Compensation in acid-base disturbances

Expected pCO2 = 1.5 × [HCO3] +8 ±2

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

Example

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

Discussion

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

Conclusion

Metabolic acidosis (likely lactic acidosis)

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

Acute

Chronic

Acute

Chronic

24 = mid-range value of HCO₃

40 = mid-range value of PaCO₂

Mechanism

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)

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

Discussion

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

Conclusion:

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

Discussion

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

Conclusion

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