Many families who have bought a non-invasive ventilator notice that the machine is labelled "BiPAP" or "CPAP" and ask themselves what the difference is, since the names differ by only one letter. They also wonder why the choice of machine cannot be made by the family. The key to answering these questions is not the brand of the device but the delivery mode of the machine, and whether the patient's problem is being unable to get air in or being unable to exchange air.
In brief: a home non-invasive ventilator (NIV) delivers pressure support for breathing through a mask, with no need for intubation. BiPAP gives two levels of pressure, one for breathing in and one for breathing out, and is mostly used for hypoventilation, neuromuscular disease and COPD with chronic hypercapnia. CPAP keeps a single continuous pressure and mainly treats sleep apnoea (McKim 2011; Ergan 2019). The role of the ventilator is to support ventilation and control symptoms, and it is not physiotherapy. All pressure settings are a medical order. The role of the physiotherapist is limited to providing safe airway clearance support, and the evidence for this is mostly of low certainty.
How a non-invasive ventilator (NIV) works
"Non-invasive" means that no tube through the mouth or windpipe is needed. Pressure is delivered into the airway by a nasal mask or full face mask that fits closely on the face, so the patient can use it intermittently at home as needed (for example only while sleeping at night). The Canadian Thoracic Society clinical practice guideline on home mechanical ventilation was written to standardise the assessment and management of such home cases. It covers many indications including motor neurone disease (ALS), spinal cord injury, muscular dystrophy, scoliosis and obesity hypoventilation syndrome, and notes that user-friendly non-invasive ventilators have become increasingly common (McKim 2011). The European Respiratory Society guideline on long-term home non-invasive ventilation likewise takes mask delivery and long-term home use as the basic framework for discussing NIV (Ergan 2019).
One point needs to be made clear. NIV provides "pressure support" and not "oxygen therapy". The device pushes air into the lungs, which helps breathing and removes carbon dioxide from the body, and it does not supply pure oxygen directly. Which mode the machine uses to deliver air is exactly what separates BiPAP from CPAP.
BiPAP (two pressures) and CPAP (one pressure): the difference in delivery mode
The most basic difference between BiPAP and CPAP is how many levels of pressure are delivered. CPAP keeps a single, fixed continuous positive pressure from start to finish, acting like a splint that holds the upper airway open. BiPAP splits breathing in and breathing out into two different pressures. A higher pressure is delivered on breathing in to help ventilation, and the pressure is lowered on breathing out to reduce the resistance the patient meets when breathing out and make the process more comfortable. This two-pressure design makes BiPAP better suited to patients who really need ventilatory support (McKim 2011; Ergan 2019).
- Delivers one fixed pressure throughout, with no distinction between breathing in and out
- Works like a splint that holds a collapsing upper airway open
- Mainly used for obstructive sleep apnoea
- Treats airway collapse more than hypoventilation
- Higher pressure on breathing in and lower pressure on breathing out, set separately
- The pressure difference between the two directly helps air move in and out
- Mostly used for hypoventilation and neuromuscular disease
- COPD with hypercapnia in the chronic stable phase is one of its applications
In other words, CPAP mainly addresses whether the airway collapses, while BiPAP concentrates on whether the lungs can exchange air effectively. This difference in mechanism corresponds to the different patient groups described below. Clinically, the indication decides which machine is chosen, and the order can never be reversed. Home ventilators and tracheostomy care, covered in the parent article, already explains that ventilatory support and airway clearance are two separate areas, and this page looks in more depth at how BiPAP and CPAP work.
Types of ventilator suited to different conditions
Whether BiPAP or CPAP should be used depends entirely on the cause behind the patient's difficulty in breathing. The European Respiratory Society has published a guideline on long-term home non-invasive ventilation for patients with hypercapnic COPD in the chronic stable phase. This is a clinical setting in which BiPAP two-pressure support is applied to selected chronic hypercapnic COPD patients, and it is a completely different indication from using CPAP for sleep apnoea (Ergan 2019). Common home pairings are as follows:
| Neuromuscular disease · hypoventilation | In patients with muscular dystrophy or motor neurone disease, for example, the strength of the breathing muscles declines, and ventilation is often worse when lying flat at night, so carbon dioxide easily builds up in the body. In clinical practice BiPAP is therefore mostly used at night to help them ventilate (McKim 2011). |
| COPD with chronic hypercapnia | Selected patients with hypercapnic COPD in the chronic stable phase can use two-pressure support such as BiPAP at home on a long-term basis. This is a specific setting listed in the ERS guideline, and not every COPD patient needs it (Ergan 2019). |
| Obstructive sleep apnoea | If the upper airway repeatedly collapses during sleep, CPAP is generally used to deliver a single continuous pressure to hold the airway open. This is a problem of airway collapse and not of hypoventilation. |
| Common precondition: a medical order | Which type of device a patient should use, how much pressure to set and when to start are all decided by the respiratory doctor from the results of a sleep study or blood gas analysis. Family members should never judge for themselves or change the settings of the machine. |
As for the question of when a patient with neuromuscular disease should start home NIV, clinical assessment must put together overnight ventilation performance, blood gas data and clinical symptoms, and cannot be decided by an individual figure alone. The section on indications for home ventilators and ventilatory support in the parent article gives a fuller description of the service, and this page concentrates on the difference in mechanism between the two devices.
The separate roles of ventilator and physiotherapy, and the limits of the evidence
The function of a ventilator is to support ventilation and ease symptoms. It does not directly clear retained secretions from the airway. As for the role of physiotherapy in airway clearance, the existing research is generally of low certainty. The methods are safe and can help to clear secretions in the short term, but no curative effect can be claimed, and it has not been shown to lower mortality or reduce hospital admissions.
The limits, stated openly: Airway clearance in patients with neuromuscular disease falls into two main groups clinically. One is proximal techniques, which strengthen the cough to bring secretions up. The other is peripheral techniques, which help to move secretions toward the large airways. This approach only manages secretions in selected patients and is not a way to cure the disease (Chatwin 2018). For mechanical insufflation-exsufflation (MI-E) in patients with neuromuscular disease, the current evidence is not enough to support or oppose it, and important short-term and long-term clinical outcomes cannot be assessed (Morrow 2013). In addition, for adult pneumonia, the Cochrane review does not recommend conventional chest physiotherapy as a routine adjunct treatment, and the certainty of evidence for its effect is very low (Chen 2022).
Airway clearance care combined with NIV is therefore not a cure-all that guarantees no pneumonia. It is built on ventilatory support, with safe secretion clearance added for specific patients who need it. The actual steps, timing and potential risks of suctioning belong to another area of practice, and the article on when suctioning is needed at home covers them. This article does not repeat that content.
The one hard marker of weak cough: PCF below 270 L/min
Across the whole plan of airway clearance care, only one objective standard has a clear clinical boundary. It is the measurement of peak cough flow (PCF) to judge whether the patient's cough is strong enough to clear secretions unaided.
Decision principle in the AARC guideline
- When the PCF of a neuromuscular patient is below 270 L/min, cough assistance techniques should be added (Strickland 2013)
- The same guideline also advises against routine airway clearance in patients with COPD in general
- If a patient has retained secretions with symptoms, it can be considered case by case according to the patient's wishes and tolerance
Clinical reference value for an effective cough
- For an adult to have an effective cough, the PCF reference value needs to be above about 160–200 L/min (Toussaint 2018)
- Based on this value, the ENMC workshop also proposed a stepwise airway clearance technique (ACT) management framework
- The actual value must be measured and interpreted by the medical team with instruments, and family members should not guess it
It must be stressed that a PCF below 270 L/min is the clear threshold for introducing cough assistance. If the patient's value has not reached this threshold, or the clinical picture is unclear, the therapist first uses safe techniques that the patient can tolerate and tells the family frankly about the limits of the evidence. The therapist will never promise to clear all secretions in one go. For how mechanical cough assist devices are operated and combined with manual techniques, see the page on cough assistance in neuromuscular disease.
Common home care situations in Hong Kong and community resources
In Hong Kong, cases that need ventilator support are most often seen in the transition period when a patient has just been discharged from a public hospital and is about to recover at home. A few common examples follow. An older adult with a neuromuscular disease (such as muscular dystrophy or motor neurone disease) wears BiPAP at night to help ventilation. If the cough is weak by day and the PCF is low, the carer or physiotherapist needs to use cough assistance techniques to help clear secretions. Another group is older adults who completed a sleep study in a public hospital and were diagnosed with obstructive sleep apnoea. They use CPAP at home every night, and the family helps to put on the mask, cleans the parts daily and follows up how consistently it is used. There are also patients with hypercapnic COPD who use NIV long term at home after discharge and combine it with the breathing and secretion clearance techniques taught by the physiotherapist. This kind of care is also an adjunct and not a cure.
Local patient organisation support
- The Hong Kong Neuro-Muscular Disease Association (香港肌健協會) is a mutual support organisation set up for patients with neuromuscular disease and their families
- It provides medical information and referral to community resources
- Telephone: +852 2338 4123; website: hknmda.org.hk
The medical system and referral routes
- Respiratory outpatient clinics and sleep study services under the Hospital Authority
- For NIV or CPAP used at home, the prescription of the device, the sleep study and the pressure settings must all be assessed clinically by a respiratory specialist in a public hospital or in private practice
- All ventilator settings are a doctor's prescription, and a home visit physiotherapist does not and should not adjust them
Many family members easily confuse ventilatory support with airway clearance care. In home care, the duty of the physiotherapist is to provide airway clearance support and to educate family members (including improving tolerance of wearing the mask, and teaching postural drainage and coughing techniques). The physiotherapist does not change the pressure settings of the ventilator. If during follow-up the patient is found to have abnormal ventilation or falling oxygen saturation, the therapist refers the patient to a doctor at once and never overreaches by adjusting the machine. For the full process of home assessment and family training, see the parent page on the home assessment and family training arrangements for home ventilators.
When should a physiotherapist be arranged to visit at home?
If an older adult or patient at home has just started BiPAP or CPAP, and the carer has no idea how to clear secretions safely or when cough assistance is needed, the safest course is to arrange for a physiotherapist with relevant clinical experience to visit and look at the situation in person. This is safer than the family working it out alone. In the home physiotherapyservice provided by registered physiotherapists at Nova Health, the measurement of peak cough flow (PCF) is used as the objective triage marker. Cough assistance is introduced when the value is below the clear threshold of 270 L/min. If the threshold has not been reached or the situation is unclear, safe techniques that the patient can tolerate are used first, and the limits of the research evidence are explained honestly to the family, with no false promise of a cure. The pressure levels and delivery mode of the ventilator are entirely the respiratory doctor's orders, and the therapist will never change them.