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Respiratory Disorders (Expert Forum)
relation of O2 to CO2
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relation of O2 to CO2
by 6Proz, Sep 14, 2004 12:00AM
After 6 years of developing symptoms, my wife has recently been diagnosed with Primary lateral sclerosis (for her purposes, much like ALS). Breathing has become a major problem. Pulmonary function test showed marginally okay results (PCO2 of 44, PO2 of 81). Pulse oxygen is usually 94- 96%, but over-night pulse-ox test showed low O2 about 15% of the time. Pulmonologist, who does not have ALS/PLS patients but talked with ALS clinic, ordered O2 supplementation. O2 supplementation has not helped much. She still has trouble breathing, and is always fatigued. Doctor says she is not close to needing NIPPV because O2 is okay. I am concerned that she may still retain too much CO2 (her expiratory volume was 86% and residual volume far above predicted). We are considering buying a home pulse-ox to help gauge her condition, and whether we need to take corrective action. We have read about CO2 problems with PLS/ALS, but I couldn’t find clear information on the measure of O2 as an indicator of CO2 retention. I would appreciate information before buying oximeter. How related are the pulse-oxygen and CO2? (Next pulmnologist appointment is not for a month)
Doctor's Answer
by National Jewish, Sep 23, 2004 12:00AM
, Sep 23, 2004 12:00AM
You are right that the arterial blood gas results are "marginally okay." At sea level the normal oxygen saturation level is 94% to 96%. During the night it is normal for this to drop to 92%. Oxygen saturation (SaO2) can be measured by an oximeter as well as by an arterial blood gas. At sea level the normal oxygen level is 95 mm Hg to 100 mm Hg. An arterial blood gas is done to measure the oxygen level (PaO2), since it cannot be measured by an oximeter. At sea level the normal carbon dioxide level is 35 mm Hg to 45 mm Hg. Just like with the oxygen level, an arterial blood gas is done to measure the carbon dioxide level (PaCO2), since it cannot be measured by an oximeter. There is no linear relationship between the oxygen saturation level and the carbon dioxide level.
Your wife’s oxygen level of 81 mm Hg is definitely low. This is presumed to be due to hypoventilation or under ventilation based upon the pulmonary function tests (PFTs) results that you noted “expiratory volume was 86% and residual volume far above predicted”. Without lung disease, a drop in oxygen saturation to 90% or less due to hypoventilation would almost always be seen with some degree of carbon dioxide retention. Your wife’s carbon dioxide level of 44 mm Hg is within the normal range.
Her fatigue may be a feature of her primary lateral sclerosis (PLS). It could also reflect poor quality sleep. Possibly this is related to neurologic changes in her breathing pattern. This could happen even without a drop in her oxygen saturation level. A formal sleep study would help to sort out all of this.
Your wife’s oxygen level of 81 mm Hg is definitely low. This is presumed to be due to hypoventilation or under ventilation based upon the pulmonary function tests (PFTs) results that you noted “expiratory volume was 86% and residual volume far above predicted”. Without lung disease, a drop in oxygen saturation to 90% or less due to hypoventilation would almost always be seen with some degree of carbon dioxide retention. Your wife’s carbon dioxide level of 44 mm Hg is within the normal range.
Her fatigue may be a feature of her primary lateral sclerosis (PLS). It could also reflect poor quality sleep. Possibly this is related to neurologic changes in her breathing pattern. This could happen even without a drop in her oxygen saturation level. A formal sleep study would help to sort out all of this.
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You should speak with the doctor about what is appropriate for her medical monitoring and needs. If he believes it is medically NECESSARY for her condition, ask him to prescribe it & submit a pre-authorization request with her insurer so it MAY be at least partially covered. A FEW patients have been able to get oximeters covered by insurers, but most of our insurers have refused on the grounds that oximeters are only approved for CLINIC use & not personal home use.
http://www.buyemp.com/dept.asp?dept_id=1030104
Good luck!
Aloha,
Starion