculty. A financial profit or loss in each mission was determined for each department by allocation of all departmental expenses and revenues to each mission. Faculty expenses (and related expenses) were allocated to the missions based on the percentage of faculty effort in each mission. This information was correlated with objective measures of mission activities. The assessment of activity allowed a better understanding of the real costs of mission activities by linking salary costs, assumed to be related to faculty time, to the missions. This was a basis for strategic planning and for allocation of institutional resources.

Electrophysiologic monitoring during surgery to repair the thoracoabdominal aorta.

The repair of aneurysms and dissections that involve the thoracoabdominal aorta represent a major stress to the cardiovascular surgery team because of the feared complication of paraplegia. Here, the etiology of this complication is explained through a description of the relevant surgical anatomy and characteristics of hemodynamic support. In addition, recent advances in the neurophysiologic assessment of the descending motor pathways and their application to perioperative monitoring are discussed.