|Marco Dotti, MD
Obstructive sleep apnea (OSA) is evolving to become one of the most common risk factors for several co-morbidities, including diabetes, heart disease, and stroke, in combination with hypertension, obesity, and hypercholesterolemia. Primary care physicians should screen every patient for OSA through appropriate questioning, physical examination, and checking blood pressure and fasting lipid panel. Sleep apnea is also evolving to a broader spectrum of “nocturnal unstable breathing,” encompassing different subtypes of nocturnal respiratory disorders.
Often, these disorders are intertwined and do not appear in isolation. For example, OSA is often associated with central sleep apnea (CSA). CSA is the archetype of unstable breathing during non-REM sleep. Lack of cortical activity during NREM sleep leaves ventilation control to be dependent on saturation of gases. Patients with CSA are particularly sensitive to oscillation of PCO2 and tend to stop breathing with relatively high levels of CO2.
CSA occurs anytime the body feels that CO2 is low; this also occurs physiologically at high altitude. It also takes place in pathological situations. For example, strokes may produce damage to the brainstem chemo receptive respiratory centers, leading to CSA. Congestive heart failure may produce slow circulation, resulting in delayed exposure of chemoreceptors to CO2, leading to overshooting and undershooting of the respiratory drive. This translates into a crescendo-decrescendo respiratory type, which is a subtype of CSA, called the classic periodic breathing of Cheyne-Stokes.
Besides CSA being related to strokes, congestive heart failure, narcotics and idiopathic causes, a new subtype of CSA has emerged, called complex sleep apnea. This is the emergence of central apneas after applying positive air pressure (PAP) to patients with OSA. The pathogenesis is likely related to sudden hypocapnia, leading to ventilatory suppression, occurring after the opening of the upper airways by applying PAP.
Complex sleep apnea occurs in roughly 5 percent of patients with moderate to severe OSA. CSA is translated clinically in a PAP failure of stabilizing the breathing and improving the sleep architecture. Nocturnal polysomnogram is the state-of-the-art tool in detecting any type of unstable nocturnal breathing disorder. This includes the classic straightforward OSA (75 percent of patients), central sleep apnea, mixed obstructive sleep apnea, central sleep apnea, complex sleep apnea and the hypoventilation syndromes (neuromuscular disease, lung and thoracic cage disorders, and most frequently, obesity/hypoventilation syndromes). Moreover, the sleep study represents the theatre to deliver the appropriate PAP therapy.
The arsenal of PAP machines has evolved from the traditional CPAP (continuous positive air pressure) and bipap (bilevel positive air pressure) to PAP devices with servo-ventilation which aim to assist the “unstable breathing of the central sleep apnea spectrum.”
During the last 30 years, sleep apnea has evolved from an uncommon disorder to a common major risk factor. The spectrum of sleep apnea has also undergone a dramatic evolution into a relative complex subclassification with a vast array of effective treatment options.