The Reverse MAS Effect: Obstruction Induced by Full Face Mask Usage.

In our lab we occasionally come across a problem we call the ‘reverse MAS effect’ (where MAS refers to the Mandibular Advancement Splint).

This occurs when a patient, undergoing a manual CPAP titration, responds well to CPAP delivered via nasal mask, but due to respiratory instability (e.g. from an intermittent mouth leak and/or residual OSA) a full face mask (FFM) is trialed. Instead of improving the situation, the FFM increases the frequency and severity of obstructive events, resulting in substantially higher pressures to overcome the OSA.

We assumed the FFM was promoting obstruction by displacing the jaw and altering the airway; since the MAS is used to alleviate obstruction by advancing the mandible, and the FFM appeared to do the opposite. We informally described the phenomenon as the ‘reverse MAS effect’.

So what is actually happening in this situation? Why do some patients have increased upper airway obstruction when using a FFM compared to a nasal mask? In short, it depends on the individual.

Kaminska et al observed 6 patients that required less pressure on nasal mask vs a FFM, hypothesizing these patients were susceptible to posterior mandibular displacement1. To overcome this the patients were given a MAS to use in combination with CPAP therapy so their jaw sat in the neutral position.  The full face mask + MAS combination decreased pressure requirements in only 50% of cases2, and for one subject, increased the required level of CPAP (Table 1)1, suggesting FFM induced mandibular displacement is only observed in some individuals.

Tech Talk 6/15 Table 1.


In a case study where the same problem was observed, the FFM was causing posterior displacement of the tongue3. For this patient 7cmH2O delivered via nasal mask was sufficient, whereas 16 cmH2O + was required on a FFM. Further investigation showed that tongue displacement was causing significant obstruction of the airway and was therefore the proposed mechanism behind the increased CPAP requirements on a FFM (Figure 1)3.

Tech Talk Fig 1.







Figure 1 – Polysomnography summary of a natural sleep continuous positive airway pressure (CPAP) titration study (top) and sleep endoscopy images (bottom) using both nasal and oronasal masks. During the first part of the natural sleep CPAP titration, a nasal CPAP of 7 cmH2O adequately controlled obstructive events. During the second part, an oronasal mask was used. Observe the persistence of obstructive respiratory events even at pressures up to 16 cmH2O. The endoscopic image taken with nasal CPAP of 7 cmH2O shows a wide-open oropharynx. In contrast, the image taken with oronasal CPAP of 16 cmH2O shows the tongue base posteriorly displaced, pushing the epiglottis and significantly narrowing the airway. Sp,O2: arterial oxygen saturation measured by pulse oximetry; CA: central apnoea; OA: obstructive apnoea; MA: mixed apnoea; HYPO: hypopnoea; PAP: pulmonary artery pressure; REM: rapid eye movement. Respiratory event time is shown in seconds

Figure reference: Schorr, Fabíola, et al. "Continuous positive airway pressure delivered by oronasal mask may not be effective for obstructive sleep apnoea." European Respiratory Journal 40.2 (2012): 503-505.

In addition to the above hypotheses’ people with OSA that use an full face mask are, by definition, more likely to mouth breath or at least sleep with their mouth open (perhaps due to poor skeletal muscle tone). This open mouth position increases the risk of airway collapsibility2,4-5. There is also a strong correlation between the prevalence of OSA and obesity:  since obese OSA sufferers have increased fat deposits in tissue surrounding the pharynx6, as well as enlarged soft tissue in the upper airway and tongue7, those that mouth breath could be at a higher risk of upper airway collapsibility, and thus require a higher pressures on an FFM.   

As common with complex medical conditions such as OSA, the contributing factors of this ‘reverse MAS’ phenomenon are patient specific. For a patient susceptible to this problem where a nasal mask is not an option, a combination of interventions may be required to effectively control their OSA (e.g. addition of MAS, weight loss or surgery to prevent detrimental structural changes).

From a technical point of view, our ability to tackle this issue is limited. We can try to optimize treatment via a nasal mask by employing positional therapy or a chinstrap to encourage mouth closure (and thus attenuate airway collapse) but if posterior displacement of the mandible or tongue is the culprit, the chinstrap may also exacerbate obstruction.  Above everything, a successful CPAP titration study is strongly dependent on what the patient tolerates. Some patients just don’t like nasal masks and may prefer a higher pressure on a FFM than any treatment administered via a nasal mask.

Unfortunately, there is no clear course of treatment available to patients that display the ‘reverse MAS effect’. The only clear outcome is that we may need a new phrase to describe the problem, as posterior mandibular displacement is only part of the explanation. As always, we just have to do our best with the little time we have during a study, and hope that the patient is a good sport.

By Melissa Cava


1 Kaminska, M., et al. "Higher effective oronasal versus nasal continuous positive airway pressure in obstructive sleep apnea: Effect of mandibular stabilization." Canadian respiratory journal: journal of the Canadian Thoracic Society 21.4 (2014): 234.

2 Teo, Ming, et al. "Equivalence of nasal and oronasal masks during initial CPAP titration for obstructive sleep apnea syndrome." Sleep 34.7 (2011): 951.

3 Schorr, Fabíola, et al. "Continuous positive airway pressure delivered by oronasal mask may not be effective for obstructive sleep apnoea." European Respiratory Journal 40.2 (2012): 503-505.

4 Kuna, Samuel T., and John E. Remmers. "Neural and anatomic factors related to upper airway occlusion during sleep." The Medical clinics of North America 69.6 (1985): 1221-1242.

5 Meurice, Jean-Claude, et al. "Effects of mouth opening on upper airway collapsibility in normal sleeping subjects." American journal of respiratory and critical care medicine 153.1 (1996): 255-259.

6Horner, R. L., et al. "Sites and sizes of fat deposits around the pharynx in obese patients with obstructive sleep apnoea and weight matched controls." European Respiratory Journal 2.7 (1989): 613-622.

7 Schwab, Richard J., et al. "Identification of upper airway anatomic risk factors for obstructive sleep apnea with volumetric magnetic resonance imaging." American journal of respiratory and critical care medicine 168.5 (2003): 522-530.