Saturday, July 16, 2011

Possible Mechanisms of Central Apneas in Obstructive Sleep Apnea Syndrome

An article in the June edition of the Sleep Journal is about the different types of apneas seen in patients with obstructive sleep apnea syndrome (OSA). This post is going to be on the technical side, so reader beware. There are two main types of apneas. Obstructive ones are the more common type, and occur when the upper airway (or throat) collapse. Central apneas occur when the throat remains open, but there is no breath due to a communication problem between the brain and lungs. An analogy might help explain it better. Imagine our respiratory system is like a garden hose attached to a spigot. If you turn the spigot on, but pinch off the hose, you get little or no water flow - that is like an obstructive apnea. If you turn off the spigot, but leave the hose alone, you get no water flow - that is like a central apnea.

In OSA, patients usually have mostly obstructive apneas, but can occasionally have central ones as well. Researchers are not clear why this occurs. There is speculation that in patients with both types of apneas, there are at least two mechanisms in place. One is that the throats of these patients collapse more easily than individuals without OSA - this leads to the obstructive apneas. The second is that there is an instability in the control of breathing in patients with OSA. What this means is that the brain likes to have a precise level of carbon dioxide (CO2) in the blood - if it gets too high, your brain will make you breathe harder and faster to bring down the CO2 level. If the CO2 level gets too low, your brain makes you breathe less or even not at all to allow the CO2 level to rise up to normal. If you have an obstructive apnea, your lungs are still trying to breathe, but can't due to the closed throat. After the obstructive apnea is over, however, some people's brain overcompensates and breathes too hard and too fast. This reduces the CO2 level and the brain reduces breathing rates to make the CO2 level rise - this compensation can lead to the central apneas seen in OSA.

In this study, researchers studied the differences between patients with OSA that had pure obstructive apneas and no central apneas, and those with predominant obstructive apneas but with some degree of central apneas. Using sophisticated tools, they determined that patients with predominant OSA had more respiratory control instability than those that had pure OSA. In other words, the patients with predominant OSA were more likely to have their spigots turned off. Both groups had the same upper airway collapsibility, so that does not explain why the predominent OSA group has central apneas. These findings could help researchers figure out ways to treat patients that have non-obstructive apneas as part of their OSA syndrome.

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