Acid-base analysis

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 Acidosis
PaCO2 > 45 mmHg
Respiratory Alkalosis
PaCO2 < 35 mmHg
Metabolic Acidosis
HCO3<22 mEq/L
Metabolic Alkalosis
HCO3>26 mEq/L
Arterial blood
gas diagram
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.
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 acidosis
The acidosis
is metabolic
Abnormally high PaCO2 (>45 mmHg) in
metabolic alkalosis indicates respiratory
compensation (hypoventilation).
PaO2 may be normal or slightly below normal (<=80 mmHg)
Uncompensated metabolic alkalosis
pH=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
There’s respiratory
The alkalosis
is metabolic
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
Abnormally low HCO3 (<22 mEq/L) in
respiratory alkalosis indicates renal compensation.
Compensated (or chronic) respiratory alkalosis
pH=7.44, PaCO2=26 mmHg, HCO3=17 mEq/L,
It’s alkalosis PaO2=53 mmHg, pneumonia
The alkalosis
is respiratory
The alkalosis is not
metabolic – quite
the opposite
'Acid-base pHysiology' , K. Brandis,
GlobalRPh: Arterial blood gases.

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