Blood Gas Analysis
Blood gases and acid base balance
Normally we do venous/capillary blood gases. Arterial blood gases are only done in the very sick children, who have been intubated/ventilated and have arterial lines in situ (usually in the theatre prior to transfer to the PICU in Newcastle.
Here is a stepwise approach to blood gas analysis.
STEP 1:
[pic]
STEP 2: Look at pCO2
[pic]
STEP 3: Look at Bicarbonate & Base excess (BE)
[pic]
Base excess (BE): Base excess is the amount mmol/l of base that is needed to correct pH back to normal when pCO2 is corrected to 5.3 kPa (40mmHg).
[pic]
STEP 4
Compensation:
Remember that long standing respiratory acidosis (CO2 retention) results in compensatory metabolic alkalosis (raised Bicarbonate) and pH moves towards near normal, similarly long standing metabolic acidosis results in compensatory respiratory alkalosis such as in DKA
[pic]
In case of metabolic acidosis, now work out Anion Gap.
Anion gap= [Na+ - (Cl- +HCO3 ) Normal values 10-14 mmol/L
Normal anion gap metabolic acidosis (Hyperchloraemic)
Causes include:
GI or renal loss of bicarbonate, but re-absorption of excess chloride, i.e.,
(GI fluid is rich in bicarb).
A useful mnemonic is DURHAM
Diarrhoea
Ureteral diversion into sigmoid colon
Renal Tubular acidosis
Hyperalimentation
Acetazolamide
Miscellaneous condition: pancreatic fistula, cholestyramine, calcium chloride ingestion
Increased anion gap metabolic acidosis
Main causes include: DKA, lactic acidosis, renal failure, toxic ingestion
[pic]
Predominantly respiratory alterations in acid-base balance are managed by controlling ventilation.
Metabolic changes are managed by treating the underlying problem, e.g.: hypoxia, shock, infection, diabetic ketoacidosis, inborn error of metabolism, etc.
The use of alkali, e.g. bicarbonate, is controversial: there is evidence that it can be detrimental. However, in severe acidosis,
Normally we do venous/capillary blood gases. Arterial blood gases are only done in the very sick children, who have been intubated/ventilated and have arterial lines in situ (usually in the theatre prior to transfer to the PICU in Newcastle.
Here is a stepwise approach to blood gas analysis.
STEP 1:
[pic]
STEP 2: Look at pCO2
[pic]
STEP 3: Look at Bicarbonate & Base excess (BE)
[pic]
Base excess (BE): Base excess is the amount mmol/l of base that is needed to correct pH back to normal when pCO2 is corrected to 5.3 kPa (40mmHg).
[pic]
STEP 4
Compensation:
Remember that long standing respiratory acidosis (CO2 retention) results in compensatory metabolic alkalosis (raised Bicarbonate) and pH moves towards near normal, similarly long standing metabolic acidosis results in compensatory respiratory alkalosis such as in DKA
[pic]
In case of metabolic acidosis, now work out Anion Gap.
Anion gap= [Na+ - (Cl- +HCO3 ) Normal values 10-14 mmol/L
Normal anion gap metabolic acidosis (Hyperchloraemic)
Causes include:
GI or renal loss of bicarbonate, but re-absorption of excess chloride, i.e.,
(GI fluid is rich in bicarb).
A useful mnemonic is DURHAM
Diarrhoea
Ureteral diversion into sigmoid colon
Renal Tubular acidosis
Hyperalimentation
Acetazolamide
Miscellaneous condition: pancreatic fistula, cholestyramine, calcium chloride ingestion
Increased anion gap metabolic acidosis
Main causes include: DKA, lactic acidosis, renal failure, toxic ingestion
[pic]
Predominantly respiratory alterations in acid-base balance are managed by controlling ventilation.
Metabolic changes are managed by treating the underlying problem, e.g.: hypoxia, shock, infection, diabetic ketoacidosis, inborn error of metabolism, etc.
The use of alkali, e.g. bicarbonate, is controversial: there is evidence that it can be detrimental. However, in severe acidosis,