Time to Ditch Neuromuscular Blockade in Cardiac Surgery?

Author: Rebecca M. Gerlach, MD

Cardiac surgery without neuromuscular blockade is safe and feasible under the right conditions, a study has found.

“Although there have been many studies in the recent literature looking at complications associated with residual neuromuscular blockade after anesthesia in noncardiac surgery, we realized that nobody has looked at this issue in cardiac patients,” said Rebecca M. Gerlach, MD, assistant professor of anesthesia at the University of Chicago.

“Given that cardiac surgical patients are at high risk for postoperative pulmonary complications and the rate of metabolism of neuromuscular blocking agents is likely altered in these individuals, we thought this a relevant subject to look at,” she said.

The effects of postoperative residual neuromuscular blockade also may be amplified in these patients due to intraoperative hypothermia and frequent dosing, leading Dr. Gerlach and her colleagues to evaluate the effect of a protocol omitting intraoperative paralysis on the incidence of early postoperative pulmonary complications and surgical tolerability.

Elective Surgery Requiring Bypass

To date, 45 patients have been enrolled in the ongoing trial, all of whom underwent elective surgery (coronary artery bypass grafting [CABG], valve replacement or CABG plus valve replacement) requiring cardiopulmonary bypass at the hands of one of two participating surgeons. The 23 patients undergoing neuromuscular blockade received 0.2 mg/kg of cisatracurium, with intraoperative redosing to maintain a train-of-four of zero to one twitch.

By comparison, the non-neuromuscular blockade arm (n=22) received only a single intubating dose of succinylcholine (1 mg/kg) and no ongoing paralysis. Each group underwent a standardized anesthetic drug protocol of 0.15 mg/kg of midazolam, 15 mcg/kg of fentanyl, and isoflurane and propofol to maintain bispectral index (BIS) less than 55.

The trial’s primary outcome is a composite of early (<72 hours after ICU admission) postoperative pulmonary complications, including extubation after more than 24 hours, reintubation, pneumonia, pulmonary aspiration, need for noninvasive respiratory support, acute respiratory distress syndrome and mortality attributed to a respiratory event. “We’re looking specifically inthe first 72 hours because we felt that we could relate that specifically back to a complication from residual neuromuscular blockade,” Dr. Gerlach said in an interview with Anesthesiology News.

The researchers also assessed surgical tolerability. “The surgeons are blinded to which group the patients are in,” she said. “At the end of the case, we asked them to rate the surgical conditions and whether they noticed any patient movement.” Anesthesiologists also recorded perioperative patient movement. These included minor things such as diaphragm movement and what they deemed major movement, including moving limbs or bucking or coughing on the ventilator.

Only Minor Movements

As reported at the 2017 annual meeting of the Society of Cardiovascular Anesthesiologists (abstract SCA162), the non-neuromuscular blockade arm featured more high-risk patients (11/22 patients with Euroscore >6, compared with eight of 23 in the blockade arm), despite randomization. Nevertheless, mean age, body mass index and mean cardiopulmonary bypass times were similar between groups.

Of note, the researchers found that the lack of neuromuscular blockade had no effect on surgical tolerability when assessed by the surgeon or anesthesiologist. “Patient movement only happened in one or two patients where the surgeons had to stop what they were doing and wait for us to deal with the issue,” Dr. Gerlach said. “But for the most part, these minor movements were not particularly bothersome.” Major movement has not been observed thus far in the trial.

“Interestingly, when you ask surgeons if they’ve noticed minor patient movement during cardiac surgery, there were a surprising number of times where they noticed it in the neuromuscular blockade group,” she said. “So we know that with monitored titration of neuromuscular blocking agents, there still may be times where we see minor patient movement during anesthesia.”

Nevertheless, the investigators also recognized that they needed to have measures in place to abate the potential for patient movement in those who did not receive neuromuscular blockade. “We have a protocol so that if there is some minor movement, we increase our dose or add additional opioid or anesthetic,” she explained. “And in the event of something that could be hazardous to the patient, then they can cross over into the muscle relaxant group, if need be.”

Postoperative pulmonary complications were found to occur in two of the 23 individuals in the neuromuscular blockade group: extubation after more than 24 hours in one patient and aspiration in one patient. On the other hand, a total of four of the 22 patients who underwent surgery without neuromuscular blockade experienced one or more postoperative pulmonary complications, including extubation after more than 24 hours (one patient), reintubation (one patient), pneumonia (three patients), aspiration (one patient) and the need for noninvasive respiratory support (two patients). The study is ongoing and has yet to tally enough patients to determine statistical significance regarding postoperative pulmonary complications.

In the end, the researchers noted that while successful cardiac surgery without neuromuscular blockade can be challenging, it has the potential for significant downstream benefits. “If you ask which is easier, it’s easier to paralyze and not have to worry about the potential for movement,” Dr. Gerlach said. “But at the same time, we’re finding that it’s definitely feasible to do it either way. Sure, it takes a little more attention to detail in terms of continuing adequate dosing of your drugs and maintaining depth of anesthesia at an appropriate level. And it obviously helps that our surgeons are tolerant if there is movement during the case.

“But we’re finding that they often don’t know which group the patients are in, which implies—at a minimum—that a lot of the neuromuscular blocking agents we’re using are likely not necessary in the high doses that are traditionally used.”

According to Hilary P. Grocott, MD, professor of anesthesia and perioperative medicine and professor of surgery at the University of Manitoba, in Winnipeg, Dr. Gerlach and her colleagues have clearly shown the feasibility of performing cardiac surgery without muscle relaxation. “In fact, this is already commonplace in many centers. However, ascribing any benefit to it will be very challenging, as there are a multitude of reasons why pulmonary complications occur after cardiac surgery. Residual muscle relaxation is just one of these.

“Interestingly, avoiding muscle relaxants can also be a reasonable means to detect an unintended inadequate depth of anesthesia that could be a precursor to intraoperative awareness, which itself is a good reason to avoid muscle relaxants,” Dr. Grocott added.