This topic was actually brought up by a medical student who worked with me last week in the operating room. After seeing a unit of packed red blood cells hanging alongside a liter of normal saline on the IV pole, he asked how anesthesiologists know how much fluid to give to a patient. We went over how to create a fluid replacement plan.
There are three major components to a fluid plan:
FLUID DEFICIT
This refers to how much the patient is already behind in volume… primarily from being NPO for surgery. The classic 4-2-1 calculation (4 cc/kg for the first 10 kilograms, 2 cc/kg for the next 10 kilograms, and 1 cc/kg for each kilogram after) is used to calculate a patient’s maintenance requirement per hour. In a 70 kg patient, this would be ~ 110 cc/hr. Multiply this by the number of hours the patient has been NPO (let’s say 10 hours for simplicity). This means the patient is already 1100 cc (1.1 liters) behind BEFORE surgery.
50% of this will be replaced in the first hour of surgery with crystalloid, 25% in the second hour, and 25% in the third hour. Now you’re all caught up with your fluid deficit.
MAINTENANCE
Using the same calculation as above (4-2-1), replace the maintenance requirement each hour. In other words, our hypothetical patient would receive an additional 110 cc of crystalloid each hour of the surgery.
ONGOING LOSSES
I like to think of surgical losses in terms of “things you can see” and “things you can’t.” In longer cases, anesthesiologist’s routinely measure the estimated blood loss (EBL) by examining the surgical field, sponges, suction canisters, etc. Replace blood 1:1 with colloid or 3:1 with crystalloid. Monitor the urine output accordingly (0.5 cc/kg/hour at a minimum in adults).
Now there are insensible losses which are difficult to calculate. These are evaporative losses from the surgical site, fluid which has shifted (“third-spaced”) into new compartments as an inflammatory response, etc. Some estimates for this kind of fluid loss are as follows:
- Minimal tissue trauma (ex. herniorrhaphy): 2-4 cc/kg/hr
- Moderate tissue trauma (ex. cholecystectomy): 4-6 cc/kg/hr
- Severe tissue trauma (ex. bowel resection): 6-8 cc/kg/hr
Replace this with crystalloid each hour. Overall, I try to err on the side of conservative fluid management (you can always give more), but by actually formulating a fluid plan, one can have a more systematic approach to resuscitation. In an ideal world, our fluid replacement strategy would try to match the tonicity and “type” of fluids which are lost (ie, replace blood with blood, replace gastric losses with gastric juice, etc.); however, we have to make do with our available resources. 🙂
