# A Novel mHealth Approach for the Monitoring and Assisted Therapeutics of Obstructive Sleep Apnea
## 2 Related Work  
The act of breathing is intrinsic to human beings, occurring mostly without us thinking or even realizing it is happening. However, in specific situations, it is possible to control it, for example when we dive underwater or when we sing a song. The respiration rate is the total number of breaths, or breathing cycles, that occur each minute. This rate can be a good indicator that something is not right, because the rate can change in the presence of an illness. The respiratory rate is controlled by the respiratory center, which is located in the brain, and which responds primarily to changes in the levels of oxygen, carbon dioxide, and pH in the blood [25]. Ventilation control results from a complex interaction of multiple regions of the brain that send a signal to the muscles used in breathing to contract. The result is, typically, a rhythmic and consistent ventilation rate that provides the body with the necessary amount of oxygen, while removing carbon dioxide. Although most mammals have rigid skeletal support of the pharyngeal airway, in humans, the upper airway permeability is maintained, primarily, by the activation of soft tissue structures and muscle activation. The performance and stability of the upper airway play a major role in obstructive sleep apnea, and its collapse has been pointed out as one of the main factors for the occurrence of this disease. However, other factors, such as a low arousal threshold and an instability in respiratory control, may contribute to the poor activation of the upper airway muscles and their collapse during sleep [26]. Subjects with OSA, for the most part, have a narrowing of the upper airway, with a much higher potential for collapse, than people without this disease. During wakefulness, to compensate for this anatomical deficiency, they manifest increased activity of the airway dilator muscles. However, this increased muscle activity is lost at the beginning of sleep, which causes the pharyngeal airway to collapse in subjects with OSA [27]. The great hypoglossal nerve is one of the 12 cranial nerves. The hypoglossal nerve, beginning at the base of the brain, near the brain stem, travels down the neck and past the carotid artery and jugular vein, extending past the back of the throat to reach the lower part of the mouth. It then branches out to connect with the different muscles that control the tongue. The muscles controlled by this nerve are subdivided into the following two groups: the extrinsic muscles and the intrinsic muscles [25,28]. The intrinsic muscles lie entirely within the tongue and are responsible for the changes in the shape of the tongue. On the other hand, the extrinsic muscles extend outside the tongue and their job is to physically move it. Most of the studies and research on these muscles focus primarily on the genioglossus muscle. This is because it is the largest dilator muscle of the upper airways and is responsible for the action of pulling the posterior portion of the tongue to the front to unblock the airway. The genioglossus muscle is said to have a phasic inspiratory activity because, during wakefulness, it is spontaneously activated, and its activity is greater during inspiration. At the onset of sleep, the inspiratory and expiratory activity declines rapidly, both in healthy people and in people with OSA, however, it is more noticeable and of greater intensity in those with the disease. When the negative pressure and respiratory drive increase, the activity of this muscle also increases in order to try to compensate for these effects, which is often insufficient until awakening occurs. Therefore, the activity of the genioglossus muscle directly alters the pharyngeal lumen, and the behavior of the airway varies during obstructive events and is distinct from patient-to-patient. In the literature, one way to innervate the muscle and promote airway patency improvement, is through the electrical stimulation of the genioglossus muscle itself, or even of the hypoglossal nerve [29,30].