Some mucolytic agents cause more problems than they solve

MARCH trial provides ample reason to discourage routine use in vented patients

Jun 25, 2026

WARNING: This post says the word “mucus” about 1,427 times.

Plugging of the small airways (and sometimes the large ones) by mucus is a major problem for many mechanically ventilated patients. Mucus plugging is a notorious source of relatively rapid worsening of ventilation and gas exchange, initiating a cascade of concern and diagnostic and therapeutic maneuvers by the care team.

In a common scenario, at 8 a.m., it looks like the patient is going to experience a respiratory-induced cardiac arrest; at lunchtime, after very little if any meaningful intervention, the patient looks like they will soon extubate themselves and walk home. (Well, maybe not home: most patients experiencing significant mucus plugging are elderly, frail, have severe chronic lung disease, or a combination of these.)

Mucus plugging is a stealthy bogeyman whose presence in ventilated patients is usually inferred. Garden-variety mucus plugging of the smaller airways can’t be easily identified on chest radiographs or CT scanning. Even bronchoscopy has major limitations in identifying or treating the issue, because the bronchoscope only reaches about a third of the distance into the tracheobronchial tree; mucus plugging can often be diffuse and compromise beds of smaller airways unreachable by the endoscope.

Severe, obvious, abundant mucus production requiring frequent suctioning helps predict extubation failure in mechanically ventilated patients. At the much more common lesser magnitudes, however, evidence is lacking as to whether the degree of clinically observable mucus production correlates with patient-centric metrics like duration of ventilation, ICU length of stay, etc.

The traditional, pragmatic response to this uncertainty in vented patients with abundant mucus production has been to instill N-acetyl-cysteine (NAC, still commonly called by its old brand name Mucomyst™ in the U.S.) down the endotracheal tube. NAC liquefies mucus through a direct chemical reaction in which its free sulfhydryl group attacks the disulfide bonds connecting mucin polymers, breaking down the viscous, stringy strands into a thinner, more liquid state. I’m sorry I had to write that.

NAC works great in test tubes but has never been demonstrated to be clearly beneficial to vented patients, either as a routine part of respiratory care or as a targeted therapy for those with high mucus production.

Further, nebulized NAC is not always benign: its chemical properties cause irritation to tracheobronchial mucosa, infrequently and unpredictably triggering bronchospasm.

In the largest randomized trial (NEBULAE, n=922, NEJM 2018), scheduled acetylcysteine (with the bronchodilator salbutamol) did not result in fewer ventilator days than as-needed mucolytics + salbutamol.

Mucolytic agents can also be delivered I.V. (NAC, bypassing its high first-pass metabolism by the liver) or enterally (carbocysteine or erdosteine).

A meta-analysis of very low-quality evidence (i.e., of many small underpowered trials of varying methodology and rigor, testing multiple delivery methods, I.V., nebulized, oral) did not suggest reduced vent days from mucoactive agents in the ICU generally (but did suggest shortened ICU stays).

A large, high-quality randomized trial was then performed.

The MARCH Trial

At 71 ICUs in the U.K., 1,956 mechanically ventilated adults who were considered by their physicians to have difficult-to-clear secretions were randomized into a 2 x 2 factorial design to receive either-or-both enteral carbocysteine 750 mg q8h, 4 mL nebulized hypertonic saline q4h, or placebo. (I.E., they could randomly get either agent and a placebo, both agents, or two placebos).

The first important finding was that there was no interaction effect between the two agents (increasing the confidence in the independent effect of each agent on the results).

Neither carbocysteine nor hypertonic saline reduced ventilator days over placebo. For both agents, the numerical direction favored placebo. There were no improvements in secondary outcomes, either.

Both agents appeared to independently harm patients:

Carbocisteine increased clinically important upper GI bleeding (1.4% with carbocisteine vs 0.2% without, P=0.01).
Hypertonic saline increased bronchoconstriction requiring bronchodilator use (2.4% with saline vs. 0.4% without, p=0.001)

Hypertonic saline is known to cause bronchoconstriction in susceptible patients, so the result was not overly surprising.

Carbocysteine has a known association with GI bleeding: in its UK product labeling, peptic ulcer disease is a contraindication, and GI hemorrhage is an explicit warning. The putative mechanism is by breaking down helpful, protective mucus in the GI tract, exposing epithelium to acid-induced ulceration with resulting bleeding.

But Remember Those Great Ads With the Mucus-Monster Guy? That Stuff Must Work

Carbocysteine is not approved by the U.S. FDA, but is (or was) in use in the U.K., Europe, Asia, and South America under brand names like Mucodyne.

Oral erdosteine has weak evidence to support its use in outpatients with moderate-to-severe COPD, based on the RESTORE trial (U.K./Italy). Erdosteine has orphan drug status in the U.S., and is not approved for use as a mucolytic in the ICU.

In the U.S., guaifenesin is sometimes deployed in vented patients in the hope that it will improve secretion clearance. Long used as an over-the-counter expectorant in the ambulatory setting, a widely cited randomized trial (in 2014) showed no effect of guaifenesin on either sputum characteristics or symptom resolution in outpatients with productive coughs. There is virtually zero evidence to support its use in mechanically ventilated patients.

Conclusion

Mucus plugging is a prominent and difficult problem affecting many mechanically ventilated patients.

Unfortunately, no therapy has been shown to be consistently beneficial, and some can have harms.

Common sense preventive and mitigation measures like adequate humidification, appropriately frequent turns and suctioning, (maybe) targeted use of chest physiotherapy and bronchoscopy (for suspected central obstructions) are the primary tools at the clinician’s disposal.

Once preventive measures are in place, airway clearance is yet another of the major physiologic functions that critically ill patients must regain on their own, largely independent of care teams’ best intentions and efforts. Whether and when a severely ill mechanically ventilated patient does so will often determine their outcome.