Phrenic nerve block caused by interscalene brachial plexus block: Effects of digital pressure and a low volume of local anesthetic

Background and Objectives. Interscalene brachial plexus block (ISB) is asso ciated with phrenic block and diaphragmatic paralysis when high volumes (40 -50 mt) of local anesthetic are injected. The goal of our study was to test if a low volume of local anesthetic administered while maintaining proxima l digital pressure might more selectively block the brachial plexus and dec rease the frequency of phrenic nerve block. Methods. Twenty healthy patient s undergoing ISB for orthopedic surgery of the upper extremity were randoml y allocated to receive either 20 mt 1.5% mepivacaine while proximal digital pressure to the site of puncture was performed, or 40 mt 1.5% mepivacaine without digital pressure. Spirometry and clinical data were evaluated at ba seline, 10, and 90 minutes after accomplishing the block and after the moto r and sensory block resolved. Diaphragmatic excursion during deep inspirati on was also evaluated 90 minutes after the block was performed, with the pa tient in the sitting position. Results. Interscalene brachial plexus block produced diaphragmatic paralysis in all patients included in the study, as demonstrated by the pulmonary function testing and the chest radiograph. No significant differences were found in any of the parameters studied. At 10 minutes, baseline functional residual capacity had diminished by 34 +/- 10 % in the 40 mt group and 37 +/- 13% in the 20 mt group. Maximum cephalad se nsory dermatome level was also similar in both groups, being C3 or above in all patients. Ipsilateral hemidiaphragmatic motion was similar in both gro ups (3.2 +/- 2.3 cm in the 40 mt group and 2.6 +/- 1.7 cm in the 20 mt grou p). However, in no case was dyspnea manifested. Conclusions. Decreasing the volume of local anesthetic and applying proximal digital pressure to the s ite of injection is not effective in reducing the cervical block spread and the frequency or intensity of diaphragmatic paralysis during interscalene ISB.