Acidosis & alkalosis: Difference between revisions
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Acidosis and Alkalosis is a difficult topic to approach, but with a little understanding it is actually fairly simple. | |||
#'''Is the pH high or low?''' - | |||
#'''Is CO<sub>2</sub> abnormal?''' - if ' | ===Glossary=== | ||
# | *'''Acidosis''' - The pH of the [[blood]] is low. This means there are a lot of H+ ions. | ||
*'''Alkalosis''' - The pH of the blood in high. This means there aren't many H+ ions. | |||
*'''Respiratory''' - The cause of the pH change is [[respiratory]]. | |||
*'''Metabolic''' - The cause of this pH change is non respiratory (usually [[kidney]]) | |||
*'''Compensatory''' means that there is a change to the body's acid-base balance but the [[buffer systems]] are attempting to deal with it. | |||
==Working out what's going on== | |||
There are basically four abnormalities with regards to the acid-base balance in blood. You should ask yourself three questions: | |||
#'''Is the pH high or low?''' - low pH= ''acidosis'' and high pH = ''alkalosis''. | |||
#'''Is CO<sub>2</sub> abnormal?''' - if so, is this in keeping with the pH change? - i.e. when ''pH↑, CO<sub>2</sub>↓'' or '''pH↓, CO<sub>2</sub>↑'''. These changes point to a ''respiratory'' acidosis/alkalosis. If there is no abnormality in CO<sub>2</sub>, it may be there has been a ''compensatory'' change. | |||
#'''Is the HCO<sub>3</sub><sup>-</sup> abnormal?''' - if so, is this in keeping with the pH change? - i.e. (opposite to CO<sub>2</sub> changes) when ''pH↑, HCO<sub>3</sub><sup>-</sup>↑'' OR ''pH↓ HCO<sub>3</sub><sup>-</sup>↓'' then the change is ''metabolic''. Otherwise, there may again have been a ''compensatory'' change. | |||
==The types of imbalance== | |||
At the end of asking these questions you should end with one of the five options below: | |||
*Metabolic acidosis with a high anion gap | |||
*Metabolic acidosis with a normal anion gap | |||
*Metabolic alkalosis | |||
*Respiratory acidosis with a high anion gap | |||
*Respiratory alkalosis | |||
===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<sup>+</sup> - (Cl<sup>-</sup> + HCO<sub>3</sub><sup>-</sup>) = 8-12mEq/l (that's the number of charges) | |||
We then split metabolic acidosis into types of anion gap: | |||
====High anion gap==== | |||
This usually occurs due to an increase in non-'measured' acids such as. The main causes are: | |||
*'''lactic acidosis''' (increased ''lactic acid'') (due to [[shock]], [[infection]] or [[hypoxia]], leading to increase in breathing muscles, producing acid). | |||
*'''[[renal failure]]''' (increased ''uric acid'' or ''urate'') | |||
*'''[[diabetic ketoacidosis]]''' (note - [[alcohol]] can also cause ketoacidosis) | |||
*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<sup>+</sup> ions are ingested (though Cl<sup>-</sup> is retained). This decreases blood pH. | |||
*'''Most common: [[diarrhoea]]''' | |||
*renal tubular acidosis (failure of the kidneys to transport H<sup>+</sup> 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<sup>+</sup> ions go somewhere other than the blood. | |||
*'''Most common: [[vomiting]]''' | |||
[[Hypokalaemia]] causes a shift of K<sup>+</sup> ions from intra- to extracellular space. To maintain electrical neutrality, cells shift H<sup>+</sup> into the cells. [[Burns]] and the ingestion of a basic substance can also cause metabolic alkalosis. | |||
===Respiratory acidosis=== | |||
*'''Most common: [[respiratory failure]]'''. | |||
The first thing to do is confirm respiratory failure by checking the ''Pa''O<sub>2</sub> is low. This is important because they may need [[non-invasive ventilation]] if they are becoming acidotic. | |||
Other causes are rare. | |||
===Respiratory alkalosis=== | |||
*Most common: hyperventilation. | |||
There are numerous causes to hyperventilation: ''CNS'' - [[stroke]], [[subarachnoid haemorrhage]], [[meningitis]]; ''others'' - anxiety, altitude, hyperthermia, pregnancy, [[PE]], drugs (e.g. salicylates). |
Latest revision as of 15:58, 12 April 2011
Acidosis and Alkalosis is a difficult topic to approach, but with a little understanding it is actually fairly simple.
Glossary
- Acidosis - The pH of the blood is low. This means there are a lot of H+ ions.
- Alkalosis - The pH of the blood in high. This means there aren't many H+ ions.
- Respiratory - The cause of the pH change is respiratory.
- Metabolic - The cause of this pH change is non respiratory (usually kidney)
- Compensatory means that there is a change to the body's acid-base balance but the buffer systems are attempting to deal with it.
Working out what's going on
There are basically four abnormalities with regards to the acid-base balance in blood. You should ask yourself three questions:
- Is the pH high or low? - low pH= acidosis and high pH = alkalosis.
- Is CO2 abnormal? - if so, 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 there is no abnormality in CO2, it may be there has been a compensatory change.
- Is the HCO3- abnormal? - if so, is this in keeping with the pH change? - i.e. (opposite to CO2 changes) when pH↑, HCO3-↑ OR pH↓ HCO3-↓ then the change is metabolic. Otherwise, there may again have been a compensatory change.
The types of imbalance
At the end of asking these questions you should end with one of the five options below:
- Metabolic acidosis with a high anion gap
- Metabolic acidosis with a normal anion gap
- Metabolic alkalosis
- Respiratory acidosis with a high anion gap
- Respiratory alkalosis
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 types of anion gap:
High anion gap
This usually occurs due to an increase in non-'measured' acids such as. The main causes are:
- lactic acidosis (increased lactic acid) (due to shock, infection or hypoxia, leading to increase in breathing muscles, producing acid).
- renal failure (increased uric acid or urate)
- diabetic ketoacidosis (note - alcohol can also cause ketoacidosis)
- 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.
- Most common: diarrhoea
- 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.
- Most common: vomiting
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
- Most common: respiratory failure.
The first thing to do is confirm respiratory failure by checking the PaO2 is low. This is important because they may need non-invasive ventilation if they are becoming acidotic.
Other causes are rare.
Respiratory alkalosis
- Most common: hyperventilation.
There are numerous causes to hyperventilation: CNS - stroke, subarachnoid haemorrhage, meningitis; others - anxiety, altitude, hyperthermia, pregnancy, PE, drugs (e.g. salicylates).