Can't Cough Up Phlegm? Chest Percussion, Cough Assist Machines and Suctioning: A Practical Guide to Airway Clearance

When caring for a family member who is bedbound long-term, has had a stroke or has a neuromuscular disease, the worry families meet most often is this: the patient often has phlegm but cannot cough it up, so what should be done? Many people first think of patting the patient's back, buying a cough assist machine, or even suctioning straight away. Before acting, a more basic idea needs to be clear: the real problem is often not that there is phlegm, but that the patient does not have the strength to cough it away.

An effective cough needs a deep breath in, then the vocal cords closing, and then the air being released forcefully. In people with neuromuscular disease, a weakened cough reflex after stroke or long-term bed rest, one of these two steps, breathing in or breathing out, often fails. The phlegm looks plentiful, but the real reason is that the patient cannot clear it. This article starts from that physiological mechanism and explains which patients each airway clearance technique suits (chest percussion, postural drainage, air stacking, manually assisted cough, cough assist machines and suctioning), and states the limits of the current medical evidence. One principle runs through the whole article: any airway clearance plan must first be assessed by a physiotherapist. The article sets out the principles and the decisions behind them, and it is not an instruction manual for families to carry out on their own.

Why phlegm cannot be coughed up: the problem of a weak cough

Having phlegm is not a disease in itself, and producing phlegm is a normal cleaning mechanism of the airway. The real problem lies in the "engine" that clears the phlegm, the cough, which has lost its strength. A study of 155 patients with neuromuscular disease (Trebbia 2005) gave a useful analysis. Among many factors, maximum inspiratory capacity alone explained about 44% of the variation in peak cough flow, while breathing-out factors accounted for a much smaller share. The conclusion of the study also states directly that "strengthening inspiration seems to be the key".

This explains a blind spot that is often overlooked. Many people think a weak cough is simply because the abdominal muscles are too weak to push the air out, but being unable to take a deep enough breath in also makes the whole cough fall apart. If the breath in is not enough, the explosive force that follows has nothing to build on. For this reason, techniques that "make up" the volume breathed in (such as air stacking, described below) make sense: expand the lungs fully first and then cough, and the cough naturally has more force.

Two peak cough flow values: 160 and 270

To judge whether a patient's cough is strong enough, there is an objective measure used in the clinic: peak cough flow (PCF), the highest speed the airflow reaches during a cough. Families should know two values from it, but they have quite different clinical meanings and must not be confused:

160 is the "floor". Bach 1996 followed extubation and decannulation attempts in 49 patients with neuromuscular disease. All 43 attempts with PCF above 160 L/min succeeded, and all 15 attempts with PCF below 160 failed. In other words, once the value is below about 160, the patient cannot clear the airway by coughing alone. The review by Winck 2015 also supports using an oral PCF above 160 L/min (or above 60 L/min when measured through a tracheal tube) as a reference for readiness for extubation or decannulation.

270 is the "trigger point". The American Association for Respiratory Care (AARC, Strickland 2013) advises that when the PCF of a patient with neuromuscular disease is below 270 L/min, cough assistance techniques should be started. The standard is set higher than 160 because PCF often falls further when the patient has an acute infection or is tired. Starting early leaves a margin, and avoids a last-minute rescue after the value has dropped through the floor.

Airway clearance methods at different levels

Airway clearance is not a single technique. It is a ladder that moves from "the patient does it" step by step to "mechanical help" and finally to the "last resort". The clinical principle is to start from the level the patient can manage with the least intervention, and to move down the ladder only as the patient's ability gets weaker:

① Patient-led

The active cycle of breathing techniques (ACBT: breathing control, thoracic expansion, huff forced expiration), and oscillating positive expiratory pressure (PEP) devices such as Acapella or Flutter. These suit patients who can still follow instructions and can blow out air alone.

② Carer-assisted

Postural drainage (using gravity to let phlegm flow towards the large airways) together with chest percussion or vibration, which can also be combined with manually assisted cough (pressing on the patient's upper abdomen or chest at the moment of the cough to help generate force). This uses gravity and hand techniques to make up for the patient's lack of strength.

③ Mechanical assistance

A cough assist machine (MI-E / mechanical insufflation-exsufflation: inflating with positive pressure and then switching rapidly to negative pressure to imitate a cough), and air stacking (using a resuscitator bag or ventilator to deliver several breaths in a row to expand the lungs before coughing). These are meant to amplify the cough in patients whose cough is clearly weak.

④ Suctioning

Using a negative-pressure catheter to remove secretions directly from the airway. This is the last resort and is used only in patients in whom all the methods above have failed to clear phlegm.

Which method suits which patient

Following this ladder, clinicians match a technique to the patient's ability and cough strength (PCF). The table below shows common matches, but the actual choice must still be assessed by a physiotherapist:

For patients with chronic obstructive pulmonary disease (COPD) or bronchiectasis, who have "a lot of phlegm but a cough that is still strong", airway clearance is only one part of overall care. COPD management is mainly pulmonary rehabilitation and breathing training, described on the COPD physiotherapy page. Overall home care for neurodegenerative disease (such as MND / ALS) is described on the physiotherapy page for neurodegenerative disease. Which technique to use for a weak cough, and when to start, is covered in the article on cough assistance in neuromuscular disease.

How chest percussion and postural drainage are done, and their contraindications

Chest percussion (cupping the palm and tapping the chest wall rhythmically, together with postural drainage to bring phlegm out by gravity) is the step carers most often try to do themselves. This technique has clear rules and contraindications, however, and doing it wrongly can hurt the patient. The common points of technique are as follows:

The tapping times and positions above are clinical operating guidance, not derived from randomised controlled trials, and they do not suit everyone. More importantly, chest percussion and postural drainage have clear contraindications, and should not be done without professional assessment:

How air stacking and manually assisted cough are done

For a patient with a weak cough, instead of patting the back again and again, it is better to make up the inspiratory volume first and then let the patient cough. Air stacking is a technique designed on this logic. It does not depend on expensive equipment, and a carer can usually learn it in one session:

1. 1
   Prepare

   Use a resuscitator bag or a ventilator, connected to a face mask or mouthpiece.
2. 2
   Deliver breaths in a row

   Deliver several breaths into the lungs in a row, without breathing out in between, so the lungs open up step by step to the maximum.
3. 3
   Hold the breath when full

   When the lungs are full, the throat (glottis) closes briefly and holds the large breath in the lungs.
4. 4
   Cough forcefully

   Then cough the air out forcefully in one go. If needed, press on the upper abdomen or chest with the hands at the same time to help generate force.

A randomised controlled trial in adults with Duchenne muscular dystrophy (Toussaint 2016) showed that air stacking raised peak cough flow by about 50% on average. A low-cost resuscitator bag worked about as well as a ventilator (186 versus 199 L/min, no significant difference), and both pushed PCF above the floor of 160. Most patients only need one session of practice to learn it. Another study (Veldhoen 2022) found that both air stacking and a cough assist machine raised forced vital capacity (FVC) in the short term, though the authors also cautioned that this brief increase in lung volume may not explain all of the clinical benefit. For manually assisted cough combined with a cough assist machine, a study in spinal cord injury (Lee 2024) found that doing the two together gave a better cough than the machine alone. This is stated here only in qualitative terms (combined is better than alone), and the specific flow values should not be applied directly in everyday communication.

Suctioning: the last line of defence

Suctioning uses a negative-pressure catheter to remove airway secretions directly. It is the last step on the airway clearance ladder, and is to be kept for patients who cannot clear phlegm even with all of the help above. It should never be used as the first move. Suctioning clears secretions effectively, but it carries real risks, including low blood oxygen, mucosal injury with bleeding, and marked discomfort for the patient. The standard safety points when suctioning at home include:

• Infection control—Keep the catheter and hands clean, and follow the aseptic method strictly.
• No suction on insertion—Do not apply suction while the catheter is being put in. Apply suction only as the catheter is withdrawn, rotating it as it comes out.
• Time limit—Each period of suction must not last longer than 10 to 15 seconds.
• Pressure limit—Keep the negative pressure to about 80 to 120 mmHg.
• Monitoring—Watch for any change in blood oxygen throughout, and be gentle to avoid injuring the mucosa and causing bleeding.

The 10 to 15 seconds and 80 to 120 mmHg above are also clinical operating guidance, and not research data. Before suctioning is considered, the earlier airway clearance methods should be mastered, and the carer should be trained in the correct technique by a therapist providing home physiotherapy. Suctioning should never be seen as the first step. For choosing equipment, deciding whether the patient really needs suctioning, and the full procedure at home, see the page on suctioning at home.

Research evidence on airway clearance and who it suits

Airway clearance techniques have been used clinically for many years, but objectively the overall level of medical evidence is low, and the actual benefit depends greatly on the patient group. The group in which these techniques are most worth using and the evidence is strongest is patients with neuromuscular disease and a weak cough (low PCF), which is the central point this article repeats. How large the benefit really is has to be stated fairly:

Frequently asked questions