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

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3. Venous blood gases in lieu of arterial blood gases for acid-base assessment. Summary: Venous blood gases from a large vein, in a stable patient, can be used to assess acid-base status. Discussion Mixed venous blood is blood in the pulmonary artery, the location where one can be certain that venous return from all the body's organs is thoroughly "mixed." Comparison with arterial blood shows that, except for oxygen pressure and saturation (PO2 and SO2, respectively), mixed venous blood gas values are close to those of arterial blood (see Table). Normal Blood Gas Values in Arterial and Mixed Venous Blood (at rest) Value Arterial Mixed Venous Typical A-V Difference PO2 (mm Hg) 70-100 35-40 PaO2 - 60 SaO2 (%) 93-98 65-75 SaO2 - 25 PCO2 (mm Hg) 35-45 42-52 6-8 pH 7.35-7.45 7.32-7.41 0.03-0.04 HCO3- (mEq/L) 22-26 24-28 2-4 The utility of this information is the observation that free flowing blood from any large vein (e.g., femoral, internal jugular) will closely reflect the patient's acid-base status as long as the patient is hemodynamically stable (i.e., not in shock). This observation should also hold for practically any free flowing vein where no tourniquet has been applied and there is no infusion of medications that might alter pH. (Unlike the values for pH, PCO2 and HCO3-, the difference between arterial and venous oxygen values is wide and highly variable, obviating using venous blood for oxygenation assessment.) This is NOT to say that you can follow venous blood from any source to assess acid-base status. However, a large central vein (internal jugular, femoral) can be sampled for acid-base status as long as the patient is not in shock. Furthermore, an abnormal acid-base blood state from any venous source likely reflects the central vein and arterial situation as well (again, provided the patient is not in shock, there is no tourniquet applied, and nothing is infusing the vein that might alter pH). Despite all the caveats, these observations have helped eliminate numerous arterial blood samples in intensive care patients, and also helped point to sitations where an arterial blood sample should be obtained, as in the case below. A physician draws a radial artery blood sample from a hospitalized patient who is in mild respiratory distress, and sends it for blood gas analysis. Because the plunger of the syringe has to be pulled back to obtain the blood, the physician is not sure if he has sampled the artery or the vein. The blood gas values return showing pH 7.25, PCO2 76 mm Hg, PO2 42 mm Hg, oxygen saturation 77%. The physician's reaction is that he must have sent a venous sample because "the patient is not that bad." Assuming this is correct, what could one still conclude from the blood gas values? In this example the measured PO2 and oxygen saturation may be arterial or venous; the value for oxygen saturation could (and should) be confirmed with a quick measurement of pulse oximetry. However, the pH and PCO2 values are very abnormal and indicate a state of respiratory acidosis irrespective of the blood's origin. This is because venous pH and PaCO2 are usually close to the arterial values. Except in states of severe hypoperfusion, they do not share the large differences that exist between arterial (PaO2 & SaO2) and venous (PvO2 & SvO2) values (Table). It would be a major pitfall to think that this pH and PCO2 are "only venous" just because the measured PO2 was so low. Return Introduction of "Non-invasive blood gas interpretation" Return to Table of Contents for All You Really Need to Know to Interpret Arterial Blood Gases