Obstructive Sleep Apnea Treatment with EPAP Nasal Devices: Physiological Principles and Limitations

Expiratory Positive airway pressure (EPAP) devices such as Provent have been used for treatment of obstructive sleep apnea without discussing their limitations. In this short review we discuss the physiological limitations of EPAP devices during inspiration and during expiration. During spontaneous breathing, when EPAP is excessive, the patient would have difficulty breathing in because lung compliance decreases at higher volumes. Furthermore excessive EPAP could lead to progressive trapping of air in the lungs. An ideal EPAP device should allow the patient to adjust the resistance to a comfortable level that would provide EPAP without a progressive buildup in pressure, without compromising tidal volume, without causing CO2 retention, and without disturbing sleep. The use of EPAP devices with adjustable resistance is essential for best results in treatment of obstructive sleep apnea and snoring. DOI : 10.14302/issn.2574-4518.jsdr-16-1333 Corresponding author : T.S. Hakim, Ph.D., Sleep Apnea Treatment Center, 10443 N Cave Creek Rd, Suite 110, Phoenix, AZ 85020, Email: salthakim@yahoo.com Tel: (602) 944-0847


Introduction:
The preferred therapy for obstructive sleep apnea (OSA) is continuous positive airway pressure (CPAP) 1  in detail, most of the studies that used Provent, and the reader is referred to such reviews. Others have also reviewed the literature and, in general, gave favorable opinions about use of Provent as an EPAP device for treatment of OSA 8,9,10,11,12 . It remains unclear why Provent is not effective in some patients 12 . Provent was usually more effective in patients with mild to moderate OSA than in patients with severe OSA 5 . One study attempted to identify if other factors determine which patient is likely to benefit from Provent 13 but did not add to our understanding of EPAP devices. Realizing that a fixed resistance in Provent may not always be equally effective for all patients, Ventus Medical offered Provent devices with higher levels of resistance with the hope that they would be more effective. Provent with fixed resistances of 50, 80, or 110 cmH2O/l/sec have been tested in clinical trials 6 . A few years later, another EPAP device called Theravent was introduced which was also cleared by the FDA for treatment of snoring 14 .
Theravent operates on the same principle as Provent, but the resistance is less than in Provent and was marketed as an over the counter device. Like Provent, Theravent was also provided with three levels of resistance to facilitate introduction of EPAP devices to the patients. Provent and Theravent devices continue to be marketed successfully and are recommended by physicians to patients with sleep disordered breathing.
There is a general agreement that EPAP devices are useful for OSA therapy 8,9,10,11 , yet it is not clear why some patients do not benefit significantly, and more importantly, doctors continue to look for improvements and for better understanding of the EPAP devices.
The idea of offering Provent with different levels of expiratory resistance was insightful 6 and was necessary. The scientists at Provent realized that level of resistance in the device plays a critical factor to the success of therapy. Indeed one patient may be more comfortable with a Provent with low resistance while another may prefer a Provent with a high resistance, but the reason remains unclear. Using Provent with a fixed resistance, is somewhat arbitrary; the resistance may be too little which makes the device ineffective or may be too excessive which causes difficulty breathing. There are physiological reasons that determine how much resistance is appropriate for a patient but were not discussed previously. One may be misled by the simple notion that a device with a higher resistance may work better for a patient because it could potentially generate more EPAP. There are factors related to physiology of lung and chest wall mechanics that place limits on how much resistance should be used. In a recent publication we discussed the theoretical changes in pressures in the nasopharyngeal region and in lung volume that are generated during CPAP or BiPAP and compared it to EPAP 15  and to expiration that will be discussed,

Limitations during Inspiration :
The pressure-volume relationship of the respiratory system (lungs and chest wall) exhibits a curvilinear shape. The relationship is linear in the low pressure range, but the slope (compliance) decreases at higher pressure and therefore, becomes more difficult to with large lungs (Fig 1a), or a patient with smaller lungs (Fig 1b). Zero on the Y-axis represents lung volume at

Limitations during Expiration:
The studies with Provent (having a fixed resistance), show that the device generated a wide range of EPAP (5 to 23 cmH2O) among patients 13 . The reason why some patients generated low EPAP while others generated much higher EPAP was not explained.
Differences in EPAP level among patients are related to elastic recoil properties of the respiratory system (lungs and chest wall). Exhalation during sleep is a passive process, driven only by elastic recoil of the respiratory system. Figure 2a illustrates During normal breathing with no expiratory resistance, the nasal pressure becomes slightly positive but remains close to zero (solid blue line #1). As the expiratory resistance increases slightly, EPAP would increase as illustrated with curves #2. When optimum resistance is reached, EPAP would increase as shown by red line #3.
Increasing the resistance beyond the optimum resistance will not cause EPAP to rise any further (curve #4), but instead, expiratory flow rate will be reduced (line #4 in Fig 2a). Therefore curve #3 represents the maximum EPAP that this patient can generate and tolerate while asleep. This maximum EPAP is determined by the expired flow rate generated by the passive elastic recoil of the lungs. Therefore optimum resistance is the expiratory resistance that generates maximum EPAP without compromising exhaled tidal volume.    will not increase but exhaled volume will be restricted and more air will be trapped in the lungs. In the example shown in Figure   There is no financial support, and no off label or investigational use. The EPAP mask was designed and patented by one of the authors (TSH), and constitutes a possible conflict of interest.