[Now for the more egregious stuff. Chapter 10, p. 159.]
Calculation of Intravenous Flow Rates, Infusion Times, Infusion Rate, and Bolus Using
Dimensional Analysis
Once the infusion is started, the nurse is responsible for regulating the flow rate (i.e., the number
of drops per minute required to infuse the IV within the time period or infusion rate specified).
In calculating the flow rate for drops per minute, one minute becomes the labeled value
that must be converted to an equivalent value: number of drops. One minute, therefore, is the
starting factor and drops is the answer unit and these, as in all dimensional analysis conversions,
form an equivalent relationship.
It is essential in calculating flow rate to know the drop factor. This refers to the size of
the drop delivered by the particular infusion set being used: the number of drops required to
make 1 mL (cc). The drop factor always will be stated in practice problems requiring calculation
of drops per minute.
Rounding Off
The rate of flow should be rounded to the nearest whole number. [I'll omit the 4 examples given.]
Calculation of IV Flow Rate When Total Infusion Time is Specified
Order: 1000 mL of D5W (5% Dextrose in water) IV to infuse over a period of 5 hr
Drop Factor: 10 gtt/mL
Starting Factor
1 min
Answer Unit
gtt
Equivalents: 1000 mL = 5 hr, 10 gtt = 1 mL, 60 min = 1 hr
Conversion Equation:
1 min x
1 hr x 1000 mL x 10 gtt = 33.3 = 33 gtt
60 min
5 hr
1 mL
Flow Rate: 33 gtt/min
[Numerous examples in the text follow this same format, and the above is representative.]
[May I do this one? First, what do I want to know? The flow rate in gtt/min, which is my
answer unit, not just gtt (drops). What do I know? I'm given that there are 10 gtt/mL and that
the infusion rate is 1000 mL/5 hr. It means absolutely nothing to say that I know "1 min"
anymore than it would to say I know "1 hotdog." The "1 min" used as a starting factor is quite
spurious and makes no sense at all to me. Since I want gtt on top and 10 gtt/mL has gtt in the
right place, 10 gtt/mL makes a perfectly good starting factor--I just need to get from mL to min.
My set up then:
10 gtt x 1000 mL x 1 hr = 33 gtt
1 mL
5 hr
60 min
min
Just omitting the "1 min" from the text's set up would also work.
So, let's see, the text manages to state an incorrect answer unit, then introduces a spurious
starting factor, which makes the set up wrong, which yields 33 gtt for an answer, which is also
wrong. But through the magic of smoke and mirrors, through some sort of mental slight-ofmind, they finally come up with the correct answer, which they simply declare to be 33 gtt/min.
Did I miss anything?
I laid awake late one night trying to figure what these people could possibly be thinking. It
occurred to me that they were following an unstated rule: all starting factors must be single
numbers with only one unit attached. I paged through the book, and, sure enough, all example
problems followed this "rule," even if a meaningless starting factor had to be pulled out of
nowhere to obey it. Their reasoning, you probably noticed, was given in the second paragraph:
"In calculating the flow rate for drops per minute, one minute becomes the labeled value that
must be converted to an equivalent value: number of drops. One minute, therefore, is the starting
factor and drops is the answer unit and these, as in all dimensional analysis conversions, form an
equivalent relationship."
But this looks like seriously confused, sham reasoning to me. Maybe you can make some sense
out of it.
The other possibly is that I'm the one who's crazy. After all, in the three years this text has been
used at my school, nobody seems to have found anything to take exception to. Just recently a
student was asked to solve a test problem on the board and they did it just like the text did above-and I seemed to be the only one rolling his eyes. I desperately need a reality check here, and I'm
counting on you, Obe Wan. Is there some higher math I just don't know anything about? Or are
we talking innumeracy here?]
[I've mentioned other errors elsewhere, but I trust you've seen enough, so mercifully that's all I
have for you.]