Pulmonary Function Changes After Interscalene Brachial Plexus Anesthesia with 0.5% and 0.75% Ropivacaine

A Double-Blinded Comparison with 2% Mepivacaine

Abstract

The purpose of this investigation was to compare, in a prospective, double-blinded fashion, 0.5% and 0.75% ropivacaine with 2% mepivacaine to determine their effects on respiratory function during interscalene brachial plexus (IBP) anesthesia. With ethical committee approval and written, informed consent, 30 healthy patients presenting for elective shoulder capsuloplastic or acromioplastic procedures were randomized to receive IBP anesthesia by 20 mL of either 0.5% ropivacaine (n = 10), 0.75% ropivacaine (n = 10), or 2% mepivacaine (n = 10). Block onset time, pulmonary function variables, ipsilateral hemidiaphragmatic motion (ultrasonographic evaluation), and first requirement of postoperative analgesic were evaluated. Surgical anesthesia (loss of pinprick sensation from C4 to C7 and motor block of the shoulder joint) was achieved later with 0.5% ropivacaine than with either 0.75% ropivacaine or 2% mepivacaine (P < 0.05), whereas the first pain medication was requested later with both ropivacaine concentrations than with mepivacaine (P < 0.0005). No differences in quality of the block or patient acceptance were observed in the three groups. All 30 patients had ipsilateral hemidiaphragmatic paresis and large mean decreases in forced vital capacity (ropivacaine 0.5%: 40% +/- 17%, ropivacaine 0.75%: 41% +/- 22%, mepivacaine 2%: 39% +/- 21%) and forced expiratory volume at 1 s (ropivacaine 0.5%: 30% +/- 19%, ropivacaine 0.75%: 38% +/- 26%, mepivacaine 2%: 40% +/- 10%). We conclude that, when performing IBP anesthesia, 0.5% ropivacaine does not decrease the incidence of ipsilateral paresis of the hemidiaphragm compared with 0.75% ropivacaine and 2% mepivacaine. Implications: During the first 30 min after placing interscalene brachial plexus anesthesia, 0.5% ropivacaine does not provide clinically relevant advantages in terms of pulmonary function changes compared with either 0.75% ropivacaine or 2% mepivacaine. However, 0.75% ropivacaine allows a short onset, similar to that of mepivacaine, with long postoperative analgesia.

(Anesth Analg 1999;88:587-92)

Section Editor: Denise J. Wedel.

The interscalene approach to brachial plexus anesthesia depresses pulmonary function as a consequence of unilateral hemidiaphragmatic paresis ^[1-3]. No positive effects on pulmonary function variables have been demonstrated by either decreasing the local anesthetic injection volume ^[4], applying digital pressure above the injection site ^[5,6], or using a nerve stimulator to facilitate the block ^[6].

Ropivacaine has a lower potential for central nervous system and cardiac toxicity ^[7-9]. Compared with earlier local anesthetics, ropivacaine also has a more marked differential in sensory/motor blockade, which is greater at concentrations <or=to0.5% ^[7,10,11].

The purpose of this investigation was to evaluate, in a prospective, double-blinded fashion, the pulmonary function changes during interscalene brachial plexus (IBP) anesthesia performed with 0.5% and 0.75% ropivacaine or 2% mepivacaine.

Methods

After the study protocol had been approved by the local ethical committee, written, informed consent was obtained from 30 ASA physical status I or II inpatients aged 18-65 yr scheduled to undergo IBP anesthesia for elective shoulder capsuloplastic or acromioplastic surgical procedures. Patients with respiratory or cardiac disease, diabetes, or peripheral neuropathy, as well as those receiving chronic analgesic therapy, were excluded.

To calculate the required study size, we determined the effects on forced vital capacity (FVC) reported after IBP blockade by previous investigators ^[1-6]. We wished to detect a 20% difference in the reduction of FVC compared with 0.5% ropivacaine 30 min after block placement, accepting a one-tailed alpha error of 5% and a beta error of 20% ^[12]. Based on these calculations, the required study size was 10 patients per group.

No premedication or sedation was given until the completion of all measurements. After a 20-gauge IV cannula had been inserted at the forearm, all patients received a 5-mL [center dot] kg^-1 [center dot] h^-1 infusion of lactated Ringer's solution. Standard monitoring was used throughout the study, including noninvasive arterial blood pressure, heart rate, and pulse oximetry.

Nerve block was performed with the aid of a nerve stimulator (Plexival, Medival, Italy) using a short-beveled, Teflon-coated stimulating needle (Locoplex, Vygon, France) (3.5 cm long, 25 gauge). The needle was introduced at or caudad to a line through the cricoid cartilage according to the method described by Winnie ^[13]. Paresthesiae were never intentionally sought, and a multiple injection technique was used, as is routine in our department ^[14], eliciting each of the three following muscular twitches to confirm the exact needle location: abduction of the shoulder and flexion and extension of the arm. To elicit each of the three muscular twitches, the stimulating needle was, in sequence, inserted and redirected, while the total volume of local anesthetic solution was divided among considered twitches as follows: 8 mL of anesthetic solution was injected at the abduction of the shoulder, 6 mL at the flexion of the arm, and 6 mL at the extension of the arm. The stimulation frequency was set at 2 Hz, and the intensity of stimulating current was initially set to deliver 1 mA, then was gradually decreased to <0.5 mA. Using sealed envelopes, patients were randomized to receive 20 mL of either 2% mepivacaine (n = 10), 0.5% ropivacaine (n = 10), or 0.75% ropivacaine (n = 10). Sterile syringes containing the local anesthetic solution were prepared in a double-blinded fashion by one of the authors, who did not take part in the further management of the patients. FVC, forced expiratory volume at 1 s (FEV₁), and peak expiratory flow (PEF) were measured immediately before interscalene block injection (baseline) and 2, 5, 10, 20, and 30 min after the anesthetic procedure. At the same time, the evolution of both sensory and motor nerve blocks was also evaluated. The start time for clinical assessments was the completion of the local anesthetic injection. Motor function was tested by asking the patient to abduct the arm at the shoulder joint against gravity and to flex the forearm at the elbow. Sensory block was assessed using pinprick (22-gauge needle). The onset of surgical anesthesia (ready for surgery) was defined as the loss of pinprick sensation at the skin dermatomes involved in the surgical field (C4-7) and inability to abduct the arm at the shoulder joint against gravity. All measurements were made with the patient lying in a 30[degree sign] head-up, supine position.

Before block placement and 10, 20, and 30 min after the anesthetic procedure, the ipsilateral and contralateral hemidiaphragmatic excursions were measured by using real-time ultrasonography using the method described by Urmey et al. ^[1]. Hemidiaphragmatic excursion was defined as the difference between the diaphragm position recorded at functional residual capacity and that measured at the end of a deep inspiration. Excursion distance was expressed in centimeters from the functional residual capacity position. Normal inspiratory caudad diaphragmatic excursion was designated as positive motion (+), and paradoxical cephalad motion as negative motion (-).

The adequacy of the block was judged according to the need for supplementary IV analgesics and sedation: adequate nerve block = neither IV sedation nor analgesics required to complete surgery; inadequate nerve block = need for additional IV analgesic (fentanyl 0.1 mg) and sedation (continuous infusion of propofol 2 mg [center dot] kg^-1 [center dot] h^-1) to complete surgery; failed nerve block = general anesthesia required to complete surgery. Patients with a failed nerve block were excluded from the study.

The time of first requirement of postoperative pain medication (100 mg of IV ketoprofen) was recorded; at the same time, the degree of pain was also measured by using a 100-mm visual analog scale (VAS). Acceptance of the anesthetic technique was assessed 24 h postoperatively. The block was considered satisfactory if the patient would accept the same anesthetic procedure again and unsatisfactory if the patient would prefer a different anesthetic procedure. Patients were questioned regarding neurological complications at discharge from the orthopedic ward and 1 wk after hospital discharge (at the first routine postoperative orthopedic examination).

Statistical analysis was performed using Stat-View 3.0 (Abacus Co., Palo Alto, CA). Demographic data, onset of the block, and duration of postoperative analgesia were analyzed by using analysis of variance with Dunnett's and Scheffe's tests for multiple comparisons. Analysis of variance for repeated measures was used to analyze changes in hemidiaphragmatic excursion, pulmonary function, and hemodynamic variables. Motor blockade, adequacy of intraoperative anesthesia, and acceptance of the anesthetic technique were analyzed using the contingency Table analysiswith Fisher's exact test. Bonferroni's correction was always used for post hoc comparisons. A value of P < 0.05 was considered significant. Continuous variables are presented as mean +/- SD, and ordinal data are presented as median (range) or number (percentage).

Results

The three groups of patients were similar with respect to age, weight, height, ASA physical status, and male to female ratio (Table 1). No significant changes in arterial blood pressure, heart rate, or hemoglobin oxygen saturation were observed during the study in any group.

Table 1: Demographic Data

(Figure 1) shows the time required to achieve adequate surgical anesthesia. Table 2 shows motor blockade characteristics in the three groups. The percentage of patients able to lift their arm against gravity was always higher in the ropivacaine 0.5% group than in the other two groups, but this difference failed to reach statistical significance. However, the mean time required to achieve adequate motor block of the shoulder joint was longer in the ropivacaine 0.5% group than in either the ropivacaine 0.75% or the mepivacaine 2% group (P < 0.05). Thirty minutes after block placement, one patient in the ropivacaine 0.5% group was still partially able to abduct the arm at the shoulder joint, whereas complete motor block of the shoulder was present in the other two groups at the same time. However, this patient did not complain of pain during surgery, and the procedure was uneventful. One patient in the mepivacaine group had an inadequate nerve block that required intraoperative analgesics and sedation; no patient required general anesthesia.

Figure 1: Time required to be judged ready for surgery (loss of pinprick sensation from C4 to C7 and inability to elevate the shoulder joint against gravity) in patients receiving interscalene brachial plexus anesthesia with 20 mL of either 0.5% and 0.75% ropivacaine or 2% mepivacaine. Data are presented as mean +/- SD. *P < 0.05 compared with ropivacaine 0.5%.

Table 2: Motor Blockade

The VAS scores at the first requirement of pain medication in the postoperative period were similar in the three groups, but the time from block placement to the first postoperative analgesic was statistically and clinically shorter in the mepivacaine group than in the other two groups (P < 0.0005) (Figure 2). During the first 24 h after surgery, two patients (20%) in the ropivacaine group 0.5% did not require pain medication, whereas all patients in both the other groups required postoperative analgesics. Acceptance of the anesthetic technique was good in all patients, and no persistent neurologic deficit was observed in any patients studied 1 wk after surgery.

Figure 2: Time from block placement to first requirement of pain medication in the postoperative (postop) period in patients receiving interscalene brachial plexus anesthesia with 20 mL of either 0.5% and 0.75% ropivacaine or 2% mepivacaine. Data are presented as mean +/- SD. sup [section sign] P < 0.0005 compared with mepivacaine 2%.

Significant decreases in all measured pulmonary function variables were observed in every patient after interscalene blockade. Figure 3 shows the reduction in FVC, FEV₁, and PEF as a percentage of baseline values in the three studied groups. Patients in the ropivacaine 0.5% group showed a smaller reduction in both FVC and PEF values than patients receiving either 0.75% ropivacaine or 2% mepivacaine at the 5- and 10-min measurement times. However, 30 min after the local anesthetic injection, no differences in the measured pulmonary function variables were observed among the three groups. Normal ipsilateral hemidiaphragmatic motion during deep inspiration measured immediately before the interscalene block changed to no motion or paradoxical motion after the induction of the interscalene block in all studied patients (Figure 4).

Figure 3: Reduction in forced vital capacity (FVC), forced expired volume at 1 s (FEV₁), and peak expiratory flow (PEF) in patients receiving interscalene brachial plexus anesthesia with 20 mL of either 0.5% and 0.75% ropivacaine or 2% mepivacaine. Data are presented as a percentage of baseline values. *P < 0.05 compared with ropivacaine 0.5%.

Figure 4: Mean +/- SD ipsilateral hemidiaphragmatic excursion measured before (baseline) and 10, 20, and 30 min after interscalene brachial plexus anesthesia with 20 mL of either 0.5% and 0.75% ropivacaine or 2% mepivacaine.

Discussion

Despite the reported differential in sensory/motor nerve block of ropivacaine, the effects of the interscalene approach to brachial plexus anesthesia on ipsilateral hemidiaphragmatic paresis and large reductions in pulmonary function variables were not influenced by decreasing the concentration of ropivacaine from 0.75% to 0.5% in the present study. Using the 0.5% solution of ropivacaine resulted in a statistically significant delay in the reduction of pulmonary function variables compared with both 0.75% ropivacaine and 2% mepivacaine; however, this delay was probably related to the slower onset of motor blockade, because 30 min after local anesthetic injection, no differences in pulmonary function tests were observed with the three anesthetic solutions.

These findings agree with previous investigations, which report a 100% ipsilateral hemidiaphragmatic hemiparesis after IBP anesthesia ^[1,2], and the effects on pulmonary function are similar in magnitude to the results of studies of patients with complete unilateral phrenic nerve paralysis ^[15]. Compared with our results, Urmey et al. ^[1] and Urmey and McDonald ^[2] reported a more marked paradoxical motion of the ipsilateral hemidiaphragm after a 30- to 50-mL interscalene injection of 1.5% mepivacaine with added epinephrine and bicarbonate. However, in the present study, the hemidiaphragmatic motion was measured in patients lying in a 30[degree sign] head-up, supine position, which may be less sensitive for diagnosing paradoxical diaphragmatic motion compared with the supine position, due to the weight of the abdominal contents pulling the paretic hemidiaphragm downward ^[16], but it better reflects the surgical position of the patient. Moreover, we did not measure the hemidiaphragmatic motion during forceful sniff, which further reduced the paradoxical hemidiaphragmatic motion compared with Urmey et al. ^[1] and Urmey and McDonald's ^[2] results.

Our findings are also consistent with the hypothesized mechanism for decreases in spirometric volumes after interscalene blockade; that is, the reduction in inspiratory strength resulting from diaphragmatic paresis leading to an inability to fully inspire to total lung capacity with resultant decreases of all pulmonary volumes and flows ^[2,3].

The ipsilateral hemidiaphragm paresis is strictly related to the motor block of the phrenic nerve ^[1-3]. Neither reducing the injected volume of local anesthetic solution ^[4] nor applying digital pressure above the injection site ^[5-6] affects the changes in respiratory function during IBP block. The differential sensory/motor nerve block of ropivacaine, which could theoretically minimize the motor block of the ipsilateral hemidiaphragm, increases as the concentration of ropivacaine is reduced ^[10,11]; however, concentrations <0.5% do not provide adequate surgical anesthesia ^[17]. For this reason, we decided to compare 0.5% and 0.75% ropivacaine concentrations. Bupivacaine may seem more appropriate as a control drug than mepivacaine; however, the wide and unpredictable latency of nerve blocks performed with bupivacaine has made it less popular ^[18], whereas mepivacaine is widely used for peripheral nerve blocks because of its short-onset/intermediate-duration characteristics ^[14,18,19]. This is the reason it was chosen.

We are unaware of previous investigations comparing either 0.5% or 0.75% ropivacaine with 2% mepivacaine for IBP anesthesia. In agreement with findings reported during axillary brachial plexus block ^[20], we also demonstrated that, when used for IBP anesthesia, 0.75% ropivacaine has an onset time similar to that provided by 2% mepivacaine and shorter than that provided by 0.5% ropivacaine.

As is routine in our department since 1988, we used a multiple injection technique using local anesthetic volumes distinctly lower than those usually reported for IBP anesthesia in the present investigation. Using the multiple injection technique with a nerve stimulator to confirm exact needle location markedly improves both the onset and quality of nerve block compared with the single-injection technique and allows a reduction in the volumes of local anesthetic solution ^[14,21]. However, Urmey and Gloeggler ^[4] failed to demonstrate any difference in the quality of IBP anesthesia performed with either 20 or 45 mL of a local anesthetic solution.

Because we compared 0.5% and 0.75% ropivacaine with 2% mepivacaine, the results of this investigation are relevant compared with mepivacaine only. However, we conclude that, because rapid onset of block and prolonged pain relief are important goals in regional anesthesia, 0.75% ropivacaine may be a suitable choice for IBP block, providing an onset similar to mepivacaine and prolonged postoperative analgesia, with effects on respiratory function during the first 30 min after block placement similar to those produced by either 0.5% ropivacaine or 2% mepivacaine.

We thank Dr. G. Fraschini and Dr. A. Camnasio (University Department of Orthopedic Surgery, IRCCS San Raffaele Hospital), as well as the staff of anesthesia nurses (University Department of Anesthesiology, IRCCS San Raffaele Hospital), without whose help and cooperation this study would not have been possible.