Some overweight individuals develop problems with their breathing such that they gradually breathe less and less. This leads to a lack of oxygen and a buildup of carbon dioxide in the blood, called ventilatory failure. As a consequence, if such a person develops a chest infection, they are more likely to become seriously ill and need intensive care. In addition they are much more likely to develop severe complications during and following operations. This problem can be treated with a machine at home used overnight to help breathing. It is interesting that ventilatory failure only happens in some overweight individuals, and the investigators do not understand what factors make this complication develop. There are a number of theories: for example the distribution of the fat, additional lung disease (such as asthma), the addition of obstructive sleep apnoea, a condition when there are periods of cessation of breathing overnight (which is more common in obese individuals), weak muscles of breathing (perhaps due to fatty infiltration of muscles or vitamin D deficiency), and other hormonal changes.

The investigators intend to measure many potential factors in a range of overweight individuals, some who have ventilatory failure, and some who do not, to try and work out which are the important factors that cause this problem. If the investigators can identify such factors, then this will help predict in advance who is at risk from chest infections and during operations; thus allowing for earlier provision of an overnight breathing machine. This should reduce complications and potentially deaths in such individuals.

To test the hypothesis that in obese patients with obesity-hypoventilation (OHS) there are specific factors related to the development of ventilatory failure, compared to obese subjects not in ventilatory failure

Inclusion Criteria:

• Obese (BMI > 30) with or without obesity hypoventilation
• (OHS) (18 - 85yrs)
• Admitted for management of their OHS
• Attending the sleep and ventilation clinic
• Being assessed for bariatric surgery
• Willing and able to give informed consent for participation in the study
• Men and women aged 18 - 85 years

Exclusion Criteria:

• Respiratory acidosis pH <7.30
• Severe untreated hypothyroidism
• Current treatment with theophylline
• Current treatment with diuretics
• Severe restrictive or obstructive lung disease (<30% predicted)
• Severe comorbidities such as moderate/severe COPD, left sided heart failure, and primary CNS or neuromuscular diseases
• Contraindications to MRI scanning
• Contraindications to DXA scanning
• Previous participant in research in the last 12 months

Obese (BMI > 30kg/m2) with or without OHS (18-85yrs)

1. Admitted for management of OHS
2. Attending the sleep and ventilation clinic
3. Being assessed for bariatric surgery