This is a medical question of type : information 
 What is (are) Obesity Hypoventilation Syndrome ? 
 Obesity hypoventilation (HI-po-ven-tih-LA-shun) syndrome (OHS) is a breathing disorder that affects some obese people. In OHS, poor breathing results in too much carbon dioxide (hypoventilation) and too little oxygen in the blood (hypoxemia).
                
OHS sometimes is called Pickwickian syndrome.
                
Overview
                
To understand OHS, it helps to understand how the lungs work. When you breathe, air passes through your nose and mouth into your windpipe. The air then travels to your lungs' air sacs. These sacs are called alveoli (al-VEE-uhl-eye).
                
Small blood vessels called capillaries (KAP-ih-lare-ees) run through the walls of the air sacs. When air reaches the air sacs, oxygen passes through the air sac walls into the blood in the capillaries. At the same time, carbon dioxide moves from the capillaries into the air sacs. This process is called gas exchange.
                
In people who have OHS, poor breathing prevents proper gas exchange. As a result, the level of carbon dioxide in the blood rises. Also, the level of oxygen in the blood drops.
                
These changes can lead to serious health problems, such as leg edema (e-DE-mah), pulmonary hypertension (PULL-mun-ary HI-per-TEN-shun), cor pulmonale (pul-meh-NAL-e), and secondary erythrocytosis (eh-RITH-ro-si-TOE-sis). If left untreated, OHS can even be fatal.
                
The cause of OHS isn't fully known. Researchers think that several factors may work together to cause the disorder.
                
Many people who have OHS also have obstructive sleep apnea. Obstructive sleep apnea is a common disorder in which the airway collapses or is blocked during sleep. This causes pauses in breathing or shallow breaths while you sleep.
                
Obstructive sleep apnea disrupts your sleep and causes you to feel very tired during the day. (For more information, go to the Health Topics Sleep Apnea article.)
                
Outlook
                
Doctors treat OHS in a number of ways. One way is with positive airway pressure (PAP) machines, which are used during sleep.
                
PAP therapy uses mild air pressure to keep your airways open. Your doctor might recommend CPAP (continuous positive airway pressure) or BiPAP (bilevel positive airway pressure).
                
If your doctor prescribes PAP therapy, you'll work with someone from a home equipment provider to select a CPAP or BiPAP machine. The home equipment provider will help you select a machine based on your prescription and the features that meet your needs.
                
Other treatments for OHS include ventilator (VEN-til-a-tor) support and medicines. (A ventilator is a machine that supports breathing.)
                
OHS occurs with obesity, so your doctor will likely recommend weight loss as part of your treatment plan. Successful weight loss often involves setting goals and making lifestyle changes, such as following a healthy diet and being physically active.
                
OHS can lead to other serious health problems, so following your treatment plan is important. Your health care team, home equipment provider, and family can help you manage your treatment.