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. [11,12]

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. [11,12]