Interpret the ABG. How might you explain the findings? Is this patient alkalotic? Is the patient acidotic? Based solely on the PaCO₂ – what would you expect the pH to be in this case? Explain why this patient’s acid/base profile is the way it is. What does chloride loss have to do with the development of alkalosis? Why would the fact that there is a salt-restricted diet add to the problem?

Is he ready for the OR? Does his acidosis/alkalosis need to be corrected prior to the OR? Where should you attempt to keep his CO₂ during the operation? Is there a problem with normalizing his CO₂? How much oxygen should he be on?

What is a “normal” bicarbonate level? How does the concentration of HCO₃− relate to pH?

What is the most common cause of alkalosis in a child coming for surgery? If a child with severe vomiting or an NG tube to suction is given H₂ receptor blockade – does that stop the development of alkalosis? Could that treatment cause them to become acidotic?

Why does K⁺ depletion cause alkalosis?

The patient’s blood pH is in the normal range between 7.35 and 7.45. He has a significant metabolic alkalosis that is largely compensated by a respiratory acidosis. There are many possible explanations for these findings. Most probably the ABG is due to the fact that this patient has poor cardiac function that has necessitated him being on long-term diuretics to avoid congestive heart failure. This has led to chronic chloride and sodium loss. Since the kidney must maintain electrical neutrality, for every positive ion excreted, a negative ion needs to be absorbed. When chloride is deficient, bicarbonate is resorbed to maintain electrical neutrality – leading to accumulation of bicarbonate in the bloodstream and the resulting alkalosis. On the other hand, when carbon dioxide levels in the blood increase, there is a rise in carbonic acid. A 1 mm Hg change in the PaCO₂ above or below 40 mmHg results in a 0.008 unit change in the pH in the opposite direction. If there was no alkalosis, we would expect the pH to be 7.4 – 24(0.008) = 7.2 (approximately). The entire problem might be avoided if the patient was not on a salt-restricted diet (and inadequate Cl in the diet). GI absorption would make up for losses, and alkalosis would be minimal since the kidneys would be able to continue to resorb chloride instead of bicarbonate.

Strictly in terms of his acid/base status, the patient is ready for the OR for this procedure. Attempting to correct his metabolic alkalosis prior to the operating room would take a long time and would almost certainly result in adverse effects (likely worsening of his cardiopulmonary status because of volume overload). During the case it would be wise to allow his CO₂ to remain moderately elevated to the degree it is at his baseline. Respiratory drive in a child like this is primarily based on the pH of the CSF in the midbrain around the respiratory centers. Normalization of the respiratory acidosis would leave this child very alkalotic and would almost certainly lead to respiratory depression.