Question

James got into a car accident. When the ambulance arrived, James was sitting on the flood next to his car with blood covering him. His car was hit a truck who came wrong way and hit face-to-face. James looks dazed, and as the paramedics approach he says with a mixture of panic and relief, “I am covered with blood!” James is only semi-lucid as he babbles on about pushing out the broken glass in his car window. The paramedic reports quickly that the patient is bleeding from multiple right-arm cuts and especially deep laceration on the right upper arm. The paramedic stopped the bleeding and move him quickly to the ambulance. James systolic blood pressure is 80 mm Hg (low), diastolic is not audible (too low to hear). His heart rate is 122 bpm (very rapid), and skin is pale and clammy, indicating peripheral vasoconstriction (narrowing of his 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 his vein).

Up on their arrival to the hospital, a hematocrit HCT was ordered by Dr. Smith and it is low but normal. Several vials of James’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 his deepest, right-upper-arm laceration. Despite no further bleeding since the paramedics treated him at the scene, James had another HCT tested one hour after the original HCT, drops to below normal. Dr. Smith didn’t order a blood transfusion though. Aside from his present health problem, James is otherwise healthy. He is admitted to the hospital for overnight observation.

Hematocrit is the percentage of total blood volume that is comprised of RBCs. Calculate James HCT if PCV for the RBCs is 1.25ml and plasma volume is 3.75 ml

Answer 1

As it is stated on the question, Hematocrit is the percentage of RBC existing on the total blood volume, this tells us how concentrated or diluted is our blood. Lets remember that our blood is composed by serum, red blood cells (RBC's), white blood cells (WBC's) and platelets circulating in our veins.

When we have a high hematocrit value, this means we have a concentrated blood, which means, there is a high number of RBC's circulating in our blood, also called polycithemia. This can be caused by either conditions that cause dehydration or that cause an increased production of RBC's, like chronic hypoximia, smoking, among others.

When we have a low hematocrit value, this means we have a diluted blood, which means, there is a low number of RBC's circulating in our blood, which can be caused by either RBC's loss, death, or administration of high levels of water, like in this case IV solutions that caused James blood to dilute and therefore, a decrease on the hematocrit value.

James HCT value first decreased because of the loss of blood from the injuries, however, later paramedics stopped the bleeding, therefore, we should expect HCT values to remain stable, but because James started to receive IV solutions, this caused its blood to dilute and low the percentage of RBC's in his total blood volume.

HCT is a calculated volume, it is obtained by multiplying Hemoglobin levels 3 times. In this case we have the Packed Cell Volume (PCV) which is obtained by centrifuging blood in a small tube, which causes blood to separate into three layers according to : 1. RBCs, 2. WBCs and Platelets and 3. Serum or plasma.
This means 1.25 ml of the total plasma volume (3.75ml) is composed by red blodd cells.
This value is expressed in ml and it represents the HCT, however the as the HCT is a percentage value, we have to convert it into percentage, like this:

PCV = 1.25ml Plasma Volume = 3.75 ml (Our 100%)

1.25 x 100 = 125  
125 / 3.75 = 33.33%

Therefore HCT is 33.33%, which is low, as normal HCT values range from 42-52% in males and 37-47% in females.