The increasing number of patients who are overweight (body mass index [BMI] ≥25 kg/m2) or obese (BMI ≥30 kg/m2) is an important public health issue worldwide.1 Obese patients are a challenge for anesthesiologists because of the increased risk of difficult airway and venous accesses, pharmacokinetics modifications, frequent coexisting morbidities, and difficult patient positioning.2
In obese patients, successful regional anesthesia, especially plexus blocks, may avoid difficult airway management, aspiration of gastric contents, and deleterious interactions between anesthetic drugs and patients’ comorbidities and treatments. Furthermore, regional anesthesia compared with general anesthesia has been shown to be time efficient, to decrease postoperative nausea and vomiting, to provide better analgesia, and to reduce postanesthesia care unit and hospital length of stay.3,4 Nevertheless, regional anesthesia in the obese patient may be technically challenging. Because patient positioning, anatomical landmarks, and depth of needle penetration differ between block techniques, it is necessary to evaluate each block technique independently. A large observational study has shown that obesity was associated with higher block failure and complication rates.5 However, this study included 22 different block techniques among which axillary plexus blocks were only 5.5% of blocks studied. In another study, the supraclavicular block technique in obese patients resulted in a slight decrease in success rate and no apparent effect on acute complications.6 We hypothesized that the success rate of the axillary brachial plexus block technique may be decreased in obese patients because the subcutaneous position of the axillary artery could be more difficult to identify precisely.
This study was approved by the hospital Ethics Committee. Because there was no randomization and only routine care was performed, authorization was given to waive written informed consent. Nevertheless, patients gave approval (verbal consent) to include their care in an investigative project.
From January 2005 to June 2006, patients scheduled for elbow, forearm, wrist, and hand surgery under axillary brachial plexus block were premedicated orally with hydroxyzine 1.5 mg/kg 2 hours before surgery. All blocks were performed in the anesthetic room by 5 senior anesthesiologists (J-LH, WG, AL, GO, and DB) who have performed regional anesthesia daily for >5 years. Blocks performed by residents in anesthesia were excluded.
After IV line placement, patients were placed supine with the arm abducted 90° and bent at the elbow with the forearm supinated. Standard monitoring was used including heart rate, pulse oximetry, and noninvasive arterial blood pressure. The pulse of the axillary artery was palpated at the level of the pectoralis major muscle crossing the axilla. Before the block, the subcutaneous tissue overlying the artery was infiltrated with 4 mL ropivacaine 0.5% to block the medial brachial cutaneous nerve. At the time of the study, ultrasound guidance was not available in our institution. Consequently, all axillary brachial plexus blocks were performed using a short-beveled insulated needle (22 gauge, 50 mm long) connected to a nerve stimulator (Stimuplex; B. Braun Medical, Nogent-le-Rotrou, France). The initial stimulating current was set to 0.7 mA and the stimulus frequency to 2 Hz. A triple-injection technique was used. First, the needle was inserted superior to the artery to locate the musculocutaneous and the median nerves that were blocked with 6 and 10 mL ropivacaine 0.5%, respectively. Second, the needle was inserted inferior to the artery to locate the radial nerve that was blocked with 20 mL ropivacaine 0.5%. For the median and radial nerves, distal motor responses of the wrist or the fingers were mandatory to perform the injection of local anesthetics. The stimulating current was gradually decreased to 0.4 mA before injection was performed. If this threshold could not be reached, the anesthesiologist noted the minimal current at which a sustained appropriate motor response was observed. The following data were collected by a medical student not involved in global patient care: age, weight, height, BMI (calculated as weight in kilograms divided by the height in square meters), the minimal intensity of stimulating current at the injection of local anesthetic among the 3 stimulations, and the time to perform the block from the first palpation of anatomical landmarks to the last withdrawal of the needle. Immediately after the block was performed, the patient was asked by the nurse to report the intensity of pain induced by the block procedure on a 100-mm visual analog scale. We defined obesity as BMI ≥30 kg/m2.1
Thirty minutes after the block was performed, the anesthesiologist who performed the block evaluated the sensory (using a 27-gauge needle) and motor (movements and strength) blocks of the musculocutaneous, median, radial, and ulnar nerves assessed at the hand and wrist. If a selective additional block was required for surgery, it was performed, and the patient was taken to the operating room.
At the end of surgery, a successful block was defined as adequate analgesia allowing surgery to be performed without any pain, IV analgesic administration, or nitrous oxide in oxygen inhalation. Incomplete analgesia requiring a selective additional block, IV analgesic administration, nitrous oxide in oxygen inhalation, or general anesthesia was considered a failed block. Acute complications related to local anesthetic toxicity (metallic taste, perioral paresthesia, tinnitus, preseizure excitation, seizures, arrhythmias, and cardiac arrest), vascular puncture, and unintentional paresthesia were noted. Just before leaving the postanesthesia care unit, patient's satisfaction with anesthesia (willingness to have the same anesthesia for future surgery) was collected by medical students not involved in the global care of the patient.
The main outcome variable was success rate. We considered 10% of the population to be obese.1 Power calculations were based on a 96% success rate reported in previous investigations,7,8 and in our own institution (unpublished data), with a multiple injection technique for axillary brachial plexus block. We considered a 10% difference in the main outcome variable (i.e., an 86% success rate of axillary blocks in the obese population) to be clinically relevant. Fifty-six obese patients and 504 non-obese patients would be required to detect a 10% difference in success rate (Fisher exact test), accepting a 1-sided α error of 5% and a β error of 10%. Accordingly, we decided to include 600 consecutive axillary blocks.
Statistical analysis was performed using the MedCalc statistical software (MedCalc Software, Mariakerke, Belgium). Continuous data are presented as mean (±SD) or median (range) for not normally distributed variables. Categorical variables are presented as number (%). Continuous variables were analyzed using the Student t test or Mann-Whitney U test according to data distribution. Categorical variables were analyzed using contingency tables analysis and the Fisher exact test. A P value <0.05 was considered significant.
Six hundred five consecutive axillary brachial plexus blocks were included prospectively during 1 year. Although 600 blocks were scheduled for the study, 605 were included. The 5 additional axillary brachial plexus blocks were recorded unintentionally on the last day of inclusion and were included in the statistical analysis. Eighty-five patients (14%) were obese (3 patients were morbidly obese; BMI ≥40 kg/m2). The demographics of groups and main characteristics of blocks are reported in Table 1.
The time to perform the axillary block was significantly longer in obese compared with non-obese patients (Table 2). The success rate was 97% overall, 91% in the obese patients and 98% in the non-obese population (P = 0.003). Obesity resulted in an increased risk of failure (relative risk = 4.08; 95% confidence interval: 1.71–9.68; P = 0.001). Figure 1 depicts the rate of failure along BMI values. Additional nerve blocks at the elbow were performed more frequently in obese (7%) than in non-obese patients (2%; P = 0.01).
Acute complications were reported in 71 (12%) axillary brachial plexus blocks. Acute complications were more frequent in obese than in non-obese patients (27% vs 9%; P < 0.001). Obesity resulted in an increased risk of acute complications (relative risk: = 4.78; 95% confidence interval: 1.89–4.56; P < 0.0001). Pain reported after block performance was not different between the obese and non-obese patients (Table 2). Patient satisfaction was 93% overall, 87% in the obese patients and 94% in the non-obese population (P = 0.03).
This study showed that obesity decreases the success rate and increases the incidence of acute complications of axillary brachial plexus block. Furthermore, there were more dissatisfied patients in the obese population. A growing body of evidence shows that obesity results in increased adverse events in regional anesthesia. The study by Nielsen et al.5 has shown that obese patients (BMI ≥30 kg/m2) were 1.6 times more likely to have failed regional anesthesia. However, this study included 22 different block techniques, among which 10% were spinal anesthesia and 58% were paravertebral, sciatic, and interscalene blocks. Although this study included a large patient population, its results may not be applicable to a single specific block technique because each block technique is based on different patient positioning, anatomical landmarks, and depth of needle penetration. It could be hypothesized that deep blocks requiring muscular or bony landmark palpation may be more prone to failure and complications in obese patients.9 However, obesity may have a small effect on superficial block performance requiring artery palpation (i.e., femoral, axillary, and humeral block). In 1468 brachial plexus blocks at the humeral canal, it was shown that the failure rate was not associated with the patients’ physical characteristics.10 Similarly, supraclavicular block in the obese population resulted in a slight decrease in success rate without an apparent effect on acute complications.6 This study shows that obesity decreases the success rate of axillary brachial plexus blocks performed using nerve stimulation. The impact of obesity on the success rate of axillary brachial plexus blocks was small considering that the block was still successful among 91% of the obese patients. However, because general anesthesia is challenging in obese patients, these results may be of importance for anesthesiologists.
In this study, the rate of acute complications in non-obese patients was small and similar to that reported for axillary brachial plexus blocks.7,11 Importantly, our results showed that the rate of acute complications in obese patients was >3 times that observed in non-obese patients. The acute complications reported were axillary artery puncture and unintentional paresthesia. Nielsen et al.5 reported a similar increase in the rate of acute complications in obese patients when evaluating several block techniques. Our study also showed that the time to perform the block was longer in obese than in non-obese patients (Table 2).
In clinical research, patient satisfaction is an important end point influenced by patient-related, care provider– related, and process of care–related determinants. The overall rate of satisfied patients was 93% (94% in non-obese patients), which is in accordance with previous studies.12,13 In addition, our study showed that patient satisfaction was lower in obese patients than in non-obese patients. This may be related, at least in part, to the increased time to perform the block, suggesting that repeated needle punctures and redirection could be more frequent in obese patients. In this study, patients did not receive any IV sedation or analgesia because it has been shown that 85% of patients would prefer receiving a block without sedation.12
There are important limitations of the study that must be considered. First, we studied only axillary brachial plexus blocks performed at a single academic institution and by a small group of experienced anesthesiologists. Consequently, our results might not be extended to residents and other institutions. Second, the multiple injection axillary brachial plexus block technique is well described and extensively performed worldwide. Third, this was an observational study that only suggested the effect of obesity. We cannot exclude the effect of obesity itself on the anesthesiologist practice (i.e., obesity itself could increase the anesthesiologist's propensity to perform additional selective blocks). Fourth, the definition of failed block used in this observational study was based on successful surgical anesthesia; we did not assess the various components associated with pain during block performance. Fifth, patient satisfaction was evaluated just before leaving the postanesthesia care unit and not the day after or before leaving the hospital. Consequently, some components of patient satisfaction may not have been evaluated.
In conclusion, this study showed that obesity decreased the success rate and resulted in more frequent immediate complications of axillary brachial plexus block. Furthermore, there were more dissatisfied obese patients.
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© 2010 International Anesthesia Research Society
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