When we anticipate a large blood loss, we might calculate the “allowable blood loss” (ABL) – not the amount we “allow” the surgeon to lose, but rather the volume at which we would likely need to transfuse.
where we use the initial and minimum acceptable hematocrits, and the estimated blood volume (Table 3.5). Unfortunately, we struggle to determine when we have reached the ABL. We report the estimated blood loss by looking at the surgical field, checking the volume in the suction canisters (subtracting any irrigation used), and examining the surgical sponges (a soaked 4×4 holds about 15 mL blood, a soaked lap sponge, 150 mL). More accurate, but generally impractical measures include weighing the sponges or washing them out and checking the color of the effluent. In lieu of an accurate measure, we use hemodynamic clues as well as serial hemoglobin concentrations.
This brings up the common misconception that we can assess blood loss by checking the hematocrit or hemoglobin concentration. Unless the patient has been carefully hydrated back to “euvolemia” (normal total blood volume), this is not true! Only if an equal volume of some other fluid is added (either from the interstitial space, or by us) does the hematocrit fall. If left to nature’s device, it may take up to 2 days to reach steady state. Depending on the fluid, often much more than the actual blood loss must be given to account for the small percentage that actually remains intravascularly. The volume that escapes is not lost though; it replenishes the interstitial space that so generously donated fluid to the blood stream before treatment could be instituted. This replenishment is vital for the transport of oxygen between the blood and tissues. Massive hemorrhage and hypotension compromise oxygen delivery to tissues; to maintain cellular integrity, these cells resort to anaerobic metabolism, with a by-product of lactic acid. We often gauge our resuscitation by the severity of the lactic acidosis.