Chronic kidney disease classification stratifies mortality risk after elective stent graft repair of the thoracic aorta

Article Outline

• Abstract
• Methods
• Results
• Discussion
• Author contributions
• References
• Copyright

Objective

Risk factors for perioperative and late mortality after thoracic endovascular aortic repair (TEVAR) remain ill-defined. In this study, we examined the prognostic significance of chronic kidney disease (CKD), a well-known predictor of death after thoracic aorta open repair, employing a stratification based on CKD stages derived from glomerular filtration rate (GFR) values.

Methods

A prospective database was evaluated for 179 consecutive patients electively submitted to TEVAR between 1999 and 2007. Preoperative GFR was estimated by using the Cockcroft-Gault equation. Patient groups were stratified into four quartiles by baseline serum creatinine (SC) and GFR values, with quartile I being the lowest, and quartile IV the highest, and into the five CKD stages in reverse order (I GFR ≥ 90 ml/min/1.73 m²; II 60-89; III 30-59; IV 15-29; V < 15). Prognostic significance of preoperative GFR values and CKD stages were investigated by means of univariate and multivariate analyses, and the Kaplan-Meier log-rank method.

Results

A primary technical success was achieved in 166 of 179 patients (92.7%), and an initial clinical success in 158 (88.3%). Thirty-day mortality was 5% (nine cases). Paraplegia or paraparesis were observed in 11 (6.1%) patients, and completely resolved in six cases after cerebrospinal fluid drainage. Preoperative GFR quartiles and CKD stages were significant predictors of 30-day mortality (P = .004 and P < .0001 respectively), whereas SC quartiles did not affect the outcome (P = .12). In particular, GFR quartile I (<60 ml/min/1.73 m²) was associated with a ten-fold greater risk of perioperative death compared with the other three quartiles (Odds Ratio 11.4, 95% Confidence Interval 2.3-57.0, P = .003). Midterm survival was 88.8% (159 of 179) at a mean follow-up of 35.6 ± 23.7 months. Actuarial survival at 60 months was 57.8%, 81.1%, 92.3%, and 100% for GFR quartiles I to IV respectively (P < .0001), and 0.0%, 66.7%, 59.2%, 88.6%, and 100% (P < .0001) for CKD stage V to I respectively. At univariate analyses, age (P = .019), preoperative SC quartiles (P = .001), GFR quartiles (P = .0002), and CKD stages (P < .0001) were all predictive of mid-term mortality. At multivariate Cox proportional hazards regression analysis, only CKD stages remained independently associated with the outcome (P = .008).

Conclusions

GFR is an accurate prognostic predictor in patients submitted to TEVAR. Also, perioperative and midterm mortality directly correlate with the severity of CKD stages, allowing a risk stratification model to be employed both for risk-adjusted preoperative evaluation, and to establish accurate matching criteria for comparative studies.

Thoracic endovascular aortic repair (TEVAR) has been proven as a feasible alternative to open repair, particularly in patients considered unsuitable for surgery due to the presence of significant comorbidities.¹, ² Owing to its unquestionable lower invasiveness, TEVAR is increasingly gaining favor also for the treatment of “low-risk” patients, and the first and so far only completed comparative multicenter study, namely the Gore TAG trial,³ showed the superiority of TEVAR over open repair in terms of aneurysm-related mortality and major adverse events at 5 years.⁴ Yet, the follow-up could not be completed for more than 25% of patients, and the control group included for the most part (53%) historically and retrospectively acquired patients. Also, TEVAR has so far failed to provide improved all-cause survival rates,⁴, ⁵ or quality of life benefits,⁶ and therefore no thorough conclusions can be drawn regarding long-term results.

Careful selection of patients for TEVAR, apart from anatomic factors, should include the risk of aneurysm-related death,⁷ and the prognosis related to concomitant medical diseases.⁸ Mean aortic growth rate is 0.10 cm per year,⁷ and therefore asymptomatic, small- to moderate-sized thoracic aortic aneurysms can be safely followed-up.⁹ This is particularly the case in patients with other debilitating comorbidities, in whom the risks of TEVAR may exceed those inherent to conservative treatment.⁸ As a result, preoperative risk stratification appears important both for indication to treatment, and evaluation of postoperative results.

Chronic kidney disease (CKD) is a well-known determinant of early mortality after thoracoabdominal and descending thoracic aortic open repair,¹⁰ and recent studies have described the higher prognostic value of glomerular filtration rate (GFR) compared with serum creatinine (SC) alone in patients submitted to open thoracoabdominal aortic aneurysm repair.¹¹

The present study was conducted to determine the impact on perioperative and midterm mortality of a stratification based on the National Kidney Foundation CKD stages,¹² derived from GFR values, in patients undergoing elective stent-graft repair of the thoracic aorta.

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Methods 

This study was designed as a single-center experience. A retrospective review was conducted on a prospectively computerized database of all patients undergoing elective TEVAR at our Institution. Between June 1999 and January 2007, 179 consecutive cases were treated (150 males, mean age, 70.1 ± 9.0 years). Patients affected with thoracoabdominal aortic aneurysm, submitted to hybrid surgery, were excluded from the analysis because of the presence of possible confounding factors for preoperative GFR, namely the involvement of the renal arteries.

Indications for intervention included: atherosclerotic aneurysm in 150 cases, chronic type B dissection in 17 cases, penetrating ulcer/intramural hematoma in five cases, and chronic post-traumatic false aneurysm in seven cases. Mean aortic diameter in case of aneurysmal disease was 61 ± 19 mm. Patients were operated on under general anesthesia in 111, spinal anesthesia in 60 cases, or local anesthesia in eight. Access site was femoral in 155 cases, and aorto-iliac in 24, including five cases of synchronous surgery for AAA.

Patients were stratified by the proximal landing Zone according to the Ishimaru's classification.¹³ Overall, there were 16 Zone 0, 13 Zone 1, 41 Zone 2, 58 Zone 3, and 51 Zone 4. Debranching of supra-aortic vessels was performed for Zone 0 and Zone 1 cases as previously described in detail.¹⁴ Selective revascularization of the left subclavian artery (LSA) was performed in Zone 2 patients.¹⁴ Cerebrospinal fluid (CSF) drainage was also selectively instituted in 21 (11.7%) patients, as described elsewhere.¹⁵

The feasibility of the endoluminal intervention and sizing of stent grafts were determined with preoperative computed tomography (CT) scans and aortography. The endografts implanted were Excluder TAG (WL Gore and Assoc., Flagstaff, Ariz), Talent and Valiant (AVE/Medtronic Inc., Santa Rosa, Calif), Endofit (Endomed Inc., Phoenix, Ariz), Zenith (WilliamCook Europe Aps, Bjaeverskov, Denmark), and Relay (Bolton Medical Inc., Sunrise, Fla). All the procedures were performed in the operating room, and a portable digital C-arm image intensifier with subtraction angiography and roadmap capabilities was used.

Patients were stratified by preoperative risk factors including etiology, proximal landing zone, previous aortic surgery, diabetes, tobacco use, hypertension, chronic obstructive pulmonary disease (COPD), hyperlipidemia, and by coronary artery disease (CAD), according to the Goldman revised cardiac risk index (RCRI).¹⁶

The preoperative GFR value was estimated by using the Cockcroft-Gault equation¹⁷ (140 – age) × weight/72 × serum creatinine (where age is in years, actual body weight is in kg, and serum creatinine is the baseline level obtained on the day of admission expressed in mg/dL; for women, the equation is multiplied by 0.85). GFR values are expressed as mL/min/1.73 m².

Patients groups were also stratified into four quartiles by baseline SC and GFR values, with quartile I being the lowest, and quartile IV the highest, and into the five CKD stages¹² in reverse order (I GFR ≥ 90 ml/min/1.73 m² ; II GFR 60-89 ml/min/1.73 m²; III GFR 30-59 ml/min/1.73 m²; IV GFR 15-29 ml/min/1.73 m²; V GFR < 15 ml/min/1.73 m²).

Morbidity and mortality were recorded. Neurologic deficits were defined as paraplegia or paraparesis according to the Modified Tarlov scale, and classified as immediate, when observed immediately or upon awakening, or delayed, when occurring after a period of normal neurologic function.¹⁵ Results were described according to the Reporting Standards for endovascular aortic aneurysm repair.¹⁸ Patients were followed-up at one, six, and 12 months, and yearly thereafter by means of office clinical evaluation and aortic imaging.

We investigated the influence of demographics and preoperative risk factors as possible predictors of postoperative outcome (Table I, Table II, Table III). Univariate statistics were computed by using contingency table methods. Continuous data were divided into quartiles for contingency table analysis. Survival outcome was evaluated along a 60-month distribution of failure times by using Kaplan-Meier estimates. The actuarial survival was computed according to the Kaplan-Meier log-rank method. Multivariate analysis was performed by using Cox proportional hazards regression. The null hypothesis for statistical tests was rejected at P < .05. All analyses were run using SAS 8.02 software (SAS Institute Inc, Cary, NC).

Table I. Univariate analyses of risk factors for 30-day mortality

Variable	Levels	No. patients (%)	No. deaths (%)	P value⁎
Overall		179(100.0)	9(5.0)
Age	<55	12(6.7)	0(0.0)	.50
	55-68	54(30.4)	2(3.7)
	69-80	99(55.6)	7(7.1)
	>80	13(7.3)	0(0.0)
Gender	Female	29(16.2)	1(3.4)	.67
	Male	150(83.8)	8(5.3)
COPD	Yes	104(58.1)	6(5.8)	.59
	No	75(41.9)	3(4.0)
CAD†	0	122(68.2)	4(3.3)	.27
	1	35(19.5)	4(11.4)
	2	20(11.2)	1(5.0)
	3	2(1.1)	0(0.0)
AAA surgery	No	141(78.8)	7(5.0)	.84
	Previous	33(18.4)	2(6.1)
	Simultaneous	5(2.8)	0(0.0)
Redo procedure	Yes	10(5.6)	0(0.0)	.45
	No	169(94.4)	9(5.3)
Zone‡	0	16(8.9)	2(12.5)	.40
	1	13(7.3)	0(0.0)
	2	41(22.9)	3(7.3)
	3	58(32.4)	3(5.2)
	4	51(28.5)	1(2.0)
Etiology	ATS	150(83.8)	8(5.3)	.48
	Dissection	17(9.5)	0(0.0)
	PAU/IMH	5(2.8)	0(0.0)
	Post-traumatic	7(3.9)	1(14.3)
Preop SC [Quartiles] (mg/dL)	<0.85	46(25.7)	2(4.3)	.12
	0.85-0.99	42(23.5)	0(0.0)
	1.00-1.20	46(25.7)	2(4.3)
	>1.20	45(25.1)	5(11.1)
Preop. GFR [Quartiles] (mL/min per 1.73 m2)	>87.0	45(25.1)	1(2.2)	.004
	73.0-87.0	44(24.6)	1(2.3)
	60.0-72.9	43(24.0)	0(0.0)
	<60.0	47(26.3)	7(14.9)
CKD Stages§	I(GFR ≥ 90)	38(21.2)	1(2.6)	<.0001
	II(GFR=60-89)	94(52.5)	1(1.1)
	III(GFR=30-59)	41(22.9)	5(12.2)
	IV(GFR=15-29)	4(2.2)	0(0.0)
	V(GFR<15)	2(1.1)	2(100.0)

AAA, Abdominal aortic aneurysm; ATS, atherosclerotic; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; GFR, glomerular filtration rate (estimated by using the Cockroft-Gault equation); IMH, intramural hematoma; PAU, penetrating aortic ulcer; SC, serum creatinine.

Table II. Univariate analyses of risk factors for mid-term mortality

Variable	Levels	60-Months mortality¥	P value⁎
Age	<55	0.0%	.019
	55-68	7.7%
	69-80	4.4%
	>80	23.1%
Gender	Female	3.6%	.47
	Male	7.1%
COPD	Yes	10.2%	.07
	No	1.4%
CAD†	0	5.9%	.29
	1	0.0%
	2	21.0%
	3	0.0%
AAA surgery	No	6.0%	.61
	Previous	8.8%
	Simultaneous	0.0%
Redo procedure	Yes	0.0%	.59
	No	6.9%
Zone‡	0	0.0%	.08
	1	15.4%
	2	5.3%
	3	9.3%
	4	4.0%
Etiology	ATS	7.8%	.69
	Dissection	0.0%
	PAU/IMH	0.0%
	Post-traumatic	0.0%
Preop SC [Quartiles] (mg/dL)	<0.85	2.4%	.001
	0.85-0.99	4.5%
	1.00-1.20	0.0%
	>1.20	20.0%
Preop GFR [Quartiles] (mL/min per 1.73 m2)	>87.0	0.0%	.0002
	73.0-87.0	0.0%
	60.0-72.9	6.7%
	<60.0	20.5%
CKD Stages§	I(GFR≥90)	0.0%	<.0001
	II(GFR=60-89)	3.2%
	III(GFR=30-59)	20.0%
	IV(GFR=15-29)	25.0%
	V(GFR<15)∫	/

AAA, Abdominal aortic aneurysm; ATS, atherosclerotic; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; GFR, glomerular filtration rate (estimated by using the Cockroft-Gault equation); IMH, intramural hematoma; PAU, penetrating aortic ulcer; SC, serum creatinine.

Table III. Multiple Cox proportional hazards regression analysis of risk factors for mid-term mortality

CI, Confidence interval; COPD, chronic obstructive pulmonary disease; SC, serum creatinine.

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Results 

Overall, a primary technical success was achieved in 166 of 179 patients (92.7%), and an initial clinical success in 158 (88.3%). Thirty-day mortality was 5.0% (9 of 179), and due to intraoperative graft migration (n = 1), stroke (n = 3), multiorgan embolization (n = 1), myocardial infarction (n = 2), and multiple organ failure (n = 2). Paraplegia or paraparesis were observed in 11 (6.1%) patients, and in eight cases the onset was delayed (range, 1 to 35 days). The neurologic deficit completely resolved in six cases after CSF drainage. Other postoperative complications included acute renal failure (ie, SC exceeding the baseline value by 30% and surpassing an absolute level of 2.0 mg/dL)¹⁹ reversed without dialysis in five (2.8%) patients, respiratory failure requiring intubation for > 48 hours in four (2.2%) patients, and acute myocardial infarction in two (1.1%) patients.

Among the variables analyzed, preoperative GFR quartiles and CKD stages were found to be significantly associated with 30-day mortality (P = .004 and P < .0001, respectively), whereas SC quartiles were not (P = .12) (Table I). In particular, GFR quartile I (<60 ml/min/1.73 m²) was associated with a ten-fold greater risk of death compared with the other three quartiles (OR 11.4, 95% CI 2.3-57.0, P = .003).

Midterm survival was 88.8% (159 of 179) at a mean follow-up of 35.6 ± 23.7 months. Eleven (6.5%) of 170 initial survivors died during follow-up due to aneurysm rupture (n = 2), aortoesophageal fistula nine months after implantation (n = 1), abdominal aortic aneurysm rupture (n = 1), myocardial infarction (n = 3), malignancies (n = 2), stroke (n = 1), and respiratory failure (n = 1). One successful surgical conversion was performed following stent-graft rupture 43 months after TEVAR.

Actuarial survival at 60 months was 57.8%, 81.1%, 92.3%, and 100% for GFR quartiles I to IV respectively (P < .0001), and 0.0%, 66.7%, 59.2%, 88.6%, and 100% (P < .0001) for CKD stage V to I respectively (Fig).

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• Fig. 

  Survival by Chronic Kidney Disease (CKD) stages. At key time points (0, 24, and 60 months), the number of patients in each CKD stage is listed above (P < .0001, Log Rank test). *In CKD Stage IV, standard error is >10%.

At univariate analyses, age, preoperative SC quartiles, GFR quartiles, and CKD stages were all predictive of mid-term mortality (Table II). Ishimaru's classification and COPD showed a mild association with late mortality, but did not reach statistical significance. Nevertheless, both factors were conservatively entered in the multivariable model to eliminate possible confounding factors.

At multivariate Cox proportional hazards regression analysis, only CKD stages remained independently associated with mortality (P = .008) (Table III).

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Discussion 

Chronic renal insufficiency is an established predictor of postoperative outcome traditionally estimated with SC. This is, however, an insensitive index, and particularly in cases of mild to moderate degrees of renal dysfunction. As a result, the National Kidney Foundation recommended the use of estimations of GFR from SC to avoid the misclassification of patients on the basis of SC alone, and defined five stages of severity based on the level of GFR.¹², ²⁰

In our study, GFR was estimated with the Cockcroft-Gault equation that is a simple and validated method,¹² requires information available in our database, and was earlier employed to stratify renal function according to the CKD stages.²¹ Also, our approach was methodologically consistent with previous studies on postoperative mortality after aortic aneurysms repair¹¹, ²², ²³ and coronary artery bypass grafting.²⁴

Our study confirmed GFR as a more accurate prognostic predictor than SC alone also in patients submitted to TEVAR. Furthermore, as hypothesized, perioperative and midterm mortality directly correlated with the severity of CKD stages, allowing a risk stratification model. The most relevant decrease in perioperative survival rates was observed in patients with preoperative GFR <60 ml/min/1.73 m² (ie, lower GFR quartile), that had a mortality ten times greater than that of patients with higher GFR values. Interestingly, this value coincides with the threshold for definition of CKD,²⁰ that represents a reduction by more than half of the normal GFR level, and is associated with the onset of laboratory abnormalities characteristic of kidney failure.²⁰

The analysis of the exact causal relationship between CKD and mortality is beyond the scope of this study, and would require ad hoc studies on a larger number of patients. However, we can speculate that poor survival rates are related to the increased risk of adverse cardiovascular events in patients affected with CKD.²⁰ In particular, both atherosclerosis and large-vessel remodeling are present in these patients, leading clinically to ischemic heart disease, cerebrovascular disease, peripheral vascular disease, heart failure, increased systolic blood pressure, and left ventricular hypertrophy.²⁰ Notably, even relatively minor renal abnormalities, which may remain unrevealed by routine preoperative evaluation, are associated with such a risk.²⁵ In this respect, the sensitivity of a GFR-based stratification appears again of great importance. In our study, overall 15 out of 20 deaths were due to myocardial infarction, stroke, multiorgan embolization, aneurysm rupture, and multiple organ failure.

Previous works demonstrated that TEVAR entails a relevant morbidity, including stroke, dialysis, and paraplegia, and mortality in high-risk patients.⁸, ²⁶, ²⁷, ²⁸, ²⁹ However, the definition of low or high-risk cohorts is practically based on whether patients are suitable candidates for conventional open repair. We believe that the development of a consistent and widely shared risk stratification model is necessary for patient selection among candidates to compassionate treatment, to identify who, in fact, may not benefit from it, and also to establish accurate matching criteria for randomized or case-control studies in patients at present grossly defined at “low-risk.”

To our knowledge, the literature does not provide specific studies on risk factors for mortality after TEVAR, with the exception of a recent work by Khoynezhadet et al.³⁰ The authors found only procedural type I endoleak as an independent risk factor of early mortality, and COPD, postoperative myocardial infarction, and acute renal failure as predictors of late death. Chronic renal insufficiency did not result a significant risk factor, but only preoperative permanent dialysis dependence was included among the analyzed variables.³⁰

In conclusion, we recognize some limitations of our study, including the intrinsic biases related to its retrospective fashion, even though data were prospectively collected. Due to the small number of patients, there may be other significant variables that remained unrevealed as a consequence of Type II errors, even though for this reason we ran a multivariate analysis including also risk factors that approached but not reached statistical significance at univariate analysis. In addition, the actuarial analysis is of limited value due to the small size of the lower CKD stages groups. Finally, although etiology, extension of aortic pathologies, and adjunctive procedures were taken into account, our data set included heterogeneous groups of patients, and therefore a comparison with other studies from the literature may result difficult.

Nevertheless, we believe that our work provides evidence that patients stratification based on CKD GFR stages is a reliable and useful prognostic tool to be employed both for risk-adjusted preoperative evaluation, and comparative studies regarding the safety and efficacy of ongoing technical developments, and next generation endografts.

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Author contributions 

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