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Kimberly B. Duke, MS Data Analysis and manuscript preparation
Michael C. Donohue, PhD Data Analysis and manuscript preparation
Continuous peripheral nerve blocks (CPNB) may induce muscle weakness, and multiple recently published series emphasize patient falls after post-arthroplasty CPNB. However, none have included an adequate control group, and therefore the relationship between CPNB and falls remains speculative.
We pooled data from 3 previously published randomized, triple-masked, placebo-controlled studies of CPNB involving the femoral nerve after knee and hip arthroplasty.
No patients receiving perineural saline (n=86) fell (0%, 95%CI = 0–5%), but there were 7 falls in 6 patients receiving perineural ropivacaine (n=85; 7%, 95%CI = 3–15%; Fisher’s exact test p=0.013).
Our analysis suggests there is a causal relationship between CPNB and the risk of falling after knee and hip arthroplasty.
More than one-million knee and hip arthroplasties are performed every year in the United States alone, and that number is expected to grow to 4-million annually within the next 20 years.1 Continuous peripheral nerve blocks (CPNB) are often used to provide postoperative analgesia for these often painful procedures. Although inhibition of pain fibers is the primary goal of CPNB, currently available local anesthetics approved for clinical use often decrease other afferent (e.g., nonpain-related sensory and proprioception) and efferent (e.g., motor) nerve fibers as well,2 resulting in undesirable side effects such as muscular weakness that is particularly undesirable in CPNB that affect the femoral nerve and quadriceps femoris function required for ambulation.3 Also, in contrast to the numerous articles in anesthesiology-centered journals emphasizing the multiple benefits of CPNB, publications of patient series emphasizing falls associated with CPNB have recently appeared within the surgical literature.4;5 For example, Feibel et al. report a 0.7% fall rate in a series of 1190 post-knee arthroplasty patients with a femoral CPNB.5 Unfortunately, none of these series included an adequate control group to determine whether the CPNB added to the risk of patient falls. Therefore, it remains unknown whether the falls, common in elderly surgical patients after joint arthroplasty without regional analgesia,6 would have occurred regardless of the perineural local anesthetic infusion.7 In the current report we pool data from 3 previously published randomized, triple-masked, placebo-controlled studies of CPNB involving the femoral nerve to determine if there is a causal relationship between lower extremity CPNB and postoperative falls after knee and hip arthroplasty.
Data available from previously published multicenter, randomized, triple-masked, placebo-controlled reports involving CPNB affecting the femoral nerve after knee and hip arthroplasty were pooled.8–10 No IRB oversight was required since the Common Rule exempts research “involving the collection or study of existing data… if these sources are publicly available or if the information is recorded by the investigator in a manner that subjects cannot be identified, directly or through identifiers linked to the subjects.”11,12 The primary outcome of the analysis was prospectively specified as the number of patient falls in each treatment group: either perineural ropivacaine 0.2% or perineural normal saline. Secondary outcomes included the association of other patient variables with the risk of falls. We first analyzed descriptive summaries of fall status for age, gender, weight, height, body mass index, surgical time, hospitalization duration, quadriceps weakness leading to local anesthetic basal infusion decrease, and ambulatory distance for each of the 3 days after surgery in the subgroup of patients that received ropivacaine. The primary end point, the comparison of proportion of falls across treatment groups, was analyzed with Fisher’s exact test applied to the pooled sample. We also checked for balance across treatment arms with respect to relevant covariates with, two-sample t-tests and Fisher's exact tests within each study and in the pooled sample. We applied the same tests to check for balance between those who fell and those who did not fall in order to identify any covariates that might also explain differential fall rates. We applied Pearson’s chi-squared test to examine differences in the fall rate across the 3 studies, and generated one sample 95% confidence intervals for the fall rate by inverting the score test and clipping the interval to 0–1.13 All analyses were performed using R version 2.11 (2010).†
Of the 3 multicenter studies included in the analysis, 2 involved femoral and one posterior lumbar plexus (blockade of the lateral femoral cutaneous and obturator nerves in addition to the femoral nerve) CPNB. No patients of the control groups (pooled n=86) fell (0%, 95% confidence interval = 0–5%) while receiving perineural normal saline, but there were 7 falls in 6 patients receiving perineural ropivacaine (pooled n=85; 7%, 95% confidence interval = 3–15%); p=0.013 (Tables 1 and and2).2). In addition, among patients who received ropivacaine, the fall rate was not different across the 3 studies to a statistically significant degree (Pearson’s Chi-squared = 1.47, df=2, p=0.48). Of the secondary outcomes (Table 3), it is noteworthy that of patients who experienced a fall (n=6), the time to discharge was less (mean [SD] = 3.0 [0.0] days) than in patients who did not experience a fall (n=165; 3.4 [1.0] days; p<0.001).
While all 3 previously published multicenter, randomized, triple-masked, placebo-controlled investigations reported patient falls during ropivacaine perineural infusion and none during saline perineural saline infusion, these between-treatment differences did not reach the level of statistical significance and no conclusions were offered regarding the association of CPNB and the risk of falling.8–10 However, when pooled for the current analysis, the 7 falls in 6 patients receiving ropivacaine versus no patients receiving saline (p=0.013) does suggest that there is a causal relationship between CPNB affecting quadriceps strength and the risk of falling after knee and hip arthroplasty (none wore a knee immobilizer). While the major limitation of this analysis is selection bias, until additional data are available practitioners may want to consider steps that may minimize the risk of falls. Such steps include, but are not limited to, minimizing the dose/mass of local anesthetic;14 providing limited-volume patient-controlled bolus doses which allow for a decreased basal dose without compromising analgesia in some cases,15;16 although not all;17 using a knee immobilizer and walker/crutches during ambulation;18 and educating physical therapists, nurses, and surgeons of possible CPNB-induced muscle weakness and necessary fall precautions. In addition, since the risk of falling was associated with earlier discharge (P<0.001) and 4 of the 7 falls occurred after discharge (after previous successful in-hospital ambulation), practitioners may wish to include the risk of falling as part of patient informed consent before ambulatory CPNB after knee and hip arthroplasty.
Financial Support: Funding for this project provided by National Institutes of Health grant GM077026 (P.I.: Dr. Ilfeld) from the National Institute of General Medical Sciences (Bethesda, Maryland, USA); National Institutes of Health grant RR000827 from the National Center for Research Resources (Bethesda, Maryland, USA); and the University of California San Diego Department of Anesthesiology, San Diego, California.
Conflict of Interest: Dr. Ilfeld received funding for the 3 previously published studies included in this analysis from Arrow International (Reading, Pennsylvania, USA), B. Braun Medical (Bethlehem, Pennsylvania, USA), and Stryker Instruments (Kalamazoo, Michigan, USA). These companies had absolutely no input into any aspect of the present study conceptualization, design, and implementation; data collection, analysis and interpretation; or manuscript preparation of the previously published investigations or the current study.
†R Software Environment for Statistical Computing, R Foundation for Statistical Computing, Vienna, Austria. Available at: http://www.r-project.org. Accessed May 17, 2010.