Acid-Base Physiology

Case 9 : A man with a post-operative cardiac arrest

History

A 69 year old patient had a cardiac arrest soon after return to the ward following an operation. Resuscitation was commenced and included intubation and ventilation. Femoral arterial blood gases were collected about five minutes after the arrest. Other results: Anion gap 24, Lactate 12 mmol/l.


Arterial Blood Gases

pH 6.85

pCO2 82 mmHg

pO2 214 mmHg

HCO3 14 mmol/l

Assessment

Firstly: Initial clinical assessment

The expected result here would be a mixed disorder with respiratory acidosis (due inadequate ventilation) and a lactic acidosis (related to poor perfusion).

Secondly: The acid-base diagnosis

  1. pH: The pH is extremely low (severe acidaemia) so a severe acidosis is present
  2. Pattern: The combination of a high pCO2 and a low bicarbonate means that a mixed disorder is present: there must be 2 or more primary acid-base disorders present. This pattern is found with a combined acidosis: metabolic acidosis (low bicarbonate) and a respiratory acidosis (high pCO2).
  3. Clues: The anion gap result confirms a high anion gap acidosis and the high lactate level confirms this as a severe lactic acidosis.
  4. Compensation: Consider the expected pCO2 for the metabolic acidosis: By the one & a half plus 8 rule (rule 5): Expected pCO2 = (1.5 x 14 + 8 ) = 29mmHg. The actual pCO2 of 82 mmHg is very much higher which confirms the presence of a co-existent respiratory acidosis. The pCO2 level of 82 mmHg is so high that a respiratory acidosis must be present. (In exceptional cases of severe metabolic alkalosis a pCO2 of 86mmHg has been recorded).
  5. Formulation: A severe mixed acidosis due to lactic acidosis and respiratory acidosis.
  6. Confirmation: Nil else is required. There should be clinical evidence to support the conclusion of poor peripheral perfusion. If not, then an ischaemic gut cause should be considered but there is no evidence of this here. Compared to standard normal values, the anion gap has increased by 12 & the bicarbonate level has decreased by 10 so the delta ratio is 12/10 = 1.2 - this is consistent with a high anion gap acidosis.

Finally: The Clinical Diagnosis

Cardiac arrest with low cardiac output and tissue hypoperfusion causing a severe lactic acidosis. Ventilation is depressed causing a respiratory acidosis.

Comments

The pCO2 of 82mmHg is too high to have developed from a level of 40 mmHg in 5 minutes. An elevated pCO2 must have been present before the arrest. Inadequate ventilation in this pre-arrest phase may have been related to several factors, in particular inadequate reversal of neuromuscular paralysis, airway obstruction in a supine sedated patient or acute pulmonary oedema.

The hypercapnia would have been associated with hypoxaemia and this would have contributed to the arrest. The high pO2 level on the gases is due to the high inspired oxygen fraction as such a level is not possible when breathing room air.