Yes, having Hyperkalemia would be the likely culprit in causing cardiac arrest in someone with DKA. Alluding to your other point about total body K stores, you correctly stated that DKA patients are actually K depleted. The main reason is that in DKA you spill your K stores into your urine, with the huge osmotic diuresis. Insuln related K uptake, and H+/K+ pumping are probably a small mechanism. You also must remember that a patient presenting to the ED in DKA will have low total body K (IC+EC) BEFORE any treatment with insulin. A general rule is if their K is <4.5 then you need to give put 10 meq K in the bag of NS because they can get hypokalemic in a hurry. If their K is 4.5.-6.5 do nothing until it gets below 4.5, and if K> 6.5 get an EKG and give Calcium. The K goes up initially due to H+/K+ pumping.
