The incidence and factors associated with graft infection after aortic aneurysm repair

Study design 

A retrospective cohort study was conducted using a statewide hospital administrative discharge database. Records of inpatient hospitalizations between 1987 and 2005 were evaluated to assess the incidence of aortic graft infections.

Data source 

Data were obtained from the Washington State Comprehensive Hospital Abstract Reporting System (CHARS). This data set is derived from all public and private hospitals in the state, excluding Veterans Affairs and United States military hospitals excluded. It contains demographic variables, admission and discharge administrative details, payer status, International Classification of Diseases, Ninth Revision (ICD-9) procedure and diagnosis codes, and hospital identifiers. CHARS also allows for tracking of subsequent hospitalizations and linkage to vital statistics records. The data are maintained by the Washington State Department of Health, Office of Hospital and Patient Data Systems. The reporting is mandatory. Data are currently collected using the Uniform Billing (UB) 92 format (previously, UB82). The CHARS system is designed to accommodate data elements from the Medicare provider-billing file. The reporting is done by the hospitals on the basis of information in the patient charts. The CHARS data are entered by hospital coders according to information in the charts. This study was granted an exception by agreement of the University of Washington Human Subject Review Committee and the Washington State Department of Health. The data set includes only anonymous data and is considered within the public domain.

Subjects 

CHARS records from 1987 to 2005 were searched to identify all cases of abdominal aortic surgery indicated by the ICD-9 procedure. Cases included in the cohort were patients aged ≥18 years who had the ICD-9 code for abdominal aortic surgical procedures. Patients were categorized by ICD-9 diagnosis codes for repair of the abdominal aortic aneurysm from procedure codes as repairs with open and endovascular techniques. Classification as elective vs nonelective (which included “urgent” and “emergency”) was based on the CHARS variable for admission type. For this study, any cases that were defined as urgent were placed into the nonelective category.

Variable definitions 

Nosocomial infection (NI) was defined as an infection occurring in a patient in a hospital or other health care facility in whom it was not present or incubating at the time of admission; or the residual of an infection acquired during a previous admission. We evaluated diagnostic codes for NI, which were postoperative pneumonia (997.3, 482.x), urinary tract infections (599.0, V13.02), blood stream septicemia (BSI; 038.x), and surgical site infection (SSI; 998.5, 998.51, 998.59), or a combination of these, at the index admission.

To identify patients with AGI, code 996.62 was used to all identify patients with many types of graft infection. This code includes infection and inflammatory reaction as a result of any internal prosthetic device, implant, or graft due to a vascular implant. This code was then combined with appropriate aortic resections associated with revision of AGI codes, including 38.34, resection of vessel with anastomosis abdominal aorta; 38.44, resection of vessel with replacement abdominal aorta; 38.64, other excision of vessels abdominal aorta.

All other patients were excluded, and our study cohort reflects only patients undergoing aortic resection with an associated code of infected graft at the same readmission. To further assure that we did not overestimate, all patients with the code for dialysis renal failure preoperative and postoperatively were excluded (v56.0) to remove any possibility of dialysis graft infections in the cohort.

A modified version of the Charlson Comorbidity Index10 was calculated for each patient. The Charlson measure, which has been adapted for use with administrative claims, is a weighted index of comorbidity status based on ICD-9 codes from inpatient records. Scores range from 0 to 3, where 0 indicates the absence of comorbid conditions and a score >0 indicates the presence of one or more comorbid conditions.

Classification as elective vs nonelective (which included “urgent” and “emergency”) was based on the CHARS variable for admission type. For this study, any cases that were defined as urgent were placed into the nonelective category.

Outcome metrics included in-hospital death, readmission, reintervention, and 30-day mortality. Readmissions classified by the presence of diagnostic codes for vascular graft infections combined with procedures associated with treatment of an infected aortic graft (such as extra-anatomic bypass or redo aortic surgery) were identified. In-hospital death is directly coded as a discharge status for the index admission. The 30-day mortality was defined as “all-cause” death ≤30 days of discharge as ascertained from Washington State Vital Records.

Statistical analysis 

Descriptive statistics were calculated for the entire cohort and subgroups. Categoric variables were compared using Pearson χ2 statistic, and continuous variables were evaluated using analysis of variance. Logistic regression was used to create a 2-year model to define risk of AIG. The adjusted model contains all covariates and includes clustering by hospital. The cumulative hazard of readmission with graft infection was estimated using Nelson-Aalen methods. The Nelson-Aalen estimator is a nonparametric estimator of the cumulative hazard function based on a sample that is subject to right censoring. The cumulative hazard is estimated by summing the hazard of, in this case, graft infection over time. This time-to-event analysis was used to account for the variable follow-up time among subject in the cohort. Statistical analysis was performed using Stata 9 statistical software (Stata Corp, College Station, Tex).

As well, to account for the introduction of the code for EVAR in 2000, the 2-year rates of infection were used when open surgery was compared with EVAR. This was done to insure that there was a fair comparison of the two treatment methods and that the incidence of AGI was not skewed by the large open experience reported in the data.

Demographic information 

Between 1987 and 2005, 13,902 patients (mean age, 71.3 ± 8.8 years; 90.8% men) underwent abdominal aortic surgery; of these, 12,626 had open repair and 1276 had EVAR (Table I). Patients undergoing EVAR had higher Charlson scores than those having open aortic surgery (P < .001; Table II). A total of 9835 of the cases in the study period were elective procedures (70.8%).

Table I. Cohort characteristics

		Total	Nonelective	Elective	P
	Subjects, No.	13,902	4067	9835
	Age, mean years	71.3	71.7	71.1	.001
	Male, %	79.3	77.8	80.0	.003
	Charlson Comorbidity Index category, %				<.001
	0	1.9	2.2	1.4
	1	58.3	61.6	56.9
	2	32.1	29.2	33.4
	3+	7.7	7.0	8.0
	Endovascular repair, %	9.2	2.9	11.8	<.001
	Any nosocomial infection, %	13.1	18.6	10.8	<.001
	Pneumonia	10.3	13.4	9.1	<.001
	Urinary tract infection	1.6	2.8	1.2	<.001
	Blood stream septicemia	1.6	3.2	0.9	<.001
	Surgical site infection	0.4	0.8	0.3	<.001
	Length of stay, mean days	9.1	11.7	8.0	<.001
	Discharge status, %				<.001
	Home	79.3	58.6	87.8
	Care facility	10.9	16.8	8.5
	In-hospital death	9.8	24.6	3.7
	Readmission with graft infection, %	0.4	0.4	0.5	.6
	Graft infection ≤2 years	0.2	0.2	0.2	.6
	Days to graft infection, median	937	540	1330	.1
	Graft infection rate/1000 person-years	0.7	0.7	0.7	.5

Table II. Cohort characteristic by procedure type

	Characteristic	Open	EVAR	P
	Subjects, No.	12,626	1276
	Age, mean years	71.0	73.5	<.001
	Male, %	79.9	83.4	<.001
	Charlson Comorbidity Index category, %			<.001
	0	2	1.1
	59.2	49.1
	2	31.5	38.4
	3+	7.4	11.4
	Elective admission, %	68.7	90.8	<.001
	Any nosocomial infection, %	14.0	4.0	<.001
	Pneumonia	11.1	2.4	<.001
	Urinary tract infection	1.7	1.2	.2
	Blood stream septicemia	1.6	0.7	.01
	Surgical site infection	0.5	0.0	.01
	Length of stay, mean days	9.6	3.6
	Discharge status, %			<.001
	Home	78.1	90.7
	Care facility	11.3	7.0
	In-hospital death	10.6	2.3
	Readmission with graft infection, %	0.4	0.2	.3
	Graft infection ≤2 years, %	0.2	0.2	.8
	Days to graft infection, median	1100	684
	Graft infection rate/1000 person-years	0.7	1.6	.07

EVAR, Endovascular aneurysm repair.

Outcomes 

The in-hospital mortality rate for the entire cohort was 9.81%, with 2.35% for EVAR vs 10.6% for open surgery (P < .001). The 30-day mortality was 3.8% for elective aortic surgery vs 25.0% for nonelective aortic surgery. The cumulative rate of AIG in the cohort was 0.44%. The in-hospital mortality rate at the first readmission for those identified with AGI was 18%, and the overall 1-year mortality rate for those identified to have AGI was 28%.

Group comparison 

Patients undergoing aortic aneurysm repair were compared evaluating elective or nonelective repair (Table I), open and EVAR procedures (Table II), NI at the index visit (Table III), and by the development of AGI (Table IV). Nonelective patients were more likely male and significantly more likely to develop NIs (18.6 vs 10.8, P < .001). Nonelective patients had longer lengths of stay (11.7 vs 8.0, P < .001) and were less likely to be discharged home (58.6% vs 87.8%, P < .001). The rate of readmission with a graft infection and the 2-year rate of graft infection were similar (0.2% vs 0.2%, P = .6).

Table III. Cohort characteristic by index nosocomial infection

	Characteristic	Nosocomial infection		P
		No	Yes
	Subjects, No.	12,080	1822
	Age, mean years	71.1	72.4	<.001
	Male, %	80	75.1	<.001
	Charlson Comorbidity Index category, %			.005
	0	1.9	1.7
	1	58.1	59.5
	2	32	33.2
	3+	8	5.7
	Elective admission, %	72.6	58.4	<.001
	Length of stay, mean days	8	16	<.001
	Discharge status, %			<.001
	Home	81.8	62.7
	Care facility	9.2	22.8
	In-hospital Death	9.1	14.5
	Readmission with graft infection, %	0.4	0.6	.4
	Graft infection ≤2 years	0.2	0.3	.2
	Days to graft infection, median	1100	511.5	.3
	Graft infection rate/1000 person-years	0.7	0.9	.2

CCI, Charlson Comorbidity Index.

Table IV. Cohort characteristic by graft infection (readmission)

	Characteristic	Aortic graft infection		P
		No	Yes
	Subjects, No.	13841	61
	Male, %	79.3	82.0	.6
	CCI category, %			.008
	0	1.9	6.6
	1	58.3	59.0
	2	32.1	34.4
	3+	7.8	0.0
	Elective admission, %	70.7	73.8	.6
	Endovascular repair, %	9.2	6.7	.5
	Length of stay, mean days	9.1	11.5	.03
	Discharge status, %			.04
	Home	79.2	88.5
	Care facility	10.9	11.5
	In-hospital death	9.9	0.0

CCI, Charlson Comorbidity Index.

Patients undergoing open procedures (Table II) were more often younger, healthier, more likely to develop NIs, and less likely to be discharged to home. Patients having open repair and EVAR had similar 2-year rates of graft infection and infection rates (0.2% vs 0.2%, P = .8). When compared using 1000 person-years, however, the rate of infection for EVAR was nearly half (0.7% vs 1.6%, P = .07), although this was not a significant difference. Patients who had open aortic procedures had greater rates of pneumonia (11.1% vs 2.4%, P < .001), BSI (1.6% vs 0.7%, P < .01), and SSI (.5% vs 0%, P < .012) compared with EVAR.

Patients who developed NIs (Table III) at their index visit were more likely to have longer index hospitalizations (8 days vs 16 days, P < .001) and less likely to have had elective surgery (72.6 vs 58.4, P < .001). Patients who developed a NI at the index admission had shorter times to developing AIG (511 vs 1100 days). Most (72.6%) of the cases without development of NI were elective cases compared with patients developing a NI, of which 58.4% were elective (P < .001).

The 2-year rate of AGI was 0.19% among open vs 0.16% in EVAR (P = .75) and 0.2% in both elective and nonelective patients. The 2-year rate of AGI was higher, although not significant, after any index NI (0.33% vs 0.17%, P = .16) or pneumonia (0.35% vs 0.18%, P = .16). Blood stream septicemia (0.93% vs 0.18%, P = .014) and SSI (1.61% vs 0.19%, P = .01) were significantly associated with the development of AGI. The median time to AGI presented earlier if a perioperative NI occurred (1100 vs 511, P = .3). This risk of developing AGI after open repair was highest in the first postoperative year, with 32% of all graft infections in the open cohort presenting in year 1. Of interest was that no 1-year AGI was seen in the EVAR group. Odds ratios (ORs) for AGI within 2 years showed that after adjustment, BSI was significantly associated with AGI (OR, 4.2; 95% confidence interval (CI), 1.5-11.8; Table V). The Fig demonstrates the difference in the cumulative incidence of AGI in patients undergoing open repair and EVAR (log-rank P = .34).

Table V. Odds ratios for graft infection ≤2 years

	Covariates	Unadjusted OR	95% CI	Adjusted OR⁎	95% CI
	EVAR index procedure	0.8	0.2-3.3	0.9	0.2-3.3
	Age 65+	1.1	0.4-2.9	1.1	0.4-3.3
	CCI category (1+)	0.5	0.1-3.7	0.5	0.1-4.8
	Income above median	1.7	0.8-3.8	1.7	0.9-3.3
	Elective admission	0.8	0.4-1.8	0.9	0.5-1.8
	High-volume hospital	1.1	0.3-4.6	1.0	0.3-3.8
	Any nosocomial infection	1.9	0.7-4.7
	Pneumonia	2.0	0.7-5.2	1.7	0.8-3.9
	Bloodstream septicemia	5.1	1.2-21.7	4.2	1.5-11.8
	Surgical site infection	8.7	1.2-65.2	6.8	0.8-61.1

CCI, Charlson Comorbidity Index; CI, confidence interval; EVAR, endovascular aneurysm repair; OR, odds ratio.

⁎ The adjusted model contains all covariates and includes clustering by hospital.

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Fig. Nelson-Aalen cumulative hazard estimates. Graft infection readmission by index nosocomial infection (NI).