Acidosis & alkalosis: Difference between revisions

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===Metabolic acidosis===
===Metabolic acidosis===
Unfortunately, life as ever, is not so simple. There are two types of metabolic acidosis - with or without an '''anion gap'''. Essentially, this is the difference in concentrations between cations (+ve ions) and anions (-ve ions). You calculate it using the following formula<br>
Unfortunately, life as ever, is not so simple. There are two types of metabolic acidosis - with or without an '''anion gap'''. Essentially, this is the difference in concentrations between cations (+ve ions) and anions (-ve ions). You calculate it using the following formula
<br>
 
Na<sup>+</sup> - (Cl<sup>-</sup> + HCO<sub>3</sub><sup>-</sup>) = 8-12mEq/l (that's the number of charges)<br>
Na<sup>+</sup> - (Cl<sup>-</sup> + HCO<sub>3</sub><sup>-</sup>) = 8-12mEq/l (that's the number of charges)
<br>
 
We then split metabolic acidosis into:
We then split metabolic acidosis into:
====High anion gap====
====High anion gap====

Revision as of 15:26, 11 January 2010

Here is a glossary of these terms, which is useful for understanding some of the blood buffer systems. There are basically four abnormalities with regards to the acid-base balance in blood. You should ask yourself three questions:

  1. Is the pH high or low? - low pH= acidosis and high pH = alkalosis.
  2. Is CO2 abnormal? - if 'yes':
    1. Is this in keeping with the pH change? - i.e. when pH↑, CO2 OR pH↓, CO2. These changes point to a respiratory acidosis/alkalosis. If the change is not there, then it is a compensatory change.
  3. Is the HCO3- abnormal? - if 'yes'
    1. Is this change in keeping with the pH change? - i.e. (opposite to CO2 changes) when pH↑, HCO3- OR pH↓ HCO3- then the change is metabolic. Otherwise, the change is again a compensatory change.

"Compensatory" means that there is a change to the body's acid-base balance but the buffer systems the body has in place are attempting to deal with it. At the end of asking these questions you should end with one of five options. They are the titles the following titles

Metabolic acidosis

Unfortunately, life as ever, is not so simple. There are two types of metabolic acidosis - with or without an anion gap. Essentially, this is the difference in concentrations between cations (+ve ions) and anions (-ve ions). You calculate it using the following formula

Na+ - (Cl- + HCO3-) = 8-12mEq/l (that's the number of charges)

We then split metabolic acidosis into:

High anion gap

This caused when there is an increase of non-measured acids. That is lactic acid (due to shock, infection or hypoxia), urate i.e. uric acid (in renal failure), ketoacidosis (diabetes, alcohol) and with drugs or toxins (salicylates, biguanides like metformin, ethylene glycol and methanol which are found in antifreeze)

Normal anion gap

Essentially, what happens is that bicarbonate is lost or H+ ions are ingested (though Cl- is retained). This decreases blood pH. It's usually caused by diarrhoea but other causes include: renal tubular acidosis (failure of the kidneys to transport H+ ions into urine); drugs, particularly acetazolamide; Addison's disease; pancreatic fistulae; and ammonium chloride ingestion.

Metabolic alkalosis

Essentially, there are lots of causes but basically the H+ ions go somewhere other than the blood. Vomiting is the most common causes. Hypokalaemia causes a shift of K+ ions from intra- to extracellular space. To maintain electrical neutrality, cells shift H+ into the cells. Burns and the ingestion of a basic substance can also cause metabolic alkalosis.

Respiratory acidosis

This almost invariably causes by respiratory failure. The first thing to do is confirm respiratory failure by checking the PaO2 is low.

Respiratory alkalosis

This usually due to hyperventilation. There are numerous causes to hyperventialtion: CNS - stroke, subarachnoid haemorrhage, meningitis; others - anxiety, altitude, hyperthermia, pregnancy, PE, drugs (e.g. salicylates).