Mechanisms of Apnea
Section snippets
Mechanisms of Obstructive Sleep Apnea
Repetitive cessation of breathing during sleep has substantial impact on the cardiovascular system, emphasizing the importance of understanding the underlying pathophysiology of these disorders. Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive upper airway collapse during sleep. The airway obstruction results in either greatly reduced (hypopnea) or absent (apnea) ventilation, despite persisting respiratory efforts, such that ventilatory requirements are not met. As
Consequences of OSA on the Cardiovascular System
There are several possible mechanisms by which OSA may lead to adverse cardiovascular outcomes. During each obstructive apnea/hypopnea, intrathoracic pressure can become very negative (up to −100 cm water), which results in increased cardiac afterload (via increased left ventricular transmural pressure and, thus, left ventricular wall stress/tension), decreased stroke volume, and increased sympathetic activity.36 Arterial oxygen saturation also falls during obstructive respiratory events
Mechanisms of CSA
The term central sleep apnea, defined by cessation of airflow without respiratory effort, is often used to describe several distinct yet interrelated conditions. These include high-altitude periodic breathing, drug- or substance-induced CSA, idiopathic CSA (ICSA), and Cheyne-Stokes breathing (CSB) (Fig 3).41 Unlike the ongoing respiratory efforts against a compromised upper airway that occur in OSA, the various forms of CSA are characterized by a lack of drive to breathe during sleep. However,
Cardiovascular Disease and Apnea Pathophysiology
The potential for sleep-disordered breathing to contribute to cardiovascular disease and vice versa and the underlying mechanisms are questions of ongoing investigation. For example, emerging animal data show an increased propensity for apnea and unstable breathing in response to acute elevations in left atrial pressure.74 The presence of intermittent hypoxia may further impair cardiac muscle75 and, therefore, increase the risk of developing CSA. Given the link between conditions such as heart
The Spectrum of Disease: Similarities and Differences Between OSA and CSA
Although we have initially attempted to separate our description of the underlying mechanisms of OSA and CSA, significant overlap exists, and most patients experience varying degrees of both. Indeed, central apneas lead to upper airway closure in most instances.88 This is likely a reflection of the reduced drive to all respiratory muscles (including genioglossus) that occurs with central apnea (Fig 2). Furthermore, patients with OSA have unstable respiratory control.30, 31, 32, 89 Thus, the
Summary and Importance of Improved Understanding of Underlying Apnea Mechanisms
The stoppages in breathing and the associated events that occur during sleep-disordered breathing clearly have a substantial impact on the cardiovascular system. The underlying mechanisms that contribute to both OSA and CSA are multifactorial. However, both may share common features. Important contributing factors likely include upper airway anatomy, upper airway muscle activity, respiratory control stability, lung volume, and arousal from sleep. Cardiovascular disease may also importantly
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