Effect of sodium nitroprusside on paraplegia during cross-clamping of the thoracic aorta

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.

Control of hypertension in the critically ill: a pathophysiological approach

Severe acute arterial hypertension can be associated with significant morbidity and mortality. After excluding a reversible etiology, choice of therapeutic intervention should be based on evaluation of a number of factors, such as age, comorbidities, and other ongoing therapies. A rational pathophysiological approach should then be applied that integrates the effects of the drug on blood volume, vascular tone, and other determinants of cardiac output. Vasodilators, calcium channel blockers, and beta-blocking agents can all decrease arterial pressure but by totally different modes of action, which may be appropriate or contraindicated in individual patients. There is no preferred agent for all situations, although some drugs may have a more attractive profile than others, with rapid onset action, short half-life, and fewer adverse reactions. In this review, we focus on the main mechanisms underlying severe hypertension in the critically ill and how using a pathophysiological approach can help the intensivist decide on treatment options.

Anesthetic consideration for descending thoracic aortic aneurysm repair

Anesthesia for surgery of the aorta poses some of the most difficult challenges for anesthesiologists. Major hemodynamic and physiologic stresses and sophisticated techniques of extracorporeal support are superimposed on patients with complex medical disease states. In this review, etiologies, natural history, and surgical techniques of thoracic aortic aneurysm are presented. Anesthetic considerations are discussed in detail, including the management of distal perfusion using partial cardiopulmonary bypass. Considerations of spinal cord protection, including management of proximal hypertension, cerebral spinal fluid drainage, and pharmacological therapies, are presented.

Use of partial cardiopulmonary bypass for coarctation repair through a left thoracotomy in children without collaterals

BACKGROUND

Paraplegia is a devastating complication of coarctation of the aorta (COA) repair. Since 1990 we have used left atrium-to-descending aorta cardiopulmonary bypass (CPB) for COA repair in patients with inadequate collaterals. We reviewed the results with that strategy and compared this CPB group with COA repairs in which CPB was not used to see whether there was any increase in morbidity or delay in recovery.

METHODS

From 1990 to 2006, 11 patients with COA were identified to have inadequate collaterals based on preoperative examination and intraoperative arterial monitoring and test clamp. Left thoracotomy with left atrium-to-descending aorta CPB was used in all. Age ranged from 4.2 to 17.4 years (mean, 8.7 +/- 4.6 years). Two were reoperations for recurrent COA, 3 patients had four prior transcatheter balloon dilatations. One patient had aberrant origin of the right subclavian artery. Operative techniques included resection with extended end-to-end anastomosis (n = 6), interposition graft (n = 4), and patch repair (n = 1). During the same period 71 patients older than 1 year of age had COA repair without CPB. Age ranged from 1.1 to 46.1 years (mean, 7.6 +/- 7.1 years; p = 0.6).

RESULTS

Preoperative imaging of CPB patients demonstrated absence of collaterals (n = 7), possible collaterals (n = 2), small collaterals (n = 1), and anomalous origin of the right subclavian artery (n = 1). Preoperative arm leg gradient in CPB patients was 36.0 +/- 9.0 mm Hg versus 49.9 +/- 15 mm Hg in non-CPB patients (p < 0.01). Mean distal femoral artery pressure with aortic test clamp was 34.3 +/- 4.8 mm Hg in CPB patients versus 49.8 +/- 12.4 mm Hg in non-CPB patients (p < 0.01). Mean CPB flow was 53% +/- 7.3% of calculated total flow. Cardiopulmonary bypass time ranged from 17 to 46 minutes (mean, 27.5 +/- 9.7 minutes). Aortic clamp time in CPB patients ranged from 15 to 33 minutes (mean, 21.6 +/- 6.3 minutes). In the non-CPB group aortic clamp time ranged from 10 to 50 minutes (mean, 23.4 +/- 7.5 minutes; p = 0.5). In the CPB group length of stay ranged from 3 to 7 days (mean, 4.9 +/- 1.3 days), and in the non-CPB group length of stay ranged from 3 to 12 days (mean, 4.7 +/- 1.4 days; p = 0.5). No patient had a neurologic complication. There were no other major complications in the CPB group (eg, bleeding, recurrent laryngeal nerve injury, re-COA).

CONCLUSIONS

Preoperative imaging and a lower arm-to-leg gradient in this series of COA patients suggested inadequate collateral circulation with the potential need for CPB. A femoral artery pressure of less than 45 mm Hg during test clamp was used as an indication for partial CPB. The use of left atrium-to-descending aorta CPB with just over 50% calculated total flow protected the spinal cord in a safe and expeditious fashion. Use of left heart bypass did not affect morbidity or recovery time as compared with COA repair in non-CPB patients.

Preoperative assessment and management to prevent complications during high-risk vascular surgery

OBJECTIVE

Most patients requiring vascular surgical reconstruction are at high risk for major morbidity and mortality, with certain vascular procedures at particularly high risk for complications. Although numerous comorbid conditions are precisely the risk factors that determine outcome, we review particular factors for each surgery that may be optimized to alter outcome and minimize postoperative complications.

DESIGN

Literature review.

RESULTS

Certain aspects of care are common to all vascular surgery procedures, including thoracoabdominal aortic aneurysm repair, pararenal and ruptured abdominal aortic aneurysm repair, mesenteric and renal revascularization, and carotid endarterectomy. Some factors that are important include careful preoperative assessment and optimization of cardiac, pulmonary, and renal function and volume status. In addition, the use of experienced teams during and after the procedure, as well as clear and continuous communication between all surgical team members, may improve outcome. Particular attention to procedural details is also crucial to achieving excellent results.

CONCLUSIONS

Patients needing vascular surgery often possess management challenges that increase the risk of perioperative complications. Meticulous attention to details during all phases of care, including preoperative optimization as well as intraoperative procedural conduct and communication, helps achieve optimal results and thus minimize the risk of complications.

Cardiac function is a risk factor for paralysis in thoracoabdominal aortic replacement

PURPOSE

We studied factors that influence paralysis risk, renal function, and mortality in thoracoabdominal aortic replacement.

METHODS

We prospectively collected preoperative demographic and intraoperative physiologic data and used univariate and multivariate analyses to correlate this data with risk factors for paralysis. A mathematical model of paraplegia risk was used to study the efficacy of paraplegia reduction strategies. We analyzed preoperative and operative factors for paralysis risk, renal function, and mortality for 217 consecutive patients surgically treated from 1984 through 1996 for 176 thoracoabdominal and 41 thoracic aneurysms at the University of Wisconsin Hospital and Clinics. No patient had intercostal reimplantation or assisted circulation. One hundred fifty patients (group A) received cerebrospinal fluid drainage (CSFD) and low-dose naloxone (1 microg/kg/hour) as adjuncts to reduce the risk of paralysis. Sixty-seven patients (group B) did not receive CSFD and naloxone.

RESULTS

Seventeen deficits occurred in 205 surviving patients: 5 of the 147 in group A (expected deficits = 31) and 12 of the 58 in group B (expected deficits = 13) (p < 0.001). In a multivariate logistic regression model, acute presentation, Crawford type 2 aneurysm, group B membership, and a decrease in cardiac index with aortic occlusion remained significant risk factors for deficit (p < 0.0001). By odds ratio analysis, group A patients had 1/40th the risk of paralysis of group B. The only significant predictor of postoperative renal function was the preoperative creatinine level (p < 0.0001); renal revascularization significantly improved renal function. The mortality rate was 1.6% (2) for patients undergoing elective treatment and 21% (19) for patients who had acute presentations. Acute presentation, age, and the preoperative creatinine level were found to be significant factors for operative mortality in a logistic regression model (p < 0.001) and defined a group at high risk for death.

CONCLUSIONS

CSFD and low-dose naloxone significantly reduce the paralysis risk associated with thoracoabdominal aortic replacement. A decrease in the cardiac index with aortic occlusion is a previously unreported variable that defines a subset of patients at higher risk for paralysis.

Prevention of spinal cord ischaemic complications after thoracoabdominal aortic surgery

Since the publication of prior reviews on this topic, substantial clinical experience with a variety of operative strategies to prevent ischaemic cord complications has been reported. The available data on angiographic localisation of critical intercostal vessels, and, in particular, the evoked potential response to cross-clamping in patients indicates that risk of paraplegia varies considerably even among patients with equivalent TAA extent. Factors such as individual development of the ASA, patent critical intercostals, and the particulars of collateral circulation when intercostal aortic ostia are already occluded likely account for this variability. Information available from SSEP monitoring relative to the dynamic course of cord ischaemia with cross-clamping, and the parallel, if not, frustrating experience with angiographic localisation and intercostal vessel reconstruction indicates that a narrow temporal threshold of cord ischaemia with clamping is present in many patients. This reinforces the importance of both expeditious clamp intervals, critical intercostal re-anastomoses, and the desirability of neuroprotective manoeuvres during cross-clamp induced cord ischemia. As suggested in compelling experimental work our contemporary clinical experience, and predicted by prior reviewers, regional cord hypothermia provides significant promise for limiting or eliminating, in particular, immediate perioperative deficits. Avoidance of postoperative hypotension, spinal cord oedema, and preservation of critical intercostal vessels are additional strategies necessary to impact the development of delayed deficits favourably.

Effect of nitroglycerin and cerebrospinal fluid drainage on spinal cord perfusion pressure and paraplegia during aortic cross-clamping

When sodium nitroprusside (SNP) is used to control proximal blood pressure (Px-BP) during cross-clamping (AXC) of the thoracic aorta, it decreases spinal cord perfusion pressure (SCPP) by reducing distal aortic pressure (Ds-BP) and increasing cerebrospinal fluid pressure (CSFP). The decrease cannot be reversed by CSF drainage (CSFD) because such drainage is limited by a reduction in compliance of the spinal canal. Nitroglycerin can also be used to control Px-BP, but its effect on CSF dynamics has not previously been investigated. In the present study we have compared the effects of NTG alone and in combination with CSFD, with SNP and CSFD. Each group (Gp) of six dogs was treated with SNP + CSFD (Gp 1), NTG alone (Gp 2), and NTG + CSFD (Gp 3). Left carotid and right femoral arteries were catheterized to monitor Px-BP and Ds-BP, respectively. CSFP was monitored and CSF was drained through a spinal needle placed in the cisterna cerebellomedullaris. The thoracic aorta was cross-clamped via a left thoracotomy for 60 min. Data were acquired at baseline, during aortic occlusion, and 24 hr after surgery. There were no significant differences in any measurements among the three groups before AXC; after AXC, Px-BP was maintained between 85 and 95 mm Hg in all groups. Ds-BP was significantly lower in Gp 1 than Gp 2 and 3 (7 +/- 2 mm Hg vs. 13 +/- 3 mm Hg and 17 +/- 2 mm Hg, respectively P < 0.05). CSFP did not differ between Gp 1 and 2 (10 +/- 3 mm Hg vs. 9 +/- 1 mm Hg, P > 0.05). CSFD effectively kept CSFP at zero values in Gp 3 during AXC, but not in Gp 1. SCPP was significantly higher in Gp 3 than in Gp 1 and 2 (17 +/- 2 mm Hg vs -3 +/- 4 mm Hg and 4 +/- 1 mm Hg, respectively, P < 0.05). All animals in Gp 1 and 2 suffered paraplegia, as opposed to none in Gp 3. NTG causes paraplegia by decreasing SCPP. When used in conjunction with CSFD, it controls Px-BP without causing paraplegia. CSFD cannot counteract the negative effects of SNP on SCPP; therefore, SNP contributes to postoperative paraplegia. The effects of NTG on cerebrospinal fluid dynamics are different from those of SNP. We caution surgeons against the use of NTG without CSFD during aortic cross-clamping.

Trimethaphan versus sodium nitroprusside for the control of proximal hypertension during thoracic aortic cross-clamping: the effects on spinal cord ischemia

Sodium nitroprusside (SNP) has been used to control the proximal hypertension associated with thoracic aortic cross-clamping (TACC) during thoracic aortic surgery. It worsens neurologic outcome, presumably by further decreasing distal arterial pressure and increasing cerebrospinal fluid (CSF) pressure, thereby worsening the spinal cord perfusion pressure (SCPP). Trimethaphan does not increase CSF pressure. Therefore, the present study investigates the effect of trimethaphan versus SNP to control proximal hypertension during TACC on neurologic outcome. Two groups, each with eight mongrel dogs, were studied. All animals underwent descending TACC for 45 min. The mean proximal aortic blood pressure was maintained at 95-100 mm Hg by the use of SNP or trimethaphan. Distal aortic pressure was allowed to vary. The dogs were neurologically evaluated 24 and 48 h later by a blinded observer. During cross-clamping, there was no difference in mean proximal aortic pressure between groups. After 10 min of cross-clamping, distal aortic pressure was higher (P < 0.01), CSF pressure was lower (P < 0.01), and SCPP was higher (P < 0.005) in the trimethaphan group as compared with the SNP group (group effect). Neurologic outcome as assessed by Tarlov's score was better at 24 and 48 h in the trimethaphan group (P < 0.05). Histopathologic injury trended with worsened neurologic outcome. We conclude that 1) trimethaphan produced higher SCPP than SNP, and 2) neurologic outcome was better in the trimethaphan group.

Effect of nitroglycerin on spinal cord ischemia after thoracic aortic cross-clamping

BACKGROUND

Thoracic aortic cross-clamping with the use of sodium nitroprusside (SNP) has been shown to cause a decrease in spinal cord perfusion pressure and an increased incidence of paraplegia. Nitroglycerin is frequently used in this setting. This study investigated the effects of nitroglycerin and SNP on spinal cord ischemia.

METHODS

Three-groups of 8 mongrel dogs underwent thoracic aortic cross-clamping for 45 minutes. Proximal pressure was maintained between 95 and 100 mm Hg with SNP, nitroglycerin, or phlebotomy. All animals were neurologically evaluated 24 hours later by a blinded observer, and the findings were confirmed by histopathologic study. Statistical analysis (p value of less than 0.05) of measured hemodynamic data was by analysis of variance and of Tarlov scores, the Mann-Whitney U test.

RESULTS

Distal aortic pressures (p < 0.001), Tarlov scores, and spinal cord perfusion pressures (p < 0.01 and p < 0.05 for SNP group and nitroglycerin group, respectively) were significantly higher in the phlebotomy group compared with the SNP and NTG groups. Cerebrospinal fluid pressures were significantly lower in the phlebotomy group compared with the SNP group (p < 0.001).

CONCLUSIONS

The use of either NTG or SNP was associated with a high incidence of paraplegia. Nitroglycerin appears to be no safer than SNP when used during thoracic aortic cross-clamping.

Isoflurane versus sodium nitroprusside for the control of proximal hypertension during thoracic aortic cross-clamping: effects on spinal cord ischemia

OBJECTIVE

This study was designed to compare the effects of isoflurane and nitroprusside on spinal cord ischemia when they are used to control proximal hypertension during thoracic aortic cross-clamping (TACC).

DESIGN

Prospective, randomized, blinded experimental study.

SETTING

Laboratory and animal research facility.

PARTICIPANTS

Adult mongrel dogs.

INTERVENTIONS

Two groups of eight dogs had TACC for 45 minutes. Proximal aortic, distal aortic, and cerebrospinal pressure was calculated as the distal mean pressure minus the CSF pressure. Group 1 received nitroprusside and group 2 received isoflurane to control proximal hypertension during cross-clamping. The dogs were neurologically evaluated 24 and 48 hours later by an observer blinded as to the study group. Spinal cord segments were obtained for histopathologic examination.

MEASUREMENTS AND MAIN RESULTS

Distal perfusion pressure and spinal cord perfusion pressure were significantly higher in the isoflurane group (p < .005). At 24 hours, seven of eight dogs in group 1 had severe neurologic injury (ie, paraplegia), with the eight having mild neurologic injury. This is in contrast to group 2, where 6 of 8 dogs had either minimal or no injury, one had mild injury, and one had severe injury. Similar results were observed at 48 hours (p < .005).

CONCLUSIONS

Isoflurane, when used to control proximal hypertension during TACC, produces a higher spinal cord perfusion pressure and is associated with a lower incidence of neurologic injury than nitroprusside in this canine model.