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Ann Thorac Surg 1999;67:1947-1952
© 1999 The Society of Thoracic Surgeons

Use of somatosensory evoked potentials for thoracic and thoracoabdominal aortic resections

^a Department of Cardiothoracic Surgery, Mount Sinai Medical Center, New York, New York, USA

Address reprint requests to Dr Galla, Department of Cardiothoracic Surgery, Mount Sinai Medical Center, One Gustave L. Levy Place, Box 1028, New York, NY 10029

Presented at the Aortic Surgery Symposium VI, April 30–May 1, 1998, New York, NY.

	Abstract

Top Abstract Introduction Patients and methods Results Discussion References

 
Background. Despite tremendous development in surgical and anesthetic techniques, resection of the thoracic and thoracoabdominal segments of the aorta remain associated with the risk of paralysis. Routine use of somatosensory-evoked potential (SEP) monitoring in patients undergoing surgery of the thoracic aorta has become a standard intra- and postoperative procedure at our institution since its first use in 1993.

Methods. One hundred forty nine (149) thoracic aortic operations were performed during January 1993 through January 1998 using SEP-directed serial sacrifice of paired intercostal arteries. Full, partial, or no cardiovascular bypass was variably used, dictated by anatomy; 49 patients required deep hypothermic circulatory arrest (DHCA). Patients were monitored during both the intraoperative procedure as well for the post-anesthesia period until neurologic stability and/or ability to reproducibly demonstrate lower extremity neurologic competency was established. Postoperative neurologic function was compared to ischemic intervals, extent of aortic resection, number of intercostal arteries sacrificed, type of perfusion, and underlying aortic pathology.

Results. Overall mortality in the group was 13 patients (8.7%), with no one cause predominating. Nine patients sustained permanent paraplegia, only 1 of whom lost SEPs during the procedure. Abnormal SEPs were seen in 19 patients, 14 of whom had normal neurologic function after awakening. Three of 19 (15.8%) developed late paraplegia that resolved with medical therapy. Eleven patients (7.4%) developed cerebrovascular accidents (CVA), with the majority (8) appearing in the group undergoing DHCA. The risk of CVA was significantly higher in DHCA patients (p < 0.01) than other patients. No patient with CVA had abnormal SEPs; 4 DHCA patients developed abnormal SEPs, 1 with permanent paralysis.

Conclusions. The routine use of SEP monitoring during thoracic and thoracoabdominal aortic surgery as well as during the postoperative period may be useful in decreasing the observed incidence of paraplegic events associated with these procedures.

	Introduction

Top Abstract Introduction Patients and methods Results Discussion References

 
Surgical procedures involving the descending thoracic or thoracoabdominal aorta continue to generate considerable consternation among the general medical community as well as the lay population. The chief concern of the majority of patients is not death or pain but rather continues to be the risk of paralysis, despite continued progress in lessening the incidence of this dreaded complication. Many factors have undoubtedly contributed to the reduction of cord injury, including improvements in anesthetic techniques and postoperative care. As ischemia is universally acknowledged as the etiology of spinal cord injury, intraoperative techniques are directed both toward reducing the severity and duration of cord ischemia and increasing the tolerance of the cord to reduced oxygen tensions. Among the latter, methods include retrograde perfusion of the distal aorta [1–3]; either mild [4] or profound [5] hypothermia; pharmacological therapies [6–8]; and identification [9] and/or preservation of critical intercostal arteries (CIAs) [10]. Additionally, improvements in monitoring of cord function allow early recognition of cord compromise and institution of appropriate therapies [11].

The concept of CIAs has been firmly rooted in aortic operation practice. Recently, question regarding the validity of this belief has been voiced [11] and previously published investigations in our laboratories suggest that retention of intercostal and lumbar arteries in the non-perfused aortic segment may contribute to cord ischemia [12].

The approach toward the thoracic aortic patient undergoing operation at the Mount Sinai Hospital is a combination of many therapies, and has resulted in a progressively decreasing incidence of neurological embarrassment. The aggressive use of SEP monitoring in the operative theater, the intensive care unit, and as a diagnostic modality for confirmation of delayed neurological events is essential to this approach, as well as the use of SEP guidance to observe for potential neurological sequelae during the careful sequential sacrificing of intercostal arteries. The results of our experience using these techniques are presented in this communication.

	Patients and methods

Top Abstract Introduction Patients and methods Results Discussion References

 
Over the 5-year interval from January 1993 through January 1998, 145 consecutive patients underwent a total of 149 descending aortic reconstructions using the methods outlined below. Ninety-two of these patients were male with an average age of 62.9 years. The remaining 57 female patients ranged in age from 13 (the youngest of all 149 patients) to 81 for an average of 63.3 years.

The etiologies of the patients’ diseases are listed in Table 1. A majority of the patients underwent surgery for aneurysms of atherosclerotic origin (57%) with dissection (31%) the second most frequent pathology. Other aortic pathologies (Marfan’s syndrome, degenerative disease, coarctation, mycotic disease, etc) were represented by only a few cases of each type.

A cerebrospinal fluid (CSF) drainage catheter was placed at the termination of the procedure after full reversal of administered heparin had been documented, and used in the postoperative period to drain CSF to maintain pressures of < 10 mm Hg. Partial bypass as well as DHCA patients were drained in this fashion; more recently, CSF catheters have been placed before operation if DHCA is not to be used.

Somatosensory-evoked potential monitoring was performed as previously described [13]. Briefly, a Cadwell Quantum 84 SEP generator/stimulator (Cadwell Labs, Kennewick, WA) was used to generate and record stimuli and signals delivered at the malleoli bilaterally and transmitted to the scalp. Scalp recordings were obtained via skin needle electrodes positioned in multiple locations, and represented averaged signals of 200 potentials cycled alternately from left to right lower extremity. SEP recordings, in both digital and analog presentations of waveform latency and amplitude, were recorded, beginning after induction of anesthesia, and continuing throughout the procedure and into the postoperative period until the patient was awake and responsive to command. If demonstrable, repeatable baseline neurological function was obtained after awakening from anesthesia, SEP monitoring was discontinued; otherwise, monitoring was continued until stable neurological function could reproducibly be documented. Hourly recordings were reviewed, and deviations from baseline measurements were aggressively treated: mild hypertension was induced, systemic anticoagulation begun, steroids administered, and more aggressive CSF drainage instituted routinely to treat altered SEPs. No patient was reexplored as the result of SEP changes.

All patients were approached via a left thoracotomy or thoracoabdominal incision and positioned accordingly. Thoracic entry varied from third to sixth intercostal space in accordance with the planned resection. The left groin was swiveled posteriorly to permit access to the femoral vessels for arterial, and, in the case of planned DHCA, venous cannulation. Venous cannulation in DHCA patients was achieved by advancing a long 28–34 Fr venous cannula into the right atrium (RA) using echocardiographic guidance. The left ventricular apex (LV) was vented as necessary, either directly through the apex or via the left atrium (LA). For partial heart bypass, the LA, LV, or inferior pulmonary vein (PV) were used for inflow, with arterial return entering either the femoral artery (FA) or the aorta directly (Ao). The distribution of these cannulation techniques for partial bypass is listed in Table 2.

Patients planned for DHCA were approached in much the same fashion. Paired intercostal arteries were sacrificed as above, reserving the area immediately adjacent to the left subclavian artery for dissection during the interval of arrest. Intercostal artery sacrifice was performed with the patient at normothermic temperatures to minimize thermal effects on SEP monitoring, unless hemodynamic instability demanded a more expeditious approach to the aortic arch.

Bypass was usually carried out using a centrifugal pump (Biomedicus, Minneapolis, MN). Flow was adjusted to maintain distal aortic pressure of 60 to 70 mm Hg and proximal pressure was regulated by the anesthesiologists with vasodilator agents to maintain mean arterial pressures (MAPs) of 80 mm Hg (Table 3). Core cooling was not employed for partial bypass patients as reflected in temperature differences between HCA patients and partial bypass patients (Table 3).

Two thoracic drainage catheters were left until postoperative bleeding ceased. Autotransfusion was routinely used for patients undergoing DHCA procedures.

Postoperative management
Patients were routinely managed according to open heart intensive care unit (ICU) protocols and SEP monitoring continued as outlined above. Occasionally, SEP monitoring had to be restarted when delayed neurological compromise was suspected. Blood pressure was regulated with vasoactive agents as required, with avoidance of sodium nitroprusside whenever possible [14, 15]. Patients were weaned from ventilatory support as well as from all indwelling monitoring lines, drainage tubes, and urinary catheters as expeditiously as possible. Whenever possible, patients were mobilized on the first postoperative day, and discharged from the ICU when their clinical situation permitted. Patients requiring prolonged ICU stays were managed in appropriate fashion with suitable specialty consultation as indicated. Patients sustaining delayed neurological events or catastrophic complication of other organ systems were returned to the ICU for management as indicated. All patients were routinely studied with computed tomographic scans and most with arteriograms before discharge.

Patient data were analyzed using SigmaStat for Windows (Jandel Scientific, San Rafael, CA).

	Results

Top Abstract Introduction Patients and methods Results Discussion References

 
A total of 149 procedures utilizing SEP monitoring were performed. Patient hospital stay averaged 20.3 days, with a range of from 0 days to in excess of 3 months; stay in the ICU varied from 0 to 30 days, averaging 5.2 days. The extent of the aortic repairs varied enormously, ranging from no replacement of the descending aorta to complete replacement from the distal arch to the bifurcation of the aorta into the iliac arteries. The distribution of these repairs, utilizing the Crawford classification of thoracoabdominal repairs [16] is included in Table 4.

Thirteen patients (8.7%) died (Table 5). Three died of multi-organ system failure, 3 from sepsis, 3 of cardiac causes and 1 each from brain death, exsanguination from uncontrollable hemorrhage and other various causes. One of the cardiac deaths resulted from acute occlusion of the left anterior descending coronary artery 1 day after having undergone angioplasty of a lesion in that vessel.

Of the CVAs developing in 11 patients, eight were permanent, with prolonged functional deficits, and three were transient, with the patients regaining full functional capacity before discharge. Each of the 3 patients with transient CVAs had CT or MRI documentation of injury. One patient had global neurological dysfunction, presumably from hypoxic encephalopathy, and another 25 had some form of temporary neurological dysfunction. This temporary dysfunction included slow awakening, slurred speech, minor cognitive dysfunction or transient motor impairment. None had associated lesions on CT scan.

There were 12 patients who sustained paraplegia in some form. Seven patients had permanent paraplegia after awakening from anesthesia. One of these 7 patients had permanently lost SEP signal during the surgery but the change in recorded signal had gone unnoticed. The remaining patients either sustained no change in SEP or experienced return of the lost tracing (a normal occurrence with open distal anastomosis) to normal with reperfusion of the lower extremity. Two patients developed late permanent paraplegia, one after a cardiopulmonary arrest after leaving the ICU.

Three patients developed late temporary paraplegia; 2 recovered after aggressive support, including repeat SEP monitoring and CSF drainage, steroid administration, and elevation of blood pressure, while the remaining patient recovered spontaneously. Another 2 patients had minor paresthesias, which were self-limiting and resolved spontaneously.

Abnormal SEPs were recorded in 19 patients. Fourteen of these demonstrated normal neurological function after surgery (78.9%), while the remaining 3 (15.8%) developed paraplegia. Six cases of paraplegia developed in patients exhibiting normal SEPs (4.6%) while 123 patients (95.3%) with normal SEPs had normal neurological function after operation. These differences, by ² analysis, were significant (p 0.04).

Six patients had intercostal arteries reimplanted. None of the reimplantations were performed for alterations in SEP recordings at the time of surgery, but instead were performed on the basis of clinical assessment of the likelihood of developing paraplegia. Of these 6 patients none awoke with paraplegia, and the only dysfunction that occurred arose later, after cardiopulmonary arrest (vide supra). One patient was slow in awakening and required prolonged intubation, and another developed an SEP loss that took 3 days to recover, but motor function was not affected in either of these 2 patients. The remaining 3 patients recovered uneventfully.

	Discussion

Top Abstract Introduction Patients and methods Results Discussion References

 
Postoperative neurological dysfunction arising from spinal cord compromise during aortic surgery continues to plague the thoracic surgeon. Despite the description of numerous factors contributing to the risk of paraplegia following thoracic aortic surgery, none has proven to be overwhelmingly important to allow a complete avoidance of this problem. The duration of aortic cross-clamping, hypotension, elevation of CSF pressure, chronicity of the disease process, and compromise of critical intercostal arteries have all been shown to contribute to the risk of paraplegia, and strategies have been developed to circumvent these possible etiologies during surgery with variable success.

The development of SEP monitoring during aortic surgery by Laschinger and associates [17] provided a means by which the surgeon could monitor distal neurological function and effect maneuvers to prevent dysfunction in a timely fashion. Since its inception, SEP monitoring has become routine in aortic surgery and we have previously shown its use in the postoperative period to be beneficial as well [11, 13]. In the present study, several patients with marginal or slowly returning SEPs were aggressively managed in the postoperative period with therapies aimed at improving spinal cord perfusion. Among these were continued CSF drainage to maintain CSF pressures of 10 mm Hg, elevation of mean arterial pressures, heparinization, and steroid administration. In some instances, these therapies were maintained for several days until favorable results were obtained or no further improvement was seen. Evoked potential monitoring was, in all instances, continued until the patient was sufficiently alert to respond to voluntary neurological assessment.

The use of SEP monitoring during surgery offers the promise of being able to avert neurological compromise by the early detection of abnormal signal transmission from distal extremities to the cerebral cortex. As we had previously shown, the routine monitoring of patients in the operating room is easily accomplished with minimal technical expertise [13]. Although several patients could be identified as having altered SEPs after completion of the procedure, not all of these patients developed paraplegia. Conversely, 2 patients that had normal SEPs during surgery later developed paraplegia; in one instance, paraplegia occurred after a perioperative cerebrovascular accident. This suggest that the intraoperative usefulness of SEP monitoring may not be as discriminatory as initially postulated, even though a positive significant correlation between abnormal SEPs and paralysis was seen.

The practice of serially sacrificing paired intercostal arteries, as espoused by Dapunt and associates, was developed to avoid an arterial steal phenomenon thought to participate in the development of paraplegia [12]. Using this technique, our observed rate of permanent paraplegia (4.7% immediate, 1.3% late) was acceptably low, comparable with that reported by others [5, 18]. The 3 patients developing late paraplegias all reverted with the aggressive management outlined above, including reinstitution of SEP monitoring to confirm amelioration of neurological dysfunction. The absence of altered SEP signals, however, should not preclude the institution of an aggressive management protocol when a clinical situation indicates its use. We consider the continued and liberal application of SEP monitoring outside the operating room a desirable adjunct to the routine management of these aortic patients. This is especially true when a large number of intercostals are planned to be sacrificed, as this has been previously shown to be a determinant of the likelihood of paralysis [11]. A similar effect was seen in this series of patients as well (Table 6).

The use of SEP monitoring and serial intercostal artery sacrifice is, however, not without limitations. The initial expense of the equipment is substantial although the per case expenditure is low. Preexisting neurological conditions may interfere with the accurate use of this monitoring technique [19–21], as can various anesthetic agents [22, 23] and physical conditions [21, 23]. Most noteworthy among the latter are hypothermia and ischemia of the lower extremity induced by cannulation of the femoral artery. Additionally, when the distal perfusion is interrupted for the lower anastomosis, SEPs commonly disappear, only to reappear after perfusion is reinstituted. Finally, the slow, monitored sacrifice adds time to the procedure, lengthening the operation by 30–60 minutes, depending upon the number of arteries sacrificed and the chosen interval of observation between each arterial division. But we feel that the benefits gained by this technique, as demonstrated by the low observed paraplegia rate, more than outweigh the limitations cited above.

The technique of thoracic aortic resection, as performed by the surgeons at the Mount Sinai Hospital and outlined above, has been found to allow the safe and reproducible resection of the descending thoracic and thoracoabdominal aorta. Despite the increased time required for the methods described, the results of these techniques are comparable to or better than those reported from other centers. Monitored serial sacrifice of the intercostal arteries aids in preventing paraplegia by avoidance of an arterial steal from spinal cord perfusion. The expanded use of SEP monitoring into the postoperative period and later has sufficient merit to warrant its continued use and to recommend its adoption as a routine standard of care.

	References

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