Acid Base Stuff in the ICU

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How is H+ determined?

[H+] = 24 * (PCO2/HCO3)

Decrease in HCO3 causes what?

Primary metabolic acidosis

Increase? Metabolic alkalosis.

Equation for secondary response to metabolic acidosis?

deltaPaCO2 = 1.2* deltaHCO3

Using normal PaCO2 of 40 mmHg and normal HCO3 of 24 mEq/L, you can rewrite the equation as:

Expected PaCO2 = 40 - (1.2 (24-current HCO3))

Expected PaCO2 response to metabolic alkalosis?

Expected PaCO2 = 40+ (0.7*(current HCO3 - 24)))

Where does the secondary response to changes in PaCO2 occur?

Kidneys.. can take 2-3 days to reach completion

Metabolic secondary response to acute changes in respiratory drive?

deltaHCO3 = 0.1 * delta PaCO2 for acute respiratory acidosis

deltaHCO3 = 0.2 * deltaPaCO2 for respriatory alkalosis

Metabolic changes to chronic changes in respiratory drive?

Increase in PaCO2, kidneys respond by increasing HCO3 reabsorption in the proximal renal tubules, which raises the plasma HCO3 conce ntration...

The opposite is the case for decrease in PaCO2...

deltaHCO3 = 0.4 * deltaPaCO2

For chronic respiratory acidosis:
Expected HCO3 = 24 + [0.4 * (current PaCO2 - 40)]

For chronic respiratory alkalosis:
Expected HCO3 = 24 + [0.4*(40-current PaCO2)]

How to identify the primary acid-base disorder?

If PaCO2 and/or pH is outside the normal range, there is a disorder

If PaCO2 and pH are both abnormal int he same direction, --> primary metabolic acid base disorder

If PaCO2 and pH are both abnormal in the opposite direction, there is a primary respiratory acid-base disorder.

If only the pH or PaCO2 is abnormal, the condition is mixed respiratory and metabolic...
--> if the PaCO2 is abnormal, the directional change in PaCO2 identifies the type of respriatory disorder... and the opposiing metabolic disorder

--> if the pH is abnormal, the directional change in pH identifies the type of metabolic disorder.. e.g. low pH is
metabolic acidosi...

For a primary metabolic disorder, if the measured PaCO2 is higher than expected, there is a secondary respiratory acidosis, and if the measured PaCO2 is less than expected, there is a secondary respiratory alkalosis.

Rule 5: For a primary respiratory disorder, a normal or near-normal HCO3 indicates that the disorder is acute.
Rule 6: For a primary respiratory disorder where the HCO3 is abnormal, determine the expected HCO3 for a chronic respiratory disorder.
6a. For a chronic respiratory acidosis, if the HCO3 is lower than expected, there is an incomplete renal response, and if the HCO3 is higher than expected, there is a secondary metabolic alkalosis.
6b. For a chronic respiratory alkalosis, if the HCO3 is higher than expected, there is an incomplete renal response, and if the HCO3 is lower than expected, there is a secondary metabolic acidosis.

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