Q Henderson-Hasselbalch equation is used to determine?
A. PaCO2
B. PaO2
C. Systemic pH
D. Anion gap
Explanation
Correct Answer: C. Systemic pH
Ref: Harrison’s 17th Ed 287
The Henderson–Hasselbalch equation relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, Ka, and the concentrations of the species in solution. To derive the equation a number of simplifying assumptions have to be made.
The Henderson-Hasselbalch equation allows for calculation of pH from HCO3- and pCO2:
pH = 6.1 + log ([HCO3-] / 0.03 x pCO2)
0.03 = solubility constant of pCO2
6.1 = pKa of carbonic acid
Start with an ABG and then proceed in the following order
Evaluate blood pH:
pH < 7.35 (acidemia) → primary disorder is an acidosis
pH > 7.45 (alkalemia) → primary disorder is an alkalosis
Evaluate pCO2 (partial pressure of carbon dioxide in blood, reference range: 33–45 mm Hg) to determine whether the primary acid-base disorder is respiratory or metabolic
pH and pCO2 change in the opposite direction → respiratory disorder
↓ pH and ↑ pCO2 → respiratory acidosis
↑ pH and ↓ pCO2 → respiratory alkalosis
pCO2 and pH change in the same direction → metabolic disorder
↓ pH and ↓ pCO2 → metabolic acidosis
In case of metabolic acidosis → calculate anion gap to identify the possible causes (see “Anion gap†below)
↑ pH and ↑ pCO2 → metabolic alkalosis
Suspect a mixed acid-base disorder if
pCO2 or HCO3- is abnormal and pH is normal (or did not change as expected, e.g., a very high pCO2 and a mild acidosis)
PCO2 and HCO3- shift towards acidosis (↑ PCO2 and ↓ HCO3-) or alkalosis (↓ PCO2 and ↑ HCO3 )
Lesser- or greater-than-expected compensatory response (see “Compensation†below)
Evaluate HCO3- (Reference range: 22–28 mEq/L)
High → metabolic alkalosis or compensated respiratory acidosis
Normal → non-compensated respiratory disorders
Low → metabolic acidosis or compensated respiratory alkalosis
Evaluate pO2
High → hyperoxemia
Low → hypoxemia
Example
pH = 7.5, pCO2 = 20 mmHg, HCO3 = 22 mEq/L, pO2 = 70 mmHg
Alkalosis (pH), respiratory disorder (↑ pH and ↓ pCO2), normal bicarbonate concentration, ↓ pO2 = non-compensated respiratory alkalosis with hypoxemia
A. PaCO2 B. PaO2 C. Systemic pH D. Anion gap Explanation
Correct Answer: C. Systemic pH
Ref: Harrison’s 17th Ed 287
The Henderson–Hasselbalch equation relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, Ka, and the concentrations of the species in solution. To derive the equation a number of simplifying assumptions have to be made.
The Henderson-Hasselbalch equation allows for calculation of pH from HCO3- and pCO2:
pH = 6.1 + log ([HCO3-] / 0.03 x pCO2)
0.03 = solubility constant of pCO2
6.1 = pKa of carbonic acid
Start with an ABG and then proceed in the following order
Evaluate blood pH:
pH < 7.35 (acidemia) → primary disorder is an acidosis
pH > 7.45 (alkalemia) → primary disorder is an alkalosis
Evaluate pCO2 (partial pressure of carbon dioxide in blood, reference range: 33–45 mm Hg) to determine whether the primary acid-base disorder is respiratory or metabolic
pH and pCO2 change in the opposite direction → respiratory disorder
↓ pH and ↑ pCO2 → respiratory acidosis
↑ pH and ↓ pCO2 → respiratory alkalosis
pCO2 and pH change in the same direction → metabolic disorder
↓ pH and ↓ pCO2 → metabolic acidosis
In case of metabolic acidosis → calculate anion gap to identify the possible causes (see “Anion gap†below)
↑ pH and ↑ pCO2 → metabolic alkalosis
Suspect a mixed acid-base disorder if
pCO2 or HCO3- is abnormal and pH is normal (or did not change as expected, e.g., a very high pCO2 and a mild acidosis)
PCO2 and HCO3- shift towards acidosis (↑ PCO2 and ↓ HCO3-) or alkalosis (↓ PCO2 and ↑ HCO3 )
Lesser- or greater-than-expected compensatory response (see “Compensation†below)
Evaluate HCO3- (Reference range: 22–28 mEq/L)
High → metabolic alkalosis or compensated respiratory acidosis
Normal → non-compensated respiratory disorders
Low → metabolic acidosis or compensated respiratory alkalosis
Evaluate pO2
High → hyperoxemia
Low → hypoxemia
Example
pH = 7.5, pCO2 = 20 mmHg, HCO3 = 22 mEq/L, pO2 = 70 mmHg
Alkalosis (pH), respiratory disorder (↑ pH and ↓ pCO2), normal bicarbonate concentration, ↓ pO2 = non-compensated respiratory alkalosis with hypoxemia