How Is Peak Cough Flow Measured? Where the 270 Threshold Comes From

How is peak cough flow measured, and why is 270 L/min used as a threshold? Peak cough flow (PCF) is measured with a peak flow meter and reflects the strength of the cough used to clear sputum. International guidelines (AARC 2013) use a PCF below 270 L/min as the threshold for considering cough assistance in people with neuromuscular disease, because PCF falls further during an infection and can drop below the roughly 160 L/min needed for an effective cough (Strickland et al. 2013). This threshold is a practical decision for selected patients and is not a promise of a cure or of lower mortality.

The types of cough assistance and how the machines work are outside the scope of this page. This page concentrates on a detail that is often skipped: how to measure PCF accurately, and how the number 270 is worked out. The measuring tool affects the reading, and the step between 270 and 160 needs some explaining.

What peak cough flow is and why it is measured

Peak cough flow is the highest airflow during one forceful cough and is used to measure objectively how strongly the patient can cough up sputum. Apart from neuromuscular disease, PCF is seldom used clinically. For patients with weak expiratory muscles, though, it is one of the few indicators that turns whether the cough is strong enough into a number that can be tracked. A PCF above 160 is needed for an effective cough, and a PCF below 270 is linked to a higher risk of retained secretions and infection (Brennan et al. 2022). This floor for an effective cough comes from studies of extubation and decannulation: almost all patients who could produce more than 160 L/min cleared the airway successfully, while those below it could not (Bach & Saporito 1996).

How peak cough flow is measured

PCF is measured with a peak flow meter. The steps look simple, but each one can distort the reading, so an accurate measurement should be demonstrated and supervised by the therapist.

The key steps for measuring PCF at home and in the clinic:

Seal against leaks—Attach the peak flow meter to a face mask or mouthpiece and seal it firmly over the nose and mouth. When measuring at home, the most common reason for a low reading is air leaking, which underestimates the true cough strength.
Breathe in deeply to close to full lung capacity—Before measuring, the patient must breathe in as fully as possible before coughing. If the breath in is not deep enough, the peak flow of the cough is pulled down directly, and the number then reflects an inadequate breath in and not weak coughing.
Cough once forcefully and read the highest value—After the deep breath in, the patient coughs once as hard as possible and the peak value is read. Three attempts are generally made and the highest taken, so that one poor attempt does not underestimate the strength (Sancho et al. 2004).
Use the same device each time—To watch how the numbers change over time, the same portable meter should be used every time, with the same posture and method, so that the readings can be compared directly (Kulnik et al. 2015).

The difference between portable peak flow meter and spirometer readings

The same patient measured with different devices can give very different PCF readings. This is often overlooked, yet it is enough to distort judgement.

How large the difference is

In the same group of people with neuromuscular disease, the portable meter read 160.9 and the spirometer read 248.5 L/min
The Bland–Altman mean bias was about 87 L/min, with the portable meter reading lower
A different measuring tool alone is enough to push the reading for the same person across a clinical dividing line

Causes of the difference

A portable meter has limits of internal resistance, measuring range and response speed
If the mask leaks or the mouthpiece is not sealed tightly, the reading is lower still
Other studies of their samples showed that the two devices read similarly, which depends on the people studied and the method of measurement

Numbers measured with different devices cannot be compared directly. One study found that readings of a portable peak flow meter were generally lower than those of a spirometer (an average difference of about 87 L/min; Rodrigues et al. 2017), but a study of a mixed sample showed that the two read similarly (Sancho et al. 2004). The point is not which device is more accurate but that when watching a trend, the same device and the same method must always be used. A number measured in hospital today and a number measured at home tomorrow should not be used directly against the 270 and 160 standards.

Why 270? The reasoning from 160 to 270

The figure of 270 is worked out from the floor for an effective cough plus a layer of safety buffer. Once these two layers are understood, it is clear why the guidelines do not wait for a fall below 160 before stepping in.

160 L/min = the floor for an effective coughIn studies of extubation and decannulation, almost all patients with a PCF above 160 cleared the airway successfully and those below it failed, so 160 became the dividing line for whether sputum clearance works at the time (Bach & Saporito 1996).

PCF drops sharply during an infectionIf a person with neuromuscular disease has only about 270 when the condition is stable, then with a respiratory infection, plenty of sputum and breathlessness the cough strength falls further and can easily drop from 270 to below 160 (Tzeng & Bach 2000).

270 = an early-warning buffer in the stable phaseClinical guidelines therefore advise that if PCF is below 270 when the condition is stable, cough assistance should start to be considered. This leaves room for the fall that happens during an infection, so there is no need to wait until the cough is completely ineffective before acting in a hurry (AARC 2013).

160 is the line for whether the cough works at the time, and 270 is the line for whether the patient has enough in reserve to cope with an infection when stable. The two numbers are used in different situations, and mixing them up leads to a wrong judgement. A reading of 250 in the stable phase does not mean sputum cannot be cleared at the time, but it does mean the risk is high at the next infection.

Assistive techniques after PCF falls below 270 and the research evidence

How to follow up after the value falls below 270 depends first on the level of evidence. The hard-outcome evidence for this step is far weaker than the decision rule of starting assistance when PCF is low.

The step the guidelines support

PCF below 270 in the stable phase → consider cough assistance (AARC 2013)
A practical decision for selected patients and not a rigid rule
The aim is to leave a buffer before an infection and not to treat disease

The step where evidence is still weak

Mechanical insufflation-exsufflation: randomised evidence is insufficient to support or oppose (Morrow 2013)
Other airway clearance techniques: not routinely recommended because of insufficient evidence (AARC 2013)
No study has shown that these techniques lower infection, hospital admission or death

What a value below 270 triggers is a safe, short-term aid to clearing sputum, and not a machine that treats disease. Which level to use (manually assisted cough, breath stacking or a machine) still has to be assessed individually according to swallowing and bulbar function. Airway clearance techniques can bring up more secretions, improve some symptoms in the short term and are broadly safe, but only a small amount of low-quality evidence supports an improvement in quality of life (Lee et al. 2015).

The limits of PCF as an indicator

The number 270 also has its limits. A recent review written by 21 international experts divides airway clearance techniques into proximal (cough augmentation) and distal (secretion mobilisation) and makes recommendations on objective measures such as PCF. Its content rests mainly on expert consensus, which reflects the lack of high-level randomised controlled trials in this field (Chatwin et al. 2018).

What PCF does not reflect

Swallowing and bulbar function are not reflected by PCF
How much sputum there is in the airways, and the overall condition, need to be assessed separately
Even if PCF reaches the target, it does not mean that every sputum clearance problem has been solved

The current research evidence in this field

The effect of conventional chest physiotherapy on mortality or cure rate in adult pneumonia is very uncertain (Chen 2022, very low certainty)
Clinical guidelines do not advise routine airway clearance for simple pneumonia or COPD (AARC 2013)
Current practice relies mainly on expert consensus and lacks large randomised controlled trials (RCTs) (Chatwin 2018)

270 should be understood as a trigger point that suggests closer monitoring, and not as a mechanical line between life and death. The current evidence does not support describing any respiratory physiotherapy as lowering infection, hospital admission or death.

Measuring PCF at home and when to seek the therapist’s help

At home, the point is to use a consistent method and watch the trend. A common situation is that a patient with motor neurone disease or muscular dystrophy catches a cold and has a lot of sputum that cannot be coughed up. The family compares the reading from a portable peak flow meter with the usual baseline to judge whether cough assistance or medical help should be arranged early. Readings from a portable meter and a hospital spirometer cannot be compared directly, so tracking should look at the fall relative to the patient’s own stable baseline, and should not apply 270 and 160 rigidly. A clear fall in the reading, or signs of infection, is a signal to seek help.

At a home physiotherapy assessment, before measuring PCF the therapist first demonstrates the movement from the deep breath in to the forceful cough, confirms that the mask or mouthpiece is sealed with no leaks, and then measures three times in a row and takes the highest. Air leaks and inadequate breath-in volume are the most common reasons for a low reading at home, and teaching the seal and the deep breath in is what lets the therapist tell whether strength has really fallen or the method was wrong. The therapist also reminds the caregiver to use the same portable peak flow meter each time and record at the same time of day, and leaves a card comparing the baseline in the stable phase with the line for seeking help, so that the family looks at the patient’s own fall and does not apply the guideline numbers rigidly.

In the public sector, the neurology and respiratory departments of the Hospital Authority provide referral for respiratory function assessment and PCF measurement in neuromuscular disease. Among patient organisations, the Hong Kong Neuromuscular Disease Association (HKNMDA) provides medical information, financial assistance for equipment and a mutual-help network for people with neuromuscular disease and their families (tel. +852 2338 4123, email [email protected]; for contact details, please refer to the latest on the Contact Us page of its website). If a patient at home has a weakening cough, an obviously wet-sounding chest, choking easily after eating or repeated chest infections, PCF should be measured and a swallowing assessment carried out. Nova Health’s registered physiotherapists use a peak flow meter to measure peak cough flow attempt by attempt and interpret it together with swallowing and bulbar function, and do not draw conclusions from a single number.

Key points

To measure PCF with a peak flow meter, seal the nose and mouth with no leaks, breathe in deeply to close to maximum lung capacity, cough once forcefully, and measure three times and take the highest. If any one of these steps is done poorly, the reading is inaccurate.
A portable peak flow meter reads generally lower than a hospital spirometer (an average difference of about 87 L/min; Rodrigues 2017). To watch a trend, the same device and method must always be used, and readings from different devices must not be compared directly.
The threshold of 270 is worked out from 160, the floor for an effective cough (Bach 1996), plus a safety buffer. A patient with only about 270 when the condition is stable has a sharp fall in PCF on infection and can easily drop below 160 (Tzeng 2000), so the guidelines advise considering cough assistance below 270 (AARC 2013).
Using an assistive device when the reading is below 270 aims to help clear sputum safely in the short term and is not a treatment of the disease. For mechanical insufflation-exsufflation, the current evidence is insufficient to support or oppose its use (Morrow 2013), and no study has shown that it reduces infection, hospital admission or death.
PCF is only a single indicator of cough strength and does not show swallowing ability or the overall condition. At home, watch how much the reading has fallen from the usual, and let the therapist judge together with other assessments (Chatwin 2018).

Frequently asked questions

How is peak cough flow (PCF) measured? Is it practical to measure at home?

A peak flow meter is attached to a face mask or mouthpiece and sealed so that no air leaks. The patient breathes in deeply (to close to full lung capacity), coughs as hard as possible once, and the highest flow is read off. Three attempts are generally made and the highest value is taken. At home, a portable peak flow meter can be used to track the trend, but in the same person with neuromuscular disease, the portable meter reads systematically lower than the hospital spirometer (a study found an average difference of about 87 L/min), so the same device and the same method must always be used for comparison (Rodrigues 2017). How to interpret a reading, and whether to start cough assistance, should be left to the physiotherapist or doctor.

Why is it 270 L/min and not 160?

160 L/min is the minimum threshold for an effective cough at the time, and below it sputum clearance is essentially ineffective (Bach 1996). 270 is an early-warning buffer for the stable phase and does not mean the cough is ineffective at the time. If a person with neuromuscular disease has only about 270 when stable, cough strength drops sharply with a respiratory infection and can easily fall below 160 (Tzeng 2000). That is why international guidelines start to consider cough assistance when PCF in the stable phase is below 270, to be prepared before an infection comes (AARC 2013).

Does a PCF below 270 always mean a cough assist machine must be used? How real is the effect?

Below 270 is the threshold in the guidelines for considering cough assistance. It is a practical decision for selected patients and is not a rigid rule, and each case needs individual assessment. For specific techniques (such as mechanical insufflation-exsufflation), randomised evidence is still insufficient to support or oppose their use (Morrow 2013), and the guidelines also state that other airway clearance techniques cannot be routinely recommended because of the lack of evidence (AARC 2013). A reasonable position is a safe, short-term aid to clearing sputum, and it should not be regarded as a cure, as lowering mortality or as a guarantee of fewer infections.

If PCF is normal, is there nothing to worry about?

PCF is only a single objective indicator of cough strength. It does not reflect swallowing, the load of airway secretions or the overall condition. The guidelines use a value below 270 as the trigger, and the actual follow-up still has to be combined with clinical assessment. A PCF that reaches the target does not mean that every sputum clearance problem has been solved. It should therefore be interpreted by the physiotherapist together with other assessments and conclusions should not be drawn from a single number (Chatwin 2018).

How Is Peak Cough Flow (PCF) Measured, and Why Is the Threshold 270 L/min? The Reasoning from 160 to 270