Venous or Arterial Blood Gas Measurement

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To the Editor.— In further reply to the question of McGee regarding the value of venous versus arterial blood as a source of measuring blood gases, I find the answers given do not clearly define what is being measured. If one is interested in acid-base balance, a simultaneous measurement of hemoglobin, pH, and carbon dioxide tension (Pco2) will define the base excess of the blood, which is a measure of the nonrespiratory or metabolic component of acid-base balance, and by definition the value should be identical in venous or arterial blood. Carbon dioxide content, pH, and hemoglobin concentrations could also be used to determine base excess, but among all of these factors the base excess is the only accurate measure of the degree of metabolic acidosis or alkalosis. The carbon dioxide content as well as the Pco2are affected by changes in ventilation, and Siggaard-Andersen defines hypoventilation as

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