The possibility of symptomatic or permanent phrenic nerve palsy (PNP) need not loom over every patient administered an interscalene nerve block.
Ki Jinn Chin, MD, associate professor in the Department of Anesthesia at Toronto Western Hospital, University of Toronto, told attendees of the 2016 International Symposium of Ultrasound for Regional Anesthesia, Pain Medicine, and Peri-operative Application that the risk can be cut by shifting the location of the block down the neck and reducing the dose, volume or concentration of the anesthesia.
To be sure, temporary PNP occurs with nearly all interscalene blocks, and most patients compensate for temporary diaphragmatic paralysis by using their intercostal accessory muscles and increasing the expansion of their contralateral diaphragm, Dr. Chin said. These patients normally experience only minimal decreases in tidal volume and small reductions in pulmonary oxygen, which are not usually clinically significant (Anesth Analg 1995;81:962-966).
However, patients with obesity, those kept in a supine position postoperatively, and individuals with a history of neuromuscular disease or preexisting lung disease can experience symptomatic shortness of breath and clinically significant hypoxemia even with temporary PNP, Dr. Chin said.
“We should consider doing everything we can to prevent PNP in these patients,” said Dr. Chin, who also noted that one in 2,000 patients given an interscalene block may experience permanent PNP (Anesthesiology 2013;119:250-252).
Addressing the Mechanics of PNP
One cause of PNP with interscalene blocks is the spread of anesthetic to the roots at C3 and C4 and the phrenic nerve, Dr. Chin said. To avoid this occurrence, he suggested shifting the location of the interscalene block lower down the neck, rather than targeting the roots at C5 and C6, as is normally done.
“This allows us to block the C5-C6 groups where they coalesce into the superior trunk, rather than injecting at the root level,” he explained, noting he and his colleagues have not documented PNP rates with this approach, but they believe it can reduce the likelihood of the complication.
Turn Down the Volume
Reducing the volume of anesthetic is another way of limiting the risk for PNP, Dr. Chin said. In one study he pointed to, researchers injected 10 mL of bupivacaine—rather than the typical 20 mL—at the level of root C7. The combination of a lower volume and lower location provided a “perfectly adequate block,” Dr. Chin said, while leading to PNP in only 13% of patients (Reg Anesth Pain Med 2009;34:498-502).
Injecting 5 mL of bupivacaine at the level of root C7 may eliminate instances of PNP altogether, according to one Dutch study (Reg Anesth Pain Med 2010;35:529-534).
“If you’re really serious about eliminating PNP, you want to go with 5 mL at a level as distal to the C5-C6 roots as possible,” Dr. Chin said, noting this is a technically challenging and potentially risky approach when done by less experienced anesthesiologists.
Administering a lower concentration of anesthetic also can mitigate the possibility of PNP, Dr. Chin continued. In one trial, a 20-mL dose of 0.1% ropivacaine, rather than 0.5% or 0.75% ropivacaine, was injected at the level of root C7 and led to PNP in 42% of patients (Pain Med 2016 Apr 13. [Epub ahead of print]). Similarly, when 20 mL of bupivacaine 0.25% were injected at the level of roots C5 and C6 in another study, only 21% of recipients experienced the complication (J Shoulder Elbow Surg 2013;22:381-386).
“The downside of using a lower volume and concentration is that it may reduce the duration of sensory blockade, may lead to higher pain scores, can lower satisfaction and quality of recovery, and increases the need for opioids,” Dr. Chin said.
Vincent Chan, MD, professor of anesthesia at the University of Toronto, who organized the meeting, commented, “Lowering the dose, volume, concentration and the location of the block are all good strategies to prevent PNP, but we need to further study how the duration of analgesia suffers with these strategies.”
One study showing analgesia is not necessarily affected with a lower dose suggests these modifications to the technique may not lead to inadequate analgesia (Anesthesiology 2013;118:863-867).
Ultimately, PNP-related hemidiaphragmatic paresis resulting in respiratory distress is an uncommon occurrence, Dr. Chan said. Nevertheless, he said it is important to find additional ways of reducing the incidence of this complication.
“As anesthesiologists, we are sometimes caught by surprise when an apparently healthy day surgery patient develops difficulty breathing and oxygen desaturation after an interscalene block, and requires unanticipated hospital admission,” Dr. Chan said.