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“For those who are still using bicarb in your prime. There is some literature that states the occurrence of acidosis at the start of bypass is an iatrogenic event, that is due to the choices of asanguineous fluids in the prime and not due to hypotension or dilutional acidosis.
Therefore, if you switch your crystalloid solutions to balanced crystalloids, you will not need to add bicarbonate to your priming fluids. The center I had worked at in Halifax has been using balanced crystalloids for more than 30 years without the use of bicarb in their crystalloid only prime and we never had any issues with acidosis after going on bypass … verified after the first 10 minute blood gas.
If you use a synthetic colloid/crystalloid prime, that is another matter because all the colloids are acidic in nature (with the exception of Hextend).
Balanced crystalloids (Normosol R, Plasmalyte A, and Plasmalyte 148) are isotonic solutions that contain either sodium hydroxide or hydrochloric acid to normalize their pH. They also contain large amounts of unmeasured anions in the form of Acetate (27 mEq/L) and Gluconate (23 mEq/L) which create a SID value of 47 mEq/L. Crystalloids with a SID value > 24 mEq/L will lead to a progressive alkalotic state after the start of bypass.
Acetate is normally produced in the gut and as a by-product of tissue metabolism. The normal value for acetate in blood is between 0.01-0.07 mmol/L, and is quickly converted into acetyl co-enzyme A and eventually oxidized in the liver into CO2 and water.
Due to its high SID value *, balanced crystalloids have a tendency for acid base balance to move on the alkaline side of the pH scale without the addition of sodium bicarbonate during bypass.
They therefore appear to be the best choice for CPB priming solutions for most CPB cases because they avoid the development of acidosis at the start of bypass and do not require the addition of a buffer like bicarbonate.”
- Morgan TJ, Ventakesh B et al: Crystalloid strong ion difference determines metabolic acid base change during acute normovolaemic hemodilution – Intensive Care Med 2004; 30:1432-37
- Alston RP, Cormack L et al : Metabolic acidosis developing during cardiopulmonary bypass is related to a decrease in strong ion difference – Perfusion 2004, May;19(3):145-52
- Liskaser F, Story DA et al: Effect of pump prime on acidosis, strong ion difference and unmeasured ions during cardiopulmonary bypass – Anesth Intensive Care September 2009;37(5):767-72
Gerard J Myers
Halifax, Nova Scotia
Acetate is a common additive to IV solutions. It is used as a bicarbonate substitute because it is stable in solution and is converted to bicarbonate in the liver. So, you can give a lot of acetate-containing solutions without worrying about dilution of the serum bicarbonate.
- One mEq of acetate is equivalent to one mEq of bicarbonate, so give it in the same mEq dose.
There are no side effects that I know of except the usual hypernatremia if you give a lot of it.
In adults, the conversion of acetate to bicarbonate takes 5-10 minutes at normothermia, so don’t expect to see the immediate buffering change like with bicarbonate use. Wait a little while to allow the conversion to take place. Of course, the colder the patient, the longer the conversion time because the liver chemistry is slowed by hypothermia. Also, patients with cirrhosis or some other liver disease may have a harder time with the conversion.
In children, particularly babies, the liver function is less efficient than an adult. The conversion in a baby may require 6 hours. So, the use of acetate in infants is not going to work very well or quickly. So, if your program does both adults and peds, save whatever sodium bicarbonate you have left for the peds and use the acetate in the adults.
Gary Grist RN CCP
Kansas City, Mo.
* SID Values