Spinal cord stimulation evoked potentials during thoracoabdominal aortic aneurysm surgery

Bypass Techniques for Descending Thoracic Aortic Surgery

The conduct of partial left heart bypass or partial car diopulmonary bypass (CPB) during surgery involving the descending thoracic aorta or thoracoabdominal aorta is one of the most unappreciated and misunder stood extracorporeal circulation procedures in cardio vascular surgery. It is different from conventional CPB, and although some uninitiated practitioners consider it simpler, it is in fact more complicated than conven tional CPB and involves different concepts. It requires expertise and skill in regulating the flow, pressure, and oxygenation of blood going to both the proximal and distal parts of the body and management of the special bypass or shunt procedures used, specialized monitor ing, and knowledge about the protection and preserva tion of organs both proximal and distal to the aortic clamping. It demands exquisite communication and un derstanding of the unique problems faced by the sur geon, anesthesiologist, and perfusionist.

Intraoperative neurophysiological monitoring of the spinal cord during spinal cord and spine surgery: a review focus on the corticospinal tracts

Recent advances in technology and the refinement of neurophysiological methodologies are significantly changing intraoperative neurophysiological monitoring (IOM) of the spinal cord. This review will summarize the latest achievements in the monitoring of the spinal cord during spine and spinal cord surgeries. This overview is based on an extensive review of the literature and the authors' personal experience. Landmark articles and neurophysiological techniques have been briefly reported to contextualize the development of new techniques. This background is extended to describe the methodological approach to intraoperatively elicit and record spinal D wave and muscle motor evoked potentials (muscle MEPs). The clinical application of spinal D wave and muscle MEP recordings is critically reviewed (especially in the field of Neurosurgery) and new developments such as mapping of the dorsal columns and the corticospinal tracts are presented. In the past decade, motor evoked potential recording following transcranial electrical stimulation has emerged as a reliable technique to intraoperatively assess the functional integrity of the motor pathways. Criteria based on the absence/presence of potentials, their morphology and threshold-related parameters have been proposed for muscle MEPs. While the debate remains open, it appears that different criteria may be applied for different procedures according to the expected surgery-related morbidity and the ultimate goal of the surgeon (e.g. total tumor removal versus complete absence of transitory or permanent neurological deficits). On the other hand, D wave changes--when recordable--have proven to be the strongest predictors of maintained corticospinal tract integrity (and therefore, of motor function/recovery). Combining the use of muscle MEPs with D wave recordings provides the most comprehensive approach for assessing the functional integrity of the spinal cord motor tracts during surgery for intramedullary spinal cord tumors. However, muscle MEPs may suffice to assess motor pathways during other spinal procedures and in cases where the pathophysiology of spinal cord injury is purely ischemic. Finally, while MEPs are now considered the gold standard for monitoring the motor pathways, SEPs continue to retain value as they provide specificity for assessing the integrity of the dorsal column. However, we believe SEPs should not be used exclusively--or as an alternative to motor evoked potentials--during spine surgery, but rather as a complementary method in combination with MEPs. For intramedullary spinal tumor resection, SEPs should not be used exclusively without MEPs.

Intraoperative monitoring during vascular surgery

The principal objectives of intraoperative monitoring are to improve perioperative outcome, facilitate surgery and reduce adverse events, using continuously collected data of cardiopulmonary,neurologic and metabolic function to guide pharmacologic and physiologic therapy. Although sophisticated and reliable apparatus may be used to collect these data they are useless, or even harmful, without proper interpretation. This article provides a comprehensive overview of recent publications on the history,philosophy, and semantics of monitoring.

Spinal cord monitoring: somatosensory- and motor-evoked potentials

Monitoring myogenic motor EPs after transcranial electrical stimulation is effective in detecting spinal cord ischemia. During thoracoabdominal aortic aneurysm surgery, this technique is sufficiently rapid to allow timely interventions aimed at correcting ischemic conditions and preserving spinal cord blood flow. If strategies are applied to protect the spinal cord during thoracoabdominal aortic aneurysm repair (e.g., distal bypass, cerebrospinal fluid drainage, reattachment of segmental arteries), motor EP monitoring should be included in this protocol to improve neurologic outcome further. Although SSEPs provide information regarding the adequacy of spinal cord blood flow, monitoring SSEPs during thoracoabdominal aortic aneurysm repair has serious limitations. The response time is too slow to be of practical use. SSEPs also do not provide information regarding anterior horn motor function and supply, whereas the motor neurons in the anterior horn are most likely to sustain ischemic injury.

Transtracheal electrical stimulation of the spinal cord for intraoperative monitoring of the motor pathway

Because of the suppressant effects of anesthetic drugs and muscle relaxants on motor responses elicited by either magnetic or electrical transcranial stimulation, intraoperative monitoring of the motor system, and especially monitoring of lower limb function, presents many difficulties. The upper part of the spinal cord was stimulated in 14 anesthetized and relaxed dogs with a cathode attached to the intratracheal tube and an anode fixed above the upper cervical spinous processes. Action potentials evoked by single and serial stimuli were recorded from the exposed right femoral nerve and quadriceps muscle Averaging was necessary for serial stimulations. Reproducible early and late responses to both single and serial stimulations were recorded during regular anesthesia. The origin of the different responses is discussed. Transtracheal stimulation of the spinal cord is easy to perform and the responses recorded from the peripheral nerve or limb muscle are well reproducible in regular anesthesia. The method seems to be appropriate for intraoperative monitoring of the thoracolumbar spine.

Spinal cord monitoring

Over the past two decades, intraoperative spinal cord monitoring has matured into a widely used clinical tool. It is used when the spinal cord is at risk for damage during a surgical procedure. This includes orthopedic, neurosurgical, and certain cardiothoracic procedures. Both somatosensory evoked potential (SEP) and direct motor pathway stimulation techniques are available. The SEP techniques are used most widely, are generally accepted, and have been shown to reduce surgical morbidity. A large multicenter study has shown that SEP monitoring reduces postoperative paraplegia by more than 50-60%. Techniques and literature on clinical applications are reviewed in this report.

Identification and selective perfusion of the spinal cord-feeding arteries by intrathecal pO2 monitoring for spinal cord protection

OBJECTIVES

to study whether spinal cord-feeding arteries could be identified by the changes in the intrathecal pO2 (I-pO2), and to examine whether selective perfusion of feeding arteries identified by this method could protect the spinal cord against ischaemia.

DESIGN

controlled animal experiments.

MATERIALS AND METHODS

in experiment 1, using 16 mongrel dogs, 18 segmental arteries were cannulated through which oxygenated saline was injected and the I-pO2 change was observed. When the I-pO2 increase was more than 0.5 mmHg, the artery was considered to be a spinal cord-feeding artery. In experiment 2, involving 10 dogs, the segmental arteries identified as spinal cord-feeding arteries were perfused with arterial blood and the recovery of I-pO2 and evoked spinal potentials (ESP) was examined.

RESULTS

of 208 segmental arteries examined, 176 (84.6%) arteries were correctly judged and 32 (15.4%) were not. It was observed that the I-pO2 recovered from 13.9 to 30.5 mmHg and the ESP recovered from 20.9% and 8.2% to 66.5% and 44.7% of each control for the first negative (N1) and second negative (N2) components, respectively.

CONCLUSION

spinal cord-feeding arteries were successfully identified using the I-pO2 monitoring method. Perfusion of these arteries with arterial blood improved the I-pO2 and ESP, which were significantly depressed by ischaemia.

Spinal cord complications of thoracoabdominal aneurysm surgery

BACKGROUND

Although rare, paralysis secondary to spinal cord ischaemia after aortic aneurysm surgery is a devastating complication. Many papers have been published on this topic but without a clear consensus on the best way of minimizing the problem. Recent articles have included advanced pharmacological approaches and the literature has been reviewed in light of these.

METHODS

Relevant papers were identified by an extensive text word search of the Medline database and a review of quoted articles.

RESULTS

Spinal cord complications are commoner after the repair of Crawford type II aneurysms than less extensive aneurysms. The presence of dissection, rupture and prolonged clamp times are associated with an increased incidence. About a quarter of all cord problems develop over 24 h after surgery and this may be due to a reperfusion type injury, although the exact mechanisms are by no means clear.

CONCLUSION

A combination of rapid surgery, left heart bypass for the repair of more extensive aneurysms, free spinal drainage and the avoidance of postoperative hypoxia and hypotension help to minimize spinal cord ischaemia. No pharmacological agent has yet been shown conclusively to improve outcome in the clinical setting.

Clinical markers in CSF for determining neurologic deficits after thoracoabdominal aortic aneurysm repairs

BACKGROUND

Spinal cord ischemia is a major cause of morbidity and mortality after thoracoabdominal aortic aneurysm operations. The incidence of paraplegia is high even in experienced institutions.

METHODS

We investigated whether neurotransmitter excitotoxicity is associated with neurologic deficits after thoracoabdominal aortic aneurysm operations. We hypothesized that patients with spinal cord injury would manifest elevated levels of excitatory amino acids in their cerebrospinal fluid. Sixteen patients undergoing thoracoabdominal aortic aneurysm operations had cerebrospinal fluid drawn through lumbar spinal drains preoperatively, intraoperatively, and postoperatively. Excitatory amino acid levels (glutamate, aspartate, glycine) were measured using high-performance liquid chromatography. Excitatory amino acid levels were compared in patients who exhibited no neurologic deficits postoperatively (group I; n = 12) with patients who had clinically evident lower extremity and cerebral neurologic deficits (group II; n = 4).

RESULTS

Significant elevations in glutamate and aspartate levels from baseline (p < 0.05) were limited to group II. Excitatory amino acid levels in group II were significantly elevated (p < 0.05) compared with those observed in group I. Glutamate levels were especially increased during aortic cross-clamping and late reperfusion, whereas aspartate levels were increased only during late reperfusion.

CONCLUSIONS

These data suggest that neurotransmitter excitotoxicity plays a significant role in central nervous system injury.

Monitoring for spinal cord ischemia by use of the evoked spinal cord potentials during aortic aneurysm surgery

PURPOSE

This clinical study was to evaluate changes of evoked spinal cord potentials (ESCPs) elicited by direct spinal cord stimulation and to determine their relation to spinal cord ischemia during aortic aneurysm surgery.

METHODS

We monitored descending ESCPs from the thoracic spinal cord and lumbar enlargement after cervical spinal cord stimulation (thoracic descending ESCP and lumbar descending ESCP), and segmental ESCP at lumbar enlargement elicited by bilateral tibial nerve stimulation in 22 aortic aneurysm surgical operations.

RESULTS

ESCP changes were classified into three types: (1) decrease of amplitude in lumbar descending ESCP and segmental ESCP; (2) decrease of amplitude in segmental ESCP alone; (3) decrease of amplitude in all ESCPs. The late negative waves of both lumbar descending ESCP and segmental ESCP were more sensitive than other components of ESCPs. Postoperative paraplegia occurred in the two cases that showed persistent diminution of amplitude in the late negative waves. Segmental ESCP was less reliable than lumbar descending ESCP, because it depended entirely on the adequate perfusion of the lower extremities.

CONCLUSIONS

Lumbar descending ESCP was the best method for the spinal cord ischemia during aortic aneurysm surgery. Spinal cord ischemia could be detected by diminution in the amplitude of the late negative wave of lumbar descending ESCP. The recovery amplitude of the late negative wave after declamping correlated with the neurologic outcome.

Moderate hypothermia, with partial bypass and segmental sequential repair for thoracoabdominal aortic aneurysm

PURPOSE

Ischemic injury to the spinal cord, kidneys, and viscera occurs in a significant number of patients undergoing surgical repair of thoracoabdominal aortic aneurysms. Partial bypass has been used to perfuse the arterial system distal to the cross-clamp, but the primary determinant of ischemic morbidity remains the duration of aortic cross-clamping. Hypothermia may favorably affect outcome during these procedures, but moderate or deep hypothermia has traditionally required full cardiopulmonary bypass with cardiac arrest.

METHODS

In a series of patients undergoing thoracoabdominal (n = 14) or thoracic (n = 4) aneurysm repair, we used moderate hypothermia (30 degrees C) and partial bypass (aortofemoral or atriofemoral) while maintaining an intrinsic cardiac rhythm. Body temperature was controlled with a heat exchanger in the bypass circuit, which allowed for rapid cooling and rewarming. In addition to hypothermia and bypass, a segmental sequential surgical repair was used to minimize the duration of ischemia to any given vascular bed.

RESULTS

All patients survived the surgical procedure, and 16 patients survived until discharge from the hospital. None of the 18 patients had paraplegia or significant renal dysfunction. The only complication related to hypothermia was atrial fibrillation, which occurred in three patients and was amenable to therapeutic measures.

CONCLUSIONS

We conclude that moderate hypothermia, partial bypass, and segmental sequential repair may reduce ischemic injury. This combination of adjuncts was not associated with significant complications in this series of patients.

Effects of cerebrospinal fluid drainage in patients undergoing thoracic and thoracoabdominal aortic surgery

Cerebrospinal fluid (CSF) drainage has been reported to protect the spinal cord during surgical procedures requiring thoracic aortic cross-clamping. In 1986, CSF pressure monitoring and drainage was begun in an attempt to reduce the incidence of paraplegia associated with surgical repair of descending thoracic and thoracoabdominal aortic aneurysms (TAAA). These Group II patients (n = 50) were retrospectively compared to Group I patients (n = 49) who had undergone similar surgical procedures in the previous 3 years before CSF monitoring was introduced into this practice. Group II patients had intrathecal catheters placed for monitoring of CSF pressure and drainage of CSF to maintain the pressure < or = 15 mmHg. Seven patients (four in Group I, three in Group II) died before recovering from the anesthetic. Of the 47 patients in Group II who survived, none had clinically apparent complications such as an epidural hematoma or meningitis from the intrathecal catheter. The mean aortic cross-clamp time was 58.6 +/- 30.5 minutes (mean +/- SD) in Group I versus 65 +/- 42.6 minutes in Group II. Twenty-three patients in Group I and 16 patients in Group II had a shunt to the distal aorta. To maintain a CSF pressure of < or = 15 mmHg in Group II, an average of 46.9 +/- 6.9 mL of CSF was withdrawn. Of the 45 survivors in Group I, 4 developed a spinal cord deficit; the number of patients with spinal cord deficit in Group II was 4 out of the 47 who survived.(ABSTRACT TRUNCATED AT 250 WORDS)