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Effect of Interscalene Brachial Plexus Block on the Pulmonary Function of Obese Patients

A Prospective, Observational Cohort Study

Melton, M. Stephen, MD*; Monroe, Hanni E., MD*; Qi, Wenjing, PhD; Lewis, Stephanie L., MD*; Nielsen, Karen C., MD*; Klein, Stephen M., MD*

doi: 10.1213/ANE.0000000000002180
Chronic Pain Medicine: Original Clinical Research Report
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BACKGROUND: The effect of interscalene block (ISB) on pulmonary function of obese participants has not been investigated. The goal of this study is to assess the association of obesity (body mass index [BMI] >29 kg/m2 vs BMI <25 kg/m2) and change in forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) after ISB in participants undergoing outpatient shoulder surgery.

METHODS: This prospective, observational cohort study compared obese (BMI >29 kg/m2) and normal-weight (BMI <25 kg/m2) groups undergoing ISB for ambulatory shoulder surgery, on preblock and postblock FVC and FEV1, at 30 minutes postblock and in the postanesthesia care unit (PACU). The primary outcome in this study was FVC% change (percentage change from preblock to postblock values of FVC) at 30 minutes postblock in the supine position. Secondary outcomes included FVC% change at PACU and in the sitting position, FEV1% change (percentage change from preblock to postblock values of FEV1), FVC, FEV1, incidence of diaphragmatic paresis, modified Borg scale for perceived dyspnea, Richmond Agitation-Sedation Scale scores for sedation, and intraoperative airway events.

RESULTS: Fourteen participants were recruited to each group. The mean (standard deviation) BMI in the normal-weight and obese groups was 23 (1.7) and 33 (3.1) kg/m2, respectively. ISB success rate was 100%. All participants demonstrated hemidiaphragmatic paresis after ISB. Compared to the normal-weight group, in the sitting position, the obese group had a significant decrease in FVC% change at 30 minutes (−30 [10.5] vs −23 [7.2], P = .046) and an FEV1% change in the PACU (−40 [12.6] vs −27 [13.9], P = .02). No difference was found for measurements taken in the supine position. A repeated-measures analysis demonstrated that, adjusted for position, there is no significant group effect on FVC% change or FEV1% change from 30 minutes to PACU. The 2 groups were not different in terms of breathlessness and sedation at 30 minutes (P = .67, P = .48, respectively) and in the PACU (P = .69, P > .99, respectively) nor in the occurrence of intraoperative airway events (P > .99).

CONCLUSIONS: ISB is associated with greater FVC and FEV1 reductions in obese participants undergoing shoulder surgery compared to normal-weight participants. Neither time (30 minutes versus PACU) nor position (sitting versus supine) affected this relationship. Despite these changes, obesity was not associated with increased clinical respiratory symptoms or events.

From the Departments of *Anesthesiology and Biostatistics and Bioinformatics, Duke University Medical Center, Durham, North Carolina.

Accepted for publication April 24, 2017.

Funding: None.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to M. Stephen Melton, MD, Department of Anesthesiology, Duke University Medical Center, DUMC 3094, Stop 4, Durham, NC 27710. Address e-mail to steve.melton@duke.edu.

Shoulder arthroscopy and rotator cuff repair are common ambulatory surgery procedures.1,2 Interscalene block (ISB) is a regional anesthetic technique frequently utilized in these cases.3–6 Advantages of ISB include excellent surgical anesthesia and/or postoperative analgesia, reduced postoperative opioid consumption and opioid-related side effects, decreased postanesthesia care unit (PACU) length of stay, and improved patient satisfaction.3 Despite these benefits, a potential disadvantage of ISB is a transient ipsilateral hemidiaphragmatic paresis from spread of local anesthetic to the phrenic nerve.7 Ipsilateral hemidiaphragmatic paresis inhibits normal lung expansion, as a result of the paralyzed hemidiaphragm moving in a paradoxical cephalad direction during inspiration.8 Previous studies have confirmed that concomitant paralysis of the ipsilateral hemidiaphragm is an unavoidable consequence of ISB when typical volumes of local anesthetic (10–45 mL) are used.7,9–11 Even ultra-low volumes of local anesthetic (5 mL) placed under ultrasound guidance are associated with a 45% incidence of hemidiaphragmatic paresis.12 Five minutes after ISB and resultant hemidiaphragmatic paresis, Urmey and McDonald8 found that nonobese participants had a 27% decrease in forced vital capacity (FVC) and a 26% decrease in forced expiratory volume in 1 second (FEV1). Subsequent investigation in nonobese participants demonstrated that the magnitude of this reduction reached a plateau at 15 minutes with no significant change in FVC and FEV1 between 15 and 30 minutes; the supine position further exacerbated these reductions; and ISB local anesthetic injection volume (20 vs 45 mL) produced no significant difference in onset or degree of pulmonary dysfunction over time.9 Hemidiaphragmatic paresis was limited to local anesthetic duration of action.

To our knowledge, the effect of body mass index (BMI) on pulmonary function after ISB has not been investigated. As more cases transition to the ambulatory setting and the incidence of obesity rises,13 an increasing number of patients presenting for outpatient shoulder surgery will be obese (BMI >29 kg/m2) and morbidly obese (BMI >39 kg/m2). These patients present unique perioperative pulmonary issues and pose increased risk for respiratory-related complications.14 While ISB for shoulder surgery minimizes opioid use and potentially the need for general anesthesia, which may be beneficial in the anesthetic care of obese and morbidly obese patients, the respiratory-related side effects and resultant pulmonary dysfunction associated with its use are poorly defined in this population.

This prospective, observational cohort study compared the effects of ISB and associated hemidiaphragmatic paresis on pulmonary function tests (PFTs: FVC, FEV1) in obese versus normal-weight participants undergoing outpatient shoulder surgery. The hypotheses were that after ISB and associated hemidiaphragmatic paresis: (1) obese participants would demonstrate a greater decrease in PFTs versus normal-weight participants; (2) the difference in this decrease between obese and normal-weight groups would be more profound in the supine position compared to the sitting position; and (3) for both normal-weight and obese groups, the decrease in PFTs from preblock values would persist postoperatively in the PACU.

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METHODS

This article adheres to the Strengthening the Reporting of Observational Studies in Epidemiology guidelines. The Duke Institutional Review Board approved this study, and informed written consent was obtained from each enrolled participant. This was a prospective, observational cohort study comparing the effects of ISB and associated hemidiaphragmatic paresis on PFT values (FVC and FEV1) in obese versus normal-weight participants who underwent elective outpatient shoulder surgery under ISB at the Duke Ambulatory Surgery Center in Durham, NC, between January 2012 and March 2013. Subjects were considered eligible if they were >17 years of age, had a BMI of <25 or >29 kg/m2, had no known preexisting lung disease, had no contraindications to regional anesthesia, and could understand and/or comply with study instructions in English. Fourteen participants with a BMI <25 kg/m2 and 14 participants with a BMI >29 kg/m2 were enrolled. Demographic data were collected for each participant, including smoking history and whether they normally participated in exercise (defined as activity >20 minutes, 3–4 times per week), and instructions were given on the use of the Micro Handheld Spirometer with a disposable AstraGuard filter (Micro Medical Limited now CareFusion, San Diego, CA).

Participants were asked to rate their baseline dyspnea according to the modified Borg scale,15 and their Richmond Agitation-Sedation Scale (RASS)16 score was recorded. A baseline ultrasound of the diaphragm on the side to be blocked was performed according to the method described by Urmey et al7 to establish normal preblock diaphragm motion. Each participant was placed in a supine and sitting position and asked to perform a forceful sniff maneuver. During this maneuver, the diaphragm was visualized with ultrasound, and normal caudad versus paradoxical cephalad movement was noted. Finally, preblock PFT values were obtained for each participant using the handheld spirometer in accordance with the standards of lung function testing of the American Thoracic Society (ATS).17 These included FVC and FEV1, which were measured 3 times in the sitting and supine positions, separately.

After preblock values were collected, an ultrasound-guided ISB ± catheter placement was performed (SonoSite, Bothell, WA) as previously described6 using 30 mL of 0.5% ropivacaine with epinephrine 1:400,000. Anxiolysis was provided with titrated amounts of midazolam (1–2 mg intravenously) before block placement if needed. The completion time of local anesthetic injection was noted (time 0).

Thirty minutes after local anesthetic injection (time 0 + 30 minutes), sensory and motor block were evaluated. Full motor (0/5) and sensory (present/absent) blockade of the upper extremity was confirmed by testing motor function (shoulder abduction, elbow flexion/extension, wrist flexion/extension, finger abduction/adduction, and thumb abduction) on a scale of 0 to 5 (0 = no visible contraction, 1 = visible contraction/no movement, 2 = some movement/can’t overcome gravity, 3 = overcome gravity/not additional force, 4 = less than normal, 5 = normal) and sensation (pinprick) in the axillary, radial, median, ulnar, and musculocutaneous nerve distributions of the upper extremity in comparison to the nonblocked extremity. Participants again rated their dyspnea using the modified Borg scale, and the RASS score for each subject was noted. Ultrasound evaluation was performed to look for evidence of hemidiaphragmatic paresis, using the same methodology as the baseline examination. Paradoxical cephalad movement of the diaphragm was taken as evidence of onset of hemidiaphragmatic paresis. PFT measurements were repeated using the same technique that was used for preblock values. The FVC and FEV1 were obtained 3 times for each participant in the sitting and supine positions, separately.

Intraoperatively, participants underwent total intravenous anesthesia with propofol infusion to achieve loss of consciousness while maintaining spontaneous and unobstructed ventilation. Any intraoperative respiratory event, defined as airway interventions excluding jaw lift and nasal airway placement, was also recorded.

The entire sequence of block confirmation, modified Borg scale, RASS score, ultrasound of diaphragm, sitting, and supine spirometry, was repeated in the PACU prior to discharge home or discharge to the Duke Ambulatory Surgery Recovery Care Center. The Recovery Care Center is an 8-bed 23-hour recovery area which is part of the Duke Ambulatory Surgery Center. Twenty-three-hour overnight stay after shoulder surgery is surgeon specific. The PACU measurements were the final spirometry values collected, regardless of overnight stay or continuous interscalene catheter placement. Interscalene continuous catheters were connected to On-Q Ambulatory Pain Pumps (Halyard, Alpharetta, GA) infusing ropivacaine 0.2% at 6 to 8 mL/h, after PFTs were obtained.

All participants received a follow-up phone call on postoperative day 1 and for patients with indwelling continuous interscalene catheters, each additional postoperative day until continuous catheter removal (50–67 hours), as part of institutional standard practice.

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Statistical Analysis

At each measurement, the largest FVC and FEV1 were used for data analysis per the ATS guidelines.17 The primary outcome in this study was FVC% change (percentage change from baseline pre- to postblock values of FVC) at 30 minutes postblock in the supine position. Secondary outcomes included FVC% change at PACU and in the sitting position, FEV1% change (percentage change from pre- to postblock values of FEV1), FVC, FEV1, incidence of diaphragmatic paresis, modified Borg scale for perceived dyspnea, RASS scores for sedation, and intraoperative airway events. In order to compare normal-weight and obese groups on demographic variables and PFTs at each time point in each position, χ2 tests (if the expected counts are <5 for no >25% of the cells of the contingency table) or Fisher exact tests (if the expected counts are <5 for >25% of the cells of the contingency table) were used for categorical variables, and 2-sample t tests (if both groups pass the normality tests) or Wilcoxon rank sum tests (if either group fails the normality test) were used for continuous variables, including FVC% change, FEV1% change, FVC, and FEV1. To test the change of pulmonary function from baseline preblock to 30-minute postblock and PACU in each position for both normal-weight and obese groups, FVC and FEV1 were compared using paired t tests (if pass the normality test) or Wilcoxon signed rank tests (if fail the normality test). To assess the relationship between obesity and pulmonary function over time and how the relationship between obesity and pulmonary function change over time, adjusted for position, repeated measures analysis was performed on FVC% change and FEV1% change using linear mixed models. The group by time interaction was first included in the model and would later be excluded if tested nonsignificant. A sample size of 14 participants per group was calculated to provide 80% power assuming a 10-point difference in FVC% change between the obese and the normal-weight groups with standard deviations at 10 for both groups.

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RESULTS

No difference was found in age, height, race, gender, surgical procedure type, surgical position, surgical time, and smoking history between the normal-weight and the obese groups. However, a difference in regular exercise was noted between groups (P = .003). The mean (±standard deviation) BMI in the nonobese and obese groups was 23 (±1.7) and 33 (±3.1) kg/m2, respectively (P < .001; Table 1). Furthermore, no difference was found in preblock PFTs (FVC, FEV1) between normal-weight and obese groups (Table 2).

Table 1

Table 1

Table 2

Table 2

All participants, in both the normal-weight and the obese groups, demonstrated paradoxical diaphragm motion, indicative of on-going hemidiaphragmatic paresis and complete motor sensory block at all postblock time points. Compared to the normal-weight group, in the sitting position, the obese group had a significant decrease in FVC% change at 30 minutes (−30 [10.5] vs −23 [7.2], P = .046) and FEV1% change in the PACU (−40 [12.6] vs −27 [13.9], P = .02; Table 2). No difference was found for measurements taken in the supine position between normal-weight and obese groups. The plots of means represented how FVC and FEV1 changed over time, suggesting there was a group effect (obese versus normal weight) but no group by position (supine versus sitting) interaction effect (Figure). A repeated-measures analysis demonstrated that, adjusted for position, there is no significant group effect on FVC% change or FEV1% change from 30-minute postblock to PACU (Table 3). The group by time interaction was first included in the model and then excluded because it tested nonsignificant.

Table 3

Table 3

Figure

Figure

Table 4

Table 4

The 2 groups were not different in terms of breathlessness and sedation at 30 minutes (P = .67, P = .48, respectively) and in the PACU (P = .69, P > .99, respectively) nor in the occurrence of intraoperative airway events (P > .99; Table 4). At no point in data collection did a participant in either group complain of somewhat severe (modified Borg scale = 4) or severe (modified Borg scale = 5) breathlessness or have a RASS score <−1. At various postblock time points, 3 subjects in the obese group reported moderate breathlessness (modified Borg scale = 3). This subjective assessment, however, did not correspond to the subjects with the largest percentage decrease in PFTs. Airway events were similar between groups (P > .99). A laryngeal mask airway (LMA®) was placed in one obese group participant secondary to movement and obstruction with sedation prior to incision. There were no associated delays in readiness for discharge, respiratory-related issues identified during overnight stays or follow-up phone calls, unanticipated admissions, or readmissions in either group.

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DISCUSSION

The results of this study support previous findings that ISB is associated with 100% incidence of hemidiaphragmatic paresis7 and significant reductions in spirometric values (FVC, FEV1) from preblock values.8,9 In addition, these data demonstrate that obesity was associated with even greater reductions in pulmonary function. Lung parameters decreased 28% to 40% in obese participants compared to 21% to 31% in normal-weight participants. Neither time (30 minutes versus PACU) nor position (sitting versus supine) affected this relationship. Changes in pulmonary function were not associated with increased breathlessness, and despite their persistence in the PACU, there were no delays in readiness for discharge or respiratory-related issues identified during overnight stays or follow-up phone calls. To our knowledge, this is the first study to investigate pulmonary function in obese participants with phrenic nerve paresis after ISB for outpatient shoulder surgery.

Despite greater reductions in PFTs, obese participants did not experience increased breathlessness compared to normal-weight participants. Diminished inspiratory strength resulting from diaphragmatic paresis leads to an inability to fully inspire to total lung capacity and a resultant decrease in FVC and other volumes and flows that constitute routine pulmonary function testing.8 Baseline obesity-related physiology, which alters numerous respiratory parameters, including lung volumes/capacities, airway resistance,18–21 and work of breathing,22 may attenuate this decrement. Decreased functional residual capacity in obesity can lead to lung volumes lower than closing capacity, causing ventilation-perfusion mismatch and hypoxemia.23 As a result, ventilation occurs in the less compliant portion of the pressure-volume curve, increasing the effort needed to overcome decreased respiratory elasticity.23 The auto-positive end-expiratory pressure secondary to airway closure during expiration contributes to increased work of breathing due to additional ventilatory effort required by the diaphragm and other inspiratory muscles during the next inspiration.23 To reduce work of breathing, obese subjects usually adopt a breathing pattern with reduced tidal volumes and higher respiratory rates,23 potentially making the inability to fully inspire after ISB less impactful. However, clinical scenarios placing additional respiratory demand on obese patients and/or preexisting pulmonary dysfunction limiting pulmonary reserve to tolerate such changes could have tremendous impact.

The point at which FVC% reduction makes an impact and becomes clinically significant is uncertain and reflects a point at which patients can no longer tolerate such a reduction. Thus, while utilized for power analysis, one might argue that an FVC% change of 10 between groups is not a clinically significant threshold. A combination of obesity-related physiology and variations in respiratory mechanics associated with hemidiaphragmatic paresis applicable to both normal-weight and obese participants may explain the narrow margin between groups. No participant, in either group, complained of anything greater than moderate breathlessness. In fact, most reported only very, very slight breathlessness or none at all. While this study was not powered to detect differences in respiratory events, the objective lack of breathlessness and absence of significant respiratory complications and/or airway events support the continued use of ISB in obese patients. Avoiding volatile anesthetics and minimizing the use of opioids likely impact these results. As such, the findings in this study may not be generalizable to an obese population undergoing shoulder surgery under general anesthesia with volatile anesthetics and ISB for postoperative analgesia alone, which may further increase respiratory demand in this population.

The similar effect of position between groups coincides with a previous investigation demonstrating a very small incremental fall in functional residual capacity and total lung capacity in obese participants compared to normal-weight participants when changing from the sitting to supine positions.19 In the current study, while normal-weight participants demonstrated a greater change on assuming the supine position, obese participants still demonstrated lower absolute FVC and FEV1 supine values, consistent with the weight of the abdominal viscera and the pressure generated in the abdomen, enhancing cephalad displacement of the diaphragm, further limiting lung expansion.24

Published effects of obesity on PFTs are extremely inconsistent.25 These differences may be methodological (inclusion of asthmatics, position), technical (difficulty in reliable measurements), and/or gender based. Previous studies in unblocked participants, in accordance with our finding, have demonstrated a negative correlation of BMI with FVC26 and FEV1,25,26 while others have not.20 A recent study by Banerjee et al25 investigating the correlation of BMI with lung function parameters in nonblocked, nonasthmatics demonstrated no correlation of BMI with FVC and a significant negative correlation of BMI with FEV1 in obese patients. However, these correlations were found in females only. There was a lack of correlation between lung function parameters (FVC, FEV1, FEV1/FVC, FEF 25%–75%) and BMI in obese male participants. In the current study, the groups were similar in gender, but the study was not powered to investigate gender disparity. Future investigation is necessary to determine gender-specific effects of BMI on lung function after ISB.

In an attempt to mitigate effort-dependent variance in FVC and FEV1 values, participants were instructed on the correct technique for spirometry use before obtaining measurements. The highest of 3 successive measurements for FEV1 and FVC taken at each time point and in each position was utilized for data analysis. Further, the primary end point of percentage change from baseline (preblock) allowed each participant to serve as his/her own control, which minimized the impact of variance between participants. However, in a study by Giner et al,27 nearly 15% of the subjects failed to fulfill ATS criteria for spirometry, even though a qualified and regularly trained technician in a hospital lung function laboratory coached them.

While all participants were premedicated with midazolam (1–2 mg) before block placement, it is unlikely that this small dose would have a significant effect on patient effort. Sedation given intraoperatively could also affect patient effort for spirometry measurements in the PACU; therefore, measurements were performed immediately before PACU discharge. The RASS scores at all data collection time points indicated no more than very mild sedation (RASS = 0 to −1), implying that participants could fully cooperate with spirometry testing. RASS scores were similar between groups.

This study compared normal-weight (BMI <25 kg/m2) participants with obese participants (BMI >29 kg/m2), excluding those categorized as overweight (BMI 25–29 kg/m2). Comparing normal-weight and morbidly obese groups (BMI >39 kg/m2) would have further differentiated the effect of BMI on pulmonary function after ISB. Future studies involving these groups may provide additional insight into the effect of BMI on pulmonary function in patients undergoing ISB. Due to the small sample size, there may be insufficient power to detect clinically important differences between the obese and the normal-weight groups. Reduced power might account for the lack of significance for the 2-sample comparison of supine FEV1% change in PACU between the obese and the normal-weight groups (−38 vs −27, P = .0503), and the 2-sample comparison of sitting FEV1% change at 30 minutes between the obese and the normal-weight groups (−29 vs −21, P = .06), as well as the group effect in the repeated-measures analyses of FVC% change (PACU: difference estimate −6.31, SE = 3.25, P = .06) and FEV1% change (PACU: difference estimate −7.53, SE = 3.71, P = .053). In addition, the detected differences between the obese and the normal-weight groups may be caused by potential confounding variables and represent confounding biases. Increasing sample size in future studies would increase the study power and reduce type II error. Multiple measurements were taken for multiple outcomes at multiple time points. However, the primary outcome is FVC% change at 30 minutes postblock in the supine position. Therefore, we did not do multiple comparison corrections in the analysis. Nevertheless, since there are multiple secondary outcomes, there is an elevated chance (>5%) that statistically significant secondary outcomes might represent false-positive findings (type I error).

Despite a significant decrease in pulmonary function secondary to hemidiaphragmatic paresis, this study demonstrated uneventful anesthetic management in obese participants undergoing ambulatory shoulder surgery with total intravenous anesthesia and ISB. The clinical implications of this decrease remain unclear. Anesthetic management of this population for ambulatory shoulder surgery does present challenges. If obese patients do not receive ISB for shoulder surgery, general anesthesia must be performed. Importantly, general anesthesia and increased opioid requirement may increase the risk for pulmonary complications in these patients. More studies on the relationships between ISB and BMI are needed to inform clinical decision-making in obese patients undergoing outpatient shoulder surgery.

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DISCLOSURES

Name: M. Stephen Melton, MD.

Contribution: This author helped design the study, enroll the patients, collect/analyze the data, and write/edit the manuscript.

Name: Hanni E. Monroe, MD.

Contribution: This author helped design the study, enroll the patients, collect/analyze the data, and write/edit the manuscript.

Name: Wenjing Qi, PhD.

Contribution: This author helped analyze the data and write/edit the manuscript.

Name: Stephanie L. Lewis, MD.

Contribution: This author helped enroll the patients and collect/analyze the data.

Name: Karen C. Nielsen, MD.

Contribution: This author helped design the study and write/edit the manuscript.

Name: Stephen M. Klein, MD.

Contribution: This author helped design the study and write/edit the manuscript.

This manuscript was handled by: Honorio T. Benzon, MD.

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