Ann Thorac Surg 2003;75:508-513
© 2003 The Society of Thoracic Surgeons
Original article: cardiovascular
Estimating group mortality and paraplegia rates after thoracoabdominal aortic aneurysm repair
Scott A. LeMaire, MDa*,
Charles C. Miller, III, PhDa,b,
Lori D. Conklin, MDa,b,
Zachary C. Schmittling, MDa,b,
Joseph S. Coselli, MDa,b
a Michael E. DeBakey Department of Surgery, Division of Cardiothoracic Surgery, Baylor College of Medicine, Houston, Texas, USA
b The Methodist DeBakey Heart Center, Houston, Texas, USA
Accepted for publication August 21, 2002.
* Address reprint requests to Dr LeMaire, Michael E. DeBakey Department of Surgery, Division of Cardiothoracic Surgery, Baylor College of Medicine, 6560 Fannin, Suite 1100, Houston, TX 77030, USA.
e-mail: slemaire{at}bcm.tmc.edu
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Abstract
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BACKGROUND: Most clinical studies regarding thoracoabdominal aortic aneurysm (TAAA) surgery are retrospective comparisons involving heterogeneous groups of patients. Risk models that evaluate susceptibility bias enhance interpretation of these intergroup comparisons. The purpose of this analysis was to derive group risk models for mortality and paraplegia after TAAA repair.
METHODS: Data regarding 1,220 consecutive patients undergoing TAAA repair were analyzed via multiple logistic regression with stepwise model selection. Categorical preoperative risk factors that predicted 30-day mortality and paraplegia were used to develop risk models.
RESULTS: Fifty-eight patients (4.8%) died within 30 days and 56 patients (4.6%) developed paraplegia or paraparesis. Predictors of mortality were rupture, renal insufficiency, symptomatic aneurysms, and Crawford extent II repairs. Extent of repair and acute presentation were predictors of paraplegia. The derived risk models estimated mortality and paraplegia rates that correlated well with actual frequencies reported in other contemporary series (regression slopes = 0.87 and 1.06, respectively).
CONCLUSIONS: The derived risk models accurately estimate paraplegia and mortality rates in groups of patients. Prospective model validation will be required to confirm their accuracy.
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Introduction
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Estimated rates of mortality and morbidity from predictive models based on risk analyses can be applied to groups of patients and serve as indices that enhance our ability to interpret intergroup comparisons. When comparing heterogeneous groups of patients undergoing thoracoabdominal aortic aneurysm (TAAA) repair, differences in outcome may be related to several varying patient characteristics—such as aneurysm extent and rupture—rather than the factor being evaluated. Statistical models clarify the impact of these confounding effects by distilling multiple independent risk factors into a global risk estimate. While these models do not eliminate the inherent limitations of retrospective studies, they do assist in interpretation of the data by addressing the issue of confounding factors.
The use of group risk models in thoracic aortic surgery was pioneered by Acher and colleagues in 1994 [1]. Using data from patients who underwent surgery between 1960 and 1991, the University of Wisconsin group developed a highly predictive formula for paraplegia and paraparesis following descending thoracic and TAAA repair. In evaluating their strategy for spinal cord protection (cerebral spinal fluid drainage combined with intravenous naloxone), the authors used the formula to estimate the risk of paraplegia in groups treated with and without adjuncts. Although the two groups were at similar overall risk—based on their relative frequencies of aneurysm extents, aortic dissection, and acute presentation—the actual incidence of paraplegia was markedly reduced in the patients treated with the adjuncts (Table 1).
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Table 1. Data From Acher and colleagues [1] With Predicted and Actual Rates of Paraplegia or Paraparesis in Patients After Descending Thoracic and Thoracoabdominal Aortic Aneurysm Repair
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Since the results of TAAA repair have improved substantially over the past decade, the applicability of Acher’s model to more recent data is uncertain. The purpose of the current study was to use contemporary results to develop updated group risk models that estimate mortality and paraplegia rates after TAAA repair.
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Patients and methods
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Patients
Over a 13-year period, data regarding 1,532 consecutive patients undergoing graft repair of descending thoracic or TAAAs by the senior author (J.S.C.) were collected prospectively and entered into a database. Of these, 1,220 patients had TAAA repairs; the patients with aneurysms limited to the descending thoracic aorta are not considered further in this study. The patient characteristics at the time of TAAA repair are detailed in Table 2. There were 721 men (59.1%) and 499 women (40.9%). Patient ages ranged from 18 to 88 years (mean 65.7 years, median 68 years). Extensive TAAAs (Crawford extents I and II) were present in 65.1% of patients.
Surgical technique
The details of the surgical technique have been recently described elsewhere [2]. With regard to spinal cord protection, a combination of moderate heparinization, permissive mild hypothermia, sequential aortic clamping, and aggressive reattachment of critical intercostal arteries (T8 to L1) was used consistently throughout the entire series. Left heart bypass was used in 381 (31.2%) cases; no blood reservoir, heat exchanger, or oxygenator was incorporated in the bypass circuit. A detailed technical report describing our methods for using left heart bypass during TAAA repair was recently published [3]. Spinal evoked potentials were not monitored. Cerebrospinal fluid drainage was not used routinely during the study period.
Study variables and definitions
All preoperative, intraoperative, and postoperative data were retrieved from the prospectively maintained database. In addition to patient age and gender, the preoperative characteristics analyzed are listed in Table 2. The aneurysms were classified based on extent as defined by Crawford and associates [4]. Aneurysms associated with aortic dissection were considered acute if surgery was performed within 14 days of the onset of pain; after 14 days, dissection was considered chronic. Patients were considered symptomatic when any symptom (acute or chronic, severe or mild) related to the aneurysm was present, including pain, hoarseness, and dysphagia. Patients with acute presentations were defined as those with acute pain, rupture, contained rupture, and complicated acute dissection [1]. Renal insufficiency was defined as serum creatinine exceeding 3.0 mg/dl or need for hemodialysis.
All postoperative deaths were categorized based on whether they occurred within 30 days after operation (30-day deaths) and whether they occurred during the initial hospitalization (in-hospital deaths). In accordance with established guidelines, operative mortality included all deaths occurring within 30-days and all deaths occurring during the initial hospitalization [5]. All patients with postoperative neurologic deficits involving the lower extremities were included in the paraplegia category, regardless of whether the deficit was weakness (paraparesis) or paralysis, immediate or delayed, transient or permanent. This included patients with unilateral lower extremity deficits, unless an associated deficit involving the ipsilateral upper extremity—indicating a stroke—was present.
Statistical analyses
The statistical analyses were performed using the SAS (release 6.10; SAS Institute, Inc, Cary, NC) and SPSS (release 6.1.3; SPSS, Inc, Chicago, IL) systems for Windows. Categorical preoperative variables were evaluated for association with 30-day mortality or paraplegia using 
2 or Fisher exact tests. Risk factors that emerged with significance levels below 0.25 were analyzed via multiple logistic regression with stepwise model selection. Associations with outcomes were considered statistically significant when p < 0.05. Factors found to be predictive of 30-day mortality and paraplegia were used to construct risk formulas. Although operative mortality is a better outcomes measure, 30-day mortality was used in the risk model because it appears more consistently in the literature. Preoperative factors were used exclusively to allow determination of a group’s inherent initial risk independent of subsequent variations in operative technique and postoperative management.
To assess the validity of the predictive formulas, 25 series of TAAA repairs published after 1993 were reviewed [6–30]. Whenever available, data regarding the prevalence of significant risk factors were used to determine the predicted incidence of 30-day mortality and paraplegia following TAAA repair in each series. Bivariate regression or scatter plots of actual versus predicted events were then constructed and analyzed to determine the slopes and correlation coefficients for each model; the ideal slope for a predictive model is 1.0, indicating a 1:1 ratio of predicted and actual events. In many of the reports that combined patients with TAAAs and patients with descending thoracic or abdominal aortic aneurysms, separate risk factor and outcome data for the TAAA patients were not reported; these series were not able to be included in the validation study.
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Results
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Operative morbidity and mortality
Four patients (0.3%) died in the operating room. The overall operative mortality rate was 7.3% (89 patients), which included 58 30-day deaths (4.8%) and 87 in-hospital deaths (7.1%); two of the 30-day deaths occurred after discharge from the hospital. Fifty-six patients developed paraplegia (4.6% of 1,206; excludes 10 patients with preoperative paralysis and 4 patients who died during operation); of these 56 patients, 33 (59%) had paraparesis. Stratified results based on aneurysm extent are listed in Table 3.
Risk analysis for 30-day mortality
Univariate analysis of categorical preoperative risk factors revealed that symptomatic aneurysms and rupture were associated with 30-day death (Table 4).
Multivariable analysis demonstrated that rupture, renal insufficiency, symptomatic aneurysms, and extent II aneurysms were predictors of 30-day mortality. The following model for estimating 30-day mortality was based on the risk coefficients for these predictive variables:
where
renal = number of patients with renal insufficiency, rupture = number of patients with ruptured aneurysms, n = total number of patients with TAAAs, symptoms = number of patients with symptomatic aneurysms, and C2 = number of patients with Crawford extent II aneurysms.
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Table 4. Results (p Values) of Univariate Analyses Regarding Factors Associated With Neurologic Deficits and Mortality After Thoracoabdominal Aortic Aneurysm Repair
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Of the 25 recently published (after 1993) series that were reviewed [6–30], only four contained specific data for TAAA patients regarding extent, symptomatic aneurysms, rupture, preoperative renal insufficiency, and 30-day mortality (Table 5).
Using the actual and predicted 30-day mortality rates from the four reports with complete data (Fig 1A),
the predictive model yielded a slope of 0.156 (p = 0.0023, correlation coefficient = 0.9374). Adding the current series of 1,220 patients to the plot (Fig 1B) reduced the influence of the extreme outlying point and increased the slope to 0.87, which was not significantly different from 1.0 (p = 0.659, correlation coefficient = 0.8916).
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Table 5. Data From Recent Series of Thoracoabdominal Aortic Aneurysm Repair With Actual and Estimated 30-Day Mortality Rates
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Fig 1. Bivariate regression or scatter plots of actual versus predicted numbers of 30-day deaths in four contemporary series of thoracoabdominal aortic aneurysm repairs [10, 11, 15, 30] constructed without (A) and with (B) data from the current series.
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Risk analysis for paraplegia
Univariate analysis of categorical preoperative variables revealed the following factors to be associated with paraplegia (Table 4): extent II aneurysms, acute presentation, and rupture. Extent IV aneurysms and prior thoracic aortic aneurysm repair were associated with reduced risk. Multivariable analysis yielded the following model for estimating risk of deficit based on TAAA extent and acute presentation:
where E1 = (C1 x 0.01) + (C2 x 0.06) + (C3 x 0.01) + (C4 x 0.003) + (acute x 0.2), C1,2,3,4 = number of patients with Crawford TAAA extents I–IV, respectively, acute = number of patients with acute presentations (acute dissection, rupture, contained rupture, or acute pain).
Nine of the 25 reviewed series [6–30] contained enough specific data for TAAA patients to allow calculation of the predicted paraplegia rates based on Acher’s formula and the revised formula (Table 6).
Bivariate regression or scatter plots (Fig 2)
comparing the revised model with Acher’s original formula demonstrates that the newer formula allows a more accurate estimation for contemporary data. When applied to the data from the nine reports (Fig 2A), the slope for Acher’s original formula is 3.43 (significantly greater than 1, p = 0.004) and the slope of our revised formula is 1.01 (p = 0.981, correlation coefficient = 0.8274). When the data from the current series is included in the plot, the slope for Acher’s formula is 5.40 (p = 0.0001) versus 1.06 (p = 0.470, correlation coefficient = 0.9828) for the revised model (Fig 2B).
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Table 6. Data From Recent Series of Thoracoabdominal Aortic Aneurysm Repair With Actual and Estimated Incidence of Paraplegia or Paraparesis
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Fig 2. Bivariate regression or scatter plots of actual versus predicted numbers of paraplegic patients in nine current series of thoracoabdominal aortic aneurysm repairs [8, 10, 12, 15, 19, 23, 26, 28, 30] demonstrating results of both Acher’s original formula [1] (open circles and dashed lines) and the updated version (solid circles and solid lines). Plots were constructed without (A) and with (B) the data from the current series.
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Comment
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Through 1995, fewer than 3,500 TAAA repairs had been reported in the English literature [31]. Although the high-risk nature of TAAA surgery has continually stimulated investigative efforts aimed at reducing associated complications, the relative rarity of these aneurysms results in limited opportunities to perform studies involving large groups of patients. Therefore, the majority of clinical studies regarding TAAA repair are retrospective comparisons of heterogeneous groups of patients. Among the many limitations of such studies, the inability to control for the influence of confounding variables makes the interpretation of results particularly difficult.
In response to this problem, Acher and colleagues [1] developed a predictive formula for paraplegia following descending thoracic and TAAA repair. This model was based on risk coefficients for extent of aortic involvement and clinical presentation derived from Crawford’s extensive experience. We and others have used this predictive formula as an index to compare subgroups in previous analyses [22, 32, 33]. As recent refinements in surgical technique and patient management have led to improved outcomes following TAAA repair, the formula, which is based on data obtained between 1960 and 1991, has lost its predictive accuracy. Although the model was extremely good at predicting the deficit rates in 16 series reported before 1993, it currently overestimates risk (Table 6, Fig 2).
In contrast, estimates of paraplegia frequency based on our revised risk model correlate well with the actual incidence reported in contemporary series. While extent II aneurysms and acute presentations—as originally defined in Acher’s model [1]—have retained their importance as risk factors, the impact of aortic dissection has been greatly reduced. The elimination of chronic dissection from the predictive formula is consistent with our recent analysis, which demonstrated that chronic dissection is no longer a risk factor for paraplegia after TAAA repair [34]. The mortality group risk formula was developed in hope of providing a similar means for comparing the risk of 30-day mortality in heterogeneous patient groups. The predictive accuracy of both formulas, however, will require validation through prospective evaluation. Standardized reporting of all significant risk factors for paraplegia and mortality in future TAAA reports will facilitate both the evaluation of these risk models and their subsequent revisions as results continue to improve.
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Acknowledgments
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The authors gratefully acknowledge Autumn Jamison for providing database management.
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References
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Top
Abstract
Introduction
