Calculation of Body Surface Area (BSA) for Blood Volume

CHAPTER 25

Calculation of Body Surface Area, Circulating Blood Volume, Requirement of Blood Products

Namita Mishra, Sudha Rawat, Vishva Nath Sharma

BODY SURFACE AREA (BSA)

Body surface area (BSA) is the area of the external surface of the body, expressed in square meters (m²). In physiology and medicine, the body surface area is the measured or calculated surface of human body. It is used to calculate metabolic, electrolyte, nutritional requirements, drug dosage, and expected pulmonary function measurements. BSA is a measurement used in many medical tasks. For many clinical purposes BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal adipose mass. Nevertheless, there have been several important critiques of the use of BSA in determining the dosage of medications with a narrow therapeutic index like many chemotherapy medications.

USES OF THE BSA

• To gain an appreciation of the true required glomerular filtration rate (GFR) renal clearance is usually divided by the BSA.
• To calculate a better approximation of the required cardiac output as for example in children, cardiac index is used.

Cardiac output = Cardiac Index / BSA

• Chemotherapy is often dosed according to the patient’s BSA.
• Glucocorticoid dosing is also expressed in terms of BSA for calculating maintenance doses or to compare high dose use with maintenance requirement.

CALCULATION OF BSA

It is difficult to actually measure the surface area of the human body so various calculations have been published to arrive at the BSA without direct measurement.

• The most widely used is the Du Bois formula:

BSA = 0.007184 X W^0.425 X H^0.725

• A commonly used and simple one is the Mosteller formula:

0R

BSA = ( [ H X W]/ 3600)^1/2

Where

H = Height

W = weight

for example : Patient’s weight = 65 Kg

Patient’s height = 165 cm

BSA = ([65 X 165])/3600)^1/2

BSA= 1.72 m²

Recently, a weight-based formula was validated in the pediatric age group that does not include a square root, making it easier to use. It is [4Wkg+7]/[90+Wkg].

AVERAGE VALUES

Average BSA for various weights:

EFFECTIVE CIRCULATING VOLUME

Blood volume is the volume of blood (both red blood cells and plasma) in the circulatory system of any individual. A typical adult has a blood volume of approximately between 4.7 and 5 liters, with females generally having less blood volume than males. Blood volume (BV) can be calculated given the hematocrit (HCT; the fraction of blood that is red blood cells) and plasma volume (PV):

BV = PV/ (1-HCT)

Diagnostic technologies are commercially available to measure human blood volume. A recent radio nucleotide study called BVA (Blood Volume Analysis)-100, provides a measure of Red Blood Cells and Plasma with 98% accuracy.

BLOOD VOLUME ESTIMATION

CIRCULATING VOLUME OF THE CPB CIRCUIT

PRIMING VOLUME: the minimum amount of fluid (hemic or non hemic fluid) used to de- air the complete cardiopulmonary bypass (CPB) circuit is called priming volume or the circulating volume of CPB circuit. Priming of the CPB circuit is an important task for the perfusionist. Generally the main objectives of priming are:

• To deair the CPB circuit
• To check for any leaks in the circuit
• To check for any mistake in the assembling of the circuit
• To meet the need for the extra volume required to prime the CPB circuit as the patient’s blood volume is not sufficient enough to prime the CPB circuit.
• For achieving sufficient hemodilution.

It is a standard practice to use a non blood CPB prime because of the benefits of hemodilution and concerns about blood borne diseases. The total priming volume is determined by the hardware selected for the circuit to be employed. Following are the tables showing the volume required to de-air various oxygenators, arterial filters and tubing.

CPB CIRCUIT AND TOTAL PRIMING VOLUME WITH VARIOUS WEIGHT

GROUP

TUBING SIZE WITH VOLUME (ml/feet)

SPECIFIC CONSIDERATIONS:

In cases where patient is deeply cyanotic the size of the oxygenator and tubing size is selected keeping in mind the requirement of higher degree of hemodilution and higher requirement of arterial blood flows because of the presence of large (or major) aorto-pulmonary collaterals (MAPCA’s). MAPCAs arise from the aorta or its large branches and supply blood to the pulmonary arteries, because of blockade of the main pulmonary arteries. These MAPCAs ‘steal’ part of the cardiac output of the aorta and this results in reduced systemic perfusion and thus increased pump flows are required during CPB in cyanosed patients with MAPCAs to compensate for this ‘stolen’ cardiac output.

CACULATION OF BLOOD AND BLOOD PRODUCT REQUIREMENT

The hematocrit (HCT), also known as packed cell volume (PCV) or erythrocyte volume fraction (EVF) is the volume percentage (%) of red blood cells in blood. It is normally about 45% for men and 40% for women. It is considered an integral part of a person’s complete blood count along with hemoglobin concentration, white blood cell count, and platelet count. Haemoglobin concentration is reduced as a normal consequence of CPB with hemodilution. Thus the hematocrit that will result from the hemodilution caused due to priming volume of the CPB circuit should be determined. Several calculations are required to assess hemodilution and blood product requirements. To determine the effects of hemodilution, the volume concentration formula is used.

C1 X Pt BV = C2 X TV_{on CPB}

Where Pt BV = patient’s blood volume ( patient’s body weight X blood volume factor)

TV_{on CPB} = total volume on CPB (total priming volume + patient’s total blood volume)

C1 = Pre bypass hematocrit of the patient (%)

C2 = calculated hemodilutional hematocrit (%)

A decision must be made initially regarding the desired hematocrit during cardiopulmonary bypass. Based on the results of the randomized clinical study from Children’s Hospital, Boston ,it seems reasonable to consider a hematocrit of 25% to be the minimal acceptable hematocrit for any cardiopulmonary bypass condition. When the desired hematocrit has been selected the amount of bank blood that must be added to the prime should be calculated.

Prime RBC vol = {[C3]x[Pt BV + PV]} – {Pt RBC vol}

Where

Prime RBC vol = volume of blood required in prime

C3 = desired HCT on bypass

Pt BV = patient’s blood volume ( patient’s body weight X blood volume factor)

PV = total priming volume of the CPB circuit to be used

Pt RBC vol = patient’s blood volume X patient’s pre bypass hematocrit

For example:

Patient’s weight = 5 Kg

Pre bypass hematocrit (C1) = 40%

Patients blood volume (Pt BV) = 5 X 85 = 425 ml (85 is blood volume factor for 5 Kg)

PV (total priming volume of the CPB circuit to be used) = 600ml

TV_{On CPB} = (600 + 425) = 1025ml

Calculated hemodilutional HCT (%) (C2) = C1 X Pt BV / TV_{on CPB}

= 40 X 425 / 1025

= 16.5 %

16.5 is the hematocrit on bypass. If there is a certain desired hematocrit, then to achieve that hematocrit, the amount packed RBCs if needed for the same patient can be calculated as follows:

C3 (desired HCT) = 30 %

Pt BV = 425 ml

PV = 600 ml

TV _{On CPB} = (Pt BV + PV) = (425 + 600) = 1025 ml

Pt RBC vol = 425 X 0.40 = 170

Prime RBC vol = {[C3]X [Pt BV + PV]}-{Pt RBC vol} = {[0.30] X [1025]}-{170} = 137.5

Volume of RBCs needed in prime = 137.5

The hematocrit of packed RBCs is 70% thus 137.5/0.70 = 196 ml

196 ml of packed RBCs are needed to achieve a hematocrit of 30%.

Thus, 196 ml of the clear prime fluid is removed from the priming volume to account for the added packed RBCs. Therefore the calculation of priming volume now has 196 ml of packed RBCs and 404 ml of prime (crystalloid or colloid).

In some cyanotic cases where the patient’s pre bypass hematocrit is more, the blood is diluted to obtain an optimal hematocrit during cardiopulmonary bypass in order to decrease the viscosity of the blood to improve tissue perfusion and to prevent hemolysis. Thus the effect of priming fluid added to dilute the blood can also be calculated as:

TV_{on CPB} X C4 = TV_{on CPB} 1 X C5

WHERE

TV_{on CPB} = total volume on CPB (total priming volume + patient’s total blood volume) = 1025ml

C4 = Hematocrit (of cyanotic patient) on bypass = 0.60

TV_{on CPB}1 = total volume on CPB after adding 500 ml of priming fluid to the CPB circuit.

TV_{on CPB}1 = (1025 + 500) = 1525 ml

C5 = the new (affected) Hematocrit

Thus

C5 = (1025 X 0.60) / 1525 = 0.40

40 % is the new hematocrit achieved after adding 500 ml of priming fluid.

FIBRINOGEN

A critical consideration is plasma fibrinogen dilution. Normal plasma fibrinogen levels are 150-400 mg./dL. The infant/ pediatric patient’s relative low blood volume with priming requirements of the ECC circuit causes the fibrinogen concentration to be adversely diluted. During CPB, it is desirable to maintain the plasma fibrinogen concentration above 100 mg./dL. in order to prevent impairment of post-CPB hemostasis.

Given an example of a 5 Kilogram patient with blood volume of (5 x 85) 425 ml, pre bypass hematocrit of 55%, hematocrit on CPB of 25%, priming volume of 800ml of the circuit to be used for CPB and fibrinogen level of 275 mg/dl. To calculate the effect of priming, patient’s plasma volume is calculated by following formula:

BV = PV/ (1-HCT)

PV = (1-HCT) X BV

Thus PV = (1-0.55) X 425 = 191ml

PV = 191ml

Patient’s fibrinogen = 191 X 275 mg/100ml = 525 mg

Number of milligrams required = (425 + 800 ) X (1.00-0.25) = 9.19 dl

If the goal is 100mg/dl, then 919 mg of fibrinogen are needed.

Amount of fibrinogen to be added = 919 – 525 = 394mg.

394 mg of fibrinogen must be added to the prime to achieve a goal of 100 mg per dl. FFP usually contains 200 mg of fibrinogen per dl.

Thus ml of FPP needed = (394/200) X 100 = 197 ml.

Now for the calculation of priming volume the 197ml of the prime fluid (crystalloid or colloid) is replaced by FPP. Thus the clear prime volume becomes 603ml.

Suggested reading

1. Jianfeng Wang, Eiji Hihara. A unified formula for calculating body surface area of humans and animal. Eur j Appl Physiol.2004;92:13-17
2. Dill DB, Costill DL. Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J. Appl. Physiol. 1974; 37(2):247-248.
3. Tarazi RC .Pulmonary blood volume. Eur Heart J.1985Oct;6SupplC:43
4. Tarazi RC . Blood volume.. Eur Heart J.1985;6SupplC:41-42