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Chapter: Clinical Anesthesiology: Perioperative & Critical Care Medicine: Acid-Base Management

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Respiratory Compensation

1. Respiratory Compensation During Metabolic Acidosis, 2. Respiratory Compensation During Metabolic Alkalosis.

RESPIRATORY COMPENSATION

 

Changes in alveolar ventilation responsible for the respiratory compensation of Paco2 are medi-ated by chemoreceptors within the brainstem . These receptors respond to changes in cerebrospinal spinal fluid pH. Minute ventila-tion increases 1–4 L/min for every (acute) 1 mm Hg increase in Paco2. In fact, the lungs are respon-sible for eliminating the approximately 15 mEq of CO2 produced every day as a byproduct of carbo-hydrate and fat metabolism. Respiratory compen-satory responses are also important in defending against marked changes in pH during metabolic disturbances.

 

Respiratory Compensation During Metabolic Acidosis

 

Decreases in arterial blood pH stimulate medullary respiratory centers. The resulting increase in alveo-lar ventilation lowers Paco2 and tends to restore arterial pH toward normal. The respiratory response to lower Paco2 occurs rapidly but may not reach a predictably steady state until 12–24 hr; pH is never completely restored to normal. Paco2 normally decreases 1–1.5 mm Hg below 40 mm Hg for every 1 mEq/L decrease in plasma [HCO 3].

 

Respiratory Compensation During Metabolic Alkalosis

 

Increases in arterial blood pH depress respiratory centers. The resulting alveolar hypoventilation tends to elevate Paco2 and restore arterial pH toward nor-mal. The respiratory response to metabolic alkalosis is generally less predictable than the respiratory response to metabolic acidosis. Hypoxemia, as a result of progressive hypoventilation, eventually activates oxygen-sensitive chemoreceptors; the latter stimulates ventilation and limits the compensatory respiratory response. Consequently, Paco2 usually does not increase above 55 mm Hg in response tometabolic alkalosis. As a general rule, Paco2 can be expected to increase 0.25–1 mm Hg foreach 1 mEq/L increase in [HCO3].

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