In addition to its use in the Henderson-Hasselbalch equation, PCO2 provides information on pulmonary alveolar gas exchange (ventilation). If PCO2 is high, there is not a sufficient degree of alveolar ventilation. This may be due to primary lung disease (inability of the lungs to ventilate properly) or to some other reason. If PCO2 is low, there is alveolar hyperventilation, again either from primary or secondary etiology.
Month: October 2009
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Summary of Acid-Base Changes
To summarize plasma pH problems, in metabolic acidosis there is eventual HCO–3 deficit, leading to decreased plasma pH and decreased CO2 content (or CO2 combining power). In respiratory acidosis there is primary H2CO3 excess, which causes decreased plasma pH, but the CO2 content is increased due to renal attempts at compensation. In metabolic alkalosis there is eventual bicarbonate excess leading to increased plasma pH and increased CO2 content. In respiratory alkalosis there is primary carbonic acid deficit, which causes increased plasma pH, but the CO2 content is decreased due to renal attempts at compensation. The urine pH usually reflects the status of the plasma pH except in hypokalemic alkalosis, where there is acid urine pH despite plasma alkalosis.
As noted, CO2 content or combining capacity essentially constitutes the numerator of the Henderson-Hasselbalch equation. PCO2 is essentially a measurement of the equation denominator and can be used in conjunction with pH to indicate acid-base changes. This is the system popularized by Astrup and Siggaard-Anderson. PCO2 follows the same direction as the CO2 content in classic acid-base syndromes. In metabolic acidosis, PCO2 is decreased, because acids other than H2CO3 accumulate, and CO2 is blown off by the lungs in attempts to decrease body fluid acidity. In metabolic alkalosis, PCO2 is increased if the lungs compensate by hypoventilation; in mild or acute cases, PCO2 may remain normal. In respiratory alkalosis, PCO2 is decreased because increased ventilation blows off more CO2. In respiratory acidosis, PCO2 is increased because of CO2 retention due to decreased ventilation.
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Respiratory Alkalosis
The other major subdivision of alkalosis is respiratory alkalosis, which occurs when the respiratory mechanism blows off more CO2 than it normally would due to respiratory center stimulation from some cause. The main conditions in which this happens are the hyperventilation syndrome caused by hysteria or anxiety, high fever, and direct stimulation of the respiratory center by drugs. Overdose of aspirin can cause respiratory alkalosis in the early stages; although later, after more of the aspirin is absorbed, a metabolic acidosis develops. In hyperventilation of whatever cause, respirations are increased and deeper, blowing off more CO2. This creates an H2CO3 deficit since it is being used up to replenish CO2 by the lung carbonic anhydrase enzymes. Therefore, the denominator of the Henderson-Hasselbalch equation is decreased, the 20:1 ratio is increased, and plasma pH increased. The CO2 content will decrease, because when H2CO3 is lost due to formation of CO2 in the lungs, HCO3– is converted to H2CO3 in the kidney to compensate secondarily for or to replace the decreasing plasma carbonic acid.