Arterial pH < 7.35: Acidosis Is it respiratory, metabolic, or both? If PaCO 2 abnormally high (>45 mmHg), acidosis has a respiratory cause. PaO 2 likely to be low (<80 mmHg). If HCO 3 abnormally low (>22 mEq/L), acidosis has a metabolic cause.
Arterial pH > 7.45: Alkalosis Is it respiratory, metabolic, or both? If PaCO 2 abnormally low (<35 mmHg), alkalosis has a respiratory cause (hyperventilation). PaO 2 likely to be high normal ( mmHg). If HCO 3 abnormally high (>26 mEq/L), alkalosis has a metabolic cause (gain of strong acid, or loss of base).
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 PaCO 2 (<35 mmHg) in metabolic acidosis indicates respiratory compensation (hyperventilation). Pure (uncompensated) metabolic acidosis seldom seen since respiratory system compensates quickly. PaO 2 is often high normal ( mmHg) when there is respiratory compensation. Compensated (or chronic) metabolic acidosis pH=7.25, PaCO 2 =25 mmHg, HCO 3 =10 mEq/L It’s acidosis There’s respiratory compensation The acidosis is metabolic
Compensation Abnormally high PaCO 2 (>45 mmHg) in metabolic alkalosis indicates respiratory compensation (hypoventilation). PaO 2 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 The alkalosis is metabolic There’s respiratory compensation
Compensation Abnormally high HCO 3 (>26 mEq/L) in respiratory acidosis indicates renal compensation. Uncompensated (or acute) respiratory acidosis pH=7.25, PaCO 2 =60 mmHg, HCO 3 =25 mEq/L Compensated (or chronic) respiratory acidosis pH=7.35, PaCO 2 =60 mmHg, HCO 3 =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 HCO 3 (<22 mEq/L) in respiratory alkalosis indicates renal compensation. Compensated (or chronic) respiratory alkalosis pH=7.44, PaCO 2 =26 mmHg, HCO 3 =17 mEq/L, PaO 2 =53 mmHg, pneumonia It’s alkalosis The alkalosis is not metabolic – quite the opposite The alkalosis is respiratory
Sources 'Acid-base pHysiology', K. Brandis, GlobalRPh: Arterial blood gases.