CLOates

04-Nov-2006

OCCC APPM 1313

Guide for Calculations
with Body Surface Area (BSA)

Body surface area
in square meters (m^{2}) 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 m^{2}. A hypothetical problem in this form is:

1a) Order: 50 mg /m^{2} of Medication-X for a
child whose body surface area is 0.5 m^{2}.

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

Child’s dosage =
0.50 m^{2} x 50 mg / m^{2}

= (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 /m^{2} 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 m^{2}. Therefore,

Child’s dosage =
0.70 m^{2} x 50 mg / m^{2}

=
(0.70 x 50) mg

=
35 mg .

Alternatively, the
order might give only the child’s weight in a phrase like “50 mg /m^{2}
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 m^{2}. Then the calculation would look like this:

1c) Child’s
dosage = 0.90 m^{2} x 50
mg / m^{2}

=
(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/m^{2}; 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 m^{2}.

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

Child’s dosage =
Child’s BSA(in m^{2}) x
Adult dose / 1.7 m^{2}

=
0.50 m^{2} x 100 mg / 1.7 m^{2}

=
(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 m^{2}. Therefore,

Child’s dosage =
0.70 m^{2} x 100 mg / 1.7 m^{2}

=
(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 m^{2}. Then the calculation would look like this:

2c) Child’s
dosage = Child’s BSA(in m^{2}) x
Adult dose / 1.7 m^{2}

=
0.90 m^{2} x 100 mg / 1.7 m^{2}

=
(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 (m^{2}). The three scales outside the box are another
nomogram relating __both__ patient height __and__ weight to BSA in square
meters (m^{2}). Be especially
careful to use the correct units for height (cm or in) and weight (lb or kg)
when using this nomogram.