An ambulance arrives at the scene of an automobile accident, having been summoned by an in-vehicle security system. What the emergency personnel find is like a scene from a horror film. Maggie Silvers, the apparent driver of the car, is sitting, slumped next to the vehicle, with blood covering her shirt and hands. Her car has clearly hit a tree: a branch is sticking into the driver's window, and the airbag has been deployed. Maggie looks dazed, and as the paramedics approach she says with a mixture of panic and relief, "There's blood everywhere!" Maggie is only semi-lucid as she babbles on about pushing out the broken glass in her car window.

Maggie, a 48-year-old woman, is, indeed, bleeding profusely from multiple left-arm cuts and an especially deep laceration on her left upper arm. The paramedics stop the bleeding and move her quickly to the ambulance, after noting no other apparent injury. Her systolic blood pressure is 80 mm Hg (low), and her diastolic is not audible (too low to hear). Her heart rate is 122 bpm (very rapid), and her skin is pale and clammy, indicating peripheral vasoconstriction (narrowing of her blood vessels, particularly in the skin) and circulatory shock-like signs. On the way to the hospital, a paramedic begins transfusing normal saline solution (NSS; water with some NaCl, similar to body fluids, given directly into her vein).

A fast hematocrit (HCT) test upon Maggie's arrival to the emergency department (ED) indicates that her HCT is low, but normal. Several vials of Maggie's blood are also sent to the lab for blood tests and typing. Two liters of NSS are transfused over the next hour while the ED physician sutures her deepest, left-upper-arm laceration. Despite no further bleeding since the paramedics treated her at the scene, Maggie's next HCT, tested one hour after the original HCT, drops to below normal. Aside from her present health problem, Maggie is otherwise healthy. She is admitted to the hospital for overnight observation.

Short Answer Questions

1. The "fast hematocrit" involves withdrawing a very small amount of blood via a finger prick into a thin capillary tube, spinning the sample in a centrifuge so that it separates into its components, and then measuring the components. In Maggie's case, the total blood volume in the capillary tube is 20 mm, the packed cell volume (red blood cells) is 7.1 mm, and the plasma portion measures 12.9 mm. Calculate Maggie's first hematocrit.
2. In the ED, blood is withdrawn from the vein and into a test tube. The packed cell volume (RBCs) is 1.45 ml, and the plasma volume is 3.55 ml. Calculate Maggie's hematocrit in the ED.
3. Explain why the HCT drops despite no further loss of blood.
4. Why do you think paramedics give normal saline solution (NSS) and not blood in the ambulance?
5. Why might a physician be reluctant to order a blood transfusion for Maggie, or for any patient for that matter, unless absolutely necessary?
6. Despite no blood transfusion, Maggie's hematocrit improves by the time she visits her physician for the removal of her sutures a week later. [See multiple choice question 3 for the calculation.] She is adequately hydrated. Explain the physiological mechanism for the improvement in her hematocrit.
7. Besides the HCT, what other component of blood could be measured to give a better understanding of oxygen-carrying capacity? Explain your answer.
8. Explain the relationship between Maggie's low blood pressure (when the paramedics first examine her) and her blood loss. How are her rapid heart rate and pale, clammy skin related to her low blood pressure?