Metabolic Acidosis and Respiratory Alkalosis

Occurs in:

  • Septic shock
  • Sepsis and renal failure
  • Salicylate overdose
  • Congestive cardiac failure and renal failure
  • Early cardiopulmonary arrest

It can be difficult to distinguish this mixed picture from a compensated primary metabolic acidosis or respiratory alkalosis.

However, several factors will point more towards a mixed disorder:

  • The clinical picture
  • The level of HCO3 (compensation from a respiratory alkalosis rarely causes a fall in HCO3 below 18)
  • The level of CO2 – if this is very low, again, it is unlikely to be from a compensatory response alone

For example, in a patient with septic shock, secondary to a urinary tract infection:

  • pH   7.37
  • pCO2   3.8 kPa
  • pO2   11.6 kPa
  • bicarbonate  12.4 mmol/L
  • lactate   4.5 mmol/L

The patient is hyperventilating, attempting to increase the amount of oxygen to underperfused tissues. The CO2 is therefore blown off, but bicarbonate is very low, indicating a metabolic component. The lactate is high reflecting sepsis and despite all this, the pH is surprisingly normal. Remember not to be falsely reassured by the normal pH – the clinical picture will reveal an unwell patient.