ACID-BASE PRIMER

Nomenclature

• pH = - log₁₀ [H⁺]
• Alkalaemia = pH > 7.45
• Acidaemia = pH < 7.35
• Metabolic Acidosis = primary process that results in decrease in [HCO₃^2-]
• Metabolic Alkalosis = primary process that results in increase in [HCO₃^2-]
• Respiratory Acidosis = primary process that results in increase in pCO₂ 
• Respiratory Alkalosis = primary process that results in decrease in pCO₂ 
• Metabolic Compensation = secondary active process that alters [HCO₃^2-] in an attempt to normalise pH
• Respiratory Compensation = secondary active process that alters pCO₂ in an attempt to normalise pH

Reference ranges

[H⁺] = 35-45 nmol/L

pH 7.35-7.45

pCO₂ 35-45mmHg

HCO₃^2- 24 mmol/L

Acid base disturbances and defence of pH

• The body attempts to maintain pH between 7.35-7.45
• A disturbance in pH can occur by an alteration in intake/production/excretion of  volatile (respiratory) or non-volatile (metabolic) acids
• Metabolic disturbances directly alter [HCO₃^2-]
• Respiratory disturbances directly alter pCO₂

Relationship between [HCO₃^2-] pCO₂ and pH

• All these variables are reversibly dependent by mass effect according to the Henderson-Hasselbach equation:

HCO₃^2-+ H⁺ = pCO₂ + H₂O 

This can be appreciated by a respiratory disturbance resulting in an immediate change in [HCO₃^2-] as per the Henderson-Hasselbach equation 

It is possible to predict the change in [HCO₃^2-]  by the following formulae:

• Respiratory acidosis  10mmHg pCO₂ increase causes increase [HCO₃^2-] 1mmol

• Respiratory alkalosis 10mmHg pCO₂ decrease causes decrease [HCO₃^2-] by 2.5mmol

 

Compensation

• [HCO₃^2-] and pCO₂ can also be independently varied by physiological mechanisms - this is called compensation
• Compensation is an active physiological process that alters the excretion of acid to normalise pH

Metabolic compensation

• A metabolic disturbance  results in an immediate compensatory effect on respiration and pCO₂ to normalise the pH
• It is possible to predict the compensatory change in pCO₂ by the following formulae:

• Metabolic acidosis pCO₂ = (1.5 x [HCO₃^2-]) + 8

• Metabolic alkalosis pCO₂ = (0.9 x [HCO₃^2-]) + 9

Respiratory compensation

• A respiratory disturbance results in an delayed compensatory change in [HCO₃^2-]
• It is possible to predict the compensatory change in[ HCO₃^2-]  by the following formulae:

• Respiratory acidosis  10mmHg pCO₂ increase causes increase [HCO₃^2-] 4mmol 

• Respiratory alkalosis 10mmHg pCO₂ decrease causes decrease [HCO₃^2-] 5mmol 

see Summary of Acid-Base Rules

CLINICAL APPROACH TO ACID BASE DISTURBANCES

• Determine if the pH disturbance is predominantly alkaemic or acidaemic
• Determine if it is predominantly a respiratory or metabolic disturbance driving the change in pH
• Using the acid base rules determine if the degree of compensation is adequate or appropriate
• If the compensation is not appropriate or adequate, consider another primary acid-base process (e.g. double or triple acid-base disturbance)
• Correlate this with the clinical data from the patient