This chapter is relevant to Section I3(ii) of the 2017 CICM Primary Syllabus, which expects the exam candidates to "describe the function, distribution, regulation and physiological importance of sodium, chloride potassium, magnesium, calcium and phosphate ions". It is only relevant because Question 4 from the first paper of 2011 asked the trainees to compare calcium chloride and calcium gluconate.
The 10% calcium gluconate injection contains 1 g of active ingredient per 10ml vial. In these 10mls of water, each divalent molecule of calcium is wed to two molecules of gluconate ion. This gluconate ion is the conjugate base of gluconic acid of which much is made of in another chapter. Owing to the huge fat conjugate base, the calcium gluconate molecule has a molar mass of 430.373 g per mol. This is why calcium chloride ampules have more calcium in them. The excess baggage is the reason why the calcium chloride ampoule (which also contains 1g of active ingredient per 10ml) actually has more calcium than the gluconate.
What does it matter, which calcium you reach for? Typically, there is no calcium chloride available where you work. Only in the enlightened ICU can one find this rare stuff. Is there a difference between calcium chloride and calcium gluconate, in terms of ionic calcium availability?
Well. In the dark ages, the prevailing opinion was that calcium chloride is more immediately bioavailable. Writing in 1984, Heining et al noted:
"Several standard textbooks of anaesthesia and resuscitation state that calcium chloride is preferable to calcium gluconate injection because it causes a more rapid rise in plasma cCa2+"
This is because it was thought that at physiological pH some of the calcium gluconate (sources disagree as to how much) remains in a non-ionised state, as if crystals of calcium gluconate circulate around and wait for hepatocytes to crack them. The pKa of calcium gluconate (according to this industrial materials safety data sheet) is actually around 3.7, so its dissociation in the body fluids should be near-complete. In spite of this, it has been a long-held belief that the liver needs to somehow metabolise the gluconate in order for the calcium to become bioavailable.
Thankfully, here is a study of patients without a liver to whom both calcium chloride and calcium gluconate were administered. It has demonstrated that, contrary to popular belief, you do not need a liver to derive rapid and maximal benefit from calcium gluconate.
So then, what properties unique to this salt of calcium could one mention when they discuss the differences between calcium chloride and calcium gluconate?