arterial blood gases

At the moment l am doing a an essay on Chronic Obstructive Pulmonary Disease.  For this essay l would like an simple explaination on arterial blood gases.  That in incorporates why they are done and what they mean.  Plus l would like some information on CPAP non invasive ventilation which incorporates why, when, how, and the benefits this has for the patient.  I would be most grateful if you can help me.  

[National Jewish]

An Arterial blood gas (ABG) is a blood test that analyzes acid-base measurements of the blood and reveals information on the functioning of the lungs. This test provides more information than oximetry, which only measures oxygen saturation. Blood is drawn from an artery in the wrist and from this the following can be measured:
1) pH
    <7.35 indicates acidemia
    >7.45 indicates alkalemia
2) Carbon Dioxide (CO2)
    <35 mmHg indicates respiratory alkalosis
    >45 mmHg indicates respiratory acidosis
3) Bicarbonate (HCO3)
    <22 mEq/liter indicates metabolic acidosis
    >26 mEq/liter indicates metabolic alkalosis
4) Oxygen, normal reading 80-100 mmHg
5) Oxygen saturation, normal reading 92-94%

The why of CPAP ventilation is to avoid an invasive situation; that is, the placement of an endotracheal tube in the windpipe, where it will remain as long as assisted ventilation is required. Complications are associated with both placement of the tube and its sustained presence in the trachea and larynx. These can be avoided, entirely, with the use of non-invasive CPAP. In addition speech is not affected with CPAP.
CPAP allows for supported ventilation, when a patient is in respiratory failure. It is also used for the treatment of obstructive sleep apnea, and is very effective. It should be used whenever it can effectively provide ventilatory support, thus obviating the need for intubation.
The benefits to the patient are obvious.

[cajun54]

Cpap is used in the spontaneously breathing person that needs support to get over a limited medical condition or during sleep apnea to keep the air way open during sleep.  Although some people using cpap when they are in respiratory distress do get better without further intervention, some do not.  The next step is intubation and mechanical ventilation.