CLOates

04-Nov-2006

OCCC APPM 1313

Guide for Calculations with Body Surface Area (BSA)

Body surface area in square meters (m2) is used in at least two different ways in pediatric medication calculations.  In what I’ll call Category 1 problems, the physician’s order contains a rate of administration of medication in terms of BSA in m2.  A hypothetical problem in this form is:

1a)     Order:  50 mg /m2 of Medication-X for a child whose body surface area is 0.5 m2.

How many mg of Medication-X should the child receive?

Child’s dosage    =    0.50 m2   x   50 mg / m2

=    (0.50 x 50) mg

=    25 mg .

Notice that this works in a way quite similar to the rate-per-kilogram of patient weight problems that we worked in Chapters 8 and 10.  There are at least two other variants of this Category 1 problem.  Instead of just handing us the patient’s body surface area, as in the above example, the order may give us only the information required to determine BSA from the nomogram in the text on page 229.  A hypothetical problem in this form is:

1b)     Order:  50 mg /m2 of Medication-X for a child whose height is 35 inches and whose weight is 40 lbs.

How many mg of Medication-X should the child receive?

This height and weight yields a body surface area of approximately 0.70 m2.   Therefore,

Child’s dosage    =    0.70 m2   x   50 mg / m2

=    (0.70 x 50) mg

=    35 mg .

Alternatively, the order might give only the child’s weight in a phrase like “50 mg /m2 of Medication-X  for a child with normal height for his/her weight of 52 lbs.”  In this case, we would use the boxed “For Children of Normal Height For Weight” nomogram on page 229 to find that the child’s estimated BSA is 0.90 m2.   Then the calculation would look like this:

1c)     Child’s dosage    =    0.90 m2   x   50 mg / m2

=    (0.90 x 50) mg

=    45 mg .

The other major category of BSA problems, let’s call it Category 2, specifies the adult dosage of the drug to be given and either the BSA or enough information to calculate the BSA.  When the adult dosage is specified, the drug has not been studied sufficiently in children to determine a dosage in mg/m2; therefore, it will be necessary to use the formula given in the text for approximating the child’s dosage.  A hypothetical order might look like this:

2a)     Order:  Medication-X whose adult dose is 100 mg for a child whose body surface area is 0.5 m2.

How many mg of Medication-X should the child receive?

Child’s dosage    =    Child’s BSA(in m2)   x   Adult dose / 1.7 m2

=    0.50 m2   x   100 mg / 1.7 m2

=    (0.50 x 100 / 1.7) mg

=    29.4 mg .

As with Category 1 BSA problems, instead of just handing us the patient’s body surface area, as in the above example, the order may give us only the information required to determine BSA from the nomogram in the text on page 229.  A hypothetical problem in this form is:

2b)    Order:  Medication-X whose adult dose is 100 mg for a child whose height is 35 inches and whose weight is 40 lbs.

How many mg of Medication-X should the child receive?

This height and weight yields a body surface area of approximately 0.70 m2.   Therefore,

Child’s dosage    =    0.70 m2   x   100 mg / 1.7 m2

=    (0.70 x 100 / 1.7) mg

=    41.1 mg .

As with Category 1 problems, the order might give only the child’s weight in a phrase like “Medication-X whose adult dose is 100 mg for a child with normal height for his/her weight of 52 lbs.”  In this case, we would use the boxed “For Children of Normal Height For Weight” nomogram on page 229 to find that the child’s estimated BSA is 0.90 m2.   Then the calculation would look like this:

2c)     Child’s dosage    =    Child’s BSA(in m2)   x   Adult dose / 1.7 m2

=    0.90 m2   x   100 mg / 1.7 m2

=    (0.90 x 100 / 1.7) mg

=    52.9 mg .

Remember that the nomogram on page 229 is two nomograms on one page.  The boxed area is a nomogram that relates the weight in pounds of a child whose height is normal for his/her weight to BSA in square meters (m2).  The three scales outside the box are another nomogram relating both patient height and weight to BSA in square meters (m2).  Be especially careful to use the correct units for height (cm or in) and weight (lb or kg) when using this nomogram.