How Is Huffing Done to Clear Phlegm? The Forced Expiration Technique (FET) and How It Differs From Coughing

Caring for a family member with a lot of phlegm often brings a frustrating situation. The phlegm has already come up to the throat, but when the patient coughs hard the face goes red, the head spins and the strength runs out, and in the end the whole mouthful of phlegm is swallowed back. If the aim is to bring phlegm up without a violent cough, there is a method in the clinic called "huffing", whose formal name is the "forced expiration technique" (FET).

In short, huffing (FET) means breathing out forcefully with the glottis open, which brings phlegm up without a violent cough. It reduces airway collapse and also takes less energy. The research shows that this approach is safe in the short term and helps particular patients to clear phlegm, but it cannot cure the disease or lower mortality, and the overall quality of evidence for airway clearance techniques is also low (low certainty, Zisi 2022; Lee 2015). This page concentrates on huffing itself, including the method, how it differs from coughing and when each approach is used. The full rhythm of the active cycle of breathing, and the whole ladder of airway clearance from the patient doing it alone to relying on mechanical help, are set out in the complete guide to airway clearance and chest percussion.

What is huffing, and how it differs from coughing

To understand how huffing works, first look at why coughing costs so much energy. A forceful cough needs a deep breath in first, then closing the glottis so that pressure in the chest rises sharply, and then a sudden burst. This explosive force is strong, but every cough uses a great deal of energy, and the very high pressure in the chest makes the soft airways collapse in an instant. Huffing breaks this process down. The glottis stays open, and the phlegm is moved by the airflow of the breath out alone, which takes much less effort. The physiotherapy guideline of the British Thoracic Society lists FET together with the active cycle of breathing techniques (ACBT) as standard airway clearance methods for adult spontaneously breathing patients (Bott 2009).

Why huffing reduces airway collapse and tiredness

The physiological mechanism lies in which section of the airway is squeezed out of shape when breathing out. When a person breathes out forcefully, an "equal pressure point" forms between the lungs and the airways. The airway downstream of this point (the end nearer the mouth) is squeezed flat easily, because the pressure outside the tube is greater than the pressure inside it. During an explosive cough with the glottis closed, the pressure in the chest is very high, and the airway near the equal pressure point collapses even more easily in an instant, which in turn blocks the phlegm behind it. In huffing, the glottis is open, so the pressure in the chest is not so extreme. The position of the equal pressure point also moves up and down with the lung volume, and this is the physiological basis for the "huffing at two lung volumes" described below (Bott 2009). In other words, a huff is not a "gentler cough". It brings phlegm up layer by layer in a way that does not flatten the airways.

This principle also explains why "coughing hard again and again" often backfires in a weak patient with a lot of phlegm. Each high-pressure explosive cough collapses the downstream airway again, so the phlegm is not cleared and the patient is exhausted and more breathless, which becomes a vicious circle. The role of huffing is to break this deadlock. Using breathing-out movements that can be repeated and do not flatten the airways, it moves the phlegm up in relay stages until it reaches the large airways, and only then is a short burst of force used to clear it. Huffing is therefore meant to "work with coughing" and not to "replace coughing", and the two work most efficiently when they share the job (Bott 2009).

How to huff to clear phlegm: at two lung volumes (low volume for the periphery, high volume for the proximal airways)

The knack of huffing is to decide, from where the phlegm is, which lung volume to start breathing out from. A huff that starts from a lower lung volume is better at moving phlegm that is deeper (peripheral), while a huff from a higher lung volume (breathing in fully and then huffing) is better at clearing phlegm in the large (proximal) airways (Bott 2009; Zisi 2022). The specific steps are as follows:

Between each huff, add one or two normal relaxed breaths, and do not keep huffing hard until dizzy.

• ① Low-volume huff (bringing peripheral phlegm up)—First take a steady breath in, without filling the lungs completely. Open the mouth and throat, and breathe out a mid to low volume of air steadily and forcefully with a "haaa", as if breathing onto a mirror. Do this two or three times in a row and feel the phlegm slowly moving upwards.
• ② Sense where the phlegm has moved to—When the phlegm is heard or felt to have reached the large airways (for example a rattling sound near the throat, or a feeling that the phlegm has loosened), get ready to bring it out with a high-volume huff or a gentle cough. Never cough with full force from start to finish.
• ③ High-volume huff or gentle cough (clearing proximal phlegm)—Take a big breath in, open the throat and give a forceful "haa!", or cough gently once, to bring out the phlegm that has been pushed up to the large airways, and catch it with a tissue or a kidney dish.
• ④ Rest, then repeat—After one set, breathe steadily a few times and rest before starting the next set, with no need to rush to cough up a lot of phlegm at once. This rhythm of working and resting is part of the active cycle of breathing.

Clinically, huffing is rarely used alone. It is mostly built into the rhythm of the active cycle of breathing (breathing control, then thoracic expansion, then huffing). To see how the whole cycle fits together and when to move from one step to the next, see how to do the active cycle of breathing technique.

Huffing versus coughing, and when to use each

Huffing and coughing are not a choice of one or the other. They come in order and each has its own job. The section below sets out the roles of the two and one safety limit that must never be crossed:

The effect of huffing and the research evidence

To ask whether huffing is effective, the question must be looked at objectively from two sides. On one side, a systematic review covering eleven studies found that the active cycle of breathing, which includes FET / huffing, has an equal or even better short-term effect on the amount of phlegm cleared and on lung function, both increasing the phlegm coughed up and easing breathlessness (Zisi 2022). In children with cystic fibrosis, the amount of phlegm coughed up before and after ACBT showed no statistically significant difference, but the character of the phlegm (that is, how easily it could be coughed up) improved significantly, which helped to clear secretions in the peripheral airways (Hristara-Papadopoulou 2007).

On the other side, these benefits appear mainly in the short term, and the differences between the various phlegm-clearing techniques are in fact limited. In a crossover trial of slow expiratory airway clearance in patients with bronchiectasis, all techniques promoted phlegm clearance during treatment, but the total amount of phlegm cleared in the twenty-four hours after treatment was similar, the improvement in quality of life was about the same, and lung function did not change (Herrero-Cortina 2016). Even during an acute exacerbation needing hospital admission, the amount of phlegm cleared and the improvement in quality of life were quite close between "walking while huffing" and ACBT (Phillips 2023). The Cochrane review of airway clearance in bronchiectasis also states directly that the overall evidence is of low quality. The techniques can make phlegm easier to cough up, reduce breathlessness and cough, and increase phlegm clearance in the short term, but they did not reduce the number of exacerbations, and no adverse events were reported during the studies (Lee 2015).

The reason for spending so many words on the "limits" of the technique is to put the facts plainly, so that families do not hold unrealistic expectations. Huffing is a supporting method of clearing phlegm in the short term that needs to be done every day with patience. It cannot clear all the phlegm in one go, and it cannot make lung disease "go away" or be cured. Recognising this actually helps carers and patients to keep going longer, and makes the approach more realistic, because everyone is clear that the aim of this work is to make phlegm easier to bring up, so the patient is not exhausted every time phlegm is coughed.

Which patients need special care

Huffing is safe and light on effort, but it is not an answer to everything. In particular, when the root of a patient's "inability to cough up phlegm" is that the breathing muscles themselves lack strength, teaching huffing alone is not enough to solve the problem. Extra care is needed in the following situations:

Two kinds of cases seen when teaching huffing at home

When a physiotherapist teaches huffing at home, the first step is to tell whether the patient "has an inherently weak cough" or simply "does not know how to relax the glottis". The second works quickly once huffing is learned. For the first, the therapist must first assess whether a cough assist device is needed, and never just tells the patient to cough harder in a one-size-fits-all way. A huff is not a "weaker cough". The knack is that the glottis must be open and the phlegm is moved by the airflow of the breath out, and not by an explosion of force. In teaching, the patient is guided to hold on to the feeling of "breathing on a mirror to make it mist up", and not to chase how loud the breath out is.

Another common picture is a patient practising huffing at home who "overdoes it", huffing hard in a row until dizzy, or triggering non-stop coughing, and ends up more tired. In practice, each set needs only two or three huffs, with steady normal breaths in between (that is, the rhythm of the active cycle of breathing), and a gentle cough is used to bring the phlegm out only once it has risen to the large airways, with no need to force a large amount of phlegm out at once. When the registered physiotherapists at Nova Health do a home physiotherapy assessment, they first test the patient's cough strength, demonstrate huffing / FET in person, and guide the family living with the patient on how to assist safely from the side on a sofa or at the bedside. For example, an older adult leaving hospital after pneumonia, chronic obstructive pulmonary disease or a stroke often has a lot of phlegm but cannot cough hard, and once huffing is learned no longer has to cough until dizzy and exhausted. For a long-term bedbound or less mobile older adult, a few sets of huffing can also be done at set times morning and evening with suitable positioning, with the phlegm caught on a tissue once it is brought up, so the family knows how to help and avoids hard back patting without purpose. If the patient needs a referral for respiratory rehabilitation after leaving hospital, follow-up can go through the respiratory outpatient clinics of the Hospital Authority clusters (https://www.ha.org.hk). The public who want more information on respiratory health can also look at the website of the Hong Kong Lung Foundation (香港胸肺基金會) (https://www.hklf.org.hk).

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