I apologize again for taking so long to put out this post, my internship keeps me busy pretty much all the time. Over the past few weeks I've learned a ton and I've become fairly proficient at analyzing the polysomnograms. Primarily, I've focused on identifying apneic events such as hypopneas (where the patient pauses their breath or as the term literally translates to, "under-breathes"). These hypopneas usually come in one of two varieties: obstructive or central.

An obstructive hypopnea is caused by a partial blockage in the airway. When we sleep, our throat muscles relax and allow for fatty tissue and the tongue to potentially fall back and block the airway. That's the noise you hear when someone snores. When this blockage is significant however, it reduces the airflow to the body and signals the brain to partially awaken from sleep (this is called an arousal) and instruct the body to breath. If you've heard someone with sleep apnea during the night, this is what is happening when they make a choking sound or gasp for air. 

A central hypopnea, on the other hand, is not a "mechanical" problem, but a "communication" problem. Central hypopneas occur when the brain temporarily fails to signal the muscles which are responsible for breathing. Central hypopneas are far less common and occur in less than 20% of apneic patients. 

In order to mark an event as an obstructive hypopnea, a 3% oxygen desaturation must be present. For medicare patients, due to the differing requirements they set, there must be a 4% desaturation. To mark an event as a central hypopnea, in addition to those criteria, there must be "no effort made to breath". Among the many things attached to a patient, there are two belts (one above the chest and one around the abdomen). These belts allow us to see whether the patient is actively trying to breath by expanding and contracting their diaphragm. If the readings from these belts indicate the patient is not moving the muscles necessary to breath while the desaturation occurs, the event will be marked as a central.

What took me by surprise the most is how common these events can be. In order to determine the severity of someone's sleep apnea, the AHI (apnea-hypopnea index) is used. This index essentially conveys how many events a patient has per hour. According to this index, it's normal for a person without any sleep apnea to undergo up to 5 events (which are marked by 10 seconds of not breathing/under breathing) per hour! Patients with very severe sleep apnea can undergo over 30 events per hour. That's 300 seconds of incomplete breathing. 5 minutes out of every hour! Knowing this information helped me understand why apneic patients are generally tired during the day, instead of resting overnight, their brain is continually awoken so that it can focus on ensuring proper breathing.