Overnight Oximetry Can Test the Effectiveness of CPAP Therapy

Underlying Lung Disease May Prompt Additional Testing

CPAP helps lung function but may still require overnight oximetry testing
CPAP improves lung function, but overnight oximetry testing may still be required. Science Picture Co/Getty Images

In some cases, overnight oximetry may be necessary to test the effectiveness of continuous positive airway pressure (CPAP) therapy. It may be used to ensure adequate oxygen levels during sleep, especially among people with underlying lung disease. How do you know if you should have your oxygen levels tested overnight? Learn what medical conditions might prompt you to have further testing with the use of CPAP and how this should be done.

Oxygen Levels in Sleep Apnea

Oxygen levels often drop in association with sleep apnea. As the upper airway collapses at the throat and base of the tongue, a decreased amount of fresh air reaches the lungs. Once the lungs have maximally extracted the oxygen from the air remaining in the lungs, the oxygen levels of the blood begin to fall. By definition, sleep apnea events cause a drop of at least 3 to 4 percent, but the oxygen levels may temporarily fall by 30 percent and more. These transient drops in oxygen can stress the body. They usually resolve fully when an awakening restores the airway – or when it is protected with therapy, most commonly CPAP.

In some cases, treatment with CPAP may not be enough. For those with underlying lung disorders, the function of the lungs may be compromised to an extent that oxygen extraction is no longer optimal. In some cases, further testing and even oxygen supplementation becomes necessary.

Lung Conditions May Prompt Overnight Oximetry Testing on CPAP

In general, it is unnecessary to have overnight oximetry testing with the use of CPAP. Modern CPAP devices are able to monitor and report the persistent of obstructive sleep apnea events. The effectiveness of therapy can be determined via the residual apnea-hypopnea index (AHI).

This can be measured by the device periodically assessing airway resistance. In general, the average AHI should be <5 on treatment, and the lower the number, the better.

Due to the possibility that oxygen levels may be inadequate during sleep, despite a normal AHI, some individuals will require additional testing. This condition is called hypoxemia and may be due to hypoventilation. This occurs with an inadequate respiratory rate (slowed breathing), reduced lung volumes (due to disease or anatomical problems), or underlying lung dysfunction.

Some of the most common medical disorders that compromise lung function include chronic obstructive pulmonary disease (COPD) and congestive heart failure (CHF). COPD includes emphysema, and is much more common among smokers. CHF is associated with ankle swelling, weight gain, and a productive cough with difficulty breathing worsened by lying flat. Restrictive lung disease due to musculoskeletal (scoliosis) or neurological disorders (ALS) also can contribute.

In these various conditions, the lungs cannot adequately inflate and oxygen exchange cannot occur.

There are conditions that overlap sleep apnea with another medical disorder. The most common is called “overlap syndrome” and includes sleep apnea and COPD. When sleep apnea occurs with obesity and leads to abnormal carbon dioxide levels, obesity hypoventilation syndrome is present.

Testing Oxygen Levels at Home

Oxygen levels can be tested simply at home with an overnight oximeter. This device includes a finger clip with a long wire that either attaches to a recorder or directly to your CPAP machine. With a red light shining through the fingertip, it is able to detect blood oxygen levels as well as pulse rate.

When reviewed by your doctor, it can be determined if your oxygen levels are maintained in the normal range. An occasional drop may not be significant. However, sustained oxygen levels below 88% may suggest the need for oxygen supplementation. If oxygen is required during sleep, it is often delivered from an oxygen concentrator via tubing connected to an adaptor on your CPAP. The amount of oxygen required can vary and is measured in liters per minute.

It is best for your oximetry data to be reviewed in the context of your CPAP compliance data. Therefore, if sleep apnea is persisting and contributing to oxygen drops, optimizing the CPAP settings may be all that is required to correct the abnormalities.

In some cases, additional formal testing may be required. An overnight in-center sleep study that includes a titration of CPAP therapy and oxygen supplementation may be indicated in select situations.

Start by speaking with your doctor about your therapy and determine whether further testing, including overnight oximetry, is required to identify any problem and ultimately improve your breathing in sleep.


Kryger, M.H. et al. "Principles and Practice of Sleep Medicine." ExpertConsult, 5th edition, 2011.

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