Interpretation of arterial blood gasesMeera Ladwa

What it measures• Measures acidity of arterial blood• Measures partial pressure of oxygen and carbon dioxide in the

blood• Often measure Hb, electrolytes (Na, K), glucose, lactate• Derives bicarbonate and base excess

Why measure?• Tells you about acid-base balance• Tells you about patient’s oxygenation and ventilation• Therefore they are very useful in patient who are critically ill,

deteriorating, have respiratory problems or are on invasive or non-invasive ventilation

Pathophysiology• The function of crucial metabolic processes depends on a

narrow range of pH (pH 7.35-7.45)• H+ ions are normal products of cellular metabolism• H+ ion concentration is INVERSELY PROPORTIONAL to pH• Therefore the body must continually dispose of acid in order

to keep pH within range

Pathophysiology

BUFFERING (carbonic acid-bicarbonate system)

H2O + CO2 ↔ H2CO3 ↔ HCO3 - + H+

Respiratory system (rapid change in pH)• If pH falls, patient can increase respiratory rate to ‘blow off’

CO2

Pathophysiology

BUFFERING (carbonic acid-bicarbonate system)

H2O + CO2 ↔ H2CO3 ↔ HCO3 - + H+

Renal or metabolic system (slower change in pH)•If pH falls, kidneys can increase reabsorption of bicarbonate (and increase secretion of H+)

Pathophysiology

Henderson-Hassalbalch equation.

• Mathematical relationship between pH, pCo2 and HCO3

Ka = [H+][A-] [HA]

pH = pKa + log [Base] [Acid]

Examples• Respiratory acidosis e.g. opiate overdose

pH 7.29 (7.35-7.45)pCO2 8.6 (4.7-6.0 kPa)pO2 9.0 (9.3- 13.3 kPa)HCO3 22.4 (22-26mmol/L)BE 1 (-2 to +2)

Examples• Metabolic acidosis (with partial respiratory compensation) e.g.

DKA, lactic acidosis, AKI, diarrhea

pH 6.90 (7.35-7.45)pCO2 3.1 (4.7-6.0 kPa)pO2 14.0 (9.3- 13.3 kPa)HCO3 13.0 (22 – 26mmol/L)BE -8 (-2 to +2)

Examples • Chronic CO2 retention with metabolic compensation e.g.

COPD

pH 7.36 (7.35-7.45)pCO2 6.8 (4.7 – 6.0 kPa)pO2 9.0 (9.3 – 13.3 kPa)HCO3 31.0 (22-26mmol/L)BE -1.0 (-2 to +2)

Examples• Mixed respiratory and metabolic acidosis (eg opiate o/d in

renal failure)

pH 7.16 (7.35-7.45)pCO2 6.8 (4.7 – 6.0 kPa)pO2 10.0 (9.3 – 13.3 kPa)HCO3 14 (22-26mmol/L)BE -7.0 (-2 to +2)

Examples• Respiratory alkalosis with metabolic acidosis (salicylate

overdose)

pH 7.47 (7.35-7.45)pCO2 4.1 (4.7 – 6.0 kPa)pO2 10.0 (9.3 – 13.3 kPa)HCO3 17 (22-26mmol/L)BE -6.0 (-2 to +2)