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Ann Thorac Surg 2001;72:80-84
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Prevention and detection of spinal cord injury during thoracic and thoracoabdominal aortic repairs

Accepted for publication March 14, 2001.

Address reprint requests to Dr Wada, Department of Thoracic and Cardiovascular Surgery, Hyogo College of Medicine, 1-1 Mukogawa-cho Nishinomiya, Hyogo, 663-8501, Japan
e-mail: wadatora{at}hyo-med.ac.jp

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. Spinal cord injury is a most dreaded and unpredictable complication. In this study, based on our experimental results in dogs and early clinical results, we reviewed the incidence of paraplegia and the detection of spinal cord injury.

Methods. Eighty-two patients who underwent elective surgical repair of the descending thoracic and thoracoabdominal aorta over 17 years were subjects for this study. Sixty-two patients were male and 20 were female. Their mean age was 61.6 years (range, 17 to 81 years). Monitoring somatosensory evoked potentials (SEP) and measurement of mean distal aortic pressure and cerebrospinal fluid pressure were performed perioperatively.

Results. Sixty patients had no ischemic change in SEP. In 17 patients with significant ischemic changes of SEP, SEP recovered by increasing spinal cord perfusion pressure to more than 40 mm Hg. Two patients with complete loss of SEP experienced paraplegia. One patient had delayed paraplegia.

Conclusions. These results strongly suggest that SEP, mean distal aortic pressure, cerebrospinal fluid pressure should be monitored during aortic cross-clamping. Maintaining spinal cord perfusion pressure at more than 40 mm Hg by increasing mean distal aortic pressure or withdrawal of cerebrospinal fluid is valuable for preventing paraplegia.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
Despite recent advances in operative techniques, anesthetic management, and postoperative supportive care, a most dreaded operative complication with surgical treatment of descending thoracic and thoracoabdominal aortic diseases, paraplegia, remains unresolved. At present, the incidence of perioperative paraplegia ranges from 3.8% to 17.6% [1]. In 1960, Miyamoto and associates [2] and Blaisdell and Cooley [3] reported that cerebrospinal fluid pressure (CSFP) is an important factor in spinal cord injury after thoracic aortic cross-clamping because elevated CSFP aggravates spinal cord ischemia by compressing the vascular system of the spinal cord. In 1982, Coles and colleagues [4] and Cunningham and associates [5] demonstrated an earlier detection of spinal cord ischemia by using somatosensory evoked potentials (SEP) that were generated by intraoperative stimulation of peripheral nerves. In 1984, Oka and Miyamoto [6, 7] investigated and reported that in a dog experimental study, the effect of spinal cord perfusion pressure (SCPP), defined as the pressure difference between mean distal aortic pressure (MDAP) and CSFP was one of the important factors for perioperative development of spinal cord injury. In 1987, Maeda and coworkers [8] reported that, in clinical experience, maintaining SCPP at more than 40 mm Hg during aortic cross-clamping by increasing MDAP or withdrawal of cerebrospinal fluid (CSF) was valuable for the prevention of paraplegia. In this study, we reviewed the incidence of paraplegia and perioperative detection of spinal cord injury in cases of descending thoracic and thoracoabdominal aortic repairs by monitoring SEP, MDAP, and CSFP during aortic cross-clamping.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Patients and methods
Between December 1, 1982, and June 31, 1999, 82 patients who underwent elective surgical repairs at Hyogo College of Medicine for descending thoracic aortic aneurysms, thoracoabdominal aortic aneurysms, and coarctation of the aorta were enrolled as subjects in this study. A retrospective review of the patients’ perioperative and postoperative outcomes was conducted using hospital records and clinic charts. Sixty-two patients were male and 20 were female. Their ages ranged from 17 to 81 years, with a mean age of 61.6 years. In this study, cases with emergent operation and using deep hypothermic circulatory arrest were excluded because we could not monitor SEP.

Perioperative monitoring system
Monitoring of SEP and measurement of MDAP and CSFP were performed perioperatively (Fig 1). Somatosensory evoked potentials were measured with a Synax 1200 clinical evoked potential system (NEC Co, Ltd, Tokyo, Japan). Somatosensory evoked potential traces were generated by electrical stimulation of the peroneal nerves bilaterally with two bipolar input channels. One hundred consecutive stimuli with a 200-V square-wave stimulation of 0.2 milliseconds at a frequency of 2 Hz were averaged for each SEP trace. When the amplitude of SEP was less than 50%, we judged this as a significant ischemic change. When the amplitude was more than 80%, we judged this as recovery of SEP. Cerebrospinal fluid pressure was measured with a spinal drainage kit (Silascon, Kaneka Medix Co, Tokyo, Japan) placed in the intrathecal space through a lumbar puncture needle. Withdrawal of CSF was performed through this catheter. Arterial pressure in the right radial artery and one of the dorsal pedis arteries was measured constantly. Spinal cord perfusion pressure was calculated as the pressure difference between MDAP and CSFP.

View larger version (22K): [in this window] [in a new window]   Fig 1. Perioperative monitoring, showing how to record somatosensory evoked potentials (SEP) and evaluate SCPP. (CSFP = cerebrospinal fluid pressure; EEG = electro-encephalogram; MDAP = mean distal aortic pressure; SCPP = spinal cord perfusion pressure.)

Type of aortic disease
The subjects included 38 patients with dissecting aneurysm (Stanford type B), 32 patients with descending thoracic aortic aneurysm, 10 patients with thoracoabdominal aortic aneurysm, and 2 patients with coarctation of the aorta.

Statistical analysis
Statistical significance was determined by Student’s t test for paired data, and all values are described as mean ± standard deviation.

	Results

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Type of operation
A woven Dacron tube graft was inserted in 29 cases of descending thoracic aortic aneurysm, in 10 cases of dissecting aneurysm with descending thoracic aortic aneurysm, in 10 cases of thoracoabdominal aortic aneurysm, and in 1 case of coarctation of the aorta. Entry closure and aortoplasty was performed in 28 cases in type B aortic dissection. Four cases involved performing patch graft repair with woven Dacron wrapping for descending thoracic aortic aneurysm and coarctation. Five of 22 cases with significant ischemic changes of SEP underwent revascularization of the intercostal-arteries (T9 to T12). The mean aortic cross-clamp time was 78.3 ± 37.9 minutes.

Mortality and morbidity
The hospital mortality rate was 7.3% (6 of 82) for elective repairs. Half (3 of 6) of the causes of death were because of thoracic empyema. The other three causes of death included respiratory failure, cerebral infarction, and acute mediastinitis.

Late survival
Actuarial analysis of long-term survival rate revealed that 90% of hospital survivors were alive 1 year after operation, 78.7% were alive 5 years, and 56.8% were alive 10 years (Fig 2). The causes of late deaths were rupture of other aortic aneurysm in 3 patients, malignant disease in 3, myocardial infarction in 2, cerebral infarction in 2, hemoptysis in 2, respiratory failure in 2, and unrelated disease in 8.

View larger version (9K): [in this window] [in a new window]   Fig 2. Actuarial survival curve (Kaplan-Meier method) showing that 90% of the hospital survivors were alive at 1 year after operation, 78.7% at 5 years, and 56.8% at 10 years.

View larger version (21K): [in this window] [in a new window]   Fig 3. Relation between somatosensory evoked potential (SEP) monitoring and incidence of paraplegia (n = 82). (F-F = femoral vein to femoral artery; SCPP = spinal cord perfusion pressure.)

View larger version (19K): [in this window] [in a new window]   Fig 4. Mean cerebrospinal fluid pressure (CSFP) during aortic cross-clamping in 60 patients with no significant ischemic somatosensory evoked potential (SEP) changes and 22 with significant ischemic SEP changes. Cerebrospinal fluid pressure rose to more than 20 mm Hg in 2 patients, who experienced paraplegia.

View larger version (17K): [in this window] [in a new window]   Fig 5. Spinal cord perfusion pressure (SCPP) during aortic cross-clamping in 60 patients with no significant ischemic somatosensory evoked potential (SEP) changes and 22 with significant ischemic SEP changes. Spinal cord perfusion pressure was less than 40 mm Hg in 5 patients. Two of 3 patients with significant ischemic SEP changes during aortic cross-clamping experienced paraplegia.

	Comment

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The reported risk of paraplegia is known to be significantly higher after repairs of traumatic aortic transection, emergency, and acute aortic dissection [9–11]. The cause of paraplegia was thought to be insufficient radicular arterial flow caused by interruption of critical intercostal arteries in the replaced aortic segment, insufficient pressure of the distal aorta, or extended aortic cross-clamp time. Other factors, such as intraoperative proximal hypertension, elevation of CSFP, and postoperative hypotension, also have implications. Combination of these factors decreases blood flow to the spinal cord and may lead to spinal cord ischemic changes.

All patients had either temporary bypass shunt or femoral vein to femoral artery bypass during aortic cross-clamping. A temporary axillofemoral shunt using a 10-mm woven Dacron graft was used in 9. Two of these 9 patients had paraplegia because the SCPP could not be maintained above 40 mm Hg during temporary bypass shunting. Femoral vein to femoral artery bypass was used in 73 patients; paraplegia did not occur. Femoral vein to femoral artery bypass grafting involving the use of a pump oxygenator has the disadvantage of requiring systemic anticoagulation with heparin, although on the contrary, the advantage of this surgical procedure for unexpected bleeding is avoiding either hypotension or hypertension at proximal and distal aortic pressure [12]. Coselli and LeMaire [13] and Schepens and associates [14] presented the use of left heart bypass as beneficial in patients who had thoracoabdominal aortic aneurysm repair. Kouchoukos and Rokkas [15] recommended that hypothermic cardiopulmonary bypass with circulatory arrest is useful in patients at highest risk for development of paraplegia.

Laschinger and coworkers [16–18] described the use of monitoring of SEPs in detecting the onset of spinal cord ischemia and assessing the adequacy of distal aortic perfusion in preserving sensory conduction of the posterior and lateral spinal column during proximal aortic cross-clamping. The major disadvantage of SEP is that it could not detect occurrence of motor deficits. False-positive tracings are likely, for example, cortical dysfunction, which can be prompted by the effects of anesthetic agents or ischemia, and peripheral nerve ischemia [19]. De Hann and colleagues [20] reported that monitoring motor-evoked potentials is an effective technique to detect spinal cord ischemia for monitoring anterior spinal cord function. Somatosensory evoked potentials may not reflect selective ischemia of the spinal motor system, because the motoneuronal system in the anterior horn receives its blood supply from the anterior spinal artery. But motor-evoked potentials are readily depressed by many commonly used anesthetics, such as nitrous oxide [21], propofol [22], benzodiazepines [23], and volatile anesthetics [24].

Paraplegia did not occur in any patient whose SEPs remained intact. In contrast, when SEPs suggested significant ischemic changes, the surgeon had to assume that spinal cord ischemia would be present. In spite of immediate changes in operative technique to elevate the distal aortic pressure, reattach the intercostal arteries to the graft, or withdraw CSF, paraplegia occurred in 2 (9.1%) of the 22 patients who experienced a significant ischemic change of SEP during aortic cross-clamping. If SEPs are lost, we recommend increasing bypass flow until the SCPP is more than 40 mm Hg and the SEP amplitude and latency improve. If the SCPP is more than 40 mm Hg and the SEP amplitude does not improve, we have to reattach intercostal arteries. Safi and associates [25] reported on the importance of intercostal artery reattachment.

The use of SEP monitoring during operation offers the promise of being able to avert neurologic compromise by early detection of abnormal signal transmission from distal extremities to the cerebral cortex [26]. In this study postoperative paraplegia occurred in 3 patients, including 1 with delayed paraplegia. Two patients’ SCPPs were 32 and 35 mm Hg, and their SEP traces were lost. We could not maintain the high MDAP for using temporary bypass shunt in these 2 patients. Conversely, 1 patient who had normal SEPs during the operation later experienced paraplegia. We reevaluated whether prolongation of SEP monitoring during the postoperative period with careful systemic aortic pressure control is needed to prevent delayed paraplegia [27].

The withdrawal of CSF was undertaken to increase SCPP and enhance blood flow to the spinal cord by decompressing collateral blood vessels to the spinal cord. The elevation of CSFP is one of the major factors causing paraplegia [28]. Increased CSFP is caused by volume changes in venous capacitance beds within the dural space [29]. Proximal hypertension during cross-clamp prompts the autoregulation mechanisms of the cerebral and spinal vasculature to reflexively increase CSFP, effectively lowering SCPP, and thereby diminishing cord blood supply [30]. Conversely, drainage of CSF to reduce CSFP will offset the gradient in favor of augmenting spinal cord perfusion. When the CSFP rises to more than 20 mm Hg, we propose that CSF should be aspirated to keep the SCPP at more than 40 mm Hg at least. Both CSFP and distal aortic pressure are important determinants of spinal cord perfusion. Safi and coworkers [31] recommended that CSFP be maintained at or below 10 mm Hg. Laschinger and associates [32] recommended maintaining a distal aortic pressure of at least 60 mm Hg.

In summary, the most important results from these data of 82 patients is that paraplegia did not occur if the SCPP was maintained at more than 40 mm Hg with no significant ischemic changes of SEP. For the prevention and detection of the perioperative paraplegia, we propose the following:

1. Somatosensory evoked potentials, MDAP, and CSFP should be monitored during aortic cross-clamping.
   
2. Maintaining SCPP at more than 40 mm Hg by increasing MDAP or withdrawal of CSF is valuable for prevention of perioperative paraplegia.
   
3. Femoral vein to femoral artery bypass is useful to control the distal aortic perfusion during aortic closs-clamping, as intraoperative adjuncts.
   

This shows that monitoring of SCPP is more reliable and effective in maintaining spinal cord perfusion.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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