In a patient with extensive burn injuries, which physiologic parameter is most directly reduced and contributes to the acute threat to life?

• A. Hematocrit
• B. Oxygen saturation
• C. Red blood cell count
• D. Blood plasma volume

Answer: (D)

In extensive burns, the immediate life-threatening change is a massive fluid shift from the bloodstream into damaged tissue and the interstitial space due to capillary leak and evaporative losses. This directly reduces circulating plasma volume, which lowers preload, decreases cardiac output, and leads to hypoperfusion of organs—the hallmark of burn shock. That is why plasma volume being reduced best explains the acute threat to life.

Hematocrit may appear elevated early because plasma is lost from the intravascular space, not because the blood cells are the primary problem. Red blood cell count may not drop acutely, and oxygen saturation depends more on lung function and oxygen uptake than on intravascular volume status in the very early phase.