Non-invasive blood gas interpretation
by Lawrence Martin, M.D.

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6. Simple lung function measurement to obviate need for checking PaCO2.

Summary: PaCO2 does not rise in patients with pulmonary impairment (e.g., asthma, COPD) until FEV-1 (or peak flow) is less than 35% of predicted. Below this value the PaCO2 may be low, normal or high, but above this value the PaCO2 should not be elevated.


This is a most useful observation in patients with mild-moderate asthma and in some cases of COPD. Above 50% of predicted FEV-1 or peak flow expiratory flow rate ("peak flow"), the degree of lung impairment is not sufficient to allow PaCO2 to rise. So, assuming there is no "primary hypoventilation syndrome" (a CNS problem), the patient should not have hypercapnea. Since you can screen for hypoxemia with pulse oximetry, such patients can usually be spared a blood gas analysis.

Actually, the observation seems to hold true down to about 35% of predicted for FEV-1 or peak flow, though if a patient remains in the 35%-50% range for these values and has any new or acute symptoms, a blood gas measurement is usually advisable.

With tests of lung function below 35% of predicted, the patient may or may not be a "CO2 retainer." Any patient with acute symptoms, or who is being evaluated for the first time and has these poor test results, probably should have an arterial blood gas to rule out hypercapnea. Note that no results from pulmonary function testing can predict if the individual patient is retaining CO2 retainer. However, patients with only mild-moderate impairment of lung function (down to about 50% of predicted for these tests), and no CNS problem, should not retain CO2.

The other caveat, of course, is that checking SpO2 with pulse oximetry (as a surrogate for direct SaO2 measurement) has the inherent potential pitfalls of pulse oximetry, including missing elevated carbon monoxide. If you suspect that condition, send a venous blood gas for co-oximetry to check for %COHb, or obtain an arterial blood sample for a full blood gas analysis (i.e., including co-oximetry measurements.

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