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

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1. Venous CO₂ as a screen for acid-base disorders.

Summary: Venous CO₂, part of the standard electrolyte panel, is ubiquitously measured in hospitalized patients. If venous CO₂ is abnormal, the patient always has some type of acid-base disorder.

Discussion

Even before calculation of the time-honored anion gap, one should recognize that if venous CO₂ (also called venous bicarbonate or HCO₃^- in some labs) is abnormal, the patient has an acid-base disorder; except for the possibility of lab error, there are no exceptions.
This point may seem obvious, but in practice the obvious is often overlooked. Of the four routinely measured electrolytes in venous blood,

Na⁺, K⁺, Cl^-, and CO₂
Na⁺ and K⁺ are perceived to be far more important than the other two, whose values are often ignored. After all, Na⁺ 20% above normal at, say, 170 mEq/L, is perceived as life threatening, whereas a venous CO2 20% above normal (e.g., 34 mEq/L) is likely to be either ignored or not considered very important.
While that last statement may be true sometimes, it is not always the case; at the very least, an out-of-range CO2 indicates some type of acid-base disorder, in the above example either respiratory acidosis or metabolic alkalosis (or both). Not to recognize this could imperil the patient.

An intern is called to the bedside of an elderly woman late at night. She was admitted to the hospital 3 days earlier for a problem unrelated to her heart or lungs. She is anxious and complains of shortness of breath. Her lung fields are clear to auscultation and vital signs are normal except for slight tachycardia and a respiratory rate of 30/min. A nurse comments that the patient "gets like thtis every night." The physician orders an anti-anxiety drug for what he describes as "hyperventilation and anxiety." About 30 minutes later the pateint's breathing slows considerably and she becomes cyanotic, whereupon she is rushed to the ICU.

At the time the sedative was ordered there were two venous CO₂ measurements in her hospital chart, both 34 mEq/L, but no arterial blood gas values. On presentation to the ICU an ABG was drawn and showed acute on chronic respiratory acidosis (pH 7.10, PCO₂ 7.22). Clearly, the intern mistakenly assumed she was hyper-ventilating when the sedative was ordered. The laboratory clue was sitting right there in her hospital chart: an elevated venous CO2 of 34 mEq/L.
Finally, note that the venous "CO₂" includes both the true bicarbonate (numerator in the H-H equation as pH and PaCO₂) and the mEq/L of CO₂ contributed by dissolved CO₂ (which is the determinant of PCO₂). For this reason what the chemistry lab measures as " CO₂" is different quantitatively, albeit slightly, from what the blood gas lab calculates as "HCO₃^-". When dissolved CO₂ exerts a partial pressure of 46 mm Hg in venous blood, its quantity in the blood is

.03 mEq/L/mm Hg x 46 mm Hg = 1.38 mEq/L.
Thus the clinical chemistry lab measures, in venous blood, both the actual bicarbonate (numerator in the H-H equation) and the quantity of dissolved CO₂ (denominator in the H-H equation), and reports the result as "CO₂" (or sometimes "total CO₂") in mEq/L. This venous value obviously should not be confused with PaCO₂, which is the partial pressure of arterial CO₂ as measured in the blood gas lab.

When using electrolytes to assess acid-base status, venous CO₂ is the first value to be checked, followed by the anion gap and then the bicabonate gap, which are calculations. The recommended pathway for diagnosing acid-base problems from venous electrolytes is shown here.

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