Spinal Versus General Anesthesia for Patients Undergoing Outpatient Total Knee Arthroplasty

A National Propensity Matched Analysis of Early Postoperative Outcomes

Mark C. Kendall; Alexander D. Cohen; Stephanie Principe-Marrero; Peter Sidhom; Patricia Apruzzese; Gildasio De Oliveira


BMC Anesthesiol. 2021;21(226) 

In This Article


The study was performed in accordance with the ethical standards of the Declaration of Helsinki (1964) and its subsequent amendments. This study was performed under an exempt status granted by the Institutional Review Board (IRB#1647940). The IRB determined that the study qualified for exemption under 45 CFR 46.101(b). The exemption was granted because the study involved a retrospective review of existing data recorded in such a manner that subjects cannot be identified, directly or through identifiers linked to the subjects. Clinical information of the subjects was obtained for the years between 2005 and 2018 from the American College of Surgeons (ACS) National Surgical Quality Improvement Program (NSQIP) database. The study is reported following the STROBE guidelines for reporting observational studies.[9]

The ACS-NSQIP database is a national prospective database that compiles voluntarily reported data from over 680 institutions in the United States. For example, over 1 million cases were submitted as part of the 2017 and 2018 update to the NSQIP database. Data is collected on over 300 variables that include preoperative risk factors, intraoperative variables and post-operative outcomes including complications up to 30 days after surgical procedures.[10] Data collection has been previously described in detail.[10,11] In brief, data are collected in 8-day cycles, with the first 40 procedures in the cycle included in the dataset. The most commonly performed procedures are capped at 5 within each cycle to increase procedure heterogeneity. Trained clinical nurses assigned at each site collect data for 30 days postoperatively using isolated telephone interviews and operative and clinical notes. Interrater reliability audits of selected participating sites help ensure the collected data are of the highest quality possible. The combined results of inter-rater reliability audits completed to date revealed an overall inter-rater disagreement rate of approximately 1.8% for all assessed program variables.[10,11]

De-identified patient information is freely available to all institutional members who comply with the ACS NSQIP Data Use Agreement. The Data Use Agreement implements the protections afforded by the Health Insurance Portability and Accountability Act of 1996 and the ACS NSQIP Hospital Participation Agreement. The ACS NSQIP and the hospitals participating in this program are the sources of the data used in this study; however, these entities have not verified and are not responsible for the statistical validity of the data analysis or the conclusions derived by the authors.[12,13]

The 2005 through 2018 NSQIP Participant Use Data Files were queried to extract all patients scheduled. Patients who underwent primary, elective, unilateral TKA were identified using the Current Procedural Terminology (CPT) code 27,447. Cases involving trauma, fracture, neoplasms, infectious diseases, or patients under 18 were excluded. Patients who qualified for the study under these criteria were then further stratified to an outpatient TKA cohort, defined as length of stay (LOS) of 0 days.[13]

Outcomes Variables and Analysis

Preoperative demographic variables such as age, sex, body mass index, functional status, American Society of Anesthesiologists physical status (ASA PS) classification, smoking status, preoperative hematocrit, hypertension, diabetes, congestive heart failure, bleeding disorder and chronic obstructive pulmonary disease were compared between the two cohorts. Surgical duration was also compared between the cohorts. The primary independent variable was the type of anesthesia performed (general anesthesia or spinal anesthesia).

Postoperative events were grouped into serious adverse events (SAE), minor adverse events (MAE), and any adverse events (AE) following the same classification of prior studies on patients undergoing TKA.[14,15] The primary outcome was serious adverse events defined as a composite that includes: (1) a return to operating room, (2) wound related infection, (3) thromboembolic event, (4) renal failure, (5) myocardial infarction, (6) cardiac arrest requiring cardiopulmonary resuscitation, (7) stroke or cerebrovascular accident, (8) on ventilator > 48 h, (9) unplanned intubation, (10) sepsis/septic shock, and (10) death. Minor adverse events included: (1) blood transfusion, (2) pneumonia, (3) wound dehiscence, (3) urinary tract infection and (4) renal insufficiency.

Statistical Analysis

Due to the observational (non-randomized) nature of this data, propensity score matching was used to minimize the effects of confounding when assessing differences in patient demographics between outpatient TKA procedures performed with spinal anesthesia and general anesthesia. In this study, the probability for undergoing an outpatient TKA procedure with general anesthesia (propensity score) was calculated for each patient based on age, sex, body mass index, ASA PS classification, functional status before surgery, smoking status, preoperative hematocrit, hypertension, diabetes, congestive heart failure, bleeding disorder, COPD, and surgical duration.[16] General anesthesia patients were one-to-one matched without replacement to a spinal anesthesia patient with the nearest propensity score, using a caliper of 0.10. If such a match was not available, the patient was eliminated. With this methodology, 1,962 general anesthesia patients were matched with patients who received spinal anesthesia.

Prior to matching, pre-operative demographics were compared using unpaired Student's t test for continuous variables, and chi-square test for binary and categorical variables. Pre-operative demographics were compared in the matched cohorts using paired t-tests for continuous variables, McNemar's Test for binary variables, and Bowker's Symmetry test for categorical variables.[17]

After propensity score matching, differences in outcome rates of the matched cohorts were assessed using McNemar's test for matched data. Relative risks were calculated, as were risk differences. The rates of events between general anesthesia and spinal anesthesia cohorts were compared at 72 h postoperative (events that occurred at any time in the 72 h postoperative period). To account for multiple endpoint testing, an adjusted p-value was calculated to correct for False Discovery Rate (FDR).

All statistical analyses were conducted with the use of SAS software version 9.4 (SAS Institute Inc., Cary, North Carolina).