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CARDIOVASCULAR ANESTHESIA

Somatosensory Evoked Potential Monitoring Used to Compare the Effect of Three Asymmetric Sternal Retractors on Brachial Plexus Function

W. Scott Jellish, MD, PhD^*, Bradford Blakeman, MD, Patricia Warf, RN^*, and Stephen Slogoff, MD^*

Departments of *Anesthesiology and Thoracic and Cardiovascular Surgery, Loyola University Medical Center, Maywood, Illinois

Address correspondence and reprint requests to W. Scott Jellish, MD, PhD, Loyola University Medical Center, Department of Anesthesiology, 2160 South First Ave., Maywood, IL 60153.

	Abstract

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We compared the effect of three different asymmetric sternal retractors on brachial plexus dysfunction using intraoperative somatosensory evoked potentials (SSEPs). We studied 60 patients undergoing coronary bypass and internal mammary harvest. Assessment of brachial plexus function was performed pre- and postoperatively. Patients were assigned the use of a Pittman^TM (MN Scientific Instruments Inc., Minneapolis, MN), Rultract^TM (Rultract Inc., Cleveland, OH) , or Delacroix-Chevalier^TM (Delacroix-Chevalier, Paris, France) asymmetric sternal retractor for internal mammary exposure. SSEP changes from baseline during asymmetric retractor use and removal were determined, and average changes were compared among the retractor groups. Patient demographics and baseline SSEP values were similar. Fewer patients in the Delacroix-Chevalier^TM group had decreases in SSEP amplitudes after retractor placement. Of the patients in the Rultract^TM and Pittman^TM groups, 45% and 25%, respectively, had amplitude decreases of >50%, compared with only 5% of the Delacroix-Chevalier^TM patients. Three patients in both the Pittman^TM and Rultract^TM groups and one patient in the Delacroix-Chevalier^TM group suffered brachial plexus symptoms postoperatively. We conclude that the Delacroix-Chevalier^TM retractor is associated with less neurophysiologic evidence of brachial plexus dysfunction during asymmetric sternal retraction compared with either the Pittman^TM or Rultract^TM sternal retractors.

Implications: We used somatosensory evoked potentials to assess the effect of several different asymmetric sternal retractors on brachial plexus dysfunction and to determine which produced the least evidence of nerve damage during surgical exposure of the internal mammary artery.

	Introduction

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Injury to the brachial plexus after median sternotomy has been reported in as many as 37% of all individuals undergoing coronary artery bypass (1). Symmetric retraction of the sternum has been postulated to produce anterior first rib fracture with subsequent laceration of the brachial plexus (2). However, the most likely mechanism for injury is excessive opening of the retractor, which causes the clavicle to push into the retroclavicular space while the first rib rotates upward (3). This stretches the neural bundle and is thought to be reduced by placing the retractor in the chest as far caudal as possible and opening it as little as necessary (4).

Asymmetric sternal retraction, as used during internal mammary artery (IMA) harvest, has also been thought to put the brachial plexus at risk (5,6). These procedures require a wide sternal opening and asymmetric retraction of the chest wall, which could cause fractures at the costotransverse articulations or an increased stretch of the nerve bundle over the humeral head. An asymmetric sternal retractor, the Delacroix-Chevalier^TM (Delacroix-Chevalier, Paris, France), which uses a mechanism different from either the Pittman^TM (MN Scientific Instruments Inc., Minneapolis, MN) or Rultract^TM (Rultract Inc., Cleveland OH) retractors to open the sternum, has been developed.

This study compares the effects of the Delacroix-Chevalier^TM retractor on brachial plexus dysfunction during IMA harvest with that of the Pittman^TM and Rultract^TM retractors. Real-time intraoperative assessments of the nerve bundle using somatosensory evoked potential monitoring (SSEP) were performed on the side of sternal retraction, and the results were correlated with postoperative neurologic assessments to determine whether any retractor offers an advantage in reducing nerve injury during IMA exposure.

	Methods

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After approval by our institutional review board and obtaining written, informed consent, 60 ASA physical status III patients undergoing elective coronary artery bypass grafting with use of an IMA were admitted into the study. On the day before surgery, a detailed neurologic assessment evaluating motor and sensory functions of the brachial plexus and a thorough history regarding upper extremity pain, parathesias, numbness, and weakness were performed by a nurse practitioner who was subsequently blinded to the intraoperative portion of the study. Patients were excluded from the study if they had a history of neurologic dysfunction or if deficits were noted on their preoperative examination. They were also excluded if they had a history of polyneuritis secondary to diabetes, transient ischemic attacks, or syncopal episodes or were >150% of their ideal body weight. This same neurologic examination was performed within 2 days of tracheal extubation, which occurred within 24 h after surgery.

On the day of surgery, the patients were assigned by lottery to the use of a Pittman^TM, Rultract^TM, or Delacroix-Chevalier^TM asymmetric sternal retractor. The Pittman^TM retractor consists of two vertical bars suspended from a horizontal bar (Figure 1A). One end of each vertical bar is attached to the sternum. The other end is attached to the horizontal bar. The side of the sternum attached to each vertical bar is then elevated by a screw mechanism at the top of the vertical bar that attaches to the horizontal stabilizing bar. The Rultract^TM asymmetric sternal retractor has two small vertical arms attached to a short, broad, horizontal beam (Figure 1B). This beam connects to a cable that runs to a winch assembly attached at the top of a large vertical pole that is fixed to the table. The chest is opened asymmetrically by inserting the two small vertical arms underneath the sternum and using the winch assembly, pulling the chest wall up and lat- eral to expose the IMA underneath. The Delacroix-Chevalier^TM asymmetric sternal retractor is composed of two short horizontal arms, one with two claw attachments and the other with a blade attachment, which are connected to a rack-toothed vertical bar (Figure 1C). The claw attachments are placed under the sternal border to be lifted while the flat blade is positioned onto the opposite sternal border. The chest wall is then asymmetrically opened by lifting one side of the chest wall while simultaneously pushing down on the other. Once opened to the desired width, the retractor is locked into position.

View larger version (86K): [in this window] [in a new window]   Figure 1. The PittmanTM (A), RultractTM (B), and Delacroix-ChevalierTM (C) retractors placed into the chest cavity and retracting the sternum.

Anesthesia consisted of IV fentanyl 40–70 µg/kg and midazolam 6–8 mg. Pancuronium 0.1 mg/kg IV was used to facilitate intubation. One hundred percent oxygen was used throughout the procedure with additional isoflurane 0.2%–0.4% end tidal, if needed. Hemodynamic variables were kept within 15% of baseline by using IV phenylephrine for hypotension and nitroprusside for hypertension. Body temperature was kept within 0.5°C of preinduction values throughout the prebypass period.

SSEPs of the brachial plexus were monitored using the Nicolet Pathfinder^TM MEGA 8 channel electrodiagnostic system (Nicolet Biomedical Instruments, Madison, WI) by a trained nurse who was blinded to the type of retractor used. Fine-gauge platinum electrodes were placed subdermally after skin infiltration with 1% lidocaine in both arms and the scalp. The median nerve was stimulated at the wrist with subcortical responses monitored at C2. This nerve was monitored because the SSEP changes that occur during sternal retraction were similar to those observed in the ulnar nerve (6,7). The superficial course of the ulnar nerve make it susceptible to peripheral injury (pressure or compression), especially with arms at side positioning. Monitoring the median nerve would negate peripheral influences and more closely reflect the effect of sternal retraction on brachial plexus function. Cortical responses were recorded at C3', C4' with an F_pz ground and a reference electrode at C_z' as defined by the international 10–20 system (8). The stimulus current was standardized at 15–20 mA, and the evoked response was induced by 250–300 stimulus repetitions. Filters were set at 30–1000 Hz, and impedance was <2 k. Latency and amplitude measurements were determined for the N₁₉ peak and the P₂₂ trough. Preincision SSEP recordings were obtained immediately after induction of anesthesia and were used as a baseline value for all further comparisons. SSEPs were continuously monitored throughout the prebypass time period and were discontinued on initiation of hypothermia during cardiac bypass. Recordings of SSEP waveforms were made preincision, at asymmetric retractor insertion, at retractor removal, and after symmetric sternal retractor placement but before the initiation of bypass.

Demographic data; total anesthesia (time patient entered operating room to admission to the cardiac intensive care unit), bypass (initiation to separation from extracorporeal circulation), aortic cross-clamp (time of clamp application to removal) times; and number of internal jugular cannulation attempts were compared among groups by using Student's t-tests. Intragroup SSEP latency and amplitude values obtained during asymmetric sternal retractor placement and removal and during symmetric retractor placement were compared with baseline values using repeated-measures multiple analysis of variance. Intergroup SSEP changes from baseline that occurred at the specific time points measured were also compared using repeated-measures multiple analysis of variance. Intergroup comparisons of patients who had significant amplitude changes during surgery (>50% decrease or increase compared with baseline) and who demonstrated postoperative neurologic findings consistent with brachial plexus injury were performed by using Fisher's exact test. P < 0.05 was considered significant, and numeric values are represented as mean ± SEM.

	Results

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No differences in demographic characteristics were noted among the groups (Table 1). Total anesthesia, cardiopulmonary bypass, and aortic cross-clamp times were also similar among the three groups (Table 1). Representative somatosensory waveform recordings during placement of the three different retractors are shown in Figure 2. No differences were noted in baseline somatosensory amplitude values among the groups (Table 2). Significant changes in latency values from baseline were not observed in any of the groups during the study period (Table 2). Of the patients using the Delacroix-Chevalier^TM retractor, 50% had somatosensory amplitudes diminish on the left side after retractor insertion, compared with 80% in the Pittman^TM and 79% in the Rultract^TM groups (P < 0.05). Of the patients in the Rultract^TM and Pittman^TM groups, 45% and 25%, respectively, had amplitude decreases >50% compared with baseline values after retractor insertion (Table 3). In contrast, only 5% of patients in the Delacroix-Chevalier^TM retractor group had amplitude decreases of this magnitude. Mean somatosensory waveform amplitudes decreased after retractor placement in both the Rultract^TM and Pittman^TM groups, but they remained unchanged with the Delacroix-Chevalier^TM retractor (Table 2). After removal of both the Pittman^TM or Rultract^TM retractors, somatosensory waveform amplitudes increased but never returned to baseline values (Table 2). However, patients in the Delacroix-Chevalier^TM retractor group demonstrated no change or a mean increase in somatosensory waveform amplitudes compared with baseline values after retractor removal (Table 2).

View larger version (25K): [in this window] [in a new window]   Figure 2. Representative somatosensory evoked potential tracings for PittmanTM (A), Delacroix-ChevalierTM (B), and RultractTM (C) sternal retractors. The first tracing was recorded at incision, and the second tracing was recorded at retractor insertion and sternal opening. A decrease in amplitude occurs with the PittmanTM (A) and RultractTM (C) retractors but is not observed with the Delacroix-ChevalierTM retractor.

	Discussion

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Upper extremity neuropathy and injury to the brachial plexus after coronary artery bypass procedures has been of continuing concern to both surgeons and anesthesiologists. Numerous mechanisms have been proposed to account for these injuries, but the most likely cause is either direct trauma to the nerve bundle or stretching of the nerves between the clavicular head and first rib. Laceration of the nerve by a fractured rib would produce severe and usually irreversible injury. Most brachial plexus-related symptoms, however, usually resolve within a week. Thus, this mechanism of injury may not provide the best explanation for these postoperative findings. During median sternotomy and symmetric sternal retraction, the first rib moves anteriorly while the clavicle rotates downward (1). This movement compresses the brachial plexus as it travels into the arm and stretches the nerves tethered to their fascial attachments. Investigators using SSEPs for intraoperative evaluation of the brachial plexus during symmetric sternal retraction have noted changes in waveform characteristics that reflect this type of stretch injury (7,9).

Standard asymmetric sternal retractors, such as the Pittman^TM and Rultract^TM, use wide sternal openings and lateral force to pull the chest wall up and away from the contralateral sternal border. This movement may push the clavicular head further down onto the brachial plexus on the side of retraction and amplify the stretch injury that may occur during symmetric retraction. The Delacroix-Chevalier^TM retractor uses a different mechanism to open the chest wall. Rather than lifting and pulling the sternal half laterally, this retractor pushes the operative side up and simultaneously pushes down on the nonoperative side. This allows enough sternal opening to effectively approach the IMA and produces little lateral traction on the sternum. These observations correlate well with the neurophysiologic data obtained in the present study. Significant SSEP changes indicative of potential nerve damage occurred in only 5% of patients in the Delacroix-Chevalier^TM group, whereas more patients had these changes in both the Pittman^TM or Rultract^TM groups. In addition, the total number of patients who demonstrated any change in SSEP waveforms with retractor placement was smaller in the Delacroix-Chevalier^TM group.

Although the patients in whom the Delacroix-Chevalier^TM retractor was used showed less neurophysiologic evidence of brachial plexus injury, no significant difference was noted in postoperative sensory or motor symptoms associated with the brachial plexus. In this study, however, we only examined the time period from the start of surgery to initiation of cardiopulmonary bypass. Damage to the plexus could have occurred during unmonitored portions of the procedure. The patients who complained of bilateral brachial plexus symptoms demonstrated no corresponding changes in SSEP amplitude, which suggests that this injury occurred from a mechanism other than nerve stretch, possibly from inadequate perfusion to the hands producing local ischemic dysathesia. This nerve deficit may also have developed during bypass or some other unmonitored period. Another possibility is that the number of subjects studied per group was not sufficient to discern a clinically significant difference in outcome. The study population size was determined using the ² statistic with 2 degrees of freedom. The maximal probability of a type I error was set at 0.05 with an equal allocation of patients to the study groups. It was assumed that the Delacroix-Chevalier^TM retractor would produce a 10% decrease in the incidence of significant SSEP changes compared with the other retractors studied. Therefore, 20 patients per group would provide an 80% probability of finding this difference if it were present. A third possibility, as mentioned by Seal et al. (10), is that intraoperative SSEP monitoring may not be useful in predicting brachial plexus injury. In their study, a positive correlation between retractor placement and SSEP changes was noted, but no positive correlation between significant SSEP changes and postoperative findings was found. This result is not surprising, however, because the incidence of subclinical brachial plexus injury may be as high as 87% after coronary artery bypass procedures (11). Seal et al. (10) also noted no correlation between SSEP changes and asymmetric sternal retraction. This is in contrast with our findings in the present study and those of previous investigators (6,7,9). If SSEP changes occur with symmetric sternal retractor placement, they should also occur during asymmetric sternal retraction because the chest opening is larger and the upward force applied to the sternum is greater compared with symmetric retraction. Their results may differ from ours for several reasons. Retractors may be used differently at different institutions by different surgeons. We used the same surgeon (BB) for the entire study so we assume that there was no variation in surgical technique. In addition, electrical interference often associated with electrocautery may have obscured changes in SSEP recordings (12). All of our responses were measured after halting electrocautery for 2–3 min to assure a quiet background.

Other mechanisms have been proposed to account for brachial plexus injury. Prolonged cardiopulmonary bypass times (>200 min) and previous neuropathies are thought to increase the risk of brachial plexus injury (1). The bypass times in the present study were never >100 min, and patients were excluded from the study if previous neuropathy existed. Traumatic injury to the plexus could occur by needle trauma during placement of a jugular catheter (13), but we found no evidence of this. Our jugular catheters were always placed on the right side. Many of our patients had left-sided or bilateral symptoms. In addition, our patients were awake but sedated during the placement of these catheters. Needle insertion into the brachial plexus would have produced pain severe enough to alert the practitioner. Ulnar nerve injury has been associated with direct crush injury due to malpositioning or operating room personnel leaning on the arm (14). Tomlinson et al. (15) demonstrated reduced brachial plexopathy when the hands were positioned above the head and away from the sides of the table. Other investigators, however, have found no benefit in hands-up positioning (2,9,16). Thus, our patients were positioned with the arms adducted and placed at the sides. Finally, older and larger individuals may be at greater risk of nerve injury after cardiac surgery than younger and smaller individuals (2,4,17). No difference in age or body habitus was noted among our groups. Abnormal cycling or positioning of the blood cuff has also been linked to brachial plexus damage (18). In our study, the cuff was placed on the right side and positioned proximal to the ulnar groove in all patients. It was cycled rarely because the intraarterial catheter was used to monitor pressure.

In conclusion, the Delacroix-Chevalier^TM retractor's unique mechanism of asymmetrically opening the chest may cause less dysfunction to the brachial plexus by reducing the lateral force applied during retraction. Although clinical outcomes were similar among the groups, these conclusions are supported by SSEP monitoring of the nerve plexus during chest wall retraction, which demonstrates neurophysiologic evidence of less injury compared with that observed with either the Pittman^TM or Rultract^TM retractors.

	Footnotes

 
Presented at the annual meeting of the American Society of Anesthesiologists, Atlanta, GA, October 21–25, 1995.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999