These rules are to see if compensation has occurred and if the disorder is mixed
In acute respiratory acidosis
--> [HCO3] increases by 1 mmol/L,
* For every 10 mmHg increase in pCO2 (above 40 mmHg)
Thus,
Comment:The increase in CO2 shifts the equilibrium between CO2 and HCO3 to result in an acute increase in HCO3. This is a simple physicochemical event and occurs almost immediately.
Example: A patient with an acute respiratory acidosis (pCO2 60mmHg) has an actual [HCO3] of 31mmol/l. The expected [HCO3] for this acute elevation of pCO2 is 24 + 2 = 26mmol/l. The actual measured value is higher than this indicating that a metabolic alkalosis must also be present.
The [HCO3] will increase by 4 mmol/l for every 10 mmHg elevation in pCO2 above 40mmHg.
Expected [HCO3] = 24 + 4 { (Actual pCO2 - 40) / 10}
Comment: With chronic acidosis, the kidneys respond by retaining HCO3, that is, renal compensation occurs. This takes a few days to reach its maximal value.
Example: A patient with a chronic respiratory acidosis (pCO2 60mmHg) has an actual [HCO3] of 31mmol/l. The expected [HCO3] for this chronic elevation of pCO2 is 24 + 8 = 32mmol/l. The actual measured value is extremely close to this so renal compensation is maximal and there is no evidence indicating a second acid-base disorder
Acute respiratory alkalosis
--> [HCO3] decreases by 2 mmol/L
* For every 10 mmHg decrease in pCO2 (below 40 mmHg)
Expected [HCO3] = 24 - 2 x (40-pCO2)/10
Comment: In practice, this acute physicochemical change rarely results in a [HCO3] of less than about 18 mmol/s. (After all there is a limit to how low pCO2 can fall as negative values are not possible!) So a [HCO3] of less than 18 mmol/l indicates a coexisting metabolic acidosis.
The arterial pCO2 at maximal compensation has been measured in many patients with a metabolic acidosis. A consistent relationship between bicarbonate level and pCO2 has been found. It can be estimated from the following equation:
Expected pCO2 = 1.5 (Actual [HCO3] ) + 8 mmHg
* Units: mmols/l for [HCO3], and mmHg for pCO2
The limiting value of compensation is the lowest level to which the pCO2 can fall - this is typically 8 to 10mmHg, though lower values are occasionally seen.
The expected pCO2(in mmHg) is calculated from the following formula:
Expected pCO2 = 0.7 [HCO3] + 20 (+/- 5)
Comment: The variation in pCO2 predicted by this equation is relatively large. (The reasons for this are discussed in section 7.5)
Check Anion Gap and Delta Ratio
An elevated Anion Gap always strongly suggests a Metabolic Acidosis.
* If AG is 20-30 then high chance (67%) of metabolic acidosis
* If AG is > 30 then a metabolic acidosis is definitely present
If a metabolic acidosis is diagnosed, then the Delta Ratio should be checked
Delta Ratio Assessment Guidelines in patients with a metabolic acidosis
* < 0.4 - Hyperchloraemic normal anion gap acidosis
* 0.4 to 0.8 - Combined high AG and normal AG acidosis
* 1 - Common in DKA due to urinary ketone loss
* 1 to 2 - Typical pattern in high anion gap metabolic acidosis
* > 2 Check for either a co-existing Metabolic Alkalosis (which would elevate [HCO3]) or a co-existing Chronic Respiratory Acidosis (which results in compensatory elevation of [HCO3])
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From emedicine (http://www.emedicine.com/med/topic2009.htm)
Acute hyperventilation with hypocapnia causes a small early reduction in serum bicarbonate due to cellular uptake of bicarbonate. Acutely, plasma pH and bicarbonate concentration vary proportionately with the PaCO2 along a range of 15-40 mm Hg. After a period of 2-6 hours, respiratory alkalosis is renally compensated by a decrease in bicarbonate reabsorption. The expected change in serum bicarbonate concentration ([HCO3-]) can be estimated as follows:
* Acute - [HCO3-] falls 2 mEq/L for each decrease of 10 mm Hg in the PaCO2 (Limit of compensation: [HCO3-] = 12-20 mEq/L)
* Chronic - [HCO3-] falls 5 mEq/L for each decrease of 10 mm Hg in the PaCO2 (Limit of compensation: [HCO3-] = 12-20 mEq/L)
The expected change in pH with respiratory alkalosis can be estimated with the following equations:
* Acute respiratory alkalosis - Change in pH = 0.008 X (40 – PaCO2)
* Chronic respiratory alkalosis - Change in pH = 0.017 X (40 – PaCO2)