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Acute kidney injury (AKI) is a serious postoperative complication and is associated with poor clinical outcomes, including the development of chronic kidney disease (CKD) and death. AKI occurs in approximately 12% of patients undergoing surgery,1 affecting one in five people during postoperative hospitalization and associated with a significant increase in morbidity and mortality.2 AKI is also associated with longer hospital stays.3 In particular, AKI occurs in 7% to 11% of patients undergoing elective total hip and knee arthroplasty.4 Important risk factors for AKI after lower extremity arthroplasty are CKD, postoperative sepsis, acute myocardial infarction, and blood transfusion. A previous study has shown that even transient AKI increases the risk of mortality, hospital costs, and length of stay.5
Degenerative spine disease is a leading cause of the loss of functional health status. Degenerative spine disease arises from a combination of micro- and macromechanical injuries, metabolic processes and risk factors such as age, sex, work environment, and genetics.6 Spinal surgery is always elective, meaning that patients’ make the decision whether or not to undergo surgery.7 The joints and ligamentous elements are part of the functional spinal unit underlying all degenerative morphological and structural changes. These changes involve the disc structure of one spinal segment, the joint-ligament at the same level, and the adjacent functional spinal unit.8 These factors affect various structures, including intervertebral discs, articular surfaces, ligaments, and spinal muscles. Degenerative spine disease accounts for extensive aspects of healthcare and may lead to substantial healthcare costs, as well as to loss of quality-adjusted life years—on top of the fact that disease incidence is increasing.9 One standard surgical treatment for lumbar degenerative disc disease is spinal fusion surgery, a surgical procedure in which two vertebrae are grafted together. The goal of fusion surgery is to reduce pain by reducing movement of the spinal segment.10 The other commonly performed surgery is lumbar decompression, which is performed to treat nerves compressed by degenerating discs in the spine; it is only performed when nonsurgical treatments have not relieved pain and nerve symptoms.10
As a result of increases in life expectancy and the widespread use of noninvasive imaging methods, clinicians, surgeons, and institutions are increasingly faced with the possibility of spinal lesions and surgical treatment of older adults.11 Surgery for most degenerative diseases is elective, and the risk-benefit of the intended surgery must be fully estimated before an individual patient undergoes surgery. Understanding the risks and benefits of surgery will help patients and physicians make clinical decisions. Spine surgery has several risk factors for the development of AKI, including increased intra-abdominal pressure due to being in a prone position, hemodynamic changes, surgical inflammation, embolic events, use of intraoperative vasoactive drugs, blood loss, and hemodilution.12,13 Although patients may develop AKI in various clinical conditions, the incidence of AKI associated with major surgery is particularly high.14 Preoperative identification of high-risk patients will enable early intervention and optimal perioperative management, leading to improved patient outcomes.
As mentioned earlier, recent studies have shown an increase in the incidence of AKI and its impact on morbidity and mortality.1,9,15,16 However, although potential risk factors for AKI have been examined in studies of orthopedic patients, only limited evidence is available to guide targeted intervention strategies aimed at reducing the risk of AKI.15,17,18 Few studies have investigated AKI after spinal surgery, especially surgery for degenerative spinal disease. The true incidence and predictors for AKI following degenerative spine surgery have yet to be evaluated. Data from administrative health records may be useful for stratifying patients at risk of perioperative AKI. This study aimed to investigate the incidence and determinants of AKI following elective surgeries for degenerative lumbar spine disease, using data from a large, nationally representative cohort.
2. METHODS 2.1. Study design and data sourceThis population-based, retrospective observational study extracted all data from the US Nationwide Inpatient Sample (NIS) database, the largest all-payer, continuous inpatient care database, including about 8 million hospital stays each year.19 Patient data include primary and secondary diagnoses, primary and secondary procedures, admission and discharge status, patient demographics, expected payment source, duration of hospital stay, and hospital characteristics (i.e., bed size/location/teaching status/hospital region). All patients are initially considered for inclusion. Data from the most recent NIS database are derived from about 1050 hospitals from 44 States in the United States, sampled to represent a 20% stratified sample of US community hospitals as defined by the American Hospital Association.
2.2. Ethical statementHCUP-NIS is a deidentified database, and the Institutional Review Board of Johns Hopkins Medical Institutions deemed that the study using HCUP-NIS database does not require institutional review board.
2.3. Study populationIn the present study, patient data were identified in the NIS database through codes of the International Classification of Diseases, Ninth and Tenth Revision (ICD-9 and ICD-10). Data of adults ≥40 years old admitted to US hospitals between 2005 and 2018 who were diagnosed with degenerative disease of the lumbar spine and were undergoing elective spinal fusion or decompression were included. Emergent admissions were excluded. Patients with missing information on age, gender, and other main study variables were also excluded. The study cohort was categorized by type of surgery, that is, decompression alone or spinal fusion.
2.4. Study endpointsThe primary study endpoint was the occurrence of AKI after elective surgical procedures for degenerative lumbar spine disease. Secondary endpoints were determinants of AKI following these elective spinal surgeries. The ICD-9 and ICD-10 codes for defining lumbar spine degenerative disease, fusion, decompression, and AKI are listed in Supplementary Table 1, https://links.lww.com/JCMA/A233. The usage of these ICD codes to identify AKI has been previously validated.20,21 (Supplementary Table 1, https://links.lww.com/JCMA/A233).
2.5. Study variablesPatients’ characteristics included age (grouped by range: 40–49, 50–59, 60–69, 70–79, and ≥80 years), gender, household income level (in quartiles), insurance status (primary payer), smoking status, major comorbidities (ischemic heart disease, congestive heart failure, atrial fibrillation, diabetes, anemia, hypertension, dyslipidemia, chronic obstructive pulmonary disease, cerebrovascular disease, peripheral vascular disease, overweight and obesity, drug abuse, alcohol abuse, autoimmune rheumatic disease, CKD, coagulopathy, or any malignancy), surgical approach for fusion (anterior, posterior, combined), ≥2 level of fusion, and whether or not having hospital-acquired infections. Long-term use of NSAIDs and aspirin/anticoagulants, identified through ICD codes, were also included as study covariates. Weekend admission, hospital-related characteristics (bed size/location/teaching status/hospital region), and hospital caseload (i.e., annual caseload of spinal metastasis) were extracted from the database as part of the comprehensive data available for all patients.
2.6. Statistical analysisGiven the complex sampling design of the HCUP-NIS data, all analyses were performed using SAS survey analysis statements (SAS Institute Inc., Cary, NC, USA). Continuous data are presented as weighted mean ± standard error. Categorical data are presented as unweighted counts (weighted percentage). Distribution of continuous and categorical data between patient groups were compared using SURVEYREG, while the Rao-Scott chi-square test was performed to examine differences in the proportions between groups using SURVEYFREQ statement for categorical variables. Univariate and multivariate logistic regression analysis was conducted using SURVEYLOGISTIC to evaluate associations between variables and AKI. To explore the potential predictors of AKI, covariates that were significantly associated with AKI in univariate analysis model were entered into the multivariate models. A two-sided p value of <0.05 was regarded as statistical significance.
3. RESULTS 3.1. Patient selectionDuring 2005 to 2018 in the HCUP-NIS database, data of 1,369,502 hospital admissions with diagnoses of lumbar degenerative disease were identified. Among these hospitalized patients, the data of 467,862 patients aged ≥40 years who had undergone or who were scheduled to undergo elective lumbar spinal surgeries for lumbar degenerative disease were included. After excluding patients who had undergone emergent surgery and those with missing information on age, gender, or main study variables, the remainder of 424,569 admissions were included for analysis. Applying the sample weights provided by the NIS, this sample size could be extrapolated back to a population of 2,099,145 hospitalized patients in the whole United States. The flow diagram of the study cohort selection process is presented in Fig. 1.
Fig. 1: Flow diagram of study selection.
3.2. Characteristics of patients undergoing surgery for degenerative lumbar spine diseaseTable 1 shows the baseline demographic, clinical, and hospital characteristics of the study population. Patients’ mean age was 61.7 ± 0.04 years. Most patients were female (56.2%) and nonsmokers (70.6%). Hypertension (56.2%), dyslipidemia (34.1%), and diabetes (19.4%) were the most common comorbidities in the study cohort. Among the study population, 55,495 (13.1%) patients received decompression alone, among whom 608 (1.1%) developed AKI. In addition, 369,074 patients (87.9%) had spinal fusion, among whom 6500 (1.8%) patients developed AKI. Age, gender, insurance status, certain comorbidities, status of weekend admission, long-term use of aspirin/anticoagulant, hospital-acquired infections, and hospital bed size were significantly different between patients with or without AKI during hospitalization among patients receiving decompression alone or fusion. Patients with AKI were older with a higher proportion of males. Most comorbidities were more frequent among patients with AKI (Table 1).
Table 1 - Characteristics of patients undergoing surgery for degenerative lumbar spine disease by surgery type Characteristics OverallP-values <0.05 are shown in bold.
AKI=acute kidney injury; CKD=chronic kidney disease; COPD=chronic obstructive pulmonary disease; NSAIDs=nonsteroidal antiinflammatory drugs.
Overall, an increasing trend was observed in AKI incidence, which had grown from 0.006% in 2005 to 0.023% in 2018, accompanied by an increase in lumbar spinal surgeries. In addition, the incidence of AKI was higher among the patients receiving spinal fusion than in those receiving decompression only, with a mean difference of about 0.006% (Fig. 2).
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