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8.5: Compensation

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  • The compensatory response is a fall in bicarbonate level

    As can be seen by inspection of the Henderson-Hasselbalch equation (below), a decreased [HCO3 -] will counteract the effect of a decreased pCO2 on the pH. Mathematically, it returns the value of the \( \frac {[HCO_{3}^{-}]} {0.03pCO_{2}} \) ratio towards normal.

    \[ pH = pKa + \log \frac {[HCO_{3}^{-}]} {0.03pCO_{2}} \]

    Key points regarding compensation in respiratory alkalosis:

    • Physicochemical effect: Initially there is an immediate physicochemical change which lowers the bicarbonate slightly.
    • Role of Kidney: The effector organ for compensation is the kidney.
    • Slow Response: The renal response has a slow onset and the maximal response takes 2 to 3 days to be achieved.
    • Outcome: The drop in bicarbonate results in the extracellular pH returning only partially towards its normal value.
    Compensation in an ACUTE Respiratory Alkalosis
    • Mechanism: Changes in the physicochemical equilibrium occur due to the lowered pCO2 and this results in a slight decrease in HCO3-. There is insufficient time for the kidneys to respond so this is the only change in an acute respiratory alkalosis. The buffering is predominantly by protein and occurs intracellularly; this alters the equilibrium position of the bicarbonate system.
    • Magnitude: There is a drop in HCO3- by 2 mmol/l for every 10mmHg decrease in pCO 2 from the reference value of 40mmHg.
    • Limit: The lower limit of 'compensation' for this process is 18mmol/l - so bicarbonate levels below that in an acute respiratory alkalosis indicate a co-existing metabolic acidosis. (Alternatively, there may be some renal compensation if the alkalosis has been present longer than realised.)
    Compensation in a CHRONIC Respiratory Alkalosis
    • Mechanism: Renal loss of bicarbonate causes a further fall in plasma bicarbonate (in addition to the acute drop due to the physicochemical effect and protein buffering).
    • Magnitude: Studies have shown an average 5 mmol/l decrease in [HCO3-] per 10mmHg decrease in pCO2 from the reference value of 40mmHg. This maximal response takes 2 to 3 days to reach.
    • Limit: The limit of compensation is a [HCO3-] of 12 to 15 mmol/l.
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