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