By Dr Garzarella

Metabolic acidosis

Expected pCO2 = 1.5 × [HCO3] +8 ±2

Metabolic alkalosis

Expected pCO2 = 0.7 × [HCO3] +20 ±5


In metabolic acid-base disturbances, chemoreceptors sense an abnormal pH and stimulate the respiratory centre to increase or decrease ventilation to compensate


1. 62F presents to the emergency department with peritonitis secondary to perforated diverticulitis. She is hypotensive, tachycardic, peripherally cool and appears distressed due to pain. An ABG is taken showing:

pH 7.20

pO2 95mmHg

pCO2 30mmHg

HCO3 18mmolL/L

Na 140 mmol/L

Cl 100 mmol/L


Acidaemia is present

Low bicarbonate and low carbon dioxide, consistent with metabolic acidosis

Anion gap = 22mmol/L (high)

Carbon dioxide is lower than expected

Expected pCO2 = 1.5 × l[HCO3] +8 ±2

= (1.5 × 15)+8 ±2

= 33 to 37


Metabolic acidosis (likely lactic acidosis)

CO2 is lower than expected indicating a superimposed respiratory alkalosis (likely hyperventilation due to pain)

Respiratory acidosis



Respiratory alkalosis



In the above formulae

24 = mid-range value of HCO3

40 = mid-range value of PaCO2


Compensation in acute respiratory acidosis/alkalosis is limited to buffering as there is not enough time for the kidneys to respond

  • This buffering occurs via the law of mass action: CO2 + H2O <-> H2CO3 <-> H+ + HCO3

Compensation in chronic respiratory acidosis/alkalosis involves renal retention/loss of bicarbonate

Remember ‘1 4, 2 5’:

Respiratory acidosis – every 10mmHg rise in CO2 should increase HCO3 by 1 (acute) or 4 (chronic)

Respiratory alkalosis – every 10mmHg decrease in CO2 should decrease HCO3 by 2 (acute) or 5 (chronic)

Example 1

57M post motor vehicle accident sustaining multiple rib fractures and a small pneumothorax. Remains hemodynamically stable. No significant past medical history. An ABG is taken showing:

pH 7.23

pO2 94mmHg

pCO2 58mmHg

HCO3 26mmol/L


Acidaemia is present

High carbon dioxide, and a new injury in an otherwise healthy patient suggests an acute respiratory acidosis

Bicarbonate has compensated as expected

= 25.8


Acute respiratory acidosis (likely due to hypoventilation secondary to pain) with appropriate metabolic compensation

Example 2:

29F G1P0 at 36 weeks gestation, presents with a 2 day history of significant vomiting.

pH 7.46

pO2 98mmHg

pCO2 31mmHg

HCO3 22mmol


Mild alkalaemia is present

Low carbon dioxide suggests a respiratory alkalosis, consistent with the state of pregnancy and has occurred over the space of many months

Bicarbonate is higher than expected

= 19.5


Chronic respiratory alkalosis (likely due to pregnancy) with superimposed metabolic alkalosis (likely due to vomiting)


  • Acid base disorders, Life in the Fast Lane Available at:
  • Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010 Apr;14(2):57-64. doi: 10.4103/0972-5229.68215. PMID: 20859488; PMCID: PMC2936733.