Editor’s Perspective
• The use of peripheral nerve blocks after ambulatory shoulder surgery is increasing

• While short-term pain control is improved by nerve blocks in this context, the relationship with postdischarge outcomes is unclear

• Peripheral nerve blocks are associated with a decrease in unplanned admissions after ambulatory shoulder surgery

• There is no associated improvement in other postoperative outcomes such as emergency department visits, readmissions, mortality, or costs

Background

Nerve blocks improve early pain after ambulatory shoulder surgery; impact on postdischarge outcomes is poorly described. Our objective was to measure the association between nerve blocks and health system outcomes after ambulatory shoulder surgery.

Methods

We conducted a population-based cohort study using linked administrative data from 118 hospitals in Ontario, Canada. Adults having elective ambulatory shoulder surgery (open or arthroscopic) from April 1, 2009, to December 31, 2016, were included. After validation of physician billing codes to identify nerve blocks, we used multilevel, multivariable regression to estimate the association of nerve blocks with a composite of unplanned admissions, emergency department visits, readmissions or death within 7 days of surgery (primary outcome) and healthcare costs (secondary outcome). Neurology consultations and nerve conduction studies were measured as safety indicators.

Results

### Sample Size

This was a population-based cohort, so all eligible participants were included. With more than 6,000 outcomes, we conservatively had 600 degrees of freedom to support logistic regression modeling.37  We did not prespecify a clinically important difference in the primary outcome.

### Missing Data

Main outcome and exposure variables were complete for all participants.

We identified 59,644 people who underwent shoulder surgery in Ontario from January 2009 to December 2016 (fig. 1) at one of 118 different hospitals. Overall, nerve blocks were placed in 31,073 patients (52.1%), 1,508 (4.9%) of which were catheters. In the first year of the study, 626 of 6,487 (9.7%) of patients received a nerve block for their shoulder surgery; subsequent years had an increasingly greater proportion of patients who received a nerve block (fig. 2). Patient characteristics are described in table 1.

Table 1.

Characteristics of the Study Cohort

Fig. 1.

Flow diagram depicting the creation of the analytical dataset.

Fig. 1.

Flow diagram depicting the creation of the analytical dataset.

Close modal
Fig. 2.

Graph displaying the number of peripheral nerve blocks placed by year of study.

Fig. 2.

Graph displaying the number of peripheral nerve blocks placed by year of study.

Close modal

In the total cohort, 6,234 of 59,644 (10.4%) experienced the primary outcome (no patients died in the 30 days after surgery). Specifically, 4.9% of patients had an unplanned admission after their surgery, 0.3% of patients were readmitted to the hospital within 7 days of their surgery, and 5.9% of patients were seen in the emergency department within 7 days of their surgery.

### Primary Outcome

Of people with a nerve block, 2,808 of 31,073 patients (9.0%) had an admission, readmission, or emergency department visit within 7 days of surgery compared to 3,424 of 28,571 patients (12.0%) without a nerve block (unadjusted odds ratio 0.73; 95% CI 0.69 to 0.77; P < 0.0001). After multilevel, multivariable adjustment, no significant difference remained (odds ratio 0.96; 95% CI 0.89 to 1.03; P = 0.243). The fully specified model is provided in Supplemental Digital Content, appendix 3 (http://links.lww.com/ALN/B993), including the calibration plot, which demonstrated good agreement between observed and expected outcomes across the risk spectrum.

When evaluating the individual components of the composite outcome, there was a significant adjusted decrease in unplanned admissions for patients with a nerve block (adjusted odds ratio 0.88; 95% CI 0.79 to 0.98; P = 0.020). There was no significant adjusted difference in readmissions or emergency department visits within 7 days between the two groups, although the directional associations for these postdischarge associations did not favor nerve blocks (table 2). Table 3 describes the most common physician-assigned primary diagnoses for patients who presented to the emergency department. When the primary outcome was measured over the 30 days after surgery, findings were similar to the 7-day outcomes (table 2).

Table 2.

Association of Peripheral Nerve Blocks with Outcomes in Ambulatory Shoulder Surgery (Primary and Secondary)

Table 3.

Most Common Etiologies of Unplanned Admissions and ED Visits within 7 Days of Surgery

Before adjustment, health system costs on the day of surgery to 7 days after surgery were significantly higher with a nerve block (median cost with a nerve block $4,681 vs.$4,391 without; ratio of means 1.07; 95% CI 1.07 to 1.07; P < 0.0001). After multilevel multivariable adjustment, costs remained significantly higher in those who received a nerve block (ratio of means 1.06; 95% CI 1.02 to 1.10; P = 0.005). We found $325 (95% CI$316 to $333) in excess health system costs attributable to provision of a nerve block. A similar cost difference was seen with the 30-day cost data (median cost with a nerve block$4,840 vs. $4,528 without; adjusted ratio of means 1.06; 95% CI 1.02 to 1.10; P = 0.007). Evaluating safety indicators between the receipt of a nerve block versus no nerve block, we found that there were no differences in the odds of either neurology consultations (adjusted odds ratio 1.04; 95% CI 0.71 to 1.53; P = 0.839) or nerve conduction studies in the 90 days after surgery (adjusted odds ratio 1.02; 95% CI 0.84 to 1.24; P = 0.834). ### Sensitivity Analyses In people more than 65 yr of age (n = 8,653 or 15% of total cohort), for whom we could add additional adjustment for receipt of preoperative opioids, there was no difference in the adjusted odds of the composite outcome between nerve blocks compared to those without a nerve block (adjusted odds ratio 0.87; 95% CI 0.75 to 1.03; P = 0.110). The propensity score analysis resulted in successful matching of 12,699 people with a nerve block to 12,699 people without (42.6% of total cohort; characteristics in Supplemental Digital Content, appendix 5, http://links.lww.com/ALN/B993). The presence of a nerve block was not associated with a difference in the composite outcome at 7 days (10.8%) compared with those without a nerve block (10.5%; adjusted odds ratio 1.04; 95% CI 0.95 to 1.13; P = 0.382). When we compared no nerve block (reference category) versus nerve block with no catheter versus nerve block with a catheter, the presence of a catheter was associated with a significant increase in the adjusted odds of the 7-day composite outcome (odds ratio 1.92; 95% CI 1.55 to 2.38; P < 0.0001) compared to no nerve block; there was no difference in the adjusted odds of composite outcome between no nerve block and nerve blocks without a catheter (odds ratio 0.93; 95% CI 0.87 to 1.00; P = 0.059). ### Subgroup Analyses There was no evidence of significant effect modification on the multiplicative scale between nerve block receipt and surgery type (P = 0.067), or nerve block and time period (P = 0.314). ### Reviewer-requested Sensitivity Analyses Results of the requested analyses are provided in Supplemental Digital Content, appendix 4 (http://links.lww.com/ALN/B993). No substantive changes in our results were identified for the primary composite outcome or 7-day health system costs. The association of increased costs persisted (although attenuated) after subtracting physician billing charges for block placement (ratio of means 1.03; 95% CI 1.03 to 1.04; P < 0.001). In this retrospective study examining nerve blocks in ambulatory shoulder surgery, there was no association between nerve blocks (measured using validated billing codes) and the composite outcome of unplanned admissions or readmissions, emergency department visits, or death within 7 days. These data suggest that the early benefits of decreased pain scores with nerve blocks, proven in randomized trials, may not translate into postdischarge health system benefits. Additionally, nerve blocks were associated with a$325 increase in health system costs up to 7 postoperative days; however, despite statistical significance, this may not be clinically significant. These findings suggest that pragmatic randomized trials focused on postdischarge patient-reported outcomes, and evaluation of processes, are needed to help extend the early benefits of nerve blocks into the postdischarge phase and to address important knowledge gaps around nerve block use in ambulatory shoulder surgery.

While using a composite outcome allowed us to assess a combination of pertinent outcomes in a manner relevant to patients and the health system, the individual components of the composite outcome warrant closer examination. Unplanned day of surgery admissions were significantly lower in the nerve block group, which is consistent with the trial data demonstrating improved early pain control and shorter postanesthesia care unit stays with nerve blocks.13  However, this early benefit did not impact postdischarge outcomes, as emergency department visits and readmissions did not differ between groups. The reasons for emergency department visits may provide some insight into this finding. In the nerve block group, acute pain was more common as the primary emergency department diagnosis (table 3). This could reflect rebound pain, a phenomenon in which profound initial analgesia from a block leads to inadequate oral analgesic consumption as the block wears off.40  These findings could inform a possible prevention strategy focusing on greater patient education or process optimization around systemic analgesia as nerve blocks wear off. In people without a block, emergency department diagnoses were more commonly related to surgical issues, which is consistent with previous research.41  Across both nerve block and no nerve block groups, it is also important to note that approximately 6% of adults having ambulatory shoulder surgery return to the emergency department or are readmitted to hospital within 7 days.

Our sensitivity analyses also provide insights into the relationship between nerve blocks and outcomes. As indication and confounding bias are also important considerations in database research,39  we assessed whether a propensity score match (as opposed to our regression model) would lead to differing results (as matching allows one to estimate the effect of an intervention in the section of the population that is most comparable at baseline, whereas regression provides an estimate of what would happen if the whole population switched from no block to a block).42  However, the results were similar (no significant difference associated with nerve blocks). Unmeasured confounders can also bias results, and in the case of nerve blocks, baseline chronic pain could influence both receipt of a nerve block and risk of an adverse outcome. However, in those older than 65 yr, where prescription opioid data was available, again we found no primary outcome difference. Finally, as an exploratory analysis, we compared no block with isolated blocks or blocks with a catheter. While we found that receipt of a catheter was associated with a 92% relative increase in the odds of unplanned admissions, readmissions, or emergency department visits, caution is needed in interpreting the result of a secondary analysis. There is little published evidence regarding postdischarge outcomes with catheters versus single shot nerve blocks.43  This finding could reflect complications specific to the catheter, or patient uncertainty associated with continuous catheters. Unmeasured confounding may contribute to this effect size; however, using the E-value44  to estimate how strong a missing variable would need to be to explain away the measured effect suggests that this is unlikely (we estimate that a missing variable would need to have an odds ratio of 3.25 to decrease the association between catheters and adverse outcomes to no effect).

Finally, our analysis also addressed the association of nerve blocks with health system costs and safety indicators. We found a statistically significant association between nerve blocks and a small increase in health system costs on the day of surgery to 7 days afterward (approximately $325). Some of this cost is attributable to the cost of the physician service in placing the block and the additional anesthesia care time (in Ontario, anesthesiologists are fee-for-service and bill for specific procedures as well as time-based billing); however, even after accounting for these charges, a 3% relative increase in costs remained. Whether these costs represent a clinically important increase is questionable. For example, one must consider whether this increased cost may still represent value in providing a nerve block. While we were unable to identify any valuation data specific to pain avoidance after ambulatory surgery, avoiding postoperative pain has been identified as the third highest priority outcome for patients after surgery, and chronic pain patients have identified that they would be willing to pay$56 to \$145 per day to avoid pain; therefore, the higher cost associated with nerve block placement may well provide value at the patient level.45–47  In terms of safety, we are not aware of validated means to identify nerve injuries in administrative data; however, we did not find any differences between groups in the number of neurologic consultations or nerve conduction studies in the 90 days after surgery. While this outcome can only be considered a proxy for true nerve injury, it is important to note that there was no strong signal that nerve blocks were associated with increased nerve injuries significant enough to require specialist consultation of diagnostic testing.

### Limitations

Our findings are at risk of several types of bias. First, there is risk of misclassification bias. We validated our exposure to confirm that blocks were accurately identified, but only know that a block was placed, not how well it worked. Therefore, our findings reflect a pragmatic approach as opposed to an explanatory study.48  Confounding bias may influence receipt of specific interventions and outcomes; if unmeasured confounders led to higher risk of adverse events and increased likelihood of a block, our findings would be biased to the null. However, we controlled for prespecified confounders, and results were consistent across all analyses that were prespecified in our protocol. We were unable to measure patient-reported outcomes such as quality of life, quality of recovery, experience/satisfaction, or return to work. Our findings do not preclude benefit, as our 95% CI included values below the null value; however, despite a large sample, we found no statistically significant impact. Cost were captured at the health system level, but were not adequately granular to capture operating room supplies, and partly rely on indirect techniques that could be associated with estimation error. We did not have specifics of each nerve block technique (e.g., ultrasound vs. landmark, dose or type of local anesthetic or additives) that may impact nerve block efficacy. Findings may not generalize to all jurisdictions.

### Implications

Receipt of a nerve block for ambulatory shoulder surgery was not associated with a difference in unplanned admissions, emergency department visits, readmissions, or deaths in the 7 days after surgery; unplanned admission rates were lower in the presence of a nerve block. Pragmatic randomized trials powered for patient-centered postdischarge outcomes, as well as process evaluation, are needed to understand how the early benefits of blocks may extend after discharge and to fully inform anesthetic care.

### Acknowledgments

We would like to acknowledge the assistance of Sascha Davis, B.Sc., Information Specialist (Learning Services, The Ottawa Hospital, Ottawa, Canada) for her assistance in developing and executing our search. Dr. McIsaac acknowledges salary support from The Ottawa Hospital Department of Anesthesiology. This study was also supported by IC/ES, Toronto, Canada, which is funded by an annual grant from the Ontario Ministry of Health and Long-Term Care. The opinions, results and conclusions reported in this paper are those of the authors and are independent from the funding sources. No endorsement by IC/ES or the Ontario Ministry of Health and Long-Term Care is intended or should be inferred. These data sets were held securely in a linked, deidentified form and analyzed at IC/ES. This study used The Johns Hopkins ACG System version 10.

### Research Support

The study was funded by a Canadian Anesthesia Society Resident Research Grant. Dr. McIsaac receives salary support from Department of Anesthesiology and Pain Medicine, The Ottawa Hospital, is a Clinical Research Chair at the University of Ottawa, Ottawa, Canada, and is supported by the Canadian Anesthesia Society Career Scientist Award.

### Competing Interests

The authors declare no competing interests.

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