Acid-base imbalance: Difference between revisions
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* Single acid-base disorders are commoner than double acid-base disorders, that are in turn commoner than triple acid-base disorders | * Single acid-base disorders are commoner than double acid-base disorders, that are in turn commoner than triple acid-base disorders | ||
* A normal pH doesn't exclude an acid base disorder as a co-existing acidosis and alkalosis can result in a normal pH. | * A normal pH doesn't exclude an acid base disorder as a co-existing acidosis and alkalosis can result in a normal pH. | ||
* When the clinical picture raises suspicion of acid-base imbalance and the pH is normal, always | * When the clinical picture raises suspicion of acid-base imbalance and the pH is normal, always check for the anion gap. For e.g. patient with diabetes ketoacidosis (metabolic acidosis) and vomiting (metabloic alkalosis) will present as a normal pH but with elevated anion gaps. | ||
==See also== | ==See also== | ||
Revision as of 15:35, 1 September 2012
| Acid-base imbalance | |
| ICD-10 | E87.2-E87.4 |
|---|---|
| ICD-9 | 276.2-276.4 |
| MeSH | D000137 |
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Acid-base imbalance has several possible causes. An excess of acid is called acidosis and an excess in bases is called alkalosis. Acidosis is much more common than alkalosis. The imbalance is compensated by negative feedback to restore normal values. Acid-base balance is maintained by normal respiratory and renal excretions of carbon dioxide and acids respectively.
Causes
Sources of acid gain:
- Carbon dioxide (since CO2 and H2O form HCO3-, bicarbonate, and H+, a proton, in the presence of carbonic anhydrase)
- Production of nonvolatile acids from the metabolism of proteins and other organic molecules
- Loss of bicarbonate in faeces or urine
- Intake of acids or acid precursors
Sources of acid loss:
- Use of hydrogen ions in the metabolism of various organic anions
- Loss of acid in the vomitus or urine
Response
Responses to acidosis:
- Bicarbonate is added to the blood plasma by tubular cells.
- Tubular cells reabsorb more bicarbonate from the tubular fluid.
- Collecting duct cells secrete more hydrogen and generate more bicarbonate.
- Ammoniagenesis leads to increased buffer formation (in the form of NH3)
Responses to alkalosis:
- Excretion of bicarbonate in urine.
- This is caused by lowered rate of hydrogen ion secretion from the tubular epithelial cells.
- This is also caused by lowered rates of glutamine metabolism and ammonia excretion.
Normal pH values based on blood sample site
Venous blood gas sampling should not replace arterial blood gas sampling, but may supplement arterial blood gas monitoring as a mechanism of trending results and minimizing arterial sampling. Central venous blood is preferable to peripheral venous blood, as it more accurately represents the arterial blood gas results. Venous blood is more acidemic than arterial blood, so venous pH is lower than arterial pH.
- Arterial sample
- pH 7.35 - 7.45
- Bicarbonate - 21 to 27 meq/L
- pCO2 - 36 to 44 mmHg
- Venous sample
- pH - 0.02 to 0.04 units lower than in arterial blood
- HCO3 - 1 to 2 meq/L higher than in arterial blood
- pCO2 - 3 to 8 mmHg higher than in arterial blood
- Central venous sample
- pH - 0.03 to 0.05 pH units lower than in arterial blood
- HCO3 - almost similar to arterial blood
- pCO2 - 4 to 5 mmHg higher than in arterial blood
Approach to a patient with acid base imbalances
The following steps can help to generate a differential diagnosis on a patient with a suspected acid/base disorder:
- Evaluate the complete clinical picture and laboratory data in patients with suspected acid base disorder.
- Single acid-base disorders are commoner than double acid-base disorders, that are in turn commoner than triple acid-base disorders
- A normal pH doesn't exclude an acid base disorder as a co-existing acidosis and alkalosis can result in a normal pH.
- When the clinical picture raises suspicion of acid-base imbalance and the pH is normal, always check for the anion gap. For e.g. patient with diabetes ketoacidosis (metabolic acidosis) and vomiting (metabloic alkalosis) will present as a normal pH but with elevated anion gaps.