Acid-Base Analysis
W. Rose
See Marieb & Hoehn 9th ed., Chapter 26
Normal Arterial Blood Gas
pH = 7.35-7.45
PaCO2 = 35-45 mmHg
HCO3- = 22-26 mEq/L
PaO2 = 80-100 mmHg
We will skip base excess and anion gap
Abnormal Blood Gases
Arterial pH<7.35: Acidosis
Arterial pH >7.45: Alkalosis
Arterial pH < 7.35: Acidosis
Is it respiratory, metabolic, or both?
If PaCO2 abnormally high (>45 mmHg), acidosis has a respiratory cause. PaO2 likely to be low (<80 mmHg).
If HCO3 abnormally low (>22 mEq/L), acidosis has a metabolic cause.
Arterial pH > 7.45: Alkalosis
Is it respiratory, metabolic, or both?
If PaCO2 abnormally low (<35 mmHg), alkalosis has a respiratory cause (hyperventilation). PaO2 likely to be
high normal (95-100 mmHg).
If HCO3 abnormally high (>26 mEq/L), alkalosis has a metabolic cause (gain of strong acid, or loss of base).
pH < 7.35: Acidosis pH > 7.45: Alkalosis
Respiratory AcidosisPaCO2 > 45 mmHg
Respiratory AlkalosisPaCO2 < 35 mmHg
Metabolic AcidosisHCO3<22 mEq/L
Metabolic AlkalosisHCO3>26 mEq/L
Arterial blood gas diagram
Compensation
There is immediate but limited compensation from the bicarbonate buffer system of blood.
Lungs can compensate (in minutes) for a metabolic disturbance. Increased breathing to compensate for metabolic acidosis
is a more robust and reliable response than decreased breathing in response to metabolic alkalosis.
Kidneys can compensate (in hours to days) for a respiratory disturbance. Renal compensation for respiratory
acidosis is slow but may be nearly complete after 3-4 days; renal compensation for respiratory alkalosis is slow and incomplete.
Compensation
Abnormally low PaCO2 (<35 mmHg) in metabolic acidosis indicates respiratory
compensation (hyperventilation).
Pure (uncompensated) metabolic acidosis seldom seen since respiratory system compensates quickly.
PaO2 is often high normal (95-100 mmHg) when there is respiratory compensation.
Compensated (or chronic) metabolic acidosis pH=7.25, PaCO2=25 mmHg, HCO3=10 mEq/L
It’s acidosisThere’s
respiratory compensation
The acidosis is metabolic
Compensation
Abnormally high PaCO2 (>45 mmHg) in metabolic alkalosis indicates respiratory
compensation (hypoventilation).
PaO2 may be normal or slightly below normal (<=80 mmHg)
Uncompensated metabolic alkalosispH=7.58, PaCO2=44 mmHg, HCO3=40 mEq/L
Compensated metabolic alkalosis pH=7.50, PaCO2=55 mmHg, HCO3=40 mEq/L
It’s alkalosis The alkalosis is metabolic
There’s respiratory compensation
Compensation
Abnormally high HCO3 (>26 mEq/L) in respiratory acidosis indicates renal compensation.
Uncompensated (or acute) respiratory acidosis pH=7.25, PaCO2=60 mmHg, HCO3=25 mEq/L
Compensated (or chronic) respiratory acidosis pH=7.35, PaCO2=60 mmHg, HCO3=32 mEq/L
Near normal Abnormally high – this person
isn’t getting rid of CO2
Higher than normal – kidneys must be compensating for
the high CO2
Compensation
Abnormally low HCO3 (<22 mEq/L) in respiratory alkalosis indicates renal compensation.
Compensated (or chronic) respiratory alkalosispH=7.44, PaCO2=26 mmHg, HCO3=17 mEq/L,
PaO2=53 mmHg, pneumoniaIt’s alkalosis The alkalosis is not metabolic – quite
the oppositeThe alkalosis is respiratory
Sources'Acid-base pHysiology' , K. Brandis, http://www.anaesthesiaMCQ.com
GlobalRPh: Arterial blood gases. http://www.globalrph.com/abg_analysis.htm
Top Related